Asexual reproduction

Asexual reproduction in liverworts: a caducuous phylloid germinating

Asexual reproduction is a form of reproduction in which an organism creates a genetically-similar or identical copy of itself without a contribution of genetic material from another individual. It does not involve meiosis, ploidy reduction, or fertilization, and only one parent is involved genetically. A more stringent definition is agamogenesis, which refers to reproduction without the fusion of gametes.

Asexual reproduction is the primary form of reproduction for single-celled organisms such the archaea, bacteria, and protists. However, while all prokaryotes reproduce asexually (without the formation and fusion of gametes), there also exist mechanisms for lateral gene transfer, such as conjugation, transformation, and transduction, whereby genetic material is exchanged between organisms. Biological processes involving lateral gene transfer sometimes are likened to sexual reproduction (Narra and Ochman 2006). The reproductive variances in bacteria and protists also may be symbolized by + and - signs (rather than being called male and female), and referred to as "mating strains" or "reproductive types" or similar appellations.

Many plants and fungi reproduce asexually as well, and asexual reproduction has been cited in some animals, including bdelloid rotifers, which only are known to reproduce asexually, and various animals that exhibit parthenogenesis under certain conditions. In parthenogenesis, such as found in some invertebrates and vertebrates, an embryo is produced without fertilization by a male. Generally, parthenogenesis is considered a form of asexual reproduction because it does not involve fusion of gametes of opposite sexes, nor any exchange of genetic material from two different sources (Mayr 2001) however, some authorities (McGraw-Hill 2004) classify parthenogenesis as sexual reproduction on the basis that it involves gametes or does not produce an offspring genetically identical to the parent (such as a female domestic turkey producing male offspring).

A wide spectrum of mechanisms may be exhibited. For example, many plants alternate between sexual and asexual reproduction (see Alternation of generations), and the freshwater crustacean Daphnia reproduces by parthenogenesis in the spring to rapidly populate ponds, then switches to sexual reproduction as the intensity of competition and predation increases. Many protists and fungi alternate between sexual and asexual reproduction.

A lack of sexual reproduction is relatively rare among multicellular organisms, which exhibit the characteristics of being male or female. Biological explanations for this phenomenon are not completely settled. Current hypotheses suggest that, while asexual reproduction may have short term benefits when rapid population growth is important or in stable environments, sexual reproduction offers a net advantage by allowing more rapid generation of genetic diversity, allowing adaptation to changing environments.

Costs and benefits

In asexual reproduction, an individual can reproduce without involvement with another individual, there is no fusion of gametes, and the new organism produced inherits all of its chromosomes from one parent and thus is a genetically-similar or identical copy of the parent.

Because asexual reproduction does not require the formation of gametes (often in separate individuals) and bringing them together for fertilization, nor involvement of another organism, it occurs much faster than sexual reproduction and requires less energy. Asexual lineages can increase their numbers rapidly because all members can reproduce viable offspring. In sexual populations with two genders, some of the individuals are male and cannot themselves produce offspring. This means that an asexual lineage will have roughly double the rate of population growth under ideal conditions when compared with a sexual population half composed of males. This is known as the two-fold cost of sex. Other advantages include the ability to reproduce without a partner in situations where the population density is low (such as for some desert lizards), reducing the chance of finding a mate, or during colonization of isolated habitats such as oceanic islands, where a single (female) member of the species is enough to start a population. There does not need to be energy spent in finding and courting a partner for reproduction.

A consequence of asexual reproduction, which may have both benefits and costs, is that offspring are typically genetically similar to their parent, with as broad a range as that individual receives from one parent. The lack of genetic recombination results in fewer genetic alternatives than with sexual reproduction. Many forms of asexual reproduction, for example budding or fragmentation, produce an exact replica of the parent. This genetic similarity may be beneficial if the genotype is well-suited to a stable environment, but disadvantageous if the environment is changing. For example, if a new predator or pathogen appears and a genotype is particularly defenseless against it, an asexual lineage is more likely to be completely wiped out by it. In contrast, a lineage that reproduces sexually has a higher probability of having more members survive due to the genetic recombination that produces a novel genotype in each individual. Similar arguments apply to changes in the physical environment. From an evolutionary standpoint, one could thus argue that asexual reproduction is inferior because it stifles the potential for change. However, there is also a significantly reduced chance of mutation or other complications that can result from the mixing of genes.

Conversely, Heng (2007) proposes that the resolution to the "paradox of sex" is that sexual reproduction actually reduces the drastic genetic diversity at the genome or chromosome level, resulting in the preservation of species identity, rather than the provision of evolutionary diversity for future environmental challenges. He maintains that while genetic recombination contributes to genetic diversity, it does so secondarily and within the framework of the chromosomally defined genome. That is, the asexual process generates more diverse genomes because of the less controlled reproduction systems, while sexual reproduction generates more stable genomes.

