Asexual reproduction
2756
225289228
2008-07-12T22:22:26Z
Mathmannix
717088
[[WP:UNDO|Undid]] revision 225288182 by [[Special:Contributions/Mathmannix|Mathmannix]] ([[User talk:Mathmannix|talk]])
[[Image:caduco.jpg|thumb|400px|Asexual reproduction in [[liverwort]]s: a caducuous phylloid germinating]]
'''Asexual reproduction''' is a form of reproduction which does not involve [[meiosis]], [[ploidy]] reduction, or [[fertilization]]. Only one parent is involved in asexual reproduction. A more stringent definition is '''agamogenesis''' which refers to reproduction without the fusion of [[gamete]]s. Asexual reproduction is the primary form of reproduction for [[single-celled organism]]s such the [[archaea]], [[bacteria]], and [[protist]]s. Many [[plant]]s and [[Fungus|fungi]] reproduce asexually as well. While all [[prokaryote]]s reproduce asexually (without the formation and fusion of gametes), mechanisms for [[lateral gene transfer]] such as [[Bacterial conjugation|conjugation]], [[Transformation (genetics)|transformation]] and [[Transduction (genetics)|transduction]] are sometimes likened to [[sexual reproduction]].<ref name="Narra">{{cite journal|author = Narra HP, Ochman H | title = Of what use is sex to bacteria? | journal = Current Biology | date = 2006 | volume = 16 | pages = R705–710 | pmid = 16950097 | doi = 10.1016/j.cub.2006.08.024}}</ref>
The lack of sexual reproduction is relatively rare among multicellular organisms, for reasons that are not completely understood. 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.
Because asexual reproduction does not require the formation of [[gamete]]s (often in separate individuals) and bringing them together for [[fertilization]], 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 [[Evolution of sex#The two-fold cost of sex|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 colonisation of isolated habitats such as oceanic islands, where a single (female) member of the species is enough to start a population.
Another 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 [[evolution]]ary 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.
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.<ref>Teresa Pawlowska and John Taylor, "Organization of genetic variation in individuals of arbuscular mycorrhizal fungi", ''Nature'', 2004, volume 427 (6976), pages 733-737.</ref>
== Types of asexual reproduction ==
=== Binary fission ===
{{main|Binary fission}}
Many single-celled organisms (unicellular), such as [[archaea]], [[bacteria]], and [[protist]]s, reproduce asexually through [[binary fission]]. An exception to the rule are unicellular fungi such as [[fission yeast]], unicellular algae such as ''[[Chlamydomonas]]'', and [[ciliate]]s and some other [[protist]]s, which reproduce both sexually and asexually. Some single-celled organisms(unicellular) rely on one or more [[Host (biology)|host]] organisms in order to reproduce, but most literally divide into two organisms.
=== Budding ===
{{main|Budding}}
Some cells split via [[budding]] (for example [[Saccharomyces cerevisiae|baker's yeast]]), resulting in a smaller 'mother' cell and larger 'daughter' cell. Offspring is larger than parent. Budding is also known on a multicellular level. An animal example is [[Hydra (genus)|hydra]], which reproduces by budding. The buds grow into fully matured individuals which eventually break away from the parent organism.
=== Vegetative reproduction ===
{{main|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 [[rhizome]]s or [[stolon]]s (for example in [[strawberry]]), or the formation of new bulbs (for example in [[tulip]]s). The resulting plants form a [[clonal colony]].
=== Spore formation ===
{{main|Sporogenesis}}
Many multicellular organisms form [[spore]]s during their [[biological life cycle]] in a process called '''''sporogenesis'''''. Exceptions are animals and some protists, who 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 [[gametophyte]]s in the case of plants) without a fertilization event. These haploid individuals give rise to [[gamete]]s 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 [[sporophyte]]s 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 [[Conidium|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 ===
{{main|Fragmentation (biology)}}
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 ''[[Gemma (botany)|gemmae]]'' in [[Marchantiophyta|liverwort]]s. Most [[lichen]]s, which are a [[Symbiosis|symbiotic]] union of a fungus and [[Photosynthesis|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 ===
{{main|Parthenogenesis}}
Parthenogenesis is a form of [[agamogenesis]] in which an unfertilized egg develops into a new individual. Parthenogenesis occurs naturally in many plants, [[invertebrate]]s (e.g. water fleas, aphids, [[stick insect]]s, some ants, bees and parasitic wasps), and [[vertebrate]]s (e.g. some reptiles, amphibians, fish, very rarely birds). In plants, [[apomixis]] may or may not involve parthenogenesis.
