INTERACTIONS OF HYBRIDIZATION AND MATING SYSTEMS: A CASE STUDY IN LEPTOSIPHON (POLEMONIACEAE)

Premise of the study: The roles of hybridization and mating systems in the evolution of angiosperms have been well studied, but less work has focused on their interactions. Self-incompatible and self-compatible species often show asymmetry in heterospecific pollen rejection. Self-fertilization can preempt ovules before opportunities for hybridization. In turn, hybridization might affect mating system evolution through selection for selfing to avoid production of low fitness hybrids. Methods: AFLP and morphological analyses were used to test for hybrids in a contact zone between species with contrasting breeding systems. Crossing experiments examined the relative contributions to reproductive isolation of pollen-pistil interactions, timing of self-fertilization, and F-1 viability and fertility. A diallel cross of siblings tested for an association between heterospecific incompatibility and S-genotype in the self-incompatible species. Key results: A low frequency of hybrids was detected in the contact zone. Pollen-pistil interactions were partially consistent with the SI x SC rule; some individuals of the self-incompatible species rejected heterospecific pollen, whereas the self-compatible species was fully receptive to it. In the selfing species, individuals with early selfing produced fewer hybrid progeny than did those with delayed self-compatibility when heterospecific pollen was applied after self-pollen. Viability of F(1)s was high but fertility was low. Variability in heterospecific pollen rejection was not related to S-genotype. Conclusions: Both self-fertilization and self-incompatibility are associated with limits to hybridization at this site. The strong effect of timing of selfing on production of low fitness F(1)s suggests that hybridization might select for early selfing in this population.