Phenotypic diversity of the big seed type subcollection of wild soybean (Glycine soja Sieb. et Zucc.) in China

Wild soybean (Glycine soja), the progenitor of soybeans, has become an important germplasm source for soybean breeding. The Chinese G. soja germplasm collection includes two subcollections, one typical wild soybean (G. soja) and the other 'big seed type' (semi-wild soybean, G. gracilis). In order to understand the nature of this big seed type subcollection, a considerable number of 1,185 accessions were investigated focusing on phenotypes, compositional structure and genetic diversity of this subcollection. The results showed that there were 617 combined morphotypes formed by six major morphological traits. The morphotype number and diversity gradually decreased among four sections from small to large 100-seed weight sizes. The big seed subcollection accumulated distinctly high frequencies for white flower, grey pubescence, seed non-bloom and four seed coat colours (green, yellow, brown and black), differing from the typical G. soja. Between the two subcollections, only the compositional structures of leaf shapes were almost in agreement. A rare strip-shaped leaf only appeared in typical G. soja and small-seeded grade (100-seed wt) of the big seed type. Seed non-bloom trait showed that the increased frequencies were concomitant with the augmenting of 100-seed weight. The data suggested that white flower accumulated rapidly in the big seed type, followed by seed non-bloom, while the slowest rate of accumulation was grey pubecence, and that the 'large-seeded type (2.51-3 g 100-seed wt)' of typical G. soja and the 'smallest-seeded type (3.01-3.5 g 100-seed wt)' of the big seed type could have a closer genetic relationship. The morphotype number and genetic diversity showed reduced values with increased 100-seed weights. The geographical distribution of genetic diversity indicated two tendencies: decreased from north to south and also from east to west. Our results suggested that Northeast China was the first centre of morphological diversity for the big seed type, and the North was the secondary centre in China.