The possible origin of thick stem in Chinese wild soybean (Glycine soja)

Abstract

Thicker, erect stem and enlarged seeds are characteristic of the domestication of cultivated soybeans (Glycine max) from its progenitor, wild soybean (G.soja). Wild soybeans have different stem thicknesses but the thick stem as defined here appears in a small number of small-seeded wild soybeans (≤2.0 g/100-seeds) in China. However, little attention has been paid to this phenomenon in considering the origin of thick stem in wild soybean. Here, we addressed this question through the study of a mixed field of wild, semi-wild and cultivated soybeans. Thick-stemmed samples had lower sensitivity to light period, higher mean genetic diversity (He = 0.090, H = 0.535) and higher mean multilocus outcrossing rate (tm = 9.77 %), while thin-stemmed plants were the opposite (He = 0.029, H = 0.416) and lower mean outcrossing rate (tm = 5.88 %). F statistics calculations indicated that there was genetic differentiation between the thin and thick stems. UPGM cluster analysis showed that not only were thick-stemmed wild soybeans genetically different from thin-stemmed ones, but they were also genetically closer to semi-wild soybean, to varying degrees completely dependent on seed size. These data strongly implied that the plants with thick stems had more complicated genetic backgrounds than the thin-stemmed ones, and that they were related to cultivated soybeans. This study suggests that if plants have distinctly thick stems (an average 2.5-fold thicker than other thin-stemmed plants) or stems similar to semi-wild plants and/or near to local soybeans in a natural wild population adjacent to farmlands, such plants could be cultivar-introgressive offspring.