Evolutionary Mechanisms of Apomixis in Higher Plants

对钩孢子蕨类的无配子品系和鳞毛蕨属的无配子系和被子植物的无配子品系的遗传变异的起源进行了研究。对紫茎泽兰的细胞学、生物化学和生物计量学研究表明，有性品系和无无精品系之间的种间杂交产生了新的无性繁殖的遗传变异。利用同工酶多态性进行遗传分析表明，无配子生殖品系起源于有性和无配子生殖物种之间的杂交。在苎麻属和鳞毛蕨属中，形态证据支持无配子生殖物种的杂交起源。这些结果支持了无配子生殖品系的遗传变异是通过种间杂交积累起来的观点。在无配子生殖品系的遗传变异程度上，无配子生殖品系和紫茎泽兰的遗传变异程度进行了对比：在日本各地只发现了四种基因类型。单边型(日本无配偶型)，而在种群内和种群间发现显著的变异。由于与有性物种的频繁杂交，后者似乎处于一种动态状态。这种多态的无融合生殖的遗传本质还需要进一步的研究。在无配子生殖物种中，无配子生殖物种的平均杂合度显著高于有性物种。这一结果与以前报道的各种孤雌生殖动物的结果一致，表明无性繁殖不仅在动物中保持着高水平的平均杂合度，在植物中也是如此。

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.