Molecular basis of mechanisms of dominance relationships among S haplotypes in self-incompatible Brassicaceae.

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Self-incompatibility (SI) systems prevent self-fertilization and promote out-crossing by rejecting pollen from plants with the same S haplotype. The Brassica SI system is controlled sporophytically by multiple alleles at the single locus, termed S locus. Because of sporophytic control of Brassica SI, dominance relationships among S haplotypes are observed in both stigma and pollen. We performed test-crosses between S heterozygotes and their respective parental S homozygotes for four of these class-II S haplotypes, and observed a linear dominance relationship on the pollen side. In order to determine how this dominance relationship is controlled, RNA gel blot analyzes were performed for six S heterozygotes and their respective parental S homozygotes by using the corresponding SP11 cDNA clone as a probe. In all six S heterozygotes, SP11 derived from a dominant S haplotype was predominantly expressed, and SP11 derived from a recessive S haplotype was repressed. Therefore, the linear dominance relationship of the SI phenotype on the pollen side is regulated by the expression of SP11 gene.We determined the nucleotide sequences of the genomic clone of three class-II S haplotypes that contained corresponding SP11. These contain the part of the 5'-flanking region of the respective SRK. The distance of SP11 and SRK is about 10kbp in each S haplotypes. This result indicates that the relative position of SP11 and SRK is conserved. Therefore, the region between SP11 and SRK was named the "S-intergenic region". Then, we cloned class-II SRK, and compared these with other class-II SRK. The predicted amino acid sequence similarity within class-II SRK is about 90 to 95%, though the hypervariable region of the S domain is diverted. These results indicate not only similarity of S genes but that the physical position of their SP11 and SRK is conserved in the class-II S haplotypes.