Cloning and Sequence Characterization of SFBB (S-Locus F-Box Brothers)-Gamma Genes in Chinese Sand Pear (Pyrus pyrifolia Nakai.)

Gametophytic self-incompatibility (GSI) is a genetically controlled mechanism to prevent inbreeding and promote out-crossing. It is controlled by a single multi-allelic S-locus which contains two separate genes: one stylar S-RNase gene and one pollen S-gene. In gametophytic self-incompatible Japanese pear (Pyrus pyrifolia Nakai.), SFBB-gamma genes have been identified as good candidates for pollen S-genes. To investigate sequence characters of the possible pollen S-gene in Chinese sand pear, degenerate primers PSFBG-F and PSFBG-R were designed for cloning the SFBB-gamma homologies in eight Chinese sand pear cultivars. By genomic PCR, sequencing and sequence analysis, a total of nine distinct sequences were identified which displayed the basic structural characterization of SFB/SLF genes, i.e. an F-box motif and four variable regions. Of the nine sequences, five equaled to reported Japanese pear SFBB-gamma genes (S-1, S-2, S-4, S-5 and S-7) and the other four were considered to be new ones which were named SFBB12-gamma, SFBB13-gamma, SFBB15-gamma and SFBB22-gamma. The four new SFBB-gamma genes were expressed specifically in pollen grains. At the deduced amino acid level, they shared 20.4% to 97.7% similarities with other SFB/SLFs of rosaceous plants. The four SFBB-gamma genes cloned in this work will be useful for studying the interaction between SFBs and S-RNases, and clarifying the mechanism of GSI at the molecular level in Chinese pear.