Effect of Population Outcrossing Rate on Inbreeding Depression in Pinus contorta var. murrayana Seedlings

Abstract

Self and cross families from three populations (A, low-density, ecologically marginal site for lodgepole pine, and B + C, normal sites) were cultured in a common outdoor nursery for 2 yrs. Previous results had shown higher natural selfing rates and lower inbreeding depression in embryo survival in A than in B + C. In the nursery test, selfing decreased means of size traits, retarded phenological development, increased variance among families within populations, increased the coefficient of within-family variance, and increased within-family skewness. The selfing depression of mean values was moderately less and the selfing inflation of within-plot variation was much less in A than in B + C. Results were compatible with the idea that the higher selfing rate in A had purged both severely deleterious alleles affecting embryo survival and severely deleterious alleles affecting later life-cycle traits related to plant size. Purging partially mitigated the genetic consequences associated with increased inbreeding in small populations, but even with purging, population A retained a large inbreeding depression in both embryonic and seedling traits.