=== Agamogenesis ===
{{main|Agamogenesis}}
Agamogenesis is any form of reproduction that does not involve a male gamete. Examples are [[parthenogenesis]] and [[apomixis]].
=== Apomixis and nucellar embryony ===
{{main|Apomixis}}
{{main|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 favourable 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 [[Domesticated turkey|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 behaviour can interfere with the incubation of eggs in turkey farming.<ref>Savage, Thomas F. {{cite web
| url = http://oregonstate.edu/instruct/ans-tparth/index.html
| title = A Guide to the Recognition of Parthenogenesis in Incubated Turkey Eggs
| date= 2005-09-12
| work = Oregon State University
| accessdate = 2006-10-11 }}</ref> Another unusual case would be certain species of sharks. <ref>{{cite web|last = Savage
| first = Juliet Eilperin
| title = Female Sharks Can Reproduce Alone, Researchers Find
| work = Washington Post
|date= 2007-05-23
| url=http://www.washingtonpost.com/wp-dyn/content/article/2007/05/22/AR2007052201405.html
| accessdate = 2008-04-27 }}</ref>
Bdelloid [[rotifers]] reproduce exclusively asexually, and all individuals in the class [[Bdelloidea]] are females. Asexuality evolved in these animals millions of years ago and has persisted since. There is evidence to suggest that asexual reproduction has allowed the animals to evolve new proteins through the [[Matthew Meselson#The Meselson effect|Meselson effect]] that have allowed them to survive better in periods of dehydration.<ref>[http://www.sciencemag.org/cgi/content/full/318/5848/268 Pouchkina-Stantcheva, N.N., McGee, B.M. et al. (2007) Functional Divergence of Former Alleles in an Ancient Asexual Invertebrate. Science 318: 268-271]</ref>
== Notes and references ==
<references/>
== Further reading ==
* Graham, L., J. Graham, & L. Wilcox. 2003. ''Plant Biology''. Pearson Education, Inc., Upper Saddle River, N.J.: pp. 258-259.
* Raven, P.H., Evert, R.F., Eichhorn, S.E. 2005. ''Biology of Plants'', 7th Edition. W.H. Freeman and Company Publishers, NY.
==See also==
* [[Alternation of generation]]
* [[Bacterial conjugation]]
* [[Biological reproduction]]
* [[Cloning]]
* [[Parthenogenesis]]
* [[Reproduction]]
* [[Sex]]
== External links ==
* [http://www.tiscali.co.uk/reference/encyclopaedia/hutchinson/m0030820.html Asexual reproduction]
* [http://www.tulane.edu/~wiser/protozoology/notes/intes.html Intestinal Protozoa]
[[Category:Reproduction]]
[[bg:Безполово размножаване]]
[[ca:Reproducció asexual]]
[[cs:Nepohlavní rozmnožování]]
[[da:Ukønnet formering]]
[[de:Ungeschlechtliche Vermehrung]]
[[et:Suguta paljunemine]]
[[es:Reproducción asexual]]
[[eo:Neseksa reproduktado]]
[[fr:Multiplication asexuée]]
[[ia:Reproduction asexual]]
[[is:Kynlaus æxlun]]
[[he:רבייה אל-זוויגית]]
[[lt:Nelytinis dauginimasis]]
[[nl:Ongeslachtelijke voortplanting]]
[[ja:無性生殖]]
[[no:Ukjønnet formering]]
[[pl:Rozmnażanie bezpłciowe]]
[[ru:Бесполое размножение]]
[[simple:Asexual reproduction]]
[[sl:Nespolno razmnoževanje]]
[[sv:Asexuell förökning]]
[[th:การสืบพันธุ์แบบไม่อาศัยเพศ]]
[[vi:Sinh sản vô tính]]
[[uk:Безстатеве розмноження]]
[[zh:无性生殖]]