Dioecy derived from distyly and pollination in Psychotria rubra ( R ubiaceae) occurring in the R yukyu I slands, J apan

Floral morphology and breeding system were examined in natural populations of Psychotria rubra ( R ubiaceae) occurring in the R yukyu I slands in southwest J apan. This species is morphologically dimorphic with long‐ and short‐styled morphs; however, these morphs are functionally dioecious. All long‐styled morphs set fruits, but their short anthers completely lack pollen grains; thus, they function as pistillate flowers. Alternately, short‐styled morphs usually have fertile pollen and never set fruit, whether after open or obligate pollinations; thus, they function as staminate flowers. However, some short‐styled morphs in M ounts Katsuu‐dake and O ppa‐dake populations have no fertile pollens. No fruit was obtained from bagging experiments, indicating that P. rubra requires pollinators for its reproduction. In our pollinator observations, flies and short‐tongued wasps were the main visitors of the flowers of P. rubra . Thus, these insects are probably effective pollinators for P. rubra in the R yukyu I slands. Considering the morphological features, the dioecy found in P. rubra is probably derived from distyly; however, no sufficient evidence is available showing that dioecy has evolved “directly” from distyly.