MOLECULAR MARKER ANALYSIS OF THE APPROACH TO HOMOZYGOSITY BY SELFING IN DIPLOID ALFALFA

Although inbreeding is used extensively in alfalfa breeding and geneti cs, the actual approach to homozygosity during inbreeding is unknown a nd may differ from that predicted by the inbreeding coefficient (F) be cause of the need to select for vigor and fertility when inbreeding. O bserved and expected values of heterozygosity were compared by means o f 40 co-dominant RFLP loci for two inbred series each consisting of fo ur independent lines derived by selfing two diploid alfalfa genotypes. Selling was continued with selection for fertility until self-sterili ty was reached at the S-2, S-3, or S-4 generation. Additional S-1 prog enies were also evaluated to identify the heterozygous RFLP loci. The observed percentage heterozygosity, calculated as the percentage of th e 40 RFLP loci still heterozygous in each plant, declined less than ex pected based on the inbreeding coefficient in each inbred series. The lowest observed percentage heterozygosity was 30% in one inbred series and 40% in the other inbred series despite expectations as low as 6%. Tests of genotype and allele ratios indicated that the deviations may be due to selection favoring heterozygotes. Our results show that inb reeding in diploid alfalfa proceeds more slowly than expected, based o n F, and that inbreeding depression may limit the level of homozygosit y obtainable.