Significant differences in outcrossing rate, self-incompatibility, and inbreeding depression between two widely hybridizing species of Geum

Geum urbanum and Geum rivale are two widely hybridizing perennial herbs. Estimation of the breeding systems of these taxa using nuclear microsatellite markers scored in mother-progeny arrays demonstrated that, in pure populations, G. urbanum is predominantly selfing (outcrossing rate, t = 0.058 to 0.177), whereas G. rivale is predominantly outcrossing (t = 0.686-0.775). Theory suggests that hybridization between inbreeding and outcrossing species can potentially generate novel inbreeding lineages. However, the establishment of such lineages may be restricted either by self-incompatibility loci or deleterious recessive alleles derived from the outcrossing parent. To assess the likelihood that hybridization between G. urbanum and G. rivale will generate novel inbreeding lineages, self-incompatibility and inbreeding depression were investigated in the two taxa. Seed set in the absence of pollinators, and after controlled self- and cross-pollination, was measured to study self-incompatibility. Inbreeding depression was measured by estimating the relative fitness of offspring from controlled self-and cross-pollinations. Geum urbanum was fully self-compatible [self-compatibility index (SCI) = 1] and bagged flowers showed full seed set. By contrast, only 3% of bagged flowers set seed in G. rivale and controlled self-pollinations showed a 60-80% reduction in seed set compared to controlled outcross pollinations (SCI = 0.28). There was no evidence for inbreeding depression in G. urbanum, although significant, albeit low levels of inbreeding depression were detected in one of two G. rivale populations (delta = 0.33). The implication of these results is that if genetic material from G. rivale was incorporated into a hybrid with a selfing morphology, the establishment of this selfing lineage could be compromised by self-incompatibility and inbreeding depression. The wider implications of these results for evolution in hybrid swarms between G. urbanum and G. rivale are discussed.