Diversity of chloroplast DNA SSRs in wild and cultivated soybeans: evidence for multiple origins of cultivated soybean

Soybean [Glycine max (L.) Merr.] is one of the major crops in the world and was domesticated from a wild progenitor, Glycine soja Sieb. & Zucc., in East Asia. In order to address the questions concerning the evolution and maternal lineage of soybean, we surveyed the variation in chloroplast DNA simple sequence repeats (cpSSR) of 326 wild and cultivated soybean accessions that were collected from various Asian countries. Twenty-three variants were detected at six cpSSRs in the accessions tested. All of the variants were found in wild soybean, whereas only 14 variants existed in the cultigen. Combining the variants at the six cpSSRs gave 52 haplotypes in the former and eight haplotypes in the latter. Both analyses indicated a considerably higher genetic diversity in the wild soybean. Around 75% of the cultivated accessions tested possessed a common haplotype (no. 49), which was detected in only seven wild accessions, six from southern Japan and one from southern China. The predominant haplotype in the cultigen may therefore have originated from a rare haplotype of the wild soybean that is presently distributed in the southern areas of Japan and China. The remaining seven haplotypes in the cultigen were distributed regionally, and except for three rare haplotypes, largely overlapped with the distributions of wild accessions with the same respective haplotypes. Our results strongly suggest that the cultivated soybeans with different cpDNA haplotypes originated independently in different regions from different wild gene pools and/or hybrid swarms between cultivated and wild forms.