Presence of phylogeographic structure among wild diploid alfalfa accessions (Medicago sativa L. subsp microcarpa Urb.) with evidence of the center of origin

The genetic structure of wild germplasm on a macrogeographic scale has implications for collection and conservation of wild genetic resources of economically important plants. Cultivated alfalfa is derived from a taxonomic group called the Medicago sativa-falcata complex, which includes a number of morphologically and genetically differentiated diploid and tetraploid subspecies representing the primary gene pool for alfalfa improvement. Although the origin of the taxa included in M. sativa-falcata complex has been described broadly, the center of diversity for each taxon is not clear. In this study, 60 individual genotypes from 16 accessions of the diploid taxon M. sativa subsp. microcarpa Urb. [M. sativa subsp. caerulea (Less. ex Ledeb.) Schmalh.] were obtained from the two proposed centers of diversity, Caucasia and Central Asia, and were genotyped with 89 simple sequence repeat markers. Phylogenetic reconstructions together with population genetics statistics (heterozygosity, allelic diversity, and F-ST) were used to describe the pattern of present genetic diversity, to deduce biogeographic history, and to infer the center of diversity. Our results revealed that the Central Asian accessions were distinct from the Caucasian accessions, and that overall accessions, genetic distance was positively correlated with geographical distance, indicating a spatial genetic structure. The accessions from the Caucasian region had higher mean F-ST values, higher allele diversity and higher heterozygosity, suggesting that the Caucasian region is the likely center of diversity for M. sativa subsp. microcarpa [M. sativa subsp. caerulea] and the accessions recolonized Central Asia from Caucasia after the glacial retreat.