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Asexual Reproduction

Table of Contents

What is Asexual Reproduction?

Characteristics of asexual reproduction, types of asexual reproduction, advantages of asexual reproduction, disadvantages of asexual reproduction, asexual reproduction examples, asexual reproduction definition.

“Asexual reproduction is the mode of reproduction that is involved in the production of offspring by a single parent.”

Asexual reproduction is a mode of reproduction in which a new offspring is produced by a single parent. The new individuals produced are genetically and physically identical to each other, i.e., they are the clones of their parents.

Asexual reproduction is observed in both multicellular and unicellular organisms. This process does not involve any kind of gamete fusion and there won’t be any change in the number of chromosomes either. It will inherit the same genes as the parent, except for some cases where there is a chance of a rare mutation occurring.

Also Read:  Reproduction

Following are the important features of asexual reproduction:

• Single parent involved.
• No fertilization or gamete formation takes place.
• This process of reproduction occurs in a very short time.
• The organisms multiply and grow rapidly.
• The offspring is genetically similar.

There are different types of asexual reproduction:

Binary Fission

Fragmentation, vegetative propagation.

• Sporogenesis

The term “fission” means “to divide”. During binary fission , the parent cell divides into two cells. The cell division patterns vary in different organisms, i.e., some are directional while others are non-directional. Amoeba and euglena exhibit binary fission.

It is one of the simplest and uncomplicated methods of asexual reproduction. The parent cell divides into two, each daughter cell carrying a nucleus of its own that is genetically identical to the parent. The cytoplasm also divides leading to two equal-sized daughter cells. The process repeats itself and the daughter cells grow and further divide.

Fragmentation is another mode of asexual reproduction exhibited by organisms such as spirogyra, planaria etc. The parent body divides into several fragments and each fragment develops into a new organism.

Also Read: Fragmentation

Regeneration

Regeneration is the power of growing a new organism from the lost body part. For eg., when a lizard loses its tail, a new tail grows. This is because the specialized cells present in the organism can differentiate and grow into a new individual. Organisms like hydra and planaria exhibit regeneration.

Budding is the process of producing an individual through the buds that develop on the parent body. Hydra is an organism that reproduces by budding. The bud derives nutrition and shelter from the parent organism and detaches once it is fully grown.

For More Information On Budding, Watch The Below Video:

Asexual reproduction in plants occurs through their vegetative parts such as leaves, roots, stems, and buds. This is called vegetative propagation. For example, potato tubers, runners/stolon, onion bulbs, etc., all reproduce through vegetative propagation .

Spore Formation

Spore formation is another means of asexual reproduction. During unfavourable conditions, the organism develops sac-like structures called sporangium that contain spores. When the conditions are favourable, the sporangium burst opens and spores are released that germinate to give rise to new organisms.

In asexual reproduction, a single cell is divided to produce offspring. Simple cell-by-cell division is not possible in multicellular organisms. Most of the multicellular organisms have a complex body design. They have a higher level of organization like tissues, organs and organ systems. Thus, they need a special mode for reproduction.

Also Read: Modes of Reproduction

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Following are the advantages of asexual reproduction:

• Mates are not required.
• The process of reproduction is rapid.
• An enormous number of organisms can be produced in very less time.
• Positive genetic influences pass on to successive generations.
• It occurs in various environments.

The major disadvantages of asexual reproduction are:

• Lack of diversity. Since the offsprings are genetically identical to the parent they are more susceptible to the same diseases and nutrient deficiencies as the parent. All the negative mutations persist for generations.
• Since only one organism is involved, the diversity among the organisms is limited.
• They are unable to adapt to the changing environment.
• A single change in the environment would eliminate the entire species.

Following are the examples of asexual reproduction:

• Bacterium undergoes binary fission in which the cell divides into two along with the nucleus.
• Blackworms or mud worms reproduce through fragmentation.
• Hydras reproduce through budding.
• Organisms such as copperheads undergo parthenogenesis.
• Sugarcane can be grown through vegetative propagation.

Also Read:   Sexual Reproduction

Frequently Asked Questions

What is asexual reproduction, what is the difference between sexual reproduction and asexual reproduction.

How sexual reproduction is advantageous over asexual reproduction?

Justify the statement ‘vegetative reproduction is also a type of asexual reproduction..

• Only one parent is involved.
• It doesn’t involve meiosis and fusion of gametes.
• The offspring are genetically similar to the parents.

What are the advantages of asexual reproduction?

• Rapid Populating: Asexual reproduction gives the ability to produce large quantities of offspring.
• No Mates Required: Asexual reproduction takes the need to find a mate away, allowing these organisms to multiply.
• In Case of Emergency: In dire situations, plants and organisms can keep themselves alive and produce others to help them without the help of a mate, or other reproductive sources. Plants are a great example of this. If no pollinator is available to pollinate, then they can clone by asexual reproduction.
• No True Investment: Asexual reproducers do not have to carry their offspring for a long amount of time and produce more than one at a time. This makes it a quick and inexpensive process for them in the terms of time.

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5 Types of Asexual Reproduction

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All living things must reproduce in order to pass down genes to the offspring and continue to ensure the survival of the species.  Natural selection , the mechanism for  evolution , chooses which traits are favorable adaptations for a given environment and which are unfavorable. Those individuals with undesirable traits will, theoretically, eventually be bred out of the population and only the individuals with the "good" traits will live long enough to reproduce and pass down those genes to the next generation.

There are two types of reproduction: sexual reproduction and asexual reproduction. Sexual reproduction requires both a male and a female gamete with different genetics to fuse during fertilization, therefore creating an offspring that is different from the parents. Asexual reproduction only requires a single parent that will pass down all of its genes to the offspring. This means there is no mixing of genes and the offspring is actually a clone of the parent (barring any sort of  mutations ).

Asexual reproduction is generally used in less complex species and is quite efficient. Not having to find a mate is advantageous and allows a parent to pass down all of its traits to the next generation. However, without diversity, natural selection cannot work and if there are no mutations to make more favorable traits, asexually reproducing species may not be able to survive a changing environment.

Binary Fission

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Almost all prokaryotes undergo a type of asexual reproduction called binary fission. Binary fission is very similar to the process of mitosis in eukaryotes. However, since there is no nucleus and the DNA in a prokaryote is usually just in a single ring, it is not as complex as mitosis. Binary fission starts with a single cell that copies its DNA and then splits into two identical cells.

This is a very fast and efficient way for bacteria and similar types of cells to create offspring. However, if a DNA mutation were to occur in the process, this could change the genetics of the offspring and they would no longer be identical clones. This is one way that variation can occur even though it is undergoing asexual reproduction. In fact, bacterial resistance to antibiotics is evidence for evolution through asexual reproduction.

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Another type of asexual reproduction is called budding. Budding is when a new organism, or the offspring, grows off the side of the adult through a part called a bud. The new baby will stay attached to the original adult until it reaches maturity at which point they break off and become its own independent organism. A single adult can have many buds and many offspring at the same time.

Both unicellular organisms, like yeast, and multicellular organisms, like hydra, can undergo budding. Again, the offspring are clones of the parent unless some sort of mutation happens during the copying of the DNA or cell reproduction.

Fragmentation

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Some species are designed to have many viable parts that can live independently all found on one individual. These types species can undergo a type of asexual reproduction known as fragmentation. Fragmentation happens when a piece of an individual breaks off and a brand new organism forms around that broken piece. The original organism also regenerates the piece that broke off. The piece may be broken off naturally or could be broken off during an injury or other life threatening situation.

The most well known species that undergoes fragmentation is the starfish, or sea star. Sea stars can have any of their five arms broken off and then regenerated into offspring. This is mostly due to their radial symmetry. They have a central nerve ring in the middle that branches out into five rays, or arms. Each arm has all the parts necessary to create a whole new individual through fragmentation. Sponges, some flatworms, and certain types of fungi can also undergo fragmentation.

Parthenogenesis

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The more complex the species, the more likely they are to undergo sexual reproduction as opposed to asexual reproduction. However, there are some complex animals and plants that can reproduce via parthenogenesis when necessary. This is not the preferred method of reproduction for most of these species, but it may become the only way to reproduce for some of them for various reasons.

Parthenogenesis is when an offspring comes from an unfertilized egg. Lack of available partners, an immediate threat on the female's life, or other such trauma may result in parthenogenesis being necessary to continue the species. This is not ideal, of course, because it will only produce female offspring since the baby will be a clone of the mother. That will not fix the issue of lack of mates or carrying on the species for an indefinite period of time.

Some animals that can undergo parthenogenesis include insects like bees and grasshoppers, lizards such as the komodo dragon, and very rarely in birds.

USDA Forest Service Pacific Southwest Research Station/Wikimedia Commons/CC BY 2.5

Many plants and fungi use spores as a means of asexual reproduction. These types of organisms undergo a life cycle called alternation of generations where they have different parts of their lives in which they are mostly diploid or mostly haploid cells. During the diploid phase, they are called sporophytes and produce diploid spores they use for asexual reproduction. Species that form spores do not need a mate or fertilization to occur in order to produce offspring. Just like all other types of asexual reproduction, the offspring of organisms that reproduce using spores are clones of the parent.

Examples of organisms that produce spores include mushrooms and ferns.

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Asexual Reproduction

Reviewed by: BD Editors

Asexual Reproduction Definition

Asexual reproduction occurs when an organism makes more of itself without exchanging genetic information with another organism through sex.

In sexually reproducing organisms, the genomes of two parents are combined to create offspring with unique genetic profiles. This is beneficial to the population because genetically diverse populations have a higher chance of withstanding survival challenges such as disease and environmental changes.

Asexually reproducing organisms can suffer a dangerous lack of diversity – but they can also reproduce faster than sexually reproducing organisms, and a single individual can found a new population without the need for a mate.

Some organisms that practice asexual reproduction can exchange genetic information to promote diversity using forms of horizontal gene transfer such as bacteria who use plasmids to pass around small bits of DNA. However this method results in fewer unique genotypes than sexual reproduction.

Some species of plants, animals, and fungi are capable of both sexual and asexual reproduction, depending on the demands of the environment.

Asexual reproduction is practiced by most single-celled organisms including bacteria, archaebacteria, and protists. It is also practiced by some plants, animals, and fungi.

Advantages of Asexual Reproduction

Important advantages of asexual reproduction include:

1. Rapid population growth. This is especially useful for species whose survival strategy is to reproduce very fast.

Many species of bacteria, for example, can completely rebuild a population from just a single mutant individual in a matter of days if most members are wiped out by a virus.

