Inbreeding depression in three generations of selfed families of silver birch (Betula pendula)

Inbreeding depression in survival and growth vigor was investigated through three generations of selfing on the basis of a 13-year-old field trial in silver birch (Betula pendula Roth). Survival showed significant decline from outcrossed (S0) to firstgeneration selfed trees (S1), and some recovery from second- (S2) to third-generation selfed trees (S3), indicating the rapid purging of lethal, sublethal, or highly deleterious alleles by selfing. The continuous decline of growth vigor with increased inbreeding suggests a low efficiency in purging mildly deleterious alleles affecting quantitative traits and a low possibility of recovery in growth with further inbreeding. Linear effects of inbreeding on survival and growth vigor (taken as height, diameter at breast height, and stem volume) explained the major variation caused by inbreeding and indicates that the additively combined dominant effects at different loci is the major source of inbreeding depression in this species. A weak and significant nonlinear effect of inbreeding on height and stem volume was also observed, suggesting the existence of epistatic effects.