What refers to the type of reproduction in which an outgrowth grows and develops from the parent animal and later separates to become a new individual?

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Learning Objectives

• Discuss sexual and asexual reproduction methods

Asexual reproduction produces offspring that are genetically identical to the parent because the offspring are all clones of the original parent. This type of reproduction occurs in prokaryotic microorganisms (bacteria) and in some eukaryotic single-celled and multi-celled organisms. Animals may reproduce asexually through fission, budding, fragmentation, or parthenogenesis.

Fission, also called binary fission, occurs in prokaryotic microorganisms and in some invertebrate, multi-celled organisms. After a period of growth, an organism splits into two separate organisms. Some unicellular eukaryotic organisms undergo binary fission by mitosis. In other organisms, part of the individual separates, forming a second individual. This process occurs, for example, in many asteroid echinoderms through splitting of the central disk. Some sea anemones and some coral polyps also reproduce through fission.

Figure \(\PageIndex{1}\): Fission: Coral polyps reproduce asexually by fission, where an organism splits into two separate organisms.

Budding is a form of asexual reproduction that results from the outgrowth of a part of a cell or body region leading to a separation from the original organism into two individuals. Budding occurs commonly in some invertebrate animals such as corals and hydras. In hydras, a bud forms that develops into an adult, which breaks away from the main body; whereas in coral budding, the bud does not detach and multiplies as part of a new colony.

Figure \(\PageIndex{1}\): Budding: Hydra reproduce asexually through budding, where a bud forms that develops into an adult and breaks away from the main body.

Fragmentation is the breaking of the body into two parts with subsequent regeneration. If the animal is capable of fragmentation, and the part is big enough, a separate individual will regrow.

Many sea stars reproduce asexually by fragmentation. For example, if the arm of an individual sea star is broken off it will regenerate a new sea star. Fishery workers have been known to try to kill the sea stars that eat their clam or oyster beds by cutting them in half and throwing them back into the ocean. Unfortunately for the workers, the two parts can each regenerate a new half, resulting in twice as many sea stars to prey upon the oysters and clams. Fragmentation also occurs in annelid worms, turbellarians, and poriferans.

Figure \(\PageIndex{1}\): Fragmentation: Sea stars can reproduce through fragmentation. The large arm, a fragment from another sea star, is developing into a new individual.

Note that in fragmentation, there is generally a noticeable difference in the size of the individuals, whereas in fission, two individuals of approximately the same size are formed.

Parthenogenesis is a form of asexual reproduction where an egg develops into a complete individual without being fertilized. The resulting offspring can be either haploid or diploid, depending on the process and the species. Parthenogenesis occurs in invertebrates such as water fleas, rotifers, aphids, stick insects, some ants, wasps, and bees. Bees use parthenogenesis to produce haploid males (drones) and diploid females (workers). If an egg is fertilized, a queen is produced. The queen bee controls the reproduction of the hive bees to regulate the type of bee produced.

Some vertebrate animals, such as certain reptiles, amphibians, and fish, also reproduce through parthenogenesis. Although more common in plants, parthenogenesis has been observed in animal species that were segregated by sex in terrestrial or marine zoos. Two Komodo dragons, a bonnethead shark, and a blacktip shark have produced parthenogenic young when the females have been isolated from males.

Sexual reproduction is the combination of (usually haploid, or having a single set of unpaired chromosomes) reproductive cells from two individuals to form a third (usually diploid, or having a pair of each type of chromosome) unique offspring. Sexual reproduction produces offspring with novel combinations of genes. This can be an adaptive advantage in unstable or unpredictable environments. As humans, we are used to thinking of animals as having two separate sexes, male and female, determined at conception. However, in the animal kingdom, there are many variations on this theme.

Hermaphroditism occurs in animals where one individual has both male and female reproductive parts. Invertebrates, such as earthworms, slugs, tapeworms and snails, are often hermaphroditic. Hermaphrodites may self-fertilize or may mate with another of their species, fertilizing each other and both producing offspring. Self fertilization is common in animals that have limited mobility or are not motile, such as barnacles and clams.

Key Points

• Asexual reproduction includes fission, budding, fragmentation, and parthenogenesis, while sexual reproduction is achieved through the combination of reproductive cells from two individuals.
• The ability of a species to reproduce through fragmentation depends on the size of part that breaks off, while in binary fission, an individual splits off and forms two individuals of the same size.
• Budding may lead to the production of a completely new adult that forms away from the original body or may remain attached to the original body.
• Observed in invertebrates and some vertebrates, parthenogenesis produce offspring that may be either haploid or diploid.
• Sexual reproduction, the production of an offspring with a new combination of genes, may also involve hermaphroditism in which an organism can self-fertilize or mate with another individual of the same species.

Key Terms

• binary fission: the process whereby a cell divides asexually to produce two daughter cells
• hermaphroditism: having sexual organs of both sexes
• parthenogenesis: a form of asexual reproduction where growth and development of embryos occur without fertilization

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

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.

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

Asexual reproduction (biology definition): a mode of reproduction in which the offspring comes from a single organism, and not from the union of gametes as it is in sexual reproduction

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.

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.

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:

1. binary fission
2. budding
3. vegetative propagation
4. spore formation (sporogenesis)
5. fragmentation
6. parthenogenesis
7. apomixis

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

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.

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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 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

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 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).

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

Bacteria

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 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)

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Summary of the different types of asexual reproduction:

Try to answer the quiz below to check what you have learned so far about asexual reproduction.

References

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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