Geographic description of genetic diversity and relationships in the USDA rice world collection

Knowledge of genetic diversity and relationships among global germplasms of rice collected by the USDA since 1866 is critical for utilization, conservation, and management of the collection. A core subset developed from this collection, including 1794 accessions obtained from 112 countries in 14 geographic regions, was genotyped with one indel and 71 simple sequence repeat (SSR) markers. Total alleles were 1005, averaging 14 alleles per locus. A great majority of genetic variance was due to within instead of among geographic regions and within instead of among countries. The regions and countries were highly and significantly differentiated using these markers. Germplasm accessions obtained from the southern Asia, Southeast Asia, and Africa were highly diversified, while those from North America and western and eastern Europe had the lowest diversity. Different measurements of genetic diversity, including average number of alleles per locus, polymorphism information content (PIC), Nei index, and average number of private alleles per locus uniformly reached this conclusion. Three main clusters were revealed by both analyses of principal coordinates and Nei genetic similarities for the 14 regions. Seventy-eight countries, from which five or more accessions were collected in the core subset, were differentiated into five main clusters. Germplasm accessions obtained from Myanmar, Indonesia, Cambodia, Malaysia, and Nepal were highly diversified, while those from France, Spain, Romania, Italy, and the United States were poorly diversified. This study proves that the USDA global collection effectively supports the U.S. rice industry with vast genetic diversity responsive to biotic and abiotic stresses.