Alfalfa synthetic cultivars are produced by intermating large numbers of individuals, often 40 or more. Large numbers of parents are used in order to achieve maximum heterozygosity and avoid inbreeding depression. Yield and heterosis in alfalfa are positively correlated with genetic diversity based on DNA markers. The objective of this study is to use DNA markers to select a small number of genetically diverse parents, to produce high yielding, narrow-based synthetic cultivars. AFLP markers were used to assess the genetic diversity among 120 parents of a ‘CUF 101’ synthetic cultivar selected for grazing tolerance. Synthetics consisting of 12 and 24 parents were assembled based on genetic dissimilarity (GD) of AFLP markers. The complement of the Dice coefficient was used as a measure of GD, and cluster analysis was used to form groups of 12 and 24 individuals with similar GD scores. From each group, the individual with the highest average number of AFLP bands was selected as a parent for the synthetic. In this way, each synthetic consisted of individuals that were the most genetically diverse, with the maximum number of marker alleles. The 12, 24 and 120 parent synthetics were intermated in the greenhouse to produce seed. Two yield trials and one grazing tolerance trial are being conducted in three locations. The three synthetics were tested for resistance to Anthracnose, Aphanomyces, Phytophthora, Verticillium, and Blue Alfalfa Aphid. In all cases, no significant loss of resistance was seen when narrowing the genetic base from 120 parents to 24 or 12 parents.

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