Читать книгу Principles of Plant Genetics and Breeding - George Acquaah - Страница 346

Apomixis is widespread in nature, and occurs in several unrelated plant families. About 10% of the estimated 400 plant families and a mere 1% of the estimated 40 000 plant species they comprise exhibit apomixis. The plant families with the highest frequency of apomixis are Gramineae (Poaceae), Compositae, Rosaceae, and Asteraceae. Many species of Citrus, mango, perennial forage grasses, and guayule reproduce apomictically.

Some species can produce both sexual and apomictic seeds and are called partial apomicts (e.g. bluegrass, Poa pratensis). Species such as bahiagrass (Paspalum notatum) reproduce exclusively or nearly so by apomixis and are called complete apomicts. There are several indicators of apomixis. When the progeny from a cross in a cross‐pollinated species fails to segregate, appearing uniform and identical to the mother plant, this could indicate apomixis. Similarly, when plants expected to exhibit high sterility (e.g. aneuploids, triploids) instead show significantly high fertility, apomixis could be the cause. Obligate apomicts may display multiple floral features (e.g. multiple stigmas and ovules per floret, double or fused ovaries) or multiple seedlings per seed. Facultative apomixis may be suspected if the progeny of a cross shows an unusually high number of identical homozygous individuals that resemble the mother plant in addition to the presence of individuals that are clearly different (hybrid products).

The indicators suggested are by no means conclusive evidence of apomixis. To confirm the occurrence of apomixis and discovery of its mechanisms requires an additional progeny test as well as cytological tests of megasporogenesis and embryo sac development.