Allelic diversity and gene genealogy at the self-incompatibility locus in the Solanaceae

The self-incompatibility (S) locus of flowering plants offers an example of extreme polymorphism maintained by balancing selection. Estimates of recent and long-term effective population size (Ne) were determined for two solanaceous species by examination of S-allele diversity. Estimates of recent Ne in two solanaceous species differed by an order of magnitude, consistent with differences in the species' ecology. In one species, the evidence was consistent with historical population restriction despite a large recent Ne. In the other, no severe bottleneck was indicated over millions of years. Bottlenecks are integral to founder-event speciation, and loci that are subject to balancing selection can be used to evaluate the frequency of this mode of speciation.