Evolutionary dynamics of self-incompatibility alleles in Brassica

Self-incompatibility in Brassica entails the rejection of pollen grains that express specificities held in common with the seed parent. In Brassica, pollen specificity is encoded at the multipartite S-locus, a complex region comprising many expressed genes. A number of species within the Brassicaceae express sporophytic self-incompatibility, under which individual pollen grains bear specificities determined by one or both S-haplotypes of the pollen parent. Classical genetic and nucleotide-level analyses of the S-locus have revealed a dichotomy in sequence and function among S-haplotypes; in particular, all class I haplotypes show dominance over all class II haplotypes in determination of pollen specificity. Analysis of an evolutionary model that explicitly incorporates features of the Brassica system, including the class dichotomy, indicates that class II haplotypes may invade populations at lower rates and decline to extinction at higher rates than class I haplotypes. This analysis suggests convergence to an evolutionarily persistent state characterized by the maintenance in high frequency of a single class II haplotype together with many class I haplotypes, each in low frequency. This expectation appears to be consistent with empirical observations of high frequencies of relatively few distinct recessive haplotypes.