Comparative analysis of the self-incompatibility (S-) locus region of Prunus mume:: identification of a pollen-expressed F-box gene with allelic diversity

Background: Self-incompatibility (SI) in the Solanaceae, Rosaceae and Scrophulariaceae is gametophytically controlled by a single polymorphic locus, termed the S-locus. To date, the only known S-locus product is a polymorphic ribonuclease, termed S-RNase, which is secreted by stylar tissue and thought to act as a cytotoxin that degrades the RNA of incompatible pollen tubes. However, understanding how S-RNase causes S-haplotype specific inhibition of pollen tubes has been hampered by the lack of a cloned pollen S-determinant gene. Results: To identify the pollen S-determinant gene, we investigated the genomic structure of the S-locus region of the S-1- and S-7-haplotypes of Prunus mume (Japanese apricot), and identified 13 genes around the S-RNase gene. Among them, only one F-box gene, termed SLF (S-locus F-box), fulfilled the conditions for a pollen S-determinant gene: (i) together with the S-RNase gene, it is located within the highly divergent genomic region of the S-locus, (ii) it exhibits S-haplotype specific diversity among three analysed S-haplotypes, and (iii) it is specifically expressed in pollen, but not in the styles or leaves. Conclusion: The results indicate that SLF is a prime candidate for the pollen S-determinant gene of SI.