Definitions

apomixis

apomixis

[ap-uh-mik-sis]

Reproduction by special tissues without fertilization. Examples include parthenogenesis in animals (in which a new individual develops from an unfertilized egg) and apogamy in plants (in which the generating tissue may be either the gametophyte or the sporophyte). Apomixis provides for the preservation of traits favourable to individual survival, but it eliminates the longer-term evolutionary advantage of genetic contribution from two parents.

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In botany, apomixis (also called apogamy) is asexual reproduction, without fertilization. In plants with independent gametophytes (notably ferns), apomixis refers to the formation of sporophytes by parthenogenesis of gametophyte cells. Apomixis also occurs in flowering plants, where it is also called agamospermy. Apomixis in flowering plants mainly occurs in two forms: In agamogenesis (also called gametophytic apomixis), the embryo arises from an unfertilized egg that was produced without meiosis. In adventitious embryony , a nucellar embryo is formed from the surrounding nucellus tissue. Apomictically produced seeds are genetically identical to the parent plant.

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.). Examples of apomixis can be found in the genera Crataegus (hawthorns), Amelanchier (shadbush), Sorbus (rowans and whitebeams), Rubus (brambles or blackberries), Hieracium (hawkweeds) and Taraxacum (dandelions). Although the evolutionary advantages of sexual reproduction are lost, apomixis does pass along traits fortuitous for individual evolutionary fitness.

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).

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