2. No mate is needed to found a new population.

This is useful for species whose members may find themselves isolated, such as fungi that grow from wind-blown spores, plants that rely on pollinators for sexual reproduction, and animals inhabiting environments with low population density.

3. Lower resource investment. Asexual reproduction, which can often be accomplished just by having part of the parent organism split off and take on a life of its own, takes fewer resources than nurturing a new baby organism.

Many plants and sea creatures, for example, can simply cut a part of themselves off from the parent organism and have that part survive on its own.

Only offspring that are genetically identical to the parent can be produced in this way: nurturing the creation of a new organism whose tissue is different from the parents’ tissue takes more time, energy, and resources.

This ability to simply split in two is one reason why asexual reproduction is faster than sexual reproduction.

Disadvantages of Asexual Reproduction

The biggest disadvantage of asexual reproduction is lack of diversity. Because members of an asexually reproducing population are genetically identical except for rare mutants, they are all susceptible to the same diseases, nutrition deficits, and other types of environmental hardships.

The Irish Potato Famine was one example of the down side of asexual reproduction: Ireland’s potatoes, which had mainly reproduced through asexual reproduction, were all vulnerable when a potato-killing plague swept the island. As a result, almost all crops failed, and many people starved.

The near-extinction of the Gros-Michel banana is another example – one of two major cultivars of bananas, it became impossible to grow commercially in the 20th century after the emergence of a disease to which it was genetically vulnerable.

On the other hand, many species of bacteria actually take advantage of their high mutation rate to create some genetic diversity while using asexual reproduction to grow their colonies very rapidly. Bacteria have a higher rate of errors in copying genetic sequences, which sometimes leads to the creation of useful new traits even in the absence of sexual reproduction.

Types of Asexual Reproduction

There are many different ways to reproduce asexually. These include:

1. Binary fission . This method, in which a cell simply copies its DNA and then splits in two, giving a copy of its DNA to each “daughter cell,” is used by bacteria and archaebacteria.

2. Budding . Some organisms split off a small part of themselves to grow into a new organism. This is practiced by many plants and sea creatures, and some single-celled eukaryotes such as yeast.

3. Vegetative propagation . Much like budding, this process involves a plant growing a new shoot which is capable of becoming a whole new organism. Strawberries are an example of plants that reproduce using “runners,” which grow outward from a parent plant and later become separate, independent plants.

4. Sporogenesis . Sporogenesis is the production of reproductive cells, called spores, which can grow into a new organism.

Spores often use similar strategies to those of seeds. But unlike seeds, spores can be created without fertilization by a sexual partner. Spores are also more likely to spread autonomously, such as via wind, than to rely on other organisms such as animal carriers to spread.

5. Fragmentation . In fragmentation, a “parent” organism is split into multiple parts, each of which grows to become a complete, independent “offspring” organism. This process resembles budding and vegetative propagation, but with some differences.

For one, fragmentation may not be voluntary on the part of the “parent” organism. Earthworms and many plants and sea creatures are capable of regenerating whole organisms from fragments following injuries that split them into multiple pieces.

When fragmentation does occur voluntarily, the same parent organism may split into many roughly equal parts in order to form many offspring. This is different from the processes of budding and vegetative propagation, where an organism grows new parts which are small compared to the parent and which are intended to become offspring organisms.

6. Agamenogenesis . Agamenogenesis is the reproduction of normally sexual organisms without the need for fertilization. There are several ways in which this can happen.

In parthenogenesis, an unfertilized egg begins to develop into a new organism, which by necessity possesses only genes from its mother.

This occurs in a few species of all-female animals, and in females of some animal species when there are no males present to fertilize eggs.

In apomoxis, a normally sexually reproducing plant reproduces asexually, producing offspring that are identical to the parent plant, due to lack of availability of a male plant to fertilize female gametes.

In nucellar embryony, an embryo is formed from a parents’ own tissue without meiosis or the use of reproductive cells. This is primarily known to occur in citrus fruit, which may produce seeds in this way in the absence of male fertilization.

Examples of Asexual Reproduction

All bacteria reproduce through asexual reproduction, by splitting into two “daughter” cells that are genetically identical to their parents.

Some bacteria can undergo horizontal gene transfer – in which genetic material is passed “horizontally” from one organism to another, instead of “vertically” from parent to child. Because they have only one cell, bacteria are able to change their genetic material as mature organisms.

The process of genetic exchange between bacterial cells is sometimes referred to as “sex,” although it is performed to change the genotype of a mature bacterium, not as a means of reproduction.

Bacteria can afford to use this survival strategy because their extremely rapid reproduction makes harmful genetic mutations – such as copying errors or horizontal gene transfer gone wrong – inconsequential to the whole population. As long as a few individuals survive mutation and calamity, those individuals will be able to rebuild the bacterial population quickly.

This strategy of “reproduce fast, mutate often” is a major reason why bacteria are so quick to develop antibiotic resistance. They have also been seen to “invent” whole new biochemistries in the lab, such as one species of bacteria that spontaneously acquired the ability to perform anaerobic respiration.

This strategy would not work well for an organism that invests highly in the survival of individuals, such as multicellular organisms.

Slime Molds

Slime molds are a fascinating organism that sometimes behave like a multicellular organism, and sometimes behave like a colony of single-celled organisms.

Unlike animals, plants, and fungi, the cells in a slime mold are not bound together in a fixed shape and dependent on each other for survival. The cells that make up a slime mold are capable of living individually and may spread or separate when food is abundant, much like individuals in a colony of bacteria.

But slime mold cells are eukaryotic, and can display a high degree of cooperation to the point of creating a temporary extracellular matrix and a “body” which may become large and complex. Slime molds whose cells are working cooperatively can be mistaken for fungi, and can perform locomotion.

Slime molds can produce spores much like a fungus, and they can also reproduce through fragmentation. Environmental causes or injury may cause a slime mold to disperse into many parts, and units as small as a single cell may grow into a whole new slime mold colony/organism.

New Mexico Whiptail Lizards

This species of lizard was created by the hybridization of two neighboring species. Genetic incompatibility between the hybrid parents made it impossible for healthy males to be born: however, the female hybrids were capable of parthenogenesis, making them a reproductively independent population.

All New Mexico whiptail lizards are female. New members of the species can be created through hybridization of the parent species, or through parthenogenesis by female New Mexico whiptails.

Possibly as a remnant of their sexually reproducing past, New Mexico whiptail lizards do have a “mating” behavior which they must go through to reproduce. Members of this species are “mated with” by other members, and the lizard playing the female role will go onto lay eggs.

It is thought that the mating behavior stimulates ovulation, which can then result in a parthenogenic pregnancy. The lizard playing the “male” role in the courtship does not lay eggs.

Related Biology Terms

• Gamete – Sexual reproductive cells, which contain half of the parent organism’s genetic material.
• Reproductive strategy – A strategy that describes how a given population uses its resources to produce offspring.
• Sexual Reproduction – A means of reproduction in which the genetic material of two parents is combined to produce offspring with a unique genetic profile.

2. Which of the following events was NOT caused by low genetic diversity due to asexual reproduction? A. The Irish Potato Famine B. The disappearance of the Gros-Michel banana C. The Black Death in England D. A and B Answer to Question #2 C is correct. Europeans survived the Black Death in England, perhaps in part because of genetic diversity due to sexual reproduction.

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Asexual reproduction

Asexual reproduction eɪˈsɛkʃuəl ɹiːpɹəˈdʌkʃən Definition: In asexual reproduction, the organism is capable of reproducing an offspring in the absence of a mate.

Table of Contents

Asexual Reproduction Definition

What is asexual reproduction? Asexual reproduction is a type of reproduction that does not entail the union of sex cells or gametes. Unlike in sexual reproduction wherein male and female gametes unite to reproduce offspring, in asexual reproduction, this union is not necessary. The organism can reproduce in the absence of a mate which, in this case, produces offspring which is usually a clone of the parent. The different types of asexual reproduction are binary fission , budding , vegetative propagation , spore formation ( sporogenesis ), fragmentation , parthenogenesis , and apomixis . The organisms that reproduce through asexual means are bacteria, archaea, many plants, fungi, and certain animals.

Forum Question: How do you know if reproduction is asexual or sexual?   Best Answer !

Reproduction is one of the biological processes that are commonly carried out by an organism. In fact, the ability to reproduce is one of the major characteristics of a living thing. There are two major modes of reproduction, sexual and asexual .

Reproduction: Asexual vs. Sexual

As mentioned earlier, there are two modes of reproduction: (1) asexual and (2) sexual . Below is the table to show the main differences between the two.

Data Source: Maria Victoria Gonzaga of Biology Online

Advantages of Asexual Reproduction

In the asexuals, producing offspring is more quickly and relatively more straightforward than in the sexuals. That’s because only one participant is needed. There is no need to wait or search for a willing mate. It skips the courtship rituals as seen in higher forms of sexual animals. The organism can reproduce many offspring of its own kind in the absence of mating. Asexual reproduction, therefore, is less costly in terms of energy and time expenditure. It also gives the asexuals the advantage to colonize a habitat faster than the slowly-reproducing sexuals.

Look at the diagram below. It shows the “two-fold cost” of sexual reproduction (first described by the mathematician, John Maynard Smith) (Ref.1). In (a), the sexual population size remains the same with each generation if each individual were to contribute to the same number of offspring. In (b), the asexual population size doubled in size with each generation, implicating that the asexual population can grow at a faster rate than the sexual population. And while sexual reproduction necessitates males and females to expend time and energy to find each other and copulate, in asexual reproduction, this is not necessary.

Disadvantages of Asexual Reproduction

If asexual reproduction is less costly, less complicated, and faster, then why is sexual reproduction so prevalent among eukaryotes ? Researchers estimate that 99.9% of eukaryotes do it. (Ref. 2) And some eukaryotes capable of asexual reproduction will only resort to it if sexual reproduction has become less feasible. For instance, the female smalltooth sawfish ( Pristis pectinata ) in captivity have been shown to reproduce asexually possibly due to pressures of finding mates in a low population density. (Ref. 3)

In pure asexuals, the parent organism reproduces offspring that is a clone of itself. It becomes a disadvantage in the long run when the genetic diversity within the species is considered. It leads to low genetic variation. Unlike sexuals that incorporate recombination and segregation during meiosis and the union of the sex cells with unique genetic materials, pure asexuals do not go through these processes. And skipping meiotic events could mean less genetic diversity, and therefore, indicate a long-term evolutionary disadvantage.

