Historical inferences from the self-incompatibility locus

Ancient polymorphism preserved at the self-incompatibility locus facilitates investigation of historical occurrences far older than extant species. We outline two ways in which studies of the S-locus can provide insights into patterns of speciation. First, we review evidence concerning the prevalence of founder events in speciation. A dramatic population size reduction is expected to reduce sequence diversity at the S-locus for millions of years. Only one potential bottleneck is preserved at the S-locus of the Solanaceae, suggesting that severe population size restrictions rarely occur in successful lineages. This must be interpreted with caution because of the restrictive conditions under which bottlenecks at the S-locus will be preserved. Second, S-locus polymorphism provides a novel opportunity to reconstruct, with considerable certainty, the presence or absence of self-incompatibility among ancestral taxa. We demonstrate this approach using a phylogenetic analysis and find that transitions from self-incompatibility to self-compatibility are common and essentially irreversible in the Solanaceae. Self-incompatibility is ancestral, but self-compatible taxa currently outnumber self-incompatible ones. Either self-incompatibility is going extinct, or the presence of incompatibility increases the diversification rate, maintaining a mixture of self-incompatible and self-compatible species in equilibrium. We outline how phylogenetic approaches can be used to determine the effect of incompatibility on diversification.