バラ科植物の自家不和合性の分子機構

Self-incompatibility(GSI) in the rosaceous species is controlled by the S locus consisting of S-RNase gene and an unidentified 'pollen S' gene. A～200kbp of cosmid contig for the Sc haplotype of almond was constructed. Genomic Southern blot analyses showed that most cosmid end probes, except those near the Sc-RNase gene, cross-hybridized with DNA fragments from different S haplotypes, implying that the cosmid contig extends to the borders of the S locus. A～70kbp segment of the Sc haplotype, the S haplotype-specific region containing the S-RNase gene, was complentely sequenced. This regin was found to contain two pollen-expressed F-box genes that are likely candidates for pollen S genes. One of them, named SFB(S haplotype-specific F-Box protein), was specifically expressed in pollen, and showed high level of S haplotype-specific sequence polymorphism, comparable to that of the S-RNases. The other is unlikely to determine the S specificity of pollen, because it showed little allelic sequence polymorphism and was expressed also in pistil. Three other S haplotypes were cloned and the pollen-expressed genes were physically mapped. In all four cases, SFBs were physically linked to the S-RNase genes, and were located at the S haplotype-specific region, suggesting that the two genes are inherited as aunit. These features are consistent with the hypothrsis that SFB is the pollen S gene. This hypothesis predicts involvement of the ubiquitin/26S proteasome proteolytic pathway in the RNase-based SI system.