For instance, the lone parent passes along the same genetic information to the clone. In the event that they have to deal with a sudden disturbance in their environment, e.g. a virulent disease , both of them may be similarly susceptible because they possess the same characteristics and genes. Or, both of them may be lacking the genes that could make them resistant or at least capable of withstanding the disease. As a result, they are at risk of getting wiped out by the disease. This makes sexual reproduction crucial in terms of increasing the odds of producing species with genes that enable them to become a better fit for a new environment. In the sexuals, higher genetic diversity is achieved through crossing over , independent assortment, and gamete fusion. Purely asexual parents can get new genetic material, for example, through mutation .

Forum Question: Could animal sperm fertilize a human egg ?    Featured Answer!

Types of Asexual Reproduction

What are the 7 types of asexual reproduction? The different types of asexual reproduction are as follows:

• binary fission
• vegetative propagation
• spore formation (sporogenesis)
• fragmentation
• parthenogenesis

Binary fission

Binary fission is a type of asexual reproduction wherein a cell divides to produce two identical cells. Each of these two cells has the potential to grow to the size of the original cell. See the diagram below.

The organisms that reproduce asexually through binary fission are the prokaryotes (bacteria and archaea) and certain protozoans. The diagram above shows the fundamental steps of binary fission in prokaryotes. In certain protozoans, binary fission can be of different types based on how the cell divides. For instance, it can be an irregular type, meaning the cell divides along any plane (as observed in certain amoeba). It can also be longitudinal, as exemplified in Euglena , transverse-type, as in Paramecium , or oblique-type, as in Ceratium .

Forum Question: Why aren’t bacteria taking over the world ?    Featured Answer!  

Budding reproduction refers to the formation of an outgrowth (or bud) from an organism that is capable of developing into a new individual. The outgrowth is genetically the same as the parent but relatively smaller. It may stay attached or eventually split off from the parent.

Budding is the mode of reproduction in certain bacteria, such as Caulobacter , Hyphomicrobium , and Stella spp., fungi ( Saccharomyces cerevisiae ), and certain asexual animals, such as hydra, corals, echinoderm larvae, and some acoel flatworms. (Ref.4) Refer to the figure below as an example of budding in hydra.

Vegetative propagation

Vegetative propagation is a form of asexual reproduction in plants. It is when a new plant emerges from vegetative parts, such as specialized stems, leaves, and roots. Then, they form their own root system and grow. This form of reproduction is used by horticulturists in propagating plants that are economically important. The process does not involve pollination. Rather, new plants are grown out of vegetative parts with a specialized reproductive function. There are many forms of vegetative propagation that can be classified into two major types: natural means and artificial means . Examples of natural means are those emerging from runners (stolons), bulbs, tubers, corms, suckers (root sprouts), and plantlets.

As for artificial means, examples are those that arise from cutting, grafting, layering, tissue culture, and offset.

Spore formation (sporogenesis)

Spore formation or sporogenesis is a form of asexual reproduction that involves spores . Spores, from “sporā” , meaning “seed” and “genesis” , meaning “birth” or “origin” , are dormant, reproductive cells that are similar to seeds by serving as dispersal units . The spores though aren’t seeds in a way that they lack the embryo produced by the fusion of male and female gametes. Spores are thick-walled and highly resistant to various unfavorable conditions, like high temperatures and low humidity. When the conditions are suitable they germinate to give rise to new individuals. Vascular plants and fungi are examples of asexual organisms that reproduce by spore formation. Below is a video of how mushrooms (fungi) propagate through spores.

Fragmentation

Fragmentation refers to the parent organism breaking into fragments and each fragment is capable of developing into a new organism. This is observed in fungi (e.g. yeasts, and lichens), molds, vascular and nonvascular plants, cyanobacteria, and animals (e.g. sponges, sea stars, planarians, and many annelid worms). This form of asexual reproduction in animals may also be not intentional. Human activity, predation, and other environmental factors may cause them to split into fragments. Below is a fascinating video showing how fragmentation works — from being a headless fragment can grow into a complete planarian.

Parthenogenesis

Parthenogenesis is an asexual reproduction wherein the offspring develops from a female gamete even without prior fertilization by a male gamete. The process may be apomictic or automictic .

• Apomictic parthenogenesis occurs when one in which the egg cells produced by mitosis do not undergo meiosis and may grow to maturity to directly give rise to embryos. The offspring will be clones of the parthenogenetic parent.
• In automictic parthenogenesis , the reproductive cells go through meiosis. Then, the mature egg cell can develop into an embryo also without prior fertilization by a sperm cell. This is a more complicated form of asexual reproduction. In some cases, the offspring are haploid whereas in other cases, the ploidy is restored by various means, e.g. by doubling the chromosomes, by the fusion of the first two blastomeres, or by the fusion of meiotic products. (Ref.5)

There are many animals that reproduce asexually through parthenogenesis. Examples of invertebrates capable of parthenogenesis are aphids, rotifers, and nematodes. Some vertebrates that can also reproduce parthenogenetically are certain lizards, snakes, birds, sharks, reptiles, and amphibians. Some of them reproduce by parthenogenesis either facultatively (i.e. they can also reproduce sexually) or obligately (i.e. they have no other means to reproduce but by parthenogenesis).

Plant Apomixis

Apomixis in plants refers to asexual reproduction without fertilization. In certain plants, such as bryophytes and certain ferns, the gametophyte may give rise to a sporophyte-looking offspring but with a ploidy level of a gametophyte. This is referred to as apogamy . Then, there is also an instance wherein their sporophyte may give rise to a gametophyte-looking offspring but with a ploidy level of a sporophyte. This, in turn, is called apospory . (Ref. 6)

In flowering plants, the seed production from unfertilized ovules is referred to as agamospermy . There are two major types: gametophytic apomixis and sporophytic apomixis . (Ref. 6)

• In gametophytic apomixis , the embryo arises from an unfertilized ovum from a gametophyte that came from a cell that did not complete meiosis. The major types of gametophytic apomixis are diplospory (where the megagametophyte arises from a cell of the archesporium) and apospory (wherein the megagametophyte arises from the other cell of the nucellus. (Ref. 6)
• In sporophytic apomixis (also called adventitious embryony or nucellar embryony ), the embryo arises not from a gametophyte but from the cells of the nucellus or of an integument . (Ref. 6)

Asexual Reproduction Examples

Many bacteria reproduce by binary fission. The parent bacterial cell produces two identical clone cells by first creating a copy of the DNA molecule. Then, this is followed by chromosome segregation wherein DNA is pulled apart toward the opposite poles of the dividing cell. The cell constricts at the equatorial plane (cytokinesis), separating the cellular contents into two new cells. The process is similar to mitosis in eukaryotes. However, there is no spindle apparatus involved. The duration varies between bacterial species. Escherichia coli , for example, reproduce typically about every 20 minutes at 37 °C. (Ref. 7)

Slime molds

When food is scarce and the conditions are not suitable, plasmodium slime molds produce stalked reproductive fruiting bodies (sporangia) that contain spores. At the apical portion of the sporangia, the cells undergo meiosis, producing haploid spores that are dispersed by wind. When the conditions become favorable again, e.g. proper moisture levels and temperatures, the spore germinates and releases a haploid cell. (The haploid cells are involved in the sexual phase of the plasmodium slime mold life cycle.)

Cellular slime molds also have asexual and sexual phases in their life cycles. However, when the conditions are not favorable, they come together as a pseudoplasmodium . They form a pseudoplasmodium because the cells remain distinct, each with a nucleus of its own. A real plasmodium in slime molds is a single mass of cytoplasm undivided by membranes and containing multiple nuclei. Nevertheless, both the cellular slime molds and plasmodium slime molds produce fruiting bodies. Some of the cellular slime molds in the colony form the stalk whereas the others form the sporangium where haploid spores are produced and released from. Each spore germinates into an individual amoeba-like cell. (Ref. 8)

New Mexico whiptail lizards

The New Mexico whiptails ( Aspidoscelis neomexicanus ) are lizards that are all females. They reproduce asexually by parthenogenesis by doubling the chromosomal number twice to restore diploidy . So to begin with, they produce eight copies of each chromosome. Thus, after two rounds of cell division, four daughter cells, each with two sets of chromosomes instead of just one. (Ref. 9)

Although they do not need a male mate, they still display mating behavior with other females. A female whiptail mounts another female whiptail. This pseudocopulation behavior seemingly promotes ovulation.

While other asexuals produce genetic clones, the New Mexico whiptails are still able to produce genetically-diverse offspring. How is that possible? That’s because they are facultatively parthenogenetic. They have a so-called “hybridization event” wherein females mate with males of another species. (Ref. 10)

Do you think humans are capable of reproducing asexually? Come and share with us what you think. Join our Forum: Advantages and Disadvantages of Asexual Reproduction

Is there a possibility that humans will naturally reproduce asexually?

Asexual reproduction is considered by many as the older or more ancient form of reproduction. The first organisms on Earth are the single-celled organisms that reproduce asexually, i.e. through binary fission and budding. Later, sexual reproduction emerged as a strategy that confers evolutionary benefits. For one, sexual reproduction appears to drive genetic diversity , which is essential in driving, in turn, the generation of “favorable” traits and characteristics that make species “fit” . Sexual reproduction, eventually, has become the dominant mode of reproduction of many complex, higher forms of multicellular, including humans.

Humans reproducing asexually by natural means is less likely. Parthenogenesis, for instance, would require genetic and biological capability for it to happen. At present, humans do not have a working biological machinery to capacitate the activation of an unfertilized egg developing into a zygote.

The human body has to have the ability to respond to stimuli, should there be one, that will activate and trigger the development of an unfertilized egg into a zygote, or the fusion of genetically similar gametes into a zygote. In humans and most mammals, certain genes are imprinted, meaning these genes will be tagged chemically (to indicate which parent they came from). Depending on how the mother’s and father’s genes interact or work together in regulating growth and development, some of the genes will be expressed while others will be silenced (or expressed differently).

