The evolution and diversification of S-locus haplotypes in the Brassicaceae family.

Self-incompatibility (SI) in the Brassicaceae plant family is controlled by the SRK and SCR genes situated at the S locus. A large number of S haplotypes have been identified, mainly in cultivated species of the Brassica and Raphanus genera, but recently also in wild Arabidopsis species. Here, we used DNA sequences from the SRK and SCR genes of the wild Brassica species Brassica cretica, together with publicly available sequence data from other Brassicaceae species, to investigate the evolutionary relationships among S haplotypes in the Brassicaceae family. The results reveal that wild and cultivated Brassica species have similar levels of SRK diversity, indicating that domestication has had but a minor effect on S-locus diversity in Brassica. Our results also show that a common set of S haplotypes was present in the ancestor of the Brassica and Arabidopsis genera, that only a small number of haplotypes survived in the Brassica lineage after its separation from Arabidopsis, and that diversification within the two Brassica dominance classes occurred after the split between the two lineages. We also found indications that recombination may have occurred between the kinase domain of SRK and the SCR gene in Brassica.