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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|year = 2006|volume = 16|pages = R705–710|pmid = 16950097|doi = 10.1016/j.cub.2006.08.024|issue = 17}}</ref> A complete lack of sexual reproduction is relatively rare among multicellular organisms, particularly animals. It is not entirely understood why the ability to reproduce sexually is so common among them. Current hypotheses <ref name="Dawson">{{cite journal|author = Dawson KJ |title = The Advantage of Asexual Reproduction: When is it Two-fold?|journal = Journal of Theoretical Biology |date = October 1995|volume = 176|pages = 341–347|url = http://www.ingentaconnect.com/content/ap/jt/1995/00000176/00000003/art00203 |issue = 3|doi = 10.1006/jtbi.1995.0203}}</ref> suggest that asexual reproduction may have short term benefits when rapid population growth is important or in stable environments, while sexual reproduction offers a net advantage by allowing more rapid generation of genetic diversity, allowing adaptation to changing environments. Developmental constraints<ref name="Engelstädter">{{cite journal|author = Engelstädter J|title = Constraints on the evolution of asexual reproduction |journal = Bioessays |date= Nov 2008|volume = 30|pages = 1138–1150|issue = 11–12 |pmid=18937362 |doi=10.1002/bies.20833}}</ref> may underlie why few animals have relinquished sexual reproduction completely in their life-cycles.
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|year = 2006|volume = 16|pages = R705–710|pmid = 16950097|doi = 10.1016/j.cub.2006.08.024|issue = 17}}</ref> A complete lack of sexual reproduction is relatively rare among multicellular organisms, particularly animals. It is not entirely understood why the ability to reproduce sexually is so common among them. Current hypotheses <ref name="Dawson">{{cite journal|author = Dawson KJ |title = The Advantage of Asexual Reproduction: When is it Two-fold?|journal = Journal of Theoretical Biology |date = October 1995|volume = 176|pages = 341–347|url = http://www.ingentaconnect.com/content/ap/jt/1995/00000176/00000003/art00203 |issue = 3|doi = 10.1006/jtbi.1995.0203}}</ref> suggest that asexual reproduction may have short term benefits when rapid population growth is important or in stable environments, while sexual reproduction offers a net advantage by allowing more rapid generation of genetic diversity, allowing adaptation to changing environments. Developmental constraints<ref name="Engelstädter">{{cite journal|author = Engelstädter J|title = Constraints on the evolution of asexual reproduction |journal = Bioessays |date= Nov 2008|volume = 30|pages = 1138–1150|issue = 11–12 |pmid=18937362 |doi=10.1002/bies.20833}}</ref> may underlie why few animals have relinquished sexual reproduction completely in their life-cycles.


== Types==
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=== Fission ===
{{Main|Fission (biology)}}
In binary fission the parent organism is replaced by two daughter organisms, because it literally divides in two. organisms, both [[prokaryote]]s (the [[archaea]] and the [[bacteria]]), and [[eukaryote]]s (such as [[protist]]s and unicellular [[fungi]]), reproduce asexually through binary fission; most of these are also capable of sexual reproduction.


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In multiple fission, the [[Cell nucleus|nucleus]] of the parent cell divides by [[mitosis]] several times, producing several nuclei. The cytoplasm then separates, creating multiple daughter cells.<ref name="ebrit">{{cite web|url=http://www.britannica.com/EBchecked/topic/498542/reproduction/76146/Multiple-fission|title=Cell reproduction|publisher=Encyclopedia Britannica}}</ref><ref name="fap">{{cite book|author=Britannica Educational Publishing|title =Fungi, Algae, and Protists|publisher =The Rosen Publishing Group |year =2011|isbn =9781615304639|url =http://books.google.com.ph/books?id=_U9MB4iUpDIC&lpg=PA102&dq=multiple%20fission&hl=en&pg=PA101#v=onepage&q=multiple%20fission&f=false}}</ref><ref name="aff">{{cite book|author=P.Puranik, Asha Bhate|title =Animal Forms And Functions: Invertebrata|publisher =Sarup & Sons|year =2007|isbn =9788176257916|url =http://books.google.com.ph/books?id=-kdq6RyyVE0C&lpg=PA87&dq=multiple%20fission&hl=en&pg=PA87#v=onepage&q=multiple%20fission&f=false}}</ref>
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=== Budding ===
{{Main|Budding}}
Some cells split via [[budding]] (for example [[Saccharomyces cerevisiae|baker's yeast]]), resulting in a 'mother' and 'daughter' cell. The offspring organism is smaller than the parent. Budding is also known on a multicellular level; an animal example is the [[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 individuals are formed without the production of seeds or spores by [[meiosis]] or [[syngamy]].<ref>{{cite web|url=http://www.ucmp.berkeley.edu/glossary/gloss6/asexual.html |title=Asexual Reproduction |publisher=Ucmp.berkeley.edu |date= |accessdate=2010-08-13}}</ref> Examples of vegetative reproduction include the formation of miniaturized plants called plantlets on specialized leaves (for example in [[kalanchoe]]) and some produce new plants out of [[rhizome]]s or [[stolon]] (for example in [[Garden strawberry|strawberry]]). Other plants reproduce by forming [[bulb]]s or [[tuber]]s (for example [[tulip]] bulbs and [[dahlia]] tubers). Some plants produce [[adventitious]] shoots and suckers that form along their lateral roots. Plants that reproduce vegetatively may form a [[clonal colony]], where all the individuals are clones, and the clones may cover a large area.<ref>{{cite web|author=Fun Fact |url=http://www.fs.fed.us/wildflowers/communities/aspen/grow.shtml |title=Celebrating Wildflowers - Fading Gold - How Aspens Grow |publisher=Fs.fed.us |date=2010-05-11 |accessdate=2010-08-13}}</ref>

=== 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 (reproduction)}}
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, [[turbellarians]] and [[sea stars]]), fungi, and plants. Some plants have specialized structures for reproduction via fragmentation, such as ''[[Gemma (botany)|gemmae]]'' in [[Marchantiophyta|liverworts]]. 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.

=== Agamogenesis ===
Agamogenesis is any form of reproduction that does not involve a male gamete. Examples are [[parthenogenesis]] and [[apomixis]].

==== 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, [[rotifer]]s, 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.

==== 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 ''[[Cupressus dupreziana]]'', 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 ==
== Alternation between sexual and asexual reproduction ==

Revision as of 17:58, 30 November 2011

Asexual reproduction in liverworts: a caducous phylloid germinating

Asexual reproduction is a mode of reproduction by which offspring arise from a single parent, and inherit the genes of that parent only, it is reproduction which does not involve meiosis, ploidy reduction, or fertilization. A more stringent definition is agamogenesis which is reproduction without the fusion of gametes. Asexual reproduction is the primary form of reproduction for single-celled organisms such as the archaea, bacteria, and protists. Many plants and fungi reproduce asexually as well.

While all prokaryotes reproduce asexually (without the formation and fusion of gametes), mechanisms for lateral gene transfer such as conjugation, transformation and transduction are sometimes likened to sexual reproduction.[1] A complete lack of sexual reproduction is relatively rare among multicellular organisms, particularly animals. It is not entirely understood why the ability to reproduce sexually is so common among them. Current hypotheses [2] suggest that asexual reproduction may have short term benefits when rapid population growth is important or in stable environments, while sexual reproduction offers a net advantage by allowing more rapid generation of genetic diversity, allowing adaptation to changing environments. Developmental constraints[3] may underlie why few animals have relinquished sexual reproduction completely in their life-cycles.

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Alternation between sexual and asexual reproduction

Some species alternate between the sexual and asexual strategies, an ability known as heterogamy, depending on conditions. Alternation is observed in several rotifer species and a few types of insects, such as aphids which will, under certain conditions, produce eggs that have not gone through meiosis, thus cloning themselves. The cape bee Apis mellifera subsp. capensis can reproduce asexually through a process called thelytoky. A few species of amphibians, reptiles, and birds have a similar ability (see parthenogenesis for examples). 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. Another example are monogonont rotifers of the genus Brachionus, which reproduce via cyclical parthenogenesis: at low population densities females produce asexually and at higher densities a chemical cue accumulates and induces the transition to sexual reproduction. Many protists and fungi alternate between sexual and asexual reproduction.

For example, the slime mold Dictyostelium undergoes binary fission (mitosis) as single-celled amoebae under favorable conditions. However, when conditions turn unfavorable, the cells aggregate and follow one of two different developmental pathways, depending on conditions. In the social pathway, they form a multicellular slug which then forms a fruiting body with asexually generated spores. In the sexual pathway, two cells fuse to form a giant cell that develops into a large cyst. When this macrocyst germinates, it releases hundreds of amoebic cells that are the product of meiotic recombination between the original two cells.[4]

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.[5] A number of plants use both sexual and asexual means to produce new plants, some species alter their primary modes of reproduction from sexual to asexual under varying environmental conditions.[6]

Inheritance of asexual reproduction in sexual species

For example, in the rotifer Brachionus calyciflorus asexual reproduction (obligate parthenogenesis) can be inherited by a recessive allele, which leads to loss of sexual reproduction in homozygous offspring.[7] Inheritance of asexual reproduction by a single recessive locus has also been found in the parasitoid wasp Lysiphlebus fabarum.[8]

Examples in animals

There are examples of parthenogenesis in the hammerhead shark[9] and the blacktip shark.[10] 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.

Reptiles use the ZW sex-determination system, which produces either males (with ZZ sex chromosomes) or females (with ZW or WW sex chromosomes). Until 2010, it was thought that the ZW chromosome system used by reptiles was incapable of producing viable WW offspring, but a (ZW) female boa constrictor was discovered to have produced viable female offspring with WW chromosomes.[11] The female boa could have chosen any number of male partners (and had successfully in the past) but on these occasions she reproduced asexually, creating 22 female babies with WW sex-chromosomes.

Polyembryony is a widespread form of asexual reproduction in animals, whereby the fertilized egg or a later stage of embryonic development splits to form genetically identical clones. Within animals, this phenomenon has been best studied in the parasitic Hymenoptera. In the 9-banded armadillos, this process is obligatory and usually gives rise to genetically identical quadruplets. In other mammals, monozygotic twinning has no apparent genetic basis, though its occurrence is common. There are at least 10 million identical human twins and triplets in the world today.

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 Meselson effect that have allowed them to survive better in periods of dehydration.[12]

Molecular evidence strongly suggest that at least two species of the stick insect genus Timema have used only asexual (parthenogenetic) reproduction for one million years, the longest period known for any insect.[13]

Some scientists, however, regard evidence for long periods of asexual reproduction in multicellular species with skepticism.[14]

See also

References

  1. ^ Narra HP, Ochman H (2006). "Of what use is sex to bacteria?". Current Biology. 16 (17): R705–710. doi:10.1016/j.cub.2006.08.024. PMID 16950097.
  2. ^ Dawson KJ (October 1995). "The Advantage of Asexual Reproduction: When is it Two-fold?". Journal of Theoretical Biology. 176 (3): 341–347. doi:10.1006/jtbi.1995.0203.
  3. ^ Engelstädter J (November 2008). "Constraints on the evolution of asexual reproduction". Bioessays. 30 (11–12): 1138–1150. doi:10.1002/bies.20833. PMID 18937362.
  4. ^ R. S. Mehrotra; K. R. Aneja (December 1990). An Introduction to Mycology. New Age International. pp. 83 ff. ISBN 9788122400892. Retrieved 4 August 2010.
  5. ^ Kathleen M. Cole; Robert G. Sheath (1990). Biology of the red algae. Cambridge University Press. pp. 469–. ISBN 9780521343015. Retrieved 4 August 2010.
  6. ^ Edward G. Reekie; Fakhri A. Bazzaz (28 October 2005). Reproductive allocation in plants. Academic Press. pp. 99–. ISBN 9780120883868. Retrieved 4 August 2010.
  7. ^ C.-P. Stelzer, J. Schmidt, A. Wiedlroither, and S. Riss (2010). Loss of Sexual Reproduction and Dwarfing in a Small Metazoan. PLoS ONE 5(9): e12854. [1]
  8. ^ C. Sandrock and C. Vorburger (2011). Curr Biol. 2011 Mar 8;21(5):433-7.
  9. ^ Savage, Juliet Eilperin (23 May 2007). "Female Sharks Can Reproduce Alone, Researchers Find". Washington Post. Retrieved 27 April 2008.
  10. ^ Chapman, D. D.; Firchau, B.; Shivji, M. S. (11 October 2008). "'Virgin Birth' By Shark Confirmed: Second Case Ever". Journal of Fish Biology. 73 (6). Sciencedaily.com: 1473. doi:10.1111/j.1095-8649.2008.02018.x. Retrieved 13 August 2010.
  11. ^ "'Boa constrictor produces fatherless babies'". CBC News - Technology&Science. 3 November 2010. Retrieved 25 May 2011.
  12. ^ Pouchkina-Stantcheva, N. N.; McGee, B. M.; Boschetti, C.; Tolleter, D.; Chakrabortee, S.; Popova, A. V.; Meersman, F.; MacHerel, D.; Hincha, D. K. (2007). "Functional Divergence of Former Alleles in an Ancient Asexual Invertebrate". Science. 318 (5848): 268–71. doi:10.1126/science.1144363. PMID 17932297.
  13. ^ Davies, Ella. "Sticks insects survive one million years without sex". BBC. Retrieved 19 July 2011.
  14. ^ Judson, Olivia P. (2000). "Sinless Originals". Science. 288 (5469): 1185–6. doi:10.1126/science.288.5469.1185. JSTOR 3075245. PMID 10841738. Asexual organisms – creatures in which this cycle has come to a stop – are widely thought to be doomed to a swift extinction... Time and again, evidence of sex has been found in supposedly ancient asexual lineages, and many evolutionary biologists do not believe that any of the apparent exceptions are real. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)

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.
  • Avise, J. 2008. Clonality: The Genetics, Ecology, and Evolution of Sexual Abstinence in Vertebrate Animals. Oxford University Press

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