Genetic Diversity in Soybean

It is the purpose of this paper to illustrate the impact of geography, climate, and humankind in shaping the present-day genetic diversity in soybean [Glycine max (L.) Merr.]. Examination of soybean germplasm collections around the globe reveals that an enormous phenotypic range in genetic traits exists in soybean, which is well beyond the phenotypic range observed in the wild progenitor (Glycine soja Seib. et Zucc.). Maturity date, seed coat color, plant height, seed size, and seed yield are noted examples of traits which have a wider phenotypic range in G. max than in the wild G. soja. The diversity found in domesticated soybean is the result of over 3,000 years of cultivation in which Chinese farmers selected more than 20,000 landraces ( defmed as cultivars that predate scientific breeding). The extensive range in phenotype embodied in landraces today is the result of the slow spread of soybean throughout geographically diverse Asia (China first, then Korea and Japan), the continual occurrence of natural mutations in the crop, and both conscious and unconscious selection for local adaptation. The more recent spread of soybean out of Asia in the past 250 years, coupled with modem breeding efforts of the past 70, has intensified and globalized the process of local adaptation and increased the phenotypic range in soybean beyond that of landraces. The increased range in phenotype for modem cultivars includes increases in seed yield, elevation of seed protein/oil concentration, and development, only within the past 20 years, of commercial cultivars that are sufficiently tall and adapted to be grown profitably near the equator. The phenotypic range and distribution observed in modem cultivars and antecedent landraces have clear biogeographical interpretations which relate primarily to genetic alteration of photoperiod response (a prerequisite to adaptation to diverse latitudes) and tolerance to climate extremes. Although the phenotypic range in genetic traits has been expanded in modem soybean through global dispersal and genetic recombination, it is perhaps surprising that that these factors have not had a corresponding positive impact on genetic diversity of modem breeding programs outide of China.. Genetic diveresity in breeding programs is important as a concept, because it is a measure of the potential of a country to develop new and substantially improved cultivars. For the purposes of this paper, genetic diversity in breeding programs is defined, as ge-