In terms of artificial means, the idea of human cloning has been thought of many years ago. However, ethical, legal, and technical issues have to be addressed first before it ever becomes a reality. In 2022, though, a team of scientists reported that they were able to make a live lab rodent (a mammal) birth offspring from unfertilized eggs. One of the offspring grew to adulthood and successfully reproduced normally with a male. While this could open opportunities in agriculture, research, and medicine,  this finding is not deemed to replace sexual reproduction:

“I think there are people who will look at this and say, ‘Oh, is this going to replace reproduction? Get rid of men?’ No, it’s not,” — Marisa Bartolomei, a molecular biologist at the University of Pennsylvania (Source: Smithsonian Magazine )

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1. Smith, J. Maynard (1978). The Evolution of Sex. Cambridge University Press. ISBN 9780521293020. 2. Otto, S. P. (2008). Sexual Reproduction and the Evolution of Sex. Nature Education 1(1):182. https://www.nature.com/scitable/topicpage/sexual-reproduction-and-the-evolution-of-sex-824/ 3. Fields, A. T., Feldheim, K. A., Poulakis, G. R., & Chapman, D. D. (2015). Facultative parthenogenesis in a critically endangered wild vertebrate. Current Biology, 25(11), R446–R447. https://doi.org/10.1016/j.cub.2015.04.018 4. Budding Definition and Examples – Biology Online Dictionary. (2020, March 3). Biology Articles, Tutorials & Dictionary Online. https://www.biologyonline.com/dictionary/budding 5. Wikipedia Contributors. (2020, June 8). Parthenogenesis. Wikipedia; Wikimedia Foundation. https://en.wikipedia.org/wiki/Parthenogenesis#Automictic 6. Wikipedia Contributors. (2020, June 19). Apomixis. Wikipedia; Wikimedia Foundation. https://en.wikipedia.org/wiki/Apomixis 7. Sezonov, G.; Joseleau-Petit, D.; D’Ari, R. (28 September 2007). “Escherichia coli (E coli) Physiology in Luria-Bertani Broth”. Journal of Bacteriology. 189 (23): 8746–8749. doi:10.1128/JB.01368-07. PMC 2168924. 8. Chapter 17: Concept 17.3. (2020). Mtchs.Org. https://bodell.mtchs.org/OnlineBio/BIOCD/text/chapter17/concept17.3.html 9. Yong, E. (2010, February 21). Extra chromosomes allow all-female lizards to reproduce without males. Discover Magazine; Discover Magazine. https://www.discovermagazine.com/planet-earth/extra-chromosomes-allow-all-female-lizards-to-reproduce-without-males ‌10. How an Asexual Lizard Procreates Alone. (2016, October 19). Nationalgeographic.Com. https://www.nationalgeographic.com/magazine/2016/11/basic-instincts-whiptail-lizard-asexual-reproduction/

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Last updated on September 26th, 2023

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Asexual Reproduction: Types and Examples

Reproduction is the process of producing offspring. It is of two types; asexual and sexual reproduction. In sexual reproduction, two egg cells (dual parents) are fused to form a zygote, whereas in asexual reproduction single parent is only required to produce a new offspring.   

This type of asexual reproduction mostly takes place in unicellular organisms. The organisms that undergo asexual reproduction include; bacteria, Archea, plants, fungi, and certain animals. There are different types of asexual reproduction. These include; binary fission, budding, fragmentation, vegetative propagation, sporogenesis, parthenogenesis, and apomixis.

Table of Contents

Types of Asexual Reproduction with Examples

Binary fission.

Types of Binary Fission  

Fragmentation and Regeneration

Regeneration is regenerating or repairing any single part of an organism. Many organisms carry this kind of ability. It is common in lizards (resurgence of tail) and octopuses (regeneration of blood vessels and tails).  

Vegetative Propagation

Vegetative propagation is a type of asexual reproduction common in plants where a plant’s vegetative or non-reproductive part gives rise to new offspring. There are commonly two types of vegetative propagation; natural and artificial.

Sporogenesis

Sporogenesis is the type of asexual reproduction by the production of spores. This type of asexual reproduction occurs in some eukaryotic and prokaryotic organisms like plants , bacteria, fungi, and algae. The reproductive spores forms in the reproductive structure called sporangia which has a sporogenous cell that undergoes cell division to give rise to spores. 

Parthenogenesis

Types of parthenogenesis.

Natural parthenogenesis is of two types; complete and incomplete.

Artificial Parthenogenesis

Apomixis is the process of asexual reproduction in plants where seed formation occurs without meiosis and fertilization. The thus-formed seed then develops into the embryo. It is a substitution for sexual reproduction without involving nucleus and cell fusion. Apomixis is also termed agamospermy. It is classified into the following types: nonrecurrent, recurrent, vegetative, and adventive apomixis.  

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Microbe Notes

Asexual Reproduction: Features, Types, Examples

Reproduction is a biological process of producing offspring of a living organism. It is the basis for continuing life from generation to generation. It is seen in every living organism; from microorganisms to larger organisms including all plants and animals. Based on the number of parents involved, and the formation and fusion of gametes, there are two types of reproduction; sexual reproduction and asexual reproduction.

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What is Asexual Reproduction?

Asexual reproduction is the type of reproduction where offspring are produced without the fusion of male and female gametes. It is a reproduction process where only one individual makes its progeny without exchanging genetic material with another individual. Generally, it is considered a primitive mode of reproduction and usually occurs in microorganisms, including bacteria, fungi, and protozoa. All of the plants also have the capacity to asexually reproduce. However, only a few plants follow it as a sole mode of reproduction, and others usually show a sexual mode. In the kingdom Animalia , only a few invertebrates show asexual reproduction.

Asexual Reproduction Features

• There is no formation and fusion of male and female gametes. Hence, no male and female are required. 
• Only a single parent is involved in the process.
• There is no exchange in genetic material, hence genetically all the offspring are identical to their parent. 
• There is no chance of variation. Only some mutations may lead to evolutionary changes. Hence, evolutionarily, asexual reproduction is of no importance. 
• The generation time is usually short, and the process occurs in very less time with minimum expense of cellular energy. 
• Offspring mature very quickly and rapidly increase their population. 
• It is usually seen in unicellular organisms. Only a few invertebrates and plants also follow this mode of reproduction.
• Influenced comparatively more than in sexual reproduction by the environmental conditions and nutrition. 

Asexual Reproduction Types

Based on the mode of producing offspring, asexual reproduction is classified into several types, viz.:

Fission simply means division. It is a mode of asexual reproduction where unicellular organisms divide into two or more individuals. It is the most primitive and simple mode of reproduction, shown by unicellular organisms only like bacteria and protozoa. Here the genetic material (nucleus) splits into two or more parts. Then the cytoplasm also divides and new daughter cells are formed; each daughter cell with one nucleus. Depending upon the number of daughter cells produced from fission, it is further classified as binary fission and multiple fission. 

1. Binary Fission

It is the process of asexual reproduction where a single individual divides into two new individuals (daughter cells). Here, first, the DNA (nucleus) is replicated and divided into two, followed by the division of cytoplasm leading to the formation of two daughter cells. It is seen in bacteria, archaebacteria, and protozoa like Amoeba, Leishmania, Plasmodium, Paramecium , etc.  

It is further classified as simple binary fission, longitudinal binary fission, transverse binary fission, and oblique binary fission.

2. Multiple Fission

It is the process of asexual reproduction where a single individual divides into more than two individuals at the same time. In this process, first, the DNA (nucleus) duplicates into several copies (nuclei) (more than two), and each copy (nucleus) is surrounded by cytoplasm leading to the formation of multiple daughter cells.   It is seen in some protozoa like Entamoeba, Plasmodium , etc. some Myxomycetes , and some algae ( Siphonales, Acetabularia, etc.). 

It is a type of asexual reproduction where an outgrowth on the surface of the parent’s body is specialized and separated and developed into a new individual. It is also called “blastogenesis”. In this process, a mature parent produces an outgrowth called ‘bud’. This bud will detach and grow as a new individual (daughter organism).  It is seen in unicellular organisms like yeasts and fungi, certain protozoa, and some bacteria, and multicellular organisms like cnidarians ( Hydra ), jellyfish, flatworms, sea anemones, corals, plants, etc. Budding is further classified as internal and external budding. 

1. Internal Budding (Endodyogeny)

It is the process where two daughter cells are produced within a mother cell, which is then consumed by the daughter cells before their separation. It is usually seen in parasites like Toxoplasma, Frenkelia , etc. 

2. External Budding (Exodyogeny)

In this type, an external outgrowth develops on the surface of a parent organism which will detach on maturation and develop into a new individual. It is seen in yeasts, Hydra, sea anemones, etc.

C. Fragmentation

It is a type of asexual reproduction where the body of a mature organism is broken into several fragments, and each fragment will then subsequently grow into a new complete organism. It is a natural process but usually occurs as a result of some damage to a parent’s body. It is seen in some multicellular animals (like; starfish, Planaria, annelids including polychaetes and oligochaetes, turbellarians, etc.) and plants (like; Spirogyra, Liverwort, etc.). 

D. Regeneration

It is a type of asexual reproduction process where a detached part of an organism develops into a fully developed individual organism. Mostly it is used for restoring damaged body parts (as in many reptiles, amphibians, crayfish, etc.), but some detached parts can grow into a complete individual like in Echinoderms, Hydra , flatworms, etc.).  

E. Vegetative Propagation

It is a type of asexual reproduction shown by plants where a fragment of a plant grows into a new complete plant. The specialized reproductive part of the plant which grows into a new plant is called the vegetative propagule. It includes all the parts of the plant except seeds, fruits, and flowers. It usually occurs via stems, leaves, branches, tuber, roots, etc.  Almost all the plants have the capacity to reproduce by the vegetative propagation method, and plants that can’t produce healthy seeds follow this method as their sole mode of reproduction. Plants like bananas, bamboo, sugarcane, strawberry, rose, tulip, potato, etc. are grown by this method. 

F. Sporogenesis

It is a type of asexual reproduction where haploid spores are produced and developed into new individuals (offspring). It is also called “monogenesis ”. Spores are reproductive cells containing haploid chromosomes which can grow into a mature individual without fertilization. It is seen in many plants, algae, fungi, protozoa, and bacteria. It usually occurs in unfavorable environmental conditions, but in some plants, algae, and fungi, it is a part of their regular lifecycle.

G. Gemmulation

It is a type of asexual reproduction where a new individual is developed from a gemmule. A gemmule is an asexually produced internal cellular mass coated with tough dormant embryonic cells. It is produced in Porifera, freshwater sponges like Spongilla, and marine sponges like sea sponges, Ficulina ficus , etc.  Vegetative totipotent cells called archaeocytes modify and develops into gemmules. These gemmules release out when the sponge body dies, and develop into a new individual when subjected to suitable environmental conditions. 

H. Agamogenesis

It is a type of asexual reproduction where offspring is developed from a female gamete without the involvement of male gametes. In this type, the development of an unfertilized egg occurs leading to the formation of a new individual. It literally means “reproduction without fertilization”, and involves two types; parthenogenesis, and apomixis.

1. Parthenogenesis

It is a type of agamogenesis where an ovum, as a normal reproduction process, develops into an embryo and a mature individual. It is also called “ virgin birth ”. 

