Genetic Diversity and Population Structure of Tetraploid Accessions of the Medicago sativa-falcata Complex

The Medicago sativa-falcata complex includes the economically important forage legume alfalfa and its primary gene pool. The complex includes both diploid and tetraploid taxa, which are usually designated as distinct subspecies. The relationships among wild diploid members of the complex have been clarified using molecular markers, but the relationship among unimproved tetraploid germplasm is poorly understood. Our aim was to investigate the population genetic structure of the tetraploid Medicago sativa-falcata complex to deduce the amount and pattern of genetic diversity using genome-wide simple-sequence repeat (SSR) markers. We used 70 tetraploid accessions (280 genotypes) from the USDA National Plant Germplasm Collection that were collected from throughout the entire natural distribution range of the species and that represented putative wild populations. Population genetic analyses were conducted to determine the patterns of demarcation among accessions and germplasm groups. Model-based cluster analysis indicated that tetraploid alfalfa has two main groups corresponding to the subspecies sativa and falcata. Medicago sativa subsp.. varia produced a hybrid pattern in between M. sativa subsp. sativa and M. sativa subsp. falcata. The studies also revealed that there is a spatial genetic structure among subsp. falcata accessions, implying that extensive sampling from different localities for curation of alfalfa genetic resources is important.