Nonself-recognition-based self-incompatibility can alternatively promote or prevent introgression

1 Summary Traditionally, we expect that self-incompatibility alleles (S-alleles), which prevent self-fertilization, should benefit from negative-frequency dependent selection and rise to high frequency when introduced to a new population through gene flow. However, the most taxonomically widespread form of self-incompatibility, the ribonuclease-based system ancestral to the core eudicots, functions through nonself-recognition, which drastically alters the process of S-allele diversification. We analyze a model of S-allele evolution in two populations connected by migration, focusing on comparisons among the fates of S-alleles originally unique to each population and those shared among populations. We find that both shared and unique S-alleles originating from the population with more unique S-alleles were usually fitter than S-alleles from the population with fewer. Resident S-alleles were often driven extinct and replaced by migrant S-alleles, though this outcome could be averted by pollen limitation or biased migration. Nonself-recognition-based self-incompatibility will usually either disfavor introgression of S-alleles or result in the whole-sale replacement of S-alleles from one population with those from another.