The S receptor kinase gene determines dominance relationships in stigma expression of self-incompatibility in Brassica

Self-incompatibility (SI) in Brassica is sporophytically controlled by the multi-allelic S locus. SI phenotypes of the stigma and pollen in an S heterozygote are determined by the two S haplotypes it carries; the two haplotypes may be co-dominant or exhibit a dominant/recessive relationship. Because the S receptor kinase (SRK) gene of the S locus was recently shown to determine the S haplotype specificity of the stigma, we wished to investigate whether SRK also plays a role in the dominance relationships between S haplotypes. We crossed plants carrying an SRK²⁸ transgene with plants homozygous for one of five S haplotypes that are either co-dominant with, or recessive to, S²⁸ haplotype in the stigma, and analyzed the SI phenotypes of the progeny. In all cases, the SI phenotype of the stigma of plants carrying the SRK²⁸ transgene could be predicted by the known dominance relationships between the S haplotype(s) and the S²⁸ haplotype. Moreover, in the S⁴³ homozygote carrying the SRK²⁸ transgene where the S⁴³ phenotype in the stigma was masked by the presence of the SRK²⁸, the transcript level of SRK²⁸ was found to be much lower than that of SRK⁴³. All these results suggest that the dominance relationships between S haplotypes in the stigma are determined by SRK, but not by virtue of its relative expression level.