Abstract
α-Gliadins are a major group of gluten proteins in wheat flour that contribute to the end-use properties for food processing and contain major immunogenic epitopes that can cause serious health-related issues including celiac disease (CD). α-Gliadins are also the youngest group of gluten proteins and are encoded by a large gene family. The majority of the gene family members evolved independently in the A, B, and D genomes of different wheat species after their separation from a common ancestral species. To gain insights into the origin and evolution of these complex genes, the genomic regions of the Gli-2 loci encoding α-gliadins were characterized from the tetraploid wild emmer, a progenitor of hexaploid bread wheat that contributed the AABB genomes. Genomic sequences of Gli-2 locus regions for the wild emmer A and B genomes were first reconstructed using the genome sequence scaffolds along with optical genome maps. A total of 24 and 16 α-gliadin genes were identified for the A and B genome regions, respectively. α-Gliadin pseudogene frequencies of 86% for the A genome and 69% for the B genome were primarily caused by C to T substitutions in the highly abundant glutamine codons, resulting in the generation of premature stop codons. Comