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Asexual reproduction

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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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