Study of the evolution of heterosporous plants with respect to pollination and breeding systems

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Land plants have the major trend of evolution from homospory to heterospory with megaspores and microspores. It is interpreted that heterospory is beneficial compared to homospory in avoidance of gametophytic selfing and that heterosporous pteridophytes evolved from zoosporous ones, followed by seed plants that evolved from heterosporous pteridophytes. In this study, we attempted to examine the validity of the working hypothesis that the common selective pressure for organisms to avoid selfing would force transitions from homospory to heterospory and to seed habit. We focused on the breeding system and pollination system and compared them among different groups of plants. From field researches in Japan and adjacent regions and publication research, we observed microspores attached on the surface of megaposres in Selaginella and Isoetes : Results suggestged "spore pollination," compared to pollination with pollen (=microspores) on ovules or stigmas. We collected megaspores of Isoetes, which would be useful to examine whether "spore pollination" occurred in geological times. In anatomical observations, we verified that aerial canals form with disintegration of endodermises in Selaginella and suggested that this terrestrial heterosporous pteridophyte might have been aquatic, like Isoetes and heterosporous water ferns. Genetic analyses were made to determine the reproductive mode of Isoetes, in which selfing might occur due to "spore pollination." Results are under investigation. A reproductive phonological observation was made to reveal the breeding system of the monoecious Pinus, one of exceptions in dioecious gymnosperms. Results suggest that it has a crossing system, due to protandry. We compared the dioecy and functional dioecy with the hermaphrodite flowers of primitive angiosperms (e.g. Ambrella, Nymphaeales) and advanced ones, and implied that remarkable changes might have taken place during the evolution from gymnosperms to angiosperms.