Apomixis
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In botany, apomixis is asexual reproduction, without fertilization. Apomixis mainly occurs in two forms: In agamogenesis, the embryo arises from an unfertilized egg via a modified meiosis. In agamospermy (also called apogamy), a nucellar embryo is formed from the surrounding embryo sac tissue. Apomictically produced seeds are genetically identical to the parent plant. Although the evolutionary advantages of sexual reproduction are lost, apomixis does pass along traits fortuitous for individual evolutionary fitness.
As apomictic plants are genetically identical from one generation to the next, each has the characters of a true species, maintaining distinctions from other congeneric apomicts, while having much smaller differences than is normal between species of most genera. They are therefore often called microspecies. In some genera, it is possible to identify and name hundreds or even thousands of microspecies, which may be grouped together as aggregate species, typically listed in Floras with the convention "Genus species agg." (e.g., the bramble, Rubus fruticosus agg.). Good examples of apomixis can be found in the genera Crataegus (hawthorns), Sorbus (rowans and whitebeams), Rubus (brambles or blackberries), Hieracium (hawkweeds) and Taraxacum (dandelions).
A unique example of male apomixis has recently been discovered in the Saharan Cypress, Cupressus dupreziana, where the seeds are derived entirely from the pollen with no genetic contribution from the female "parent" (Pichot, et al., 2000, 2001).
In zoology parthenogenesis is the animal equivalent of apomixis. Recently, Matthew Meselson won the Lasker Award 2004. He and his students are probing why sex is necessary for evolution. Some small aquatic animals, bdelloid rotifers, are apomictic and have survived for millions of years without sex. They serve as an experimental model system. Meselson assumes that the advantage of sex may lie in its ability to reduce what he calls "genetic parasites" (i.e.transposable elements). These are pieces of DNA that multiply on their own and can cause genetic damage. Bdelloid rotifers don't appear to have such parasites.
[edit] See also
[edit] Further reading
- Arkhipova I. & Meselson M. (2000). Transposable elements in sexual and ancient asexual taxa. Proceedings of the National Academy of Sciences of the United States of America. 97:14473-14477
- Arkhipova I. & Meselson M. (2005). Deleterious transposable elements and the extinction of asexuals. BioEssays. 27:76-85
- Normark BB, Judson OP, & Moran NA (2003). Genomic signatures of ancient asexual lineages. Biological Journal of the Linnean Society. 79:69-84.
[edit] References
- Gvaladze G.E. (1976). Forms of Apomixis in the genus Allium L. In: S.S. Khokhlov (Ed.): Apomixis and Breeding, Amarind Pub., New Delhi-Bombay-Calcutta-New York pp. 160-165
- Pichot, C., Fady, B., & Hochu, I. (2000). Lack of mother tree alleles in zymograms of Cupressus dupreziana A. Camus embryos. Ann. For. Sci. 57: 17–22.
- Pichot, C., El Maátaoui, M., Raddi, S. & Raddi, P. (2001). Surrogate mother for endangered Cupressus. Nature 412: 39.
