Evolution under strong balancing selection:: how many codons determine specificity at the female self-incompatibility gene SRK in Brassicaceae?

Background: Molecular lock-and-key systems are common among reproductive proteins, yet their evolution remains a major puzzle in evolutionary biology. In the Brassicaceae, the genes encoding self-incompatibility have been identified, but technical challenges currently prevent detailed analyses of the molecular interaction between the male and female components. In the present study, we investigate sequence polymorphism in the female specificity determinant SRK of Arabidopsis halleri from throughout Europe. Using a comparative approach based on published SRK sequences in A. lyrata and Brassica, we track the signature of frequency-dependent selection acting on these genes at the codon level. Using simulations, we evaluate power and accuracy of our approach and estimate the proportion of codon sites involved in the molecular interaction.