| Cereals belonging to Poaceae, are the most important crops in the world, including the rice, wheat, barley, rye, maize and sorghum, and their grain offers the majority of human diet. The protein in the seeds was mainly prolamins, which comprise of big gene family, and most protein can be soluted in ethnol. As an important component of Triticeae seeds, gliadins played an important contribution for wheat breadmaking quality. In accordance with their mobility in A-PAGE (acid-PAGE), they are divided into four groups:α- (fastest mobility),β-,γ-, andω-gliadins (slowest mobility). Due to extensive allele polymorphism, these proteins have been widely used for genetic resources and cultivar identification in wheat and its relatives.The complete amino acid sequence from a number ofα-gliadins shows that the conserved primary structure of these proteins is divided into several domains, consisting of signal peptides, polyglutamine, repetitive and non-repetitive regions. In their structure,α-gliadins have six cysteine residues, form three disulfide bonds. Their conserved structures ofα-gliadin genes provide opportunities to clone and sequences the genes with respect to investigate the molecular mechanisms of genetics and polymorphism.At present, theα-gliadin genes were identified from the species of cultivated wheat (Triticum aestivum, AABBDD), tetraploid wheat (T. durum, AABB), spelta wheat (T. spelta, AABBDD), and the wheat ancestral donor species of T. monococcum (AA), Aegilops speltoides (SS) and Ae. tauschii (DD). However, there was lack of reports on the isolation and sequences ofα-gliadin genes from Triticeae species. In the present study, we first identified 20α-gliadin genes from the species from the genus of Secale, Dasypyrum, Thinopyrum, Agropyron, Pseudoroengeria, Eremopyrum, Australopyrum and Triticum-Aegilops complex, which includes the species with important contributions for wheat breeding. The results were summarized as following:(1) We first isolated theα-gliadin genes from wild Secale species of Secale sylivestre and S. africanum, which exhibited the character of putative ancestry ofα-gliadin gene family. The sequences lack of the repetitive regions and with eight cysteine residues. The specific molecular markers also produced in order to trace the Secale chromatin in wheat background.(2) Total 71α-gliadin sequences were identified firstly from the genus of Dasypyrum, including the species of diploid Dasypyrum breviaristatum, tetraploid D. breviaristatum and diploid D. villosum species. We found the diploid D. breviaristatum contained the ancestralα-gliadin sequences of Dasypyrum, and they emerged about 21MY and evolved twice. Theα-gliadin sequences from tetraploid D. breviaristatum were diverged from diploid D. breviaristatum at about 6MY, while the D. villosum occurred as late as 3MY from diploid D. breviaristatum. The results support that the genome of tetraploid D. breviaristatum was not diverged from D. villosum. We also found the length polymorphism of the promoter regions in Dasypyrumα-gliadin sequences, and produced several specific primers to target the Dasypyrumα-gliadin sequences and localized on D. villosum 6V chromosomes. The markers were used to identify the wheat-Dasypyrum introgression lines effectively.(3) Theα-gliadin gene sequences from Thinopyrum genus and the putative ancestral species, Lophopyrum elongatum, Ps. spicata, Th. bessiaberium and Th. intermedium were isolated. The phylogenetic analysis indicated that Lophopyrum elongatum and Ps. spicata displayedα-gliadin genes divergence several times, and the Th. intermedium genomes have occurred genetic rearrangement. The results suggested that the genomes of Dasypyrum and Thinopyrum were closed related, and the D. villosum, not D. brevaristatum were directly joined in the formation of Th. intermedium about 3.5MY, which also supported by the genomic in situ hybridization between species. We also observed a MITE insertion in Th. intermedium, indicating the accelerated genomic evolution of polyploid Thinopyrum species.(4) We cloned 48α-gliadin gene sequences from representing species Australopyrum retrofractum, Agropyron desetorum and Eremopyrum bonaepartis. The phylogenetic analysis indicated that Australopyrum retrofractumα-gliadin gene sequences displayedα-gliadin genes divergence several times, and that both Australopyrum retrofractum and Agropyron desetorum genomes lack of toxic epitops, suggesting the toxic epitops evolved from the divergence of Agropyron species. (5) Theα-gliadin gene sequences from Ae. tauschii native to Henan China and a wheat cultivar were isolated. It is found that high diversity existed in the Aegilops taushii genome, including some sequences have extra cysteine residues which can be used as donor for wheat quality improvement.(6) Combining with the sequences diversity ofα-gliadin gene sequences and the speciation of Triticeae species, we deduced that theα-gliadin gene sequences may originate from the wild Secale species from the ancestor of gliadin genes about 21MY. The sequences have occurred great changes during the speciation of diploid Dasypyrum, Astralopyrum, Pseudorengeria and Lophopyrum species. The accelerated evolutionary change also occurred during the polyploidization including the formation of hexaploid Thinopyrum species or common wheat. The toxic epitops Glia-αofα-gliadin gene sequences may emerged aabout 12-15MY, while the rapid increase of Glia-α-2, 9, 20 epitops after the wheat allopolyploid, which may be largely related to natural and artificial selection.In conclusion, we isolated 192α-gliadin gene sequences from diversified Triticeae species systematically, and revealed the relationship between gene divergence and speciation. The study supported that the accelerate evolution of Triticeae species and obtained specific molecular markers, which can be used for assisting wheat breeding. It is thus to note that the results are both important for triticeae genome evolution and breeding.
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