Comparative analysis of S haplotypes with very similar SLG alleles in Brassica rapa and Brassica oleracea

Self-incompatibility in Brassica is controlled by a single multi-allelic locus (the S locus) which harbors at least two highly polymorphic genes, SLG and SRK. SRK is a putative transmembrane receptor kinase and its amino acid sequence of the extracellular domain of SRK (the S domain) exhibits high homology to that of SLG. The amino acid sequences of the SLGs of S⁸ and S⁴⁶ haplotypes of B. rapa are very similar and those of S²³ and S²⁹ haplotypes of B. oleracea were also found to be almost identical. In both cases, SLG and the S domain of SRK of the same haplotype were loss similar. This seems to contradict the idea that SLG and SRK of the same haplotype have the same self-recognition specificity. In the transmembrane-kinase domain, the SRK alleles of the S⁸ and S⁴⁶ haplotypes had almost identical nucleotide sequences in spite of their lower homology in the S domain. Such a cluster of nucleotide substitutions is probably due to recombination or related events, although recombination in the S locus is thought to be suppressed. Based on our observations, the recognition mechanism and the evolution of self-incompatibility in Brassica are discussed.