Population improvement in lucerne (Medicago sativa L.): components of inbreeding depression are different in original and improved populations

Abstract

Summary. Inbreeding depression, the lowered fitness of inbred individuals compared with their non-inbred counterparts, is an important concept in lucerne improvement; but is poorly understood. Two-allele autotetraploid populations are suitable for studying inbreeding depression, especially when the population improvement strategy involves inbreeding, because they are derived from chromosome-doubling of hybrid diploid plants. They have a maximum of 2 alleles and a single allelic interaction per locus. Inbreeding depression was compared in original 2-allele autotetraploid populations and populations that had undergone inbreeding and selection. The original and improved (selected) populations were produced by intercrossing 2 single-cross lines from the original and improved 2-allele autotetraploid populations respectively. Herbage yield of the S₁ and intercrossed generations derived from these populations was evaluated in field trials at Arlington, Wisconsin, USA, and used to estimate inbreeding depression. Herbage yield of the S₁ and intercrossed generations derived from the improved population were significantly (P<0.01) higher, by 13.3 and 24%, respectively, than those derived from the original population. Selection during inbreeding probably decreased the frequency of deleterious alleles and accumulated favourable alleles. Inbreeding depression values were higher in the improved compared with the original population. Genetic load of deleterious alleles may account for much of the inbreeding depression observed in the original population, whereas, in the improved population, loss of heterozygosity or non-additive gene interactions between favourable alleles on linked chromosome segments may account for the substantial inbreeding depression. Therefore, in a population improvement program, the causes of inbreeding depression seem to be more important than their estimated value.