It is reported in over 2000 species, mainly in invertebrates (e.g., some bees, wasps, rotifers, few ant species, aphids, water fleas, etc.), and few vertebrates (turkeys, few shark species, some amphibians like frogs and salamanders, some reptiles like rock lizards, Komodo dragons, whiptails, pythons, boas, rattlesnakes, filesnakes, etc.). In plants, it is a part of the apomixis process.

It can be either obligate or facultative parthenogenesis. Some salamanders, geckos, aphids, etc., follow parthenogenesis as the only mode of reproduction. While, parthenogenesis in sharks snakes, Komodo dragons, bees, etc. are facultative types. 

Based on the type of cell division involved during the process, parthenogenesis is of two types;

Apomictic parthenogenesis ; here, egg cells are produced by mitotic division and develop directly into a diploid embryo. This leads to the production of offspring which are full clones of the mother. It is seen in aphids and plants. In plants, it is a component of apomixis. 

Automictic parthenogenesis ; here, egg cells undergo meiotic division before developing into a zygote. This leads to the formation of haploid individuals, but in some, the offspring are reestablished as diploid in several ways. Usually, the egg cell fuses with polar bodies to restore their chromosome number. This causes the production of offspring which are half clones of the mother. It is seen in ants, bees, wasps, amphibians, reptiles, etc.         

2. Apomixis  

It is a type of agamogenesis occurring in plants, where a sporophyte is formed without fertilization. It is seen in plants like hawthorn, blackberries, ferns, rose, meadow grass, dandelions, etc. It is also seen in normally sexually reproducing plants when there is no male plant nearby for pollination.    

Asexual Reproduction in Plants

Several species of plants naturally show asexual reproduction as an obligate or facultative mode of reproduction. Artificially, several economically important plant species are asexually reproduced to rapidly increase their number. Leaves, tubers, bulbs, stems, branches, buds, rhizomes, roots, spores, etc. are used for asexual reproduction in plants. 

A. Natural Methods

• Budding : e.g., in potato, banana, bamboo, sugarcane, apple, pear, cherry, etc., can grow from a bud.
• Vegetative Propagation : e.g. in Bryophyllum , strawberry, sugarcane, roses, banana, sweet potato, yam, onion, garlic, money plant, etc. are cultivated via this method.
• Sporulation: it is seen in fern, moss, liverwort, and algae.
• Fragmentation : it is seen in algae like Spirogyra .
• Apomixis : it is seen in hawthorn, blackberries, ferns, dandelion, etc.

B. Artificial Methods

• Micropropagation and Tissue Culture : these modern techniques include the use of plant cells and/or tissues for the production of a large number of offspring in a laboratory. It is based on the principle of vegetative totipotency of plant cells. Several flowers, ornamental plants, crops like bananas, strawberries, pineapple, etc, are grown by tissue culture technique.  
• Grafting : it is a technique of uniting two different related plants and growing them together as a single plant. It is commonly used in horticulture for cultivating fruits like pear, apple, cherry, almonds, oranges, etc. 
• Layering : it is a technique where a stem/branch of a plant is covered with soil and influenced for root formation. It is a traditional method used for propagating flowers and fruits like strawberry, raspberry, mango, lemon, camellia, etc. 
• Cutting : it is a process where cut pieces of plants are used for propagation. Plants like money plants, sugarcane, coleus, bamboo, rose plant, etc. are cultivated by this method.   

Asexual Reproduction in Animals  

Several species of animals, especially invertebrates, follow the asexual mode of reproduction. Mostly, it is used as a facultative mode of reproduction.  Common types of asexual reproduction in animals include:

• Fission : it is seen in unicellular animals i.e. protozoans like Paramecium, Entamoeba, Leishmania, Plasmodium, Euglena , etc. 
• Fragmentation : it is seen in starfish, Planaria , a few annelids, etc.
• Budding : it is seen in Hydra, Jellyfishes, sea anemones, etc.
• Regeneration : it is seen in crayfish, Echinoderms, Hydra , flatworms, etc.
• Gemmulation : it is seen in sponges. 
• Parthenogenesis : it is seen in wasps, bees, ants, aphids, some sharks, etc.

Asexual Reproduction Advantages

• It helps in rapid reproduction and increases population size. 
• The produced offspring mature within a short time. 
• There is no variation, hence the traits of the parents are preserved and inherited by the offspring. 
• There is comparatively less expense of cellular energy as there is no need for gametogenesis and fertilization. 
• A single parent can produce all the offspring. Hence there is no need for a mate. Also, there is no need for migration for the search for a mate and courtship display. 
• Plants that can’t produce healthy seeds, and require a longer time to sexually mature can be cultivated using the asexual reproduction method. 
• It is not affected by environmental stresses as in sexual reproduction. Also, it occurs as an alternative in case of emergency and harsh environmental conditions. 
• Helps in passing on and replicating the advantageous characteristics in a certain individual inserted by genetic engineering or mutation to a large number of offspring in a short time. 

Asexual Reproduction Disadvantages

• It does not include the exchange of genetic materials hence there is no chance of variation, and it hinders genetic diversity. 
• Harmful traits of parents will continue passing over to the offspring. It will cause the transfer of genetic diseases and other disorders. 
• As there is no evolution, there is the least chance of adaptation to changing environment and a high chance of extinction of the species. 
• There is a rapid increase in population size and intraspecific competition. 
• The offspring mature rapidly and usually have a short lifespan. 

Asexual Reproduction References

• What Is Reproduction? – Definition & Types Of Reproduction (byjus.com)
• Britannica, T. Editors of Encyclopaedia (2020, February 7). budding. Encyclopedia Britannica. https://www.britannica.com/science/budding-reproduction
• What is multiple fission? How does it occur in an organism? Explain briefly. Name one organism which exhibits this type of reproduction. (toppr.com)
• Budding: Definition, Types and Examples (collegedunia.com)
• Budding – Javatpoint
• Endodyogeny of Toxoplasma gondii. A morphological analysis. (cabdirect.org)
• Vegetative Propagation – Meaning, Types, Examples and FAQ (vedantu.com)
• Britannica, T. Editors of Encyclopaedia (2019, February 7). spore. Encyclopedia Britannica. https://www.britannica.com/science/spore-biology
• Regeneration- Types of Regeneration, Regeneration in Hydra (byjus.com)
• Gemmule – Structure formation and Sponge reproduction (byjus.com)
• Gemmules – Formation and Structure of Gemmules and Its Characteristics ( vedantu.com )
• What is the Difference Between Budding and Gemmule Formation | Compare the Difference Between Similar Terms
• Essay on Classification of Asexual Reproduction (preservearticles.com)
• Asexual Reproduction in animals: Features, Types, Advantages & Disadvantages (collegedunia.com)
• Asexual Reproduction in Animals – Examples, Advantages & Disadvantages (byjus.com)
• Two Types Of Reproduction – Asexual Reproduction And Sexual Reproduction (byjus.com)
• Asexual Reproduction – Javatpoint
• Asexual Reproduction – The Definitive Guide | Biology Dictionary
• Agamogenesis – definition of agamogenesis by The Free Dictionary
• Britannica, T. Editors of Encyclopaedia (2019, February 7). apomixis. Encyclopedia Britannica. https://www.britannica.com/science/apomixis
• Parthenogenesis Definition & Meaning – Merriam-Webster
• Parthenogenesis – Types And Significance Of Parthenogenesis (byjus.com)
• Parthenogenesis – an overview | ScienceDirect Topics
• Parthenogenesis Definition and Examples – Biology Online Dictionary
• Examples of plants that make spores and their characteristic (mammothmemory.net)
• Asexual Reproduction in Plants – Definition, Types, Methods, and FAQs (vedantu.com)
• What Is Asexual Reproduction in Plants? Modes, Examples, Diagrams (testbook.com)
• Asexual Reproduction In Plants- Types and Methods (byjus.com)

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Sexual versus Asexual Reproduction: Distinct Outcomes in Relative Abundance of Parthenogenetic Mealybugs following Recent Colonization

1 National Institute for Agro-Environmental Sciences, 3-1-3 Kannondai, Tsukuba, Ibaraki 305–8604, Japan

Ryoko T. Ichiki

2 Japan International Research Center for Agricultural Sciences, 1–1 Ohwashi, Tsukuba, Ibaraki 305–8686, Japan

Hirotaka Tanaka

3 Tottori Prefectural Museum, 2–124 Higashi-machi, Tottori, Tottori 680–0011, Japan

Daisuke Kageyama

4 National Institute of Agrobiological Sciences, 1–2 Ohwashi, Tsukuba, Ibaraki 305–8634, Japan

Conceived and designed the experiments: JT RTI HT. Performed the experiments: JT RTI. Analyzed the data: JT DK. Contributed reagents/materials/analysis tools: JT HT. Wrote the paper: JT RTI DK.

Associated Data

DNA sequence data are available from the DDBJ database with the accession numbers LC121493-LC121517. All other relevant data are within the paper.

Asexual reproduction, including parthenogenesis in which embryos develop within a female without fertilization, is assumed to confer advantages over sexual reproduction, which includes a “cost of males.” Sexual reproduction largely predominates in animals, however, indicating that this cost is outweighed by the genetic and/or ecological benefits of sexuality, including the acquisition of advantageous mutations occurring in different individuals and the elimination of deleterious mutations. But the evolution of sexual reproduction remains unclear, because we have limited examples that demonstrate the relative success of sexual lineages in the face of competition from asexual lineages in the same environment. Here we investigated a sympatric occurrence of sexual and asexual reproduction in the pineapple mealybug, Dysmicoccus brevipes . This pest invaded southwestern Japan, including Okinawa and Ishigaki Islands, in the 1930s in association with imported pineapple plants. Our recent censuses demonstrated that on Okinawa sexually reproducing individuals can coexist with and even dominate asexual individuals in the presence of habitat and resource competition, which is considered to be severe for this nearly immobile insect. Molecular phylogeny based on partial DNA sequences in the mitochondrial and nuclear genomes, as well as the endosymbiotic bacterial genome, revealed that the asexual lineage diverged from a common sexual ancestor in the relatively recent past. In contrast, only the asexual lineage exhibiting obligate apomictic thelytoky was discovered on Ishigaki. Co-existence of the two lineages cannot be explained by the results of laboratory experiments, which showed that the intrinsic rate of increase in the sexual lineage was not obviously superior to that of the asexual lineage. Differences in biotic and/or abiotic selective forces operating on the two islands might be the cause of this discrepancy. This biological system offers a unique opportunity to assess the relative success of sexual versus asexual lineages with an unusual morphology and life cycle.

