Identification and characterization of self incompatibility genes in the Fuji apple

Abstract

Self incompatibility (SI) is a mechanism that encourages outbreeding by preventing production of zygotes after self-pollination. We present the cloning and characterization of two SI genes from the Fuji apple (Mains x domestica Borkh.). A cDNA library from the carpels of mature flowers was constructed and two clones, SIF1 and SIF2, which exhibit a strong homology to previously cloned SI genes, were isolated. The nucleotide sequence of SIF1 is identical to that of the S9 gene of an apple cultivar, Wellington. Whereas, the SIF2 sequence is unique and shows a significant homology to the reported SI genes of other apple cultivars. The SIF2 cDNA clone consists of 937 nucleotides which contain a coding region of 226 amino acid residues, 47 nucleotides of the 5' untranslated region, 192 nucleotides of the 3' untranslated region, and a poly(A)(+) tail. The expression pattern of the clones was studied by cDNA blot analysis using cDNA Libraries of various organs. The result showed that the apple SI clones were expressed in the carpel but not in other organs. Using these clones, we have studied the pedigree groups of various apple cultivars. A portion of the SI genes, amplified from the genomic DNA of 9 different apple cultivars and four related species, was hybridized with either SIF1 or SIF2. This experiment revealed that the Hokuto cultivar hybridized to both SI genes of the Fuji cultivar, suggesting that these two cultivars are closely related to each other. M. baccata and Chukwang strongly hybridized to SIF1, whereas Rall's Janet, M. sieboldii, Mutsu, and M. prunifolia hybridized with SIF2. The rest of the apple cultivars did not show a significant homology to either of the SI genes.