Nearly 85% of angiosperms are hermaphrodites and selling occurs in most of them. This inbreeding enhances genetic homogeneity and narrows down the genetic base, limiting their adaptability and evolutionary potential. The limitation in them is negated by outbreeding, which is enforced by the development of forms having either a mixture of male and female flowers (monoecy) or of hermaphrodite and female (mst) flowers (gynomonoecy) or of hermaphrodites with male (fst) flowers (andromonoecy). The conjoint occurrence of hermaphrodites and female (mst) plants in natural populations was detected by Darwin (1877), who designated the phenomenon as gynodioecy, and the species having such populations as gyno- dioecious species. In some, hermaphrodites occur in conjunction with female sterile male plants. Such species are known as androdioecious species. Whereas gynodioecy is prevalent, androdioecy is extremely rare. Since mst’s are common in gynodioecious plants, their constant maintenance is possible through outbreeding, as they must be pollinated by hermaphrodites. Thus gynodioecy constitutes an outbreeding mechanism whose breeding system is comparable to that of dioecy and self-incompatibility. Unlike them, however, it involves interbreeding of mst’s and hermaphrodites. The hermaphrodites contribute pollen grains which fertilize their own ovules as well as those of the mst plants. Consequently they contribute their genes mostly via pollen and to a lesser extent through ovules. On a similar basis, female fertile plants of androdioecious populations transmit their genes mainly via ovules and to a lesser extent through pollen. Thus, the plants of dimorphic androdioecious and gynodioecious populations differ from those of monomorphic (hermaphrodite and monoecious) populations, as the former forms possess an outbreeding advantage which the latter lack. However, because of unidirectional pollen transfer, androdioecy cannot be maintained by an outbreeding advantage, whereas gynodioecy can. This difference may be one of the principal reasons why gynodioecy is much more common than androdioecy. Furthermore, the inability of cytoplasmically determined female sterile mutants to be maintained in an androdioecious system provides another reason to account for high frequency of gynodioecy (up to 3.5%) than that of androdioecy, which is extremely rare. In about 120,000 species, only ten are androdioecious (Yampolski and Yampolski 1922) and even these might be the result of year-to-year variation in sex, expression (Bertin 1982).
KeywordsInbreeding Depression Plantago Lanceolata Dioecious Species Heterozygote Advantage Cytoplasmic Gene
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