A clarification of pollen discounting and its joint effects with inbreeding depression on mating system evolution

Given the predominance of outcrossing by angiosperms, large costs must often overwhelm the genetic benefit of selfing derived from contributing two haploid genomes to each off-spring rather than one. In addition to the well-studied genetic cost of inbreeding depression, selfing imposes a mating cost whenever self-pollination reduces opportunities for pollen export. Because self-pollination is a heterogeneous process, pollen discounting and its evolutionary consequences vary with pollination conditions. In this article we model self-pollination as comprising discounting and nondiscounting components, and we consider the consequences of this heterogeneity for outcross siring success. Aided by this depiction of pollination, we then compare previous theoretical representations of pollen discounting and consider their relative virtues. Finally, we consider conditions that would allow a population to be invaded by a variant with different pollination characteristics. This analysis exposes the pollination conditions implicit in standard results of mating system theory. It also identifies associations between four possible changes in pollination expected in different reproductive environments, including the incidence of positive or negative correlations between self-pollination and pollen export. These results emphasize the benefits of expanding the theory of plant reproduction to recognize explicitly when and how pollination mechanisms affect mating outcomes.