Fewer S-alleles are maintained in plant populations with sporophytic as opposed to gametophytic self-incompatibility

Plants use self-incompatibility to reject pollen bearing alleles in common at the S-locus. These systems are classified as gametophytic (GSI) if recognition involves haploid pollen or sporophytic (SSI) if recognition involves diploid paternal genotypes. Dominance in SSI systems reduces the number of S-alleles, but it has not been clear which system should maintain greater diversity when all else is equal. We simulated finite populations to compare the equilibrium number of S-alleles in populations with either GSI or a co-dominant SSI system. When population size was constant, SSI systems maintained more S-alleles than GSI systems. When populations fluctuated in response to an S-Allee effect, fewer S-alleles were observed in SSI systems when S-allele diversity was low, and SSI populations were vulnerable to extinction over a broader range of parameters. Turnover rates at the S-locus were also faster in SSI populations experiencing strong S-Allee effects. Given the variable expectations concerning S-allele diversity in these systems, we reviewed published estimates of S-allele diversity. GSI populations have significantly more S-alleles on average than SSI populations (GSI=25.70 and SSI=16.80). Dominance likely contributes to this pattern, although the demographic consequences of the S-Allee effect may be important in populations with fewer than 10 S-alleles.