Definition: In asexual reproduction, the organism is capable of reproducing an offspring in the absence of a mate.
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Asexual Reproduction Definition
What is asexual reproduction? Asexual reproduction is a mode 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 in 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.
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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.
|Table 1: Differences between Asexual Reproduction and Sexual Reproduction|
|Asexual Reproduction||Sexual Reproduction|
|One parent is involved||Two parents are involved: paternal and maternal|
|Syngamy is absent||Syngamy is present; sperm cell (male gamete) and ovum or egg cell (female gamete) unite during fertilization|
|Meiosis usually is not needed to complete the process||Meiosis is a required step to produce gametes|
|Offspring is often genetically identical to or a clone of the parent||Offspring is genetically unique, meaning it is genetically different from its parents|
|Types: binary fission, budding, vegetative propagation, spore formation (sporogenesis), fragmentation, parthenogenesis, and apomixis||Types: syngamy and conjugation|
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 in 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, may poise as a long-term evolutionary disadvantage.
For instance, the lone parent passes along the same genetic material 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.
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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)
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.
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 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 seed 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 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 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 is 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).
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)
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 cycle. 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)
Summary of the different types of asexual reproduction:
|Types of Asexual Reproduction||Description||Examples|
|Binary fission||A cell divides to produce two identical cells. Each cell has the potential to grow to the size of the original cell.||Many bacteria, protists, unicellular fungi|
|Budding||The formation of an outgrowth (or bud) from an organism capable of developing into a new individual. The outgrowth is genetically the same as the parent but relatively smaller.||Yeasts, hydra, certain bacteria (Caulobacter, Hyphomicrobium, and Stella spp.)|
|Vegetative propagation||A new plant emerges from vegetative parts, such as specialized stems, leaves, and roots, and then they take root and grow.||Various plants, e.g. those naturally emerging from stolons, bulbs, tubers, corms, suckers (root sprouts), and plantlets, and those artificially grown by cutting, grafting, layering, tissue culture, and offset.|
|Spore formation||An asexual reproduction wherein spores are produced to germinate into new individuals||Fungi, slime molds, and vascular plants|
|Fragmentation||The parent organism breaks into fragments. Each fragment is capable of developing into a new organism.||Certain fungi (e.g. yeasts, and lichens), molds, vascular and nonvascular plants, cyanobacteria, and certain animals (e.g. sponges, sea stars, planarians, and many annelid worms)|
|Parthenogenesis||The offspring develops from a female gamete even without prior fertilization by a male gamete.||Certain invertebrates (e.g. aphids, rotifers, and nematodes) and certain vertebrates (e.g. some lizards, snakes, birds, sharks, reptiles, and amphibians).|
|Plant apomixis||Reproduction in plants without fertilization||Bryophytes, certain ferns, and flowering plants|
Try to answer the quiz below to check what you have learned so far about asexual reproduction.
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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
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