Dep. of Agronomy, Univ. of Wisconsin, Madison, WI 53706; *current address: Dep. of Crop and Soil Science, Washington State Univ., Pullman, WA 99164

More heterozygosity in alfalfa cultivars could improve forage yields by increasing the number of loci with favorable dominant alleles and the level of complimentary gene interactions. We previously found that genetic distances of pairs of tetraploid alfalfa genotypes based on molecular markers were significantly correlated with heterozygosity and with forage yields of the single-cross progenies (Kidwell et al. 1994b). Since alfalfa cultivars are synthetic populations developed by intermating many parents, we tested the use of molecular markers for selecting a subset of genetically diverse parents from a population of 93 parents of a commercial cultivar (Kidwell et al. 1998). Based on average pairwise genetic distances between genotypes, four synthetic populations (samples) were developed using 2, 4, 8, 12, 16, or 24 parents selected for genetic dissimilarity (DIS) or similarity (SIM). Forage yields of the synthetic populations varied significantly among samples within parent number and diversity group. However, there was no significant difference between forage yields of DIS and SIM groups averaged over parent number and samples. We concluded that molecular marker selection was generally ineffective due to linkage equilibrium in the population used for selecting parents and the inability to target heterozygosity to specific genome regions affecting yield.

It may be very difficult to extract complementary gene pools that fully exploit heterozygosity from cultivar germplasm because of linkage equilibrium. Two accessions representing the Peruvian and Medicago falcata germplasm sources are very unique based on molecular markers (Kidwell et al. 1994a) and could be used to form a novel genetic complement to cultivar germplasm. However, Peruvian is not adapted to northern climates. We obtained some genetic information on adaptive characteristics of Peruvian using two backcross populations from a cross of Peruvian and cv. Blazer XL. These populations were analyzed for molecular markers and a map was constructed of the four homologous cosegregation groups from the F1 for seven of the eight alfalfa linkage groups. The progeny genotypes, replicated by clonal propagation, were evaluated for winterhardiness, fall dormancy and freezing tolerance in two years. For the backcross to Blazer XL, genomic regions affecting all three traits were identified on linkage groups 5 and 8, but regions affecting only fall growth and freezing tolerance were identified on linkage groups 1 and 3, respectively. Multilocus models for winterhardiness in each of two years included 4-5 regions and explained 36-38% of the genetic variation. We are now developing a genetically unique population by intermating Peruvian and M. falcata genotypes, and this population will be used to initiate a hybrid selection experiment.