Center of Origin and Centers of Diversity in an Ancient Crop, Brassica rapa (Turnip Rape).

Brassica rapa is the most widely distributed and has the longest history of domestication of the agricultural Brassica species. Molecular genetic diversity, based on 51 simple sequence repeat primer pairs and 715 alleles at polymorphic loci, was used to predict the center of origin and centers of diversity in a global collection of 173 B. rapa accessions. The accessions were separated into 3 molecular genetic groups based on STRUCTURE analysis-group 1 from the classical Old World (Europe and west Asia-north Africa), group 2 from east Asia, and group 3 from east, central, south, and southeast Asia. Accessions classified as "wild" (B. rapa var. sylvestris) were found only in group 1 and this group had the highest number and richness of private alleles. Each group included a diverse range of agricultural morphotypes (oilseed, root, or leafy vegetable types), flowering habit (winter, semi-winter, or spring type), self-compatibility or incompatibility, and seed color. The Old World and east, south, and central Asia were distinct subpopulations based on analysis of shared unique alleles. This study supports the theory that the classical Old World is the center of origin of B. rapa, with centers of diversity in east Asia and along ancient trade routes in Asia, with recent migration to the New World.