RFLP EVALUATION OF 9 MEDICAGO ACCESSIONS REPRESENTING THE ORIGINAL GERMPLASM SOURCES FOR NORTH-AMERICAN ALFALFA CULTIVARS

All alfalfa (Medicago sativa L.) cultivars grown in North America originate from nine germplasm sources introduced into the USA from different geographic regions. Nine accessions representing the original germplasm sources have been developed; however, the level of genetic diversity within and among these accessions is not known. Genetic diversity of these accessions was estimated by screening DNAs from 12 individuals of each accession, along with bulked samples containing equal amounts of DNA from 4, 6, or all 12 individuals, for nuclear RFLPs revealed by hybridization to 35 cloned alfalfa sequences. One hundred and eighty restriction fragments were scored as present or absent across all individuals from all accessions, and genetic similarity estimates (GSEs) of all pair-wise comparisons were calculated from these data. The data were also examined by cluster analysis and principal components analysis (PCA). Individuals within accessions were highly polymorphic, whereas very few accession-specific polymorphisms were identified. Although individuals from two sources, M. falcata and Peruvian, formed distinct clusters, genotypes from the other seven accessions were not clearly discriminated by analyses of these data. The nondistinctiveness of most of these plant introductions (PIs) could be due to a similar genetic composition of the initial introductions, intercrossing of the different sources during maintenance, or genetic shift during maintenance. Based on PCA, bulked DNAs underestimated the level of genetic diversity both within and between accessions, as compared with estimates derived from analysis of individual genotypes from each source.