The genus Trillium is one of the representative temperate woodland elements which currently show disjunct distribution between North America and eastern Asia including Japan. The genus consists of 46 species and 10 species are known to occur in eastern Asia. Of those eastern Asiatic, species, eight species occur in Hokkaido, Japan. Furthemore, based on the chromosome studies, it has been also known that the eight species consist of three species and five hybrid and/or hybrid derivative species from putative hybridization among the three basic species, and they form polyploid series, 2x, 3x, 4x, 6x. By contrast, all of the North America species are known to be diploid.
In order to clarify the ecological mechanisms of natural hybridization and speciation in the eastern Asiatic Trillium, I focused on T.*hagae (triploid species), which is a natural hybrid species between T.kamtschaticum (diploid species) and T.tschonoskii (tetraploid species). This hybrid species commonly occurs where both parental species grow sympatrically. In this study, breeding system, spatial distribution patterns of flowering individuals and flowering phenology of the three Trillium species were examined in the field, and RFLP analysis of cytoplasmic DNA was also employed. As a result, it was suggested that T.hagae is predominantly derived from the hybridization between T.kamtschaticum (maternal) and T.tschonoskii (paternal).