Apomixis

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In botany, apomixis was defined by Winkler as replacement of the normal sexual reproduction by asexual reproduction, without fertilization[1]. This definition notably does not mention meiosis. Thus "normal asexual reproduction" of plants, such as propagation from cuttings or leaves, has never been considered to be apomixis, but replacement of the seed by a plantlet, or replacement of the flower by bulbils are types of apomixis. Apomictically produced offspring are genetically identical to the parent plant.

In flowering plants, the term "apomixis" is commonly used in a restricted sense to mean agamospermy, i.e. asexual reproduction through seeds.

Apogamy is a related term that has had various meanings over time. In plants with independent gametophytes (notably ferns), the term is still used interchangeably with "apomixis", and both refer to the formation of sporophytes by parthenogenesis of gametophyte cells.

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Apomixis and evolution

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.). In some plant families, genera with apomixis are quite common, e.g. in Asteraceae, Poaceae, and Rosaceae. Examples of apomixis can be found in the genera Crataegus (hawthorns), Amelanchier (shadbush), Sorbus (rowans and whitebeams), Rubus (brambles or blackberries), Poa (meadow grasses), Hieracium (hawkweeds) and Taraxacum (dandelions).

Although the evolutionary advantages of sexual reproduction are lost, apomixis can pass along traits fortuitous for evolutionary fitness. As Clausen eloquently put it[2] (page 470) "The apomicts actually have discovered the effectiveness of mass production long before Mr Henry Ford applied it to the production of the automobile. ... Facultative apomixis ... does not prevent variation; rather, it multiplies certain varietal products." Facultative apomixis means that apomixis does not always occur, i.e. sexual reproduction also can happen. It appears likely[3] that in plants all apomixis is facultative, i.e. that "obligate apomixis" is an artifact of the observation methods.

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