Post-pollination barriers do not explain the persistence of two distinct Antirrhinum subspecies with parapatric distribution

Abstract

Empirical studies of post-pollination barriers to gene flow between recently diverged plant species are important to understand ecological processes underlying speciation. Using greenhouse and common garden experiments, we investigated the strength of post-pollination barriers that restrict or prevent gene flow between two subspecies of Antirrhinum: Antirrhinum majus pseudomajus and A. m. striatum. The two are distributed parapatrically but share the same major pollinators (bumblebees), and form narrow hybrid zone in many areas of southern France and northern Spain where they come into close contact. We assessed the strength and symmetry of mating barriers and their homogeneity among populations by comparing fruit set, seed set and offspring performances between intra- and inter-subspecific crosses performed in parental and hybrid populations. Although all populations showed high levels of self-incompatibility, we found very little evidence for barriers to gene flow once pollen had been transferred to the stigma. We suggest that reproductive isolation in this system mostly involves barriers related to processes occurring before pollination, with little or no role of post-pollination barriers.