Recognition specificity of self-incompatibility maintained after the divergence of Brassica oleracea and Brassica rapa

The determinants of recognition specificity of self-incompatibility in Brassica are SRK in the stigma and SP11/SCR in the pollen, respectively, In the pair of S haplotypes BrS46 (S-46 in B. rapa) and BoS(7) (S-7 in B. oleracea), which have highly similar SRK alleles, the SP11 alleles were found to be similar, with 96.1% identity in the deduced amino acid sequence. Two other pairs of S haplotypes, BrS47 and BoS(12), and BrS8 and BoS(32), having highly similar SRK and SP11 alleles between the two species were also found. The haplotypes in each pair are considered to have been derived from a single S haplotype in the ancestral species. The allotetraploid produced by interspecific hybridization between homozygotes of BrS46 and BoS(15) showed incompatibility with a BoS(7) homozygote and compatibility with other B. oleracea S haplotypes in reciprocal crossings. This result indicates that BrS46 and BoS(7) have maintained the same recognition specificity after the divergence of the two species and that amino acid substitutions found in such cases in both SRK alleles and SP11 alleles do not after the recognition specificity. DNA blot analysis of SRK, SP11, SLG and other S-locus genes showed different DNA fragment sizes between the interspecific pairs of S haplotypes. A much lower level of sequence similarity was observed outside the genes of SRK and SP11 between BrS46 and BoS(7). These results suggest that the DNA sequences of the regions intervening between the S-locus genes were diversified after or at the time of speciation. This is the first report demonstrating the presence of common S haplotypes in different plant species and presenting definite evidence of the trans-specific evolution of self-incompatibility genes.