| The U.S. annual Medicago germplasm collection is under-utilized due to lack of agronomic information about the species. Development of a core collection, a subset of a large germplasm collection that contains accessions chosen to represent the genetic variability of the germplasm collection, could improve utilization and management of this germplasm collection. Core collections are assembled by grouping and then selecting accessions from these groups. The number of accessions selected per group is constant or related to group size. These selection procedures maximize diversity among groups but ignore diversity within groups. The Relative Diversity Method is an alternative procedure suggested here which selects the number of accessions within a group based on the group's phenotypic or genetic diversity. Using the Relative Diversity Method, a core collection of 211 accessions of annual Medicago species was selected from a subset of 1240 accessions, and evaluated for sixteen agronomic and morphological traits. This core collection was compared to ten other cores assembled using methods which differed in their use of evaluation and passport data, and selection strategies. Core collections assembled based on evaluation data and cluster analysis better represented the variability within the germplasm collection than cores assembled based solely on passport data and random selection of accessions. The Relative Diversity and Logarithm methods were the best methods used in the assemblage of the annual Medicago species core collection. To determine the influence of sample size on the core collection, cores of 5, 10 and 17% of the germplasm collection were compared. The 5 and 10% core collections were found insufficient to represent the initial subset. The core collection was evaluated for four agronomic traits in eight diverse environments across the U.S. to determine the stability and study the genotype X environment interaction of the core, and three agronomically important species: M. lupulina, M. polymorpha, and M. scutellata. Differences among environments were more important than differences among the species and the genotype by environment interaction. Overall, the core collection was unstable across environments, indicating that phenotypic evaluations performed in one location and year were insufficient for the development of a good core collection. |
