Abstract:

Five Chinese White Pear (Pyrus bretschneideri) cultivars， with known S-genotypes， were used in this study. The genomic DNAs were extracted and amplified by PCR with three pairs of primers corresponding to pear SFBB-alpha, SFBB-beta and SFBB-gamma genes, respectively. Results showed that only one primer pair PSFBG-F/PSFBG-R successfully amplified a fragment of approximately 1 300 bp from ‘Jinhua’ (S₁₃S₁₈)，‘Jinhuasihao’ (S₁₃S₁₈) and ‘Eli’ (S₁₃S₃₄) corresponding to pear SFBB-gamma gene. This gene was named PbSFBB13-gamma (Pyrus bretschneideri SFBB13-gamma) and deposited under GenBank accession No. EU081892. RT-PCR revealed that the PbSFBB13-gamma was expressed specifically in the pollen grains. The coding region of the PbSFBB13-gamma was 1 191 bp in length encoding 396 amino acids with a predicted molecular weight of 45.4 ku and isoeletric point of 4.63. The SFBB13-gamma displayed the typical structural characterization of SFB/SLF genes, I.e. an F-box motif and four variable regions. At the deduced amino acid level, it shared 17.8% to 97.7% similarities with other SFB/SLFs of rosaceous plants. These characteristics of PbSFBB13-gamma fully demonstrate that it is a candidate of pollen S-gene although its function in gametophytic self-incompatibility (GSI) response is not confirmed yet. Phylogenetic analysis revealed that 34 rosaceous SFB/SLFs were dived into two subfamily-specific groups, but did not further form species-specific subgroup. The evolutionary pattern of SFB/SLFs concurs with that of rosaceous S-Rnases, suggesting that SFB/SLFs occur after divergence of subfamily but before the divergence of species as S-Rnases do in Rosaceae. The present study could provide a scientific base for fully clarifying the mechanism of pear GSI at the molecular level.

Key words: Chinese White Pear (Pyrus bretschneideri), gametophytic self-incompatibility, pollen S-gene, S-locus-linked F-box gene, phylogenetic tree