Interspecific Hybridization among Four Species of the Genus Vigna Savi

Abstract

Crosses with three lines of mung bean (V. radiata) and two lines each of adzuki bean (V. angularis), black gram (V. mungo) and rice bean (V. umbellata) were made in order to determine the barriers to hybridization between species, and to assess the possibility of overcoming these barriers. This would provide useful information for exploiting wide germplasm resources in breeding these valuable pulse crops. Reciprocal differences were common in all the species combinations. These differences were observed in pod set, embryo abortion, embryo culture, and seedling lethality. No crosses were successful in both directions. The results of six interspecific combinations are summarized as follows: (1) Adzuki bean and black gram was the least successful combination. Although pod set was normal when adzuki bean was used as the female, embryo abortion was complete and no seedlings obtained from embryo culture survived beyond the seedling stage. (2) Black gram and rice bean was also a very difficult combination to produce. Some seedlings obtained by embryo culture from crosses using black gram as the pistillate parent reached adult stage, but died before flowering. (3) Adzuki bean and mung bean was a somewhat more successful combination. Hybrids from mung bean produced by embryo culture flowered profusely but were completely sterile. Lack of chromosome pairing, shown by an average of 2.39pi + 17.22p seems likely to be a principal cause of sterility. (4) The mung bean and rice bean combination was somewhat easier to make than the last. Embryo abortion is not complete so that some crinkled but viable seeds are produced on mung bean used as the pistillate parent. However, the hybrids, with average chromosome pairing of 0.09jy + 4.69jj + 12.26j, are still completely sterile. Colchicine-induced amphidiploids showed regular meiosis with 22 bivalents and greatly improved fertility, indicating that at least in this combination much of the sterility of the diploid hybrid is caused by chromosomal factors. However, low pod set and defective seeds in the amphidiploid indicate that some adverse genic interactions are still found. (5) The mung bean and black gram combination (the former used as the pistillate parent) produced hybrid seeds with good germination. However, the F^ hybrids were only partially fertile. One quadrivalent in 37% of the cells examined indicates that one reciprocal translocation has occurred during chromosome differentiation of these two species. Segregation for morphological characters and the appearance of highly fertile individuals in the F2 and BCi demonstrate the possibility of gene exchange between these two species. (6) The adzuki bean and rice bean combination obtained by culture of embryos from rice bean used as the pistillate parent was highly fertile with completely normal bivalent formation. Segregation for morphological characters and high fertility in the F2 and BC-^ indicate the possibility of gene exchange between these species also. On the basis of cross ability relationships, and on the assumption that cytoplasm which prevents the development of a hybrid embryo is the evolved cytoplasm, while cytoplasm which permits development of a hybrid embryo is the original type, evolutionary relationships of these four species are proposed. Mung bean and rice bean are assumed to have been derived independently from a common ancestor, possibly V. radiate var. soblobata which is a wild species. Black gram and adzuki bean are considered to be later derivatives from mung bean and rice bean, respectively. Thus, these four species can be divided into two subgroups, mung bean with black gram and rice bean with adzuki bean, in which the two species of each group retain enough chromosomal homology to allow germplasm exchange within the group.