Molecular and genetic analysis of pollen S gene controlled self-compatibility in Japanese pear

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Rosaceae fruit exhibit a gametophytic self-incompatibility (GSI) controlled by the S-haplotypes, which contains pistil and pollen S genes. The S-RNase encoded by the pistil S gene degraded rRNA of self-pollen tube, arresting pollen tube growth. Self/non-self recognition in GSI is caused between S-RNase and a product of pollen S gene. The pollen S gene in Prunus species (sweet cherry, Japanese apricot, almond) encoded F-box protein (SFB/SLF) being located from 380 by to 38 kb downstream of the S-RNase, but in Maloideae species (apple, Japanese pear) have not been identified yet.To isolate a pollen S gene of Japanese pear, in this study, we constructed a bacterial artificial chromosome (BAC) library from an S_4-homozygote segregated from 'Nijisseiki' (S2S_4), and initiated chromosome walking from S4-RNase. A BAC contig was assembled spanning 395 kb upstream and 615 kb downstream of S4-RNase. A self-compatible cultivar `Osa-Nijisseiki', a bud mutant of 'Nijisseiki' (S2S_4), has a stylar-part mutant S4^<sm>-haplotype, which deletes S_4-RNase gene but remains pollen S allele. Genomic PCR of DNA from S_4-and S_4^<sm>-homozygotes and DNA sequence of BAC contigs allowed the identification of a deletion of 236 kb around of S4-RNase. This suggested that a pollen S gene is delineated out of the deletion breakpoint; 48 kb upstream and 188 kb downstream of S_4-RNase. Out of the deletion region, five F-box genes were found and exhibit pollen specific expression, proposing that these are pollen S gene candidates. S^<sm>-haplotype specific marker was developed for early selection of self-compatible cultivars haboring S^<sm>-haplotype. Toward the fully understanding of self-incompatibility, analysis of pollen tube inhibition is required in relation to the cellular interaction with style. Ultra structural studies of the pollen tube penetrated the stylar tissue were carried by handling the serial of semi-thin sections where to detect the appropriate position of specimens.