Genome Analysis of Expressed α-gliadin Genes in Common Wheat

The α-gliadins are one of seed storage proteins and their genes have especially higher copy numbers than other seed storage proteins in wheat genome. Additionally, they are located on the group 6 chromosomes, while the other genes encoding seed storage proteins are located on the group 1 chromosomes. In order to understand genome structure and expression of α-gliadin multigenes in hexaploid wheat, BAC clones containing &alpha-gliadin genes were screened. Based on their restriction fragment patterns, we focused on five BAC clones, namely, two clones for Gli-A2, two clones for Gli-B2, and one clone for Gli-D2, to fully sequence. Approximately 200 kb was sequenced for each locus. In total, 15 α-gliadin genes were found, and transposable elements existed in each BAC clone. We calculated time since duplication of each set of α-gliadin genes, and the insertion time of retrotransposons. All adjacent gene duplication within the same BAC took place before or after allotetrapolyploidization, but duplication of certain genes occurred before diploid differentiation of wheat species. Furthermore, translocation of α-gliadin genes from chromosome 1 to chromosome 6 apparently occurred before the diversification of wheat. Duplication of genome segments containing α-gliadin genes and retrotransposons brought about through unequal crossing-over or saltatory replication. Furthermore, α-gliadin genes per se were duplicated without any suggestion of recombination events. Out of 15 α-gliadin genes, ten were expressed, although their expression patterns were distinct. Since they have similar promoter structures, epigenetic regulation(s) underlying their distinct gene expression are suggested.