Floral Biology of Heterotropa tamaensis (Aristolochiaceae) in Japan

Abstract The floral biology of Heterotropa tamaensis (Makino) F. Maekawa was investigated in native populations. The flowers were protogynous, and the stigmas and anthers were distant from one another. These floral characters are significant for cross-pollination, but breeding experiments showed that the flowers are fairly self-compatible and their pollination is obligately dependent upon animal pollen vectors. The most efficient pollinator for H. tamaensis is a fungus gnat, a member of Mycetophylidae. The fungus gnat visited the flowers relatively frequently and often carried many pollen grains on their hairy bodies. Furthermore, their eggs were frequently found within flowers collected from native habitats. The fungus gnats actively moved about within the flowers and often touched the stigmas and anthers with their bodies. Therefore, H. tamaensis seems to be predominantly self-pollinated, though cross-pollination may occur rarely. The presence of fungus gnat eggs in many flowers strengthened Vogel's hypothesis that the flowers of Heterotropa species mimic the basidiomycetes, in which fungus gnats usually oviposit.