Evidence for inbreeding depression in the tree Robinia pseudoacacia L. (Fabaceae)

The magnitude of inbreeding depression within populations is important for the evolution and maintenance of mixed mating systems. However, data are sparse on the magnitude of inbreeding depression in Robinia pseudoacacia. In this study, we compared differences in the mature seed set per fruit, seed mass, germination success, and seedling growth between self- and cross-pollination treatments and estimated the inbreeding depression at 3 stages: seed maturation, seedling emergence, and seedling growth at 10 and 20 weeks. We found that progenies resulting from cross-pollination treatments showed significantly higher fitness than progenies resulting from self-pollination, causing high levels of inbreeding depression. Inbreeding depression was not uniformly manifested, however, over the 3 stages. Inbreeding depression was the greatest between fertilization and seed maturation stage (δ = 0.5419), and the seedling emergence (0.3654) stage was second. No significant differences in seedling growth were observed between selfed and crossed progenies. The cumulative inbreeding depression (δ) across all 3 stages averaged 0.7452. Inbreeding depression may promote outcrossing in R. pseudoacacia by acting as a post-pollination barrier to selfing. The large difference in the seed set between self- and cross-pollination that we detected indicated that inbreeding depression would probably be a reasonable explanation for the high abortion and low seed set in R. pseudoacacia.