Polymorphism of Self-Incompatibility Genes

Radish (Raphanus sativus L.) has a self-incompatibility system similar to those of Brassica rapa and Brassica oleracea. Self-recognition specificities of pollen and stigmas are determined by multiple alleles of SP11 and SRK, respectively, both of which are located on the S locus. As in B. rapa and B. oleracea, stigmas in radish also have S glycoprotein encoded by SLG closely linked to SP11 and SRK. The set of alleles of SP11, SRK, and SLG is called S haplotype, and S haplotypes are numbered as S-1, S-2, S-3, etc. Although radish belongs to a genus different from Brassica, nucleotide sequences of SP11, SRK, and SLG alleles of radish and Brassica are intermingled in phylogenetic trees of SP11, SRK, and SLG, respectively, indicating that diversification of these alleles predates speciation of these genera. SP11, SRK, and SLG alleles of some S haplotypes in radish are highly similar to those of some S haplotypes in Brassica, and one S haplotype in radish has been revealed to have the same recognition specificity as that of one S haplotype in B. rapa. Comparison of nucleotide sequences of SP11 and SRK alleles and recognition specificities between similar S haplotypes of radish and Brassica may provide information useful for understanding the molecular structures of SP11 and SRK proteins. The self-incompatibility system is used for seed production in F1 hybrid cultivars of radish, in which characterization of each S haplotype, especially the strength of the self-incompatibility phenotype, is required. However, the numbering of S haplotypes in radish varies by researchers, and nucleotide sequence information on S haplotypes is thus confusing. The importance of unification of S haplotype nomenclature is herein discussed.