Striking sequence similarity in inter- and intra-specific comparisons of class I SLG alleles from Brassica oleracea and Brassica campestris : Implications for the evolution and recognition mechanism

Self-incompatibility inBrassica is controlled by a single multi-allelic locus (S locus), which contains at least two highly polymorphic genes expressed in the stigma: anS glycoprotein gene (SLG ) and anS receptor kinase gene (SRK ). The putative ligand-binding domain of SRK exhibits high homology to the secretory protein SLG, and it is believed that SLG and SRK form an active receptor kinase complex with a self-pollen ligand, which leads to the rejection of self-pollen. Here, we report 31 novelSLG sequences ofBrassica oleracea andBrassica campestris . Sequence comparisons of a large number ofSLG alleles andSLG -related genes revealed the following points. (i ) The striking sequence similarity observed in an inter-specific comparison (95.6% identity betweenSLG 14 ofB. oleracea andSLG 25 ofB. campestris in deduced amino acid sequence) suggests thatSLG diversification predates speciation. (ii ) A perfect match of the sequences in hypervariable regions, which are thought to determineS specificity in an intra-specific comparison (SLG 8 andSLG 46 ofB. campestris ) and the observation that the hypervariable regions of SLG and SRK of the sameS haplotype were not necessarily highly similar suggests that SLG and SRK bind different sites of the pollen ligand and that they together determineS specificity. (iii ) Comparison of the hypervariable regions ofSLG alleles suggests that intragenic recombination, together with point mutations, has contributed to the generation of the high level of sequence variation inSLG alleles. Models for the evolution ofSLG /SRK are presented.