Introduction

Asexual reproduction, in which offspring arise from a single female organism, occurs in a variety of eukaryotes including plants, fungi, and animals. It is assumed to confer some advantages over a sexual reproduction, in which individuals of two genders, females and males, must be involved but only females can give birth to new individuals [ 1 – 10 ]. Despite the assumption that there is a “cost of males” in sexual reproduction, this system largely predominates in animals, indicating that this cost is outweighed by the benefits of sexual reproduction [ 1 – 3 , 7 – 9 ]. Although a variety of theories have been proposed to explain the genetic and ecological fitness associated with sexual reproduction, the evolution of sexuality and reproductive systems remains as one of the major unresolved puzzles in biology [ 4 ]. In particular, there are few examples that demonstrate the success and persistence of sexual lineages in the face of competition from asexual lineages in natural environments [ 11 , 12 ].

Scale insects (Insecta: Hemiptera: Coccoidea), which include mealybugs (Pseudococcidae), are interesting for studies of the evolution and ecology of sexuality and asexuality, because this taxon exhibits various systems that control sex determination, sexual development, sex ratio, and mode of reproduction [ 13 , 14 ]. Such an extraordinary diversity of genetic systems is considered to be, at least partly, associated with their unusual morphology ( Fig 1 ) and life cycle [ 13 , 15 ]. Scale insects are plant sap feeders closely related to aphids and whiteflies and are characterized by their unusual shapes. Adult females show development with retention of juvenile physical characters (neoteny) and are relatively immobile, lacking wings and often even legs. They produce body-covering secretions that act like protective shells, and they can be long-lived (sometimes up to several months) [ 15 ]. In contrast, adult males are winged and mobile, but they are tiny and fragile and have a limited life span of a few days at most [ 15 , 16 ]. Such extreme sexual dimorphism exposes the immobile females to high levels of competition over mate resources represented by the fragile and short-lived males [ 17 ], which may lead to the evolution of reproductive systems that depend less on males [ 14 ]. In fact, males are either very rare or unknown in many scale insects, particularly in mealybugs: 10% of the species reproduce either partially or completely by parthenogenesis [ 18 ], a form of asexual reproduction in which embryos develop without fertilization.

Adult males and females exhibit completely different appearances and biology.

The pineapple mealybug, Dysmicoccus brevipes (Cockerell), is one such parthenogenetic species. It is a known vector of pineapple wilt–associated viruses, which severely reduce pineapple yields [ 19 ], and this species also attacks many other agricultural crops [ 20 , 21 ]. Dysmicoccus brevipes was first described in Jamaica and is apparently native to the New World [ 22 ], but it now has a cosmopolitan distribution associated with pineapple transportation and cultivation. Many previous studies [ 23 – 27 ] conducted in a variety of areas indicated that this species reproduces by obligate apomictic thelytokous parthenogenesis [ 18 ], although the presence of adult males has been reported [ 22 ]. At least on the Hawaiian Islands, only parthenogenetic females are found; males have yet to be discovered and are probably not present [ 22 ]. These findings suggest that both sexual and asexual reproductive systems are included in the same or a very closely related lineage, and thus D . brevipes may provide a good opportunity to examine the evolutionary consequences of competition between sexual and asexual lineages. However, little is known about the distributions of sexual and asexual D . brevipes lineages as well as their phylogenetic relationship.

Therefore, in the present study we first surveyed the occurrences and frequencies of sexual and asexual individuals in D . brevipes populations sampled on two islands in southwestern Japan. The sexual and asexual lineages co-exist on one of the islands, whereas only the asexual lineage is present in the other. We then measured their fecundity and growth rates under laboratory conditions to examine the basic elements of competition between the lineages. Finally, we attempted to elucidate their molecular phylogeny by using partial sequences of genomes of mitochondria, nuclei, and the primary endosymbiotic bacterium, Candidatus Tremblaya princeps, which is ubiquitously present in the cytoplasm of mealybug bacteriocytes and is maternally inherited [ 28 , 29 ]. Based on these results, we discussed the potential factors that may have influenced the maintenance and/or competitive elimination of sexual/asexual reproductive systems in this unique insect.

Materials and Methods

Gravid ovoviviparous females of D . brevipes were collected in pineapple fields of Okinawa Prefectural Agricultural Research Center (OPARC) on Okinawa Island (26.6°N, 128.0°E; Nago city, Okinawa Prefecture, Japan) and Ishigaki Island (24.4°N, 124.2°E; Ishigaki city, Okinawa Prefecture, Japan) with permissions and helps of Drs. S. Ohno, I. Yonaha, K. Yonamine, and C. Moromizato (OPARC). Each mealybug was collected from a different colony in order to avoid sampling bias. They were placed individually in a tight-sealed laboratory dish (5.5 cm diameter × 2.5 cm height) in a rearing room (16:8 light:dark; 23°C; 50% relative humidity), fed with a germinated broad bean plant, allowed to produce offspring for 1 week, and then soaked in ethanol and stored at –20°C for DNA extraction and sequencing, as described below. The offspring were transferred to a larger tight-sealed laboratory dish (9.0 cm diameter × 3.0 cm height) with germinated broad bean plants and reared as a brood. For sexual mealybugs, the adults that emerged were allowed to copulate for 1 day. Pregnant females were transferred to a new dish with fresh food.

Assessment of developmental and reproductive performances

Development times, pre-parturition durations, parturition durations, and numbers of offspring were assessed using laboratory-reared mealybugs under the conditions described above. For the assessment of development times, freshly born nymphs were transferred individually to a germinated broad bean plant placed in a tight-sealed dish and were monitored daily. Males made cocoons and became pupae after three molts, whereas females matured to adulthood after three molts without pupal metamorphosis. To assess reproductive performance parameters, a fresh adult female just after the final molt was placed individually in a tight-sealed dish and fed with a germinated broad bean plant. Each female of the sexual lineages was housed with one adult male that was randomly chosen from stock culture, and copulation was confirmed visually. Offspring borne by each female were counted every day until the female stopped parturition.

DNA extraction and sequencing

Total genomic DNA was extracted from the whole body of mealybug females using a DNeasy Blood and Tissue Kit (Qiagen, Tokyo, Japan). The mitochondrial cytochrome oxidase subunit I ( CO1 ) gene (ca. 1.4 kbp) and the endosymbiont bacterial RNA polymerase β subunit ( rpoB ; ca. 1.2 kbp) gene were subsequently amplified by PCR with Taq DNA polymerase (Takara Ex Taq; Takara Bio Inc., Shiga, Japan; 0.5 units in 20 μl of reaction mixture) and a set of primers (0.5 μM in final concentration), namely CO1 : 5′-tatttaatatttggattttgatcagg-3′ (forward) and 5′-caatgcatattattctgccatatt-3′ (reverse); rpoB : 5′-cgacgtggatgacatgagta-3′ (forward) and 5′-gatattctgcccacgatgat-3′ (reverse). We used a temperature profile of 35 cycles of 95°C for 1 min, 55°C for 1 min, and 72°C for 1 min. The PCR products were purified using a PCR Purification Kit (Qiagen) and served as a template for a direct sequencing reaction using a BigDye Terminator Kit ver. 3.1 (Applied Biosystems, Tokyo, Japan). The following internal sequencing primers were used in the sequencing reaction: CO1 : 5′-ttttaccaggttttggagctat-3′ (forward) and 5′-aaaaattttaattcttgttggaatagc-3′ (reverse); rpoB : 5′-gtacacccctcggaggtaat-3′ (forward) and 5′-gcttacgtggatagcagcat-3′ (reverse). The sequencing was performed using a genetic analyzer (ABI 3130; Applied Biosystems). In addition, the internal transcribed spacer between ribosomal RNA genes (ITS2, ca. 0.7 kbp) of the nuclear genome was partially amplified by the primers ITS-M-F ( 5′-ctcgtgaccaaagagtcctg-3′ ) and ITS-M-R ( 5′-tgcttaagttcagcgggtag-3′ ) [ 30 ] and sequenced. For comparisons, CO1 and rpoB were amplified and sequenced from the genomic DNA of other mealybug species: Planococcus citri , Planococcus minor , Planococcus kraunhiae , Pseudococcus comstocki , Pseudococcus cryptus , Crisicoccus matsumotoi , and Dysmicoccus neobrevipes . The DNA solutions and the voucher specimens are stored in the National Institute for Agro-Environmental Sciences (Tsukuba, Japan).

Phylogenetic analyses

The gene sequences were aligned using ClustalX software [ 31 ]. The final alignment was inspected and corrected manually, and only unambiguous nucleotide sites were used for analyses. Maximum likelihood trees with bootstrap values based on 1000 resamplings were constructed using TreeFinder [ 32 ]. ModelTest 3.7 [ 33 ] and PAUP* were used to determine appropriate evolutionary models by Akaike information criterion with a correction for finite sample sizes (AICc); the TIM + G model, TVM + I model, and TVM + G model were selected for the tree searches of CO1 , rpoB , and ITS2, respectively.

Reproductive systems of the pineapple mealybug

Two completely different reproductive systems were observed in the pineapple mealybugs collected on Okinawa: 59 wild females produced both male and female offspring (29.3% female offspring on average), whereas 24 females produced only female offspring with no males ( Fig 2 ). Females from the female-only broods produced only female offspring without copulation, indicating thelytokous parthenogenesis. Females from the broods with both sexes were able to produce both male and female offspring only after copulation and never produced offspring without copulation, indicating sexual reproduction. No copulations were observed in pairs of a male and a female of an asexual brood (>10 pairs examined), whereas copulation was observed within a few minutes when a male was placed with a virgin female of a sexual brood. As far as we observed, both of the reproductive systems were maternally hereditable and completely obligatory. The sex ratios of offspring in some matrilines are shown in S1 Fig .

Co-existence of sexually and asexually reproducing lineages

Both sexually and asexually reproducing individuals were consistently observed in three censuses in a pineapple field on Okinawa; the proportion accounted for by the sexual lineage was always predominant (60–85%) and, in total, significantly deviated from the expectation of 1:1 co-existence of the two lineages ( χ 2 = 14.8, df = 1, P < 0.001; Fig 2 ). On the other hand, only an asexual lineage was found in a pineapple field on Ishigaki; the 103 matrilines collected during three censuses were examined but no sexual reproduction was observed.

Performances with respect to the development of the two lineages of mealybugs from Okinawa were generally similar except for the sex ratios in their offspring. However, the pre-parturition duration (i.e., the time until the adult females started producing nymphs) was significantly shorter in the sexual lineage (Mann-Whitney U -test, P < 0.001; Table 1 ). The detailed data for developmental and reproductive performances are shown in S1 Table .

