The subgenus Ceratotropis of the genus Vigna which originated in Asia includes the cultigens mungbean, blackgram, adzuki bean, rice bean and moth bean. We collected their wild related species oh the Ryukyu islands of Japan as well as in the northern part of Thailand and in Peninsular Malaysia.
Analysis of cross-compatibility using wild and cultivated Ceratotropis species showed that V.nakashimae and V.riukiuensis are cross-compatible with adzuki bean. Hybridization between adzuki bean and rice bean cannot be easily achieved. However, since V.riukiuensis is also cross-compatible with rice bean, V.riukiuensis could be useful as a bridge species between adzuki bean and rice bean.
V.reflexo-pilosa and V.glabrescens are both tetraploid species (2 n = 44) and are cross-compatible with each other. Cytogenetical analysis by interspeciffic hybridization involving diploid species showed that V.glabrescens has a homologous genome with rice bean but not with either mungbean or blackgram.
Isozyme analysis suggested that V.reflexo-pilosa originated from interspecific hybridization between V.minima and V.trinervia followed by natural chromosome doubling. V.glabrescens is considered to be derived from V.reflexo-pilosa as cultivated type with erect growth habit. Isozyme analysis also suggested that V.trinervia is phylogenetically intermediate between the "mungbean group" showing an epigeal germination habit and the "adzuki bean group" showing a hypogeal germination habit.