Evolutionary Mechanisms of Apomixis in Higher Plants

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The origin of genetic variations in apogamous strains were studied for Asplenium and Dryopteris of leptosporangiate ferns and Boehmeria and Eupatorium of angiosperms. Cytological, biochemical and biometrical studies on Eupatorium showed that interspecific hybridizations between sexual and agamospermous strains generate new genetic variants reproducing agamospermously. In Asplenium, genetic analysis using isozyme polymorphism showed that an apogamous strain was originated from hybridization between sexual and apogamous species. In Boehmeria and Dryopteris, morphological evidence supported hybrid origin of apogamous species. These results support the view that genetic variations of apogamous strains are accumulated through interspecific hybridization.For degree of genetic variations in apogamous strains, contrastive results were obtained from Asplenium and Eupatorium: only four genotypes were found throughout Japan in A. aff. unilaterale (Japanese apogamous type), wheras notable intra- and interpopulational variations were found in E. scabridum. The latter seems to be in a dynamic state as a result of frequent hybridizations with sexual species. Further studies are needed to elucidate genetic nature of such polymorphic apomicts.In Asplenium, mean heterozygosity of apogamous species was notably higher than that of sexual species. This results agree well with the previously reported results on various parthenogenetic animals and suggests that asexual reproduction maintains high level of mean heterozygosity not only animals but also in plants.