Phenotypic Diversity of Modern Chinese and North American Soybean Cultivars

to expand the genetic base of NA breeding. In that regard, China has released 651 cultivars from 1923 to 1995 Chinese and North American (NA) soybean breeding programs (Cui et al., 1999). Through pedigree analysis, Cui et al. have a 70-yr history of genetic progress in relative isolation from each other. Because both programs rest upon a genetic base that is primarily (2000a,b) established that Chinese cultivars were deChinese in origin, the actual genetic distinctness of Chinese and NA rived from a far greater number of ancestors than were breeding is not clear. The objectives of this study were to (i) develop NA cultivars (339 vs. 80 ancestors) and that the two rea phenotypic similarity (PS) index for a large group of Chinese and NA gions had few identifiable ancestors in common. Carter cultivars, on the basis of biochemical, morphological, and agronomic et al. (2000) also demonstrated that several Chinese cultraits, (ii) compare Chinese and NA cultivars for PS through cluster tivars performed well agronomically in North America. analysis, and (iii) use results to develop guidelines for management In China, soybean breeders have recognized the poof the contrasting Chinese and NA breeding programs as reservoirs tential importance of China and NA as mutual reserof diversity. Chinese (47) and NA (25) cultivars were evaluated for voirs of diversity. They have released 67 cultivars since 25 traits in growth chambers. Traits pleiotropic to maturity were 1974 with at least 25% of the pedigree derived from avoided. Significant (P 0.05) differences between Chinese and NA cultivars were noted for leaf and seed traits. Multivariate analysis capmodern NA cultivars (Cui et al., 1999, 2000a). Many tured 79% of the total genotypic variation among the 72 cultivars and of these cultivars are high yielding, a finding that is was used to develop PS estimates. Cluster analysis of PS showed a consistent with the general notion that modern Chinese much greater phenotypic diversity among Chinese than among NA and NA cultivars may contrast in a beneficial way. cultivars and a striking distinctness between the two groups. The Despite important signals pointing to the desirability contrasting nature of Chinese and NA cultivars in this study is theoof exotic cultivars in Chinese and NA soybean breeding, rized to reflect that (i) the NA cultivars may trace to a subset of the the true utility of cross breeding these two cultivar pools, Chinese cultivar genetic base, and/or (ii) Chinese and NA cultivars especially in terms of NA yield improvement, remains may have diverged phenotypically via breeder selection pressure. an open question. No NA cultivar has been developed Cluster results here, based on PS, agreed roughly with previous cluster from modern Chinese cultivars. No unique yield genes analyses, which were derived from pedigree analysis. The physical distinctness of NA and Chinese cultivars shows that introgression of have been identified in these two breeding pools. Chinese cultivars into NA breeding should broaden NA germplasm’s The uniqueness of Chinese and NA breeding pools agronomic, morphological, and biochemical diversity. Introgression have been elucidated to date primarily through pedigree may be accomplished most effectively by avoiding matings of Chinese analysis. The value of cross-breeding Chinese with NA and NA cultivars from the same phenotypic cluster. cultivars will be greater than otherwise if the two groups of cultivars are distinct not only in terms of published pedigree, but in the underlying genetics as well. A limG diversity is important to applied crop iting factor in the pedigree analyses of diversity for breeding, because diversity may reduce vulneraChinese and NA cultivars is that, although the genetic bility to pests and, at the same time, accelerate breeding bases of North America and China contrast in size and progress for an agronomic trait such as yield. In China, specific members, most members of these two bases the continuing use of diverse landraces and exotic modtrace their origins to China. Thus, it is possible that ern cultivars as parental stock in recent decades has Chinese and NA cultivars may be derived from a combeen associated with improved yield and pest resistance mon preexisting genetic base despite the lack of supportin soybean [Glycine max (L.) Merr.] (Gai, 1997; Cui et al., ing pedigree evidence. In that regard, as many as 20 00