Evolutionary Mechanisms of Apomixis in Higher Plants

研究了细孢子蕨类植物无花蕨属（Asplenium）和毛蕨属（Dryopteris）和被子植物（Boehmeria）和泽兰属（Eupatorium）的无花系遗传变异的来源。对泽兰的细胞学、生化和生物统计学研究表明，有性系和无精子系之间的种间杂交产生新的无精子系遗传变异。用同工酶多态性进行的遗传分析表明，无配子菌株是有性和无配子杂交产生的。在Boehmeria和dryopteria中，形态学证据支持无配花物种的杂交起源。这些结果支持了非配花菌株的遗传变异是通过种间杂交积累的观点。对于无配系菌株的遗传变异程度，从阿斯勒姆和泽兰得到了对比的结果：在日本全地区只发现了4个基因型（日本无配系），而在剑兰中发现了显著的种群内和种群间变异。后者似乎是一种动态状态，由于频繁的杂交与有性物种。需要进一步的研究来阐明这种多态单聚体的遗传性质。在梨属植物中，无配花种的平均杂合度显著高于有性种。这一结果与以往报道的各种孤雌生殖动物的结果一致，表明无性生殖不仅在动物中而且在植物中也保持着较高的平均杂合度。

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.