Sequence and structural diversity of the S locus genes from different lines with the same self-recognition specificities in Brassica oleracea

Self-incompatibility (SI) is a mechanism for preventing self-fertilization in flowering plants. In Brassica, it is controlled by a single multi-allelic locus, S, and it is believed that two highly polymorphic genes in the S locus, SLG and SRK, play central roles in self-recognition in stigmas. SRK is a putative receptor protein kinase, whose extracellular domain exhibits high similarity to SLG. We analyzed two pairs of lines showing cross-incompatibility (S(2) and S(2-b); S(13) and S(13-b)). In S(2) and S(2-b), SRKs were more highly conserved than SLGs. This was also the case with S(13) and S(13-b). This suggests that the SRKs of different lines must be conserved for the lines to have the same self-recognition specificity. In particular, SLG(2-b) showed only 88. 5% identity to SLG(2), which is comparable to that between the SLGs of different S haplotypes, while SRK(2-b) showed 97.3% identity to SRK(2) in the S domain. These findings suggest that the SLGs in these S haplotypes are not important for self-recognition in SI.