Genetic base of Japanese soybean cultivars released during 1950 to 1988.

Plant breeding success is dependent, in part, upon the genetic diversity found within applied breeding programs. To characterize genetic diversity in applied breeding, plant breeders have invoked the concept of genetic base, which can be defined as the ancestral pool from which breeding is derived. The genetic base of modern Japanese soybean [Glycine max (L.) Merr.] cultivars is not well characterized. The objective of this study was to quantify the genetic base of Japanese soybean cultivars by coefficient of parentage (CP) analysis, to compare the genetic bases of major growing regions and release eras in Japan, and to compare the Japanese base with that of other countries. Seventy-four ancestors were identified in the pedigrees of 86 public Japanese cultivars registered from 1950 to 1988. Ancestors originating from Japan contributed 76% of the genes to the Japanese breeding, while exotic ancestors from the USA and Canada (US-CAN), China, and Korea contributed 2, 5, and 2%, respectively. The remaining portion of the base was of unknown, but presumed Japanese origin. Three major growing regions of Japan displayed very distinct genetic bases with at least 50% of the ancestral contribution unique to each region. Comparisons revealed that the Japanese base was more diverse than that of the US-CAN. The more diverse genetic base was exemplified by (i) more ancestors accounting for 50 and 80% of the genes in Japanese breeding; (ii) a continual expansion of the genetic base since 1950, while the US-CAN base remained relatively static; and (iii) a higher ratio of ancestors employed to cultivars released. The number of ancestors contributing to breeding in Japan was much smaller than that for China in terms of number of ancestors, even though both genetic bases expanded with time. The long history of soybean breeding in Japan, its diverse genetic base and its relative isolation from US-CAN and China suggest that Japanese, Chinese, and North American breeding pools may serve as important reservoirs of diversity for each other. Twelve Japanese cultivars released from 1950 through 1988 derived at least 25% of their pedigree from improved U.S. or Chinese breeding materials.