Biogeography, Local Adaptation, Vavilov, and Genetic Diversity in Soybean

Abstract

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 (defined 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 modern 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 modern 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 modern 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 modern 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 modern 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 genetic variation among cultivars found within a particular country or country subregion. Empirical analysis of DNA marker and pedigree diversity in modern cultivars indicates that diversity is greatest in cultivars developed in China, less in Japan and least in North America. Phenotypic analysis of modern Chinese and North American cultivars follows the same pattern of diversity. Pedigree analyses of Latin American breeding programs, although incomplete, show that these programs are derived primarily from a subset of North American breeding stock and are, thus, likely to be less diverse than the North American breeding program. Decreased diversity of cultivars outside of China was also correlated with a reduction in the number of founding stock used to establish the breeding programs from which the cultivars arose.

Although conscious breeding choices, the high economic costs of breeding, and historical factors can be used to explain the reduced diversity in breeding programs outside of China vs. within, it is important to note that these results, obtained from modern breeding programs, are consistent with (1) Vavilov’s principle of crop domestication, which states that genetic diversity will be greatest at the center of domestication (China in the case of soybean), and (2) the concept of Darwinian genetic drift which can be used to infer that genetic relatedness or uniformity will increase within breeding populations that are derived from relatively few founding members. A precaution gleaned from the observed trend in diversity is that all soybean breeding programs outside China, regardless of the phenotypic superiority of their genetic breeding materials, should be examined to determine the adequacy of genetic diversity. The impact of the transgenic glyphosate resistance on genetic diversity in soybean is assessed briefly.