A 2004 article in the journal Nature reported that the modern arbuscular mycorrhizas fungi, which reproduces asexually, is identical to fossil records dating back to the Ordovician period, 460 million years ago (Pawlowska and Taylor 2004).

Types of asexual reproduction

Binary fission

Binary fission involves the reproduction of a living cell by division into two parts, which each have the potential to grow to the size of the original cell. Many single-celled organisms (unicellular), such as archaea, bacteria, and protists, reproduce asexually through binary fission. Exceptions are unicellular fungi such as fission yeast, unicellular algae such as Chlamydomonas, and ciliates and some other protists, which reproduce both sexually and asexually. Some single-celled organisms (unicellular) rely on one or more host organisms in order to reproduce, but most literally divide into two organisms. In addition, mitochondria and chloroplasts of eukaryote cells divide by binary fission. (See also the description under sexual reproduction.)

Budding

Some cells split via budding (for example baker's yeast), resulting in a "mother" and "daughter" cell. Budding is the formation of a new organism by the protrusion of part of another organism, with the protrusion normally staying attached to the primary organism for a while, before becoming free. The new organism is naturally genetically identical to the primary one (a clone) and initially is smaller than the parent. Budding is also known on a multicellular level; an animal example is the hydra, which reproduces by budding. The buds grow into fully matured individuals, which eventually break away from the parent organism.

Vegetative reproduction

Vegetative reproduction is a type of asexual reproduction found in plants where new independent individuals are formed without the production of seeds or spores. Examples for vegetative reproduction include the formation of plantlets on specialized leaves (for example in kalanchoe), the growth of new plants out of rhizomes or stolons (for example in strawberry), or the formation of new bulbs (for example in tulips). The resulting plants form a clonal colony.

Spore formation

Many multicellular organisms form spores during their biological life cycle in a process called sporogenesis. Exceptions are animals and some protists, which undergo gametic meiosis immediately followed by fertilization. Plants and many algae on the other hand undergo sporic meiosis, where meiosis leads to the formation of haploid spores rather than gametes. These spores grow into multicellular individuals (called gametophytes in the case of plants) without a fertilization event. These haploid individuals give rise to gametes through mitosis. Meiosis and gamete formation therefore occur in separate generations or "phases" of the life cycle, referred to as alternation of generations. Since sexual reproduction is often more narrowly defined as the fusion of gametes (fertilization), spore formation in plant sporophytes and algae might be considered a form of asexual reproduction (agamogenesis) despite being the result of meiosis and undergoing a reduction in ploidy. However, both events (spore formation and fertilization) are necessary to complete sexual reproduction in the plant life cycle.

Fungi and some algae can also utilize true asexual spore formation, which involves mitosis giving rise to reproductive cells called mitospores that develop into a new organism after dispersal. This method of reproduction is found, for example, in conidial fungi and the red alga Polysiphonia, and involves sporogenesis without meiosis. Thus, the chromosome number of the spore cell is the same as that of the parent producing the spores. However, mitotic sporogenesis is an exception and most spores, such as those of plants, most Basidiomycota, and many algae, are produced by meiosis.

Fragmentation

Fragmentation is a form of asexual reproduction where a new organism grows from a fragment of the parent. Each fragment develops into a mature, fully grown individual. Fragmentation is seen in many organisms, such as animals (some annelid worms and starfish), fungi, and plants. Some plants have specialized structures for reproduction via fragmentation, such as gemmae in liverworts. Most lichens, which are a symbiotic union of a fungus and photosynthetic algae or bacteria, reproduce through fragmentation to ensure that new individuals contain both symbionts. These fragments can take the form of soredia, dust-like particles consisting of fungal hyphae wrapped around photobiont cells.

Parthenogenesis

Parthenogenesis is a form of agamogenesis in which an unfertilized egg develops into a new individual. Parthenogenesis occurs naturally in many plants, invertebrates (e.g. water fleas, aphids, stick insects, some ants, bees and parasitic wasps), and vertebrates (e.g. some reptiles, amphibians, fish, very rarely birds). In plants, apomixis may or may not involve parthenogenesis.

Parthenogenesis is one example of agamogenesis, the term for any form of reproduction that does not involve a male gamete. An example is apomixis.

Apomixis and nucellar embryony

Apomixis in plants is the formation of a new sporophyte without fertilization. It is important in ferns and in flowering plants, but is very rare in other seed plants. In flowering plants, the term "apomixis" is now most often used for agamospermy, the formation of seeds without fertilization, but was once used to include vegetative reproduction. An example of an apomictic plant would be the triploid European dandelion.