Molecular characterization and phylogeny of sexual and asexual lineages

Partial sequences of mitochondrial and nuclear genomes, as well as the primary endosymbiotic bacterial genome, were examined individually in 40 mealybugs collected from Okinawa (32 sexual females and 8 asexual females) and 45 mealybugs collected from Ishigaki (all asexual).

Six haplotypes were found based on the mitochondrial CO1 sequences (1449 bp). These haplotypes were clearly divided into two groups corresponding to the sexual lineage and the asexual lineage ( Fig 3a ). Among the haplotypes of the sexual lineage, haplotype A1 was predominant (87.5%). In the asexual lineage, haplotype P1 was similarly predominant both on Okinawa (87.5%) and Ishigaki (66.7%). Each haplotype represented a group that was the most closely related among the CO1 sequences of all mealybug species examined and clustered into clades with high bootstrap values ( Fig 3a ). A total of 35 nucleotide sites were substituted (2.4%; 16 transitions and 19 transversions involving eight amino acid substitutions) between haplotypes A1 and P1.

The trees were constructed by the maximum likelihood method using unambiguously aligned nucleotide sites. The trees of rpoB and ITS2 are rooted on each midpoint. The bootstrap values (>50%) obtained from 1000 resamplings are given at the nodes. Sequences of the taxa with the DDBJ/EMBL/GenBank accession numbers in brackets were obtained from the database.

Partial sequences of a single-copy protein-coding gene ( rpoB ; 1350 bp) of the primary endosymbiont, Candidatus Tremblaya princeps, harbored in the cytoplasm of mealybugs were very similar in the two lineages, but one nucleotide mutation involving an amino acid substitution was found. No differences were discovered in the sequences of the samples examined within each lineage. Like the CO1 sequences, the rpoB sequence-based phylogenetic tree showed that the two lineages of D . brevipes were closely allied and formed a reliable clade ( Fig 3b ).

The ITS2 region in the mealybug nuclear genome was successfully determined for 36 and 9 individuals collected from Okinawa and Ishigaki, respectively. Sequences of the PCR products from the other samples could not be determined by direct sequencing, probably because of intra-individual variations of the ITS2 region. The ITS2 sequences also included a fixed difference: one sequence was obtained from the samples of the sexual lineage (688 bp) and two sequences from the asexual lineage (692 bp). The two sequences of the asexual lineage had only one transition mutation, whereas two gaps and 14 nucleotide substitutions (7 transitions and 7 transversions) were found between the sexual lineage and the asexual lineage. Phylogenetic analysis of the ITS2 sequences of the genus Dysmicoccus using the maximum likelihood method demonstrated that the two lineages of D . brevipes were the most closely related of any of the mealybugs examined and formed a monophyletic clade ( Fig 3c ).

In the CO1 tree, we used the sequence from Phenacoccus solenopsis , which belongs to a different subfamily (Phenacoccinae), as the outgroup to show the phylogenetic position of D . brevipes (Pseudococcinae) among mealybug species. No reliable alignment results were generated in analyses of rpoB and ITS2 when sequences of Phenacoccinae samples were used, and therefore the trees rooted on each midpoint were shown to demonstrate only the phylogenetic relationship of the two lineages of D . brevipes . The partial DNA sequences determined in the present study were deposited in the DDBJ database with the accession numbers LC121493-LC121517.

The present study demonstrated that distinct reproductive modes—sexual and asexual—coexist in a local, non-native population of the pineapple mealybug, D . brevipes (Figs ​ (Figs1 1 and ​ and2). 2 ). The mealybugs with the distinct reproductive modes are behaviorally and genetically isolated and are likely to represent diverged lineages, although there are no morphological diagnostic features. The molecular genotyping data support this idea. DNA sequences of mitochondrial and nuclear genomes as well as the endosymbiotic bacterial genome were closely related between the lineages, but clearly separated based on the reproductive mode of the mealybug ( Fig 3 ). This study offers a unique example to investigate the evolution and ecology of sexual and asexual reproduction.

Several haplotypes with less than 0.5% nucleotide substitutions of the mitochondrial CO1 gene were discovered in each lineage, although the haplotypes A1 and P1 were predominant in the sexual and asexual lineages, respectively ( Fig 3a ). Based on the previously reported pairwise divergence rate of CO1 sequences in several arthropod groups, which was found to cluster approximately 1.5% per million years [ 34 , 35 ], the CO1 genes of the two lineages are estimated to have diverged about 1.3 million years ago. The difference between the mitochondrial DNA sequences of the two lineages was relatively small compared to intraspecific variations in some insect taxa [ 36 ], suggesting that the lineages are very closely related, although they are reproductively isolated and genetically different. This finding is consistent with the partial sequences of ITS2 in the nuclear genome, with its higher evolutionary rate, and the endosymbiotic bacterial genome ( rpoB ); the two lineages were the most closely related and formed a monophyletic group in the genus Dysmicoccus ( Fig 3 ).

Other members of the genus Dysmicoccus employ sexual reproduction [ 37 , 38 ] and obligate thelytokous parthenogenesis is reported only in D . brevipes in this genus [ 18 ], indicating that the common ancestor of the two lineages of D . brevipes would have reproduced sexually. Scale insects display a remarkable variety of genetic systems for sex determination and reproduction including parthenogenesis [ 13 ], and the ancestral system in mealybugs is assumed to have been sexual reproduction with males, the paternally derived genome of which is deactivated through heterochromatinization (paternal genome elimination: PGE) [ 39 ]. Two types of parthenogenetic reproduction, obligate apomictic thelytoky and obligate automictic thelytoky, are found in mealybugs and these systems are likely to have evolved independently multiple times from the PGE system [ 18 ]. In the PGE system with sexual reproduction, paternal genomes are not included during spermatogenesis and thus are not inherited, and therefore a female-biased sex ratio in offspring is favored by a male parent [ 13 , 14 ]. Moreover, female scale insects are more robust than males with respect to physiology and morphology [ 15 ]. In addition, females benefit more than males from mutualistic relationships with ants, which care for and guard females in exchange for payment of honeydew [ 13 , 15 ]. These features could potentially drive mealybugs to evolve parthenogenetic reproduction that does not depend on males.

It is therefore difficult to understand how sexual reproduction with the PGE genomic system has been stable and prevalent in scale insects, including some lineages of D . brevipes despite strong selective pressures against fragile males. The present case on Okinawa clearly shows that the sexual lineage can simultaneously and sympatrically exist with and even dominate the asexual lineage ( Fig 2 ) in the face of habitat and resource competition, which is considered to be severe for the nearly immobile scale insects. This suggests that advantages offered by sexual reproduction compensate for the cost of males on Okinawa. One of the advantages may be associated with a significantly shorter pre-parturition period in the sexual lineage, although it does not directly lead to better intrinsic population growth rate, because only a portion of the offspring (≈46%) are female and thus produce offspring ( Table 1 ). Copulation can be a trigger for ovarian maturation and oviposition in insects [ 40 – 42 ], which may explain the difference between pre-parturition duration in the two reproductive systems. Advanced parturition would reduce the risk of female mealybugs encountering natural enemies or biotic/abiotic accidents, leading to an increased probability of reproductive success under natural conditions. Because our assessment of developmental and reproductive performances made use of a substituted diet (germinated broad bean plants) under laboratory conditions, we may have underestimated the differences between the lineages in nature. Moreover, the two lineages may have diverged enough to have some different ecological niches, although they occur simultaneously and sympatrically in pineapple fields. Further comparisons and investigations of various host plants including pineapples and other crops are necessary to elucidate their degree of competition.

The two reproductive lineages co-exist in a part of a non-native population (Okinawa), whereas only the asexual lineage is found on another island (Ishigaki) in the same archipelago ( Fig 2 ). One simple explanation for this difference is that different colonies of the mealybug lineages may have been introduced in association with pineapples; that is, both lineages reached Okinawa but only the asexual lineage reached Ishigaki. This is unlikely, however, because these two islands belong to and are managed by the same prefectural government (Okinawa Prefecture); therefore, pineapple transportation and cultivation are assumed to have similar histories on the two islands. Pineapple cultivation started in the 1920s and 1930s in Japan [ 43 ]. The mealybugs are reported to have first invaded a district including Ishigaki along with imported pineapple plants from Taiwan at an early stage of cultivation, and they subsequently dispersed to Okinawa via pineapple seedling exchange [ 44 ]. If this scenario is true, it implies that the asexual lineage has outcompeted the sexual lineage on Ishigaki, their original area of invasion, where no sexual individuals are currently observed.

Thus, an alternative hypothesis is that the mealybugs on Okinawa and Ishigaki may have been exposed to different environmental stresses, such as those imposed by a different climate or natural enemy fauna, which would lead to differences in relative fitness of the sexual and asexual lineages and in frequencies of the sexual and asexual lineages on each island. For example, climate conditions in winter are more severe on Okinawa (winter daily minimum temperatures ≈13°C) than on Ishigaki (≈17°C) [ 45 ]. Lower temperatures would impose acquisition of cold hardiness on this tropical insect and the genetic diversity offered by sexual reproduction might have been favored on Okinawa, whereas Ishigaki has a relatively moderate winter climate and asexual mealybugs with higher growth rates might have outcompeted their sexual relatives. Further surveys of the two reproductive lineages of this unusual insect across a broad range of native and non-native populations and detailed investigations of their habitats may provide more insight into the evolution and ecology of the sexual and asexual reproductive systems in scale insects.

Supporting Information

Each box indicates offspring borne by a single female.

Acknowledgments

We appreciate the help of Drs. S. Ohno, I. Yonaha, K. Yonamine, and C. Moromizato (OPARC) in undertaking mealybug collection. JT thanks Dr. Yutaka Narai (Shimane Agricultural Technology Center) for essential advice pertaining to this study.

Funding Statement

This study was supported by the grant-in-aid for scientific research (H24-3) from the Showa Seitoku Memorial Foundation ( http://www.f-showa.or.jp ) and the grant-in-aid for scientific research (16K08103) from the Japan Society for the Promotion of Science ( https://www.jsps.go.jp ) to JT. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.

Data Availability

Plant and Animal Reproduction

While all organisms reproduce, not all organisms reproduce the same way. Explore the similar and different ways that plants and animals pass on their genes.

Biology, Genetics

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What Is Reproduction? All organisms reproduce, including plants and animals. The biological process involves an organism producing and/or giving birth to another organism. Just because all organisms reproduce doesn’t mean the methods of reproduction are the same, however. Plants and animals occupy different phylogenetic kingdoms, but they have evolved reproductive systems that overlap and diverge from each other in several ways. Even within the same kingdom, different species may have different methods of reproduction. Types of Reproduction There are two types of reproduction: asexual reproduction and sexual reproduction . The former involves a single parent that produces a genetically identical offspring, whereas the latter involves two parents of the opposite sex, each of whom contributes genetic material to produce a diverse offspring. Different plants and animal can reproduce either asexually or sexually; however, a sexual reproduction is more common among plants than animals. Asexual and sexual reproduction each have benefits and drawbacks. Organisms that reproduce asexually have the advantage of producing several genetically identical offspring quickly and with little energy. On the other hand, the lack of genetic diversity among asexual offspring means they have a lower chance of acclimating to an unstable environment. By contrast, organisms that reproduce sexually have the advantage of producing a genetically diverse offspring, which is able to adapt to its environment. But sexual reproduction comes at a cost, requiring more time and energy to produce an offspring than a sexual reproduction . Fertilization One difference between plant and animal sexual reproduction concerns fertilization . In flowering plants, the fertilization of an egg is achieved by cross- pollination . This process involves an insect like a bee that transfers the pollen grains from the anther , the male part of a flower, to the stigma , the female part of a flower. Once the pollen lands on the stigma , it passes through a long, tube-like structure called a style to reach the ovaries where fertilization takes place. It should be noted that some plants, called hermaphrodites , have male and female parts on the same plant, and are able to self-pollinate. Animals, by contrast, do not depend on third parties like insects in order to mate. As mobile creatures, animals reproduce by physically interacting with each other and often perform various mating rituals in order to woo potential partners. Embryonic Development Despite differences in the fertilization process, the embryonic development of plants and animals is similar. Once a plant egg is fertilized, it starts developing into a multicellular organism in a way similar to an animal embryo. The only major difference between the two is that a plant embryo is contained within a seed, which provides the nutrients it needs to grow, while an animal embryo develops within an egg, outside the organism, or within a uterus, inside the female parent organism. Birth and Germination Plants and animals also differ with respect to how they give birth. Animals exit their mother’s uterus as a newborn or hatch from an egg that has already left the mother’s body. A plant, by contrast, arises by germinating from a seed. The plant releases the seed, which begins to grow once it is in soil and the conditions are right for germination. After the seed has germinated into a plant, it can collect additional nutrients through its roots. Growth Rates The growth rates of plants and animals also vary. Plants have what is called indeterminate growth, meaning there is nearly no limit to how much they can grow. The extent to which a plant can grow is largely determined by its environment. Consequently, plants do not have a size or age that is deemed normal or mature. The growth rate of mammals, such as humans, is also influenced by environmental factors, like nutrition, but animals cease growing once they have reached adulthood. Asexual Reproduction As noted earlier, many plants reproduce asexually. There are a variety of ways plants can reproduce without a partner. For example, some nonflowering plants, such as moss and algae, reproduce by spore formation. These plants form several spores , which break off and grow into separate organisms. Other plants, such as strawberries, are able to reproduce asexually through vegetative propagation , either naturally or artificially. This process involves using a vegetative part of a plant, such as a root or stem, to produce a new plant. Alternative artificial methods, such as grafting , involve combining two plants into one by attaching the top part of a plant, called a scion , to the lower part of a plant, called a rootstock .

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• Basic Genetics

Sexual vs. Asexual Reproduction

Living things use lots of different strategies for producing offspring, but most strategies fall neatly into the categories of either sexual or asexual reproduction. Asexual reproduction generates offspring that are genetically identical to a single parent. In sexual reproduction , two parents contribute genetic information to produce unique offspring.

Sexual and asexual reproduction have advantages and disadvantages—which is why some organisms do both!

Click or tap an organism below . After reading a description, you'll get to vote on whether you think the organism reproduces sexually, asexually, or both. How well do you know your reproductive strategies?

Funding provided by grant 51006109 from the Howard Hughes Medical Institute, Precollege Science Education Initiative for Biomedical Research.

Asexual Reproduction ( Cambridge O Level Biology )

Revision note.

Asexual Reproduction

• Asexual reproduction does not involve sex cells or fertilisation
• Only one parent is required so there is no fusion of gametes and no mixing of genetic information
• As a result, the offspring are genetically identical to the parent and to each other (clones)
• Asexual reproduction is defined as a process resulting in genetically identical offspring from one parent

Examples of Asexual Reproduction

Bacteria produce exact genetic copies of themselves in a type of asexual reproduction called binary fission :

Binary Fission Diagram

• Plants can reproduce asexually using bulbs and tubers
• Old potatoes stored for too long start budding or 'chitting' as this process begins

Budding of Bulbs & Tubers Diagram

• Some plants grow side shoots called runners that contain tiny plantlets on them
• A good example of this is strawberry plants
• These will grow roots and develop into separate plants , again being genetically identical to the parent plant

Runners in Asexual Plant Reproduction Diagram

Some plants grow side shoots called runners that contain tiny plantlets on them. These will grow roots and develop into separate plants.

Advantages & Disadvantages of Asexual Reproduction

• Specifically in crop plants , asexual reproduction can be advantageous as it means that a plant that has good characteristics (high yield, disease-resistant, hardy) can be made to reproduce asexually and the entire crop will show the same characteristics

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• Asexual & Sexual Reproduction
• Sexual Reproduction in Plants
• Sexual Reproduction in Humans
• Inheritance
• Biotechnology
• Genetic Modification
• Energy Flow

Author: Phil

Phil has a BSc in Biochemistry from the University of Birmingham, followed by an MBA from Manchester Business School. He has 15 years of teaching and tutoring experience, teaching Biology in schools before becoming director of a growing tuition agency. He has also examined Biology for one of the leading UK exam boards. Phil has a particular passion for empowering students to overcome their fear of numbers in a scientific context.

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• Introduction

Molecular replication

Molecular reproduction.

• Binary fission
• Multiple fission
• Reproduction of organisms
• Life cycles of plants
• Life cycles of animals
• The evolution of reproduction
• The evolution of variation control

reproduction

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• Biology LibreTexts - Reproduction
• Khan Academy - Types of reproduction review
• Milne Library - Reproduction: development and physiology
• Merck Manuals - Professional Version - Conception and Prenatal Development
• National Center for Biotechnology Information - PubMed Central - Research in Reproduction: Challenges, Needs, and Opportunities
• Michigan State University Libraries - An Interactive Introduction to Organismal and Molecular Biology, 2nd ed. - Reproduction
• BCCampus Publishing - Reproduction Methods
• Table Of Contents

reproduction , process by which organisms replicate themselves.

In a general sense reproduction is one of the most important concepts in biology : it means making a copy, a likeness, and thereby providing for the continued existence of species . Although reproduction is often considered solely in terms of the production of offspring in animals and plants, the more general meaning has far greater significance to living organisms. To appreciate this fact, the origin of life and the evolution of organisms must be considered. One of the first characteristics of life that emerged in primeval times must have been the ability of some primitive chemical system to make copies of itself.

At its lowest level, therefore, reproduction is chemical replication. As evolution progressed, cells of successively higher levels of complexity must have arisen, and it was absolutely essential that they had the ability to make likenesses of themselves. In unicellular organisms, the ability of one cell to reproduce itself means the reproduction of a new individual; in multicellular organisms, however, it means growth and regeneration . Multicellular organisms also reproduce in the strict sense of the term—that is, they make copies of themselves in the form of offspring—but they do so in a variety of ways, many involving complex organs and elaborate hormonal mechanisms.

Levels of reproduction

The characteristics that an organism inherits are largely stored in cells as genetic information in very long molecules of deoxyribonucleic acid ( DNA ). In 1953 it was established that DNA molecules consist of two complementary strands, each of which can make copies of the other. The strands are like two sides of a ladder that has been twisted along its length in the shape of a double helix (spring). The rungs, which join the two sides of the ladder, are made up of two terminal bases. There are four bases in DNA : thymine, cytosine, adenine, and guanine. In the middle of each rung a base from one strand of DNA is linked by a hydrogen bond to a base of the other strand. But they can pair only in certain ways: adenine always pairs with thymine, and guanine with cytosine. This is why one strand of DNA is considered complementary to the other.

The double helices duplicate themselves by separating at one place between the two strands and becoming progressively unattached. As one strand separates from the other, each acquires new complementary bases until eventually each strand becomes a new double helix with a new complementary strand to replace the original one. Because adenine always falls in place opposite thymine and guanine opposite cytosine, the process is called a template replication —one strand serves as the mold for the other. It should be added that the steps involving the duplication of DNA do not occur spontaneously; they require catalysts in the form of enzymes that promote the replication process.

The sequence of bases in a DNA molecule serves as a code by which genetic information is stored. Using this code, the DNA synthesizes one strand of ribonucleic acid ( RNA ), a substance that is so similar structurally to DNA that it is also formed by template replication of DNA. RNA serves as a messenger for carrying the genetic code to those places in the cell where proteins are manufactured. The way in which the messenger RNA is translated into specific proteins is a remarkable and complex process. (For more detailed information concerning DNA, RNA, and the genetic code, see the articles nucleic acid and heredity: Chromosomes and genes ). The ability to synthesize enzymes and other proteins enables the organism to make any substance that existed in a previous generation. Proteins are reproduced directly; however, such other substances as carbohydrates, fats, and other organic molecules found in cells are produced by a series of enzyme-controlled chemical reactions, each enzyme being derived originally from DNA through messenger RNA. It is because all of the organic constituents made by organisms are derived ultimately from DNA that molecules in organisms are reproduced exactly by each successive generation.

Cell reproduction

The chemical constituents of cytoplasm (that part of the cell outside the nucleus) are not resynthesized from DNA every time a cell divides. This is because each of the two daughter cells formed during cell division usually inherits about half of the cellular material from the mother cell (see cell: Cell division and growth ), and is important because the presence of essential enzymes enables DNA to replicate even before it has made the enzymes necessary to do so.

Cells of higher organisms contain complex structures, and each time a cell divides the structures must be duplicated . The method of duplication varies for each structure, and in some cases the mechanism is still uncertain. One striking and important phenomenon is the formation of a new membrane . Cell membranes, although they are very thin and appear to have a simple form and structure, contain many enzymes and are sites of great metabolic activity. This applies not only to the membrane that surrounds the cell but to all the membranes within the cell. New membranes, which seem to form rapidly, are indistinguishable from old ones.

Thus, the formation of a new cell involves the further synthesis of many constituents that were present in the parent cell. This means that all of the information and materials necessary for a cell to reproduce itself must be supplied by the cellular constituents and the DNA inherited from the parent cell.