Apomixis mainly occurs in two forms. In gametophytic apomixis, the embryo arises from an unfertilized egg within a diploid embryo sac that was formed without completing meiosis. In nucellar embryony, the embryo is formed from the diploid nucellus tissue surrounding the embryo sac. Nucellar embryony occurs in some citrus seeds. Male apomixis can occur in rare cases, such as the Saharan cypress where the genetic material of the embryo are derived entirely from pollen.

The term "apomixis" is also used for asexual reproduction in some animals, notably water-fleas, Daphnia.

Alternation between sexual and asexual reproduction

Some species alternate between the sexual and asexual strategies, an ability known as heterogamy, depending on conditions. For example, the freshwater crustacean Daphnia reproduces by parthenogenesis in the spring to rapidly populate ponds, then switches to sexual reproduction as the intensity of competition and predation increases.

Many protists and fungi alternate between sexual and asexual reproduction. For example, the slime mold Dictyostelium undergoes binary fission as single-celled amoebae under favorable conditions. However, when conditions turn unfavorable, the cells aggregate and switch to sexual reproduction leading to the formation of spores. The hyphae of the common mold (Rhizopus) are capable of producing both mitotic as well as meiotic spores.

Many algae similarly switch between sexual and asexual reproduction. Asexual reproduction is far less complicated than sexual reproduction. In sexual reproduction one must find a mate.

Examples in animals

A number of invertebrates and some less advanced vertebrates are known to alternate between sexual and asexual reproduction, or be exclusively asexual. Alternation is observed in a few types of insects, such as aphids (which will, under favorable conditions, produce eggs that have not gone through meiosis, essentially cloning themselves) and the cape bee Apis mellifera capensis (which can reproduce asexually through a process called thelytoky). A few species of amphibians and reptiles have the same ability (see parthenogenesis for concrete examples). A very unusual case among more advanced vertebrates is the female turkey's ability to produce fertile eggs in the absence of a male. The eggs result in often sickly, and nearly always male turkeys. This behavior can interfere with the incubation of eggs in turkey farming (Savage 2008).

There are examples of parthenogenesis in the hammerhead shark (Eilperin 2007) and the blacktip shark (Chapman et al. 2008). In both cases, the sharks had reached sexual maturity in captivity in the absence of males, and in both cases the offspring were shown to be genetically identical to the mothers.

Bdelloid rotifers reproduce exclusively asexually, and all individuals in the class Bdelloidea are females. Asexuality arose in these animals millions of years ago and has persisted since. There is evidence to suggest that asexual reproduction has allowed the animals to develop new proteins through the Meselson effect that have allowed them to survive better in periods of dehydration (Pouchkina-Stantcheva et al. 2007).

References

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• Chapman, D. D., B. Firchau, and M. S. Shivji. 2008. Parthenogenesis in a large-bodied requiem shark, the blacktip Carcharhinus limbatus. Journal of Fish Biology 73(6): 1473. See report in Science Daily: "Virgin birth" By shark confirmed: Second case ever. Retrieved January 15, 2009.

• Eilperin, J. 2007. Female sharks can reproduce alone, researchers find. Washington Post May 23, 2007, p. A02. Retrieved January 16, 2008.

• Graham, L., J. Graham, and L. Wilcox. 2003. Plant Biology. Upper Saddle River, NJ: Pearson Education. ISBN 0130303712.

• Heng, H. H. 2007. Elimination of altered karyotypes by sexual reproduction preserves species identity. Genome 50: 517-524.

• Mayr, E. 2001. What Evolution Is. New York: Basic Books. ISBN 0465044255.

• McGraw-Hill (Publisher). 2004. McGraw-Hill Concise Encyclopedia of Science and Technology, 5th Edition. McGraw Hill Professionals. ISBN 0071429573.

• Narra, H. P., and H. Ochman. 2006. Of what use is sex to bacteria? Current Biology 16: R705–710. PMID 16950097.

• Pawlowska, T., and J. Taylor. 2004. Organization of genetic variation in individuals of arbuscular mycorrhizal fungi. Nature 427(6976): 733-737.

• Pouchkina-Stantcheva, N. N., B. M. McGee, C. Boschetti, et al. 2007. Functional divergence of former alleles in an ancient asexual invertebrate. Science 318: 268-271. Retrieved January 15, 2009.

• Raven, P. H., R. F. Evert, and S. E. Eichhorn. 2005. Biology of Plants, 7th edition. New York: W.H. Freeman and Company. ISBN 0716710072.

• Savage, T. F. 2008. [http://oregonstate.edu/instruct/ans-tparth/index.html A guide to the recognition of parthenogenesis in incubated turkey eggs. Oregon State Universtiy. Retrieved January 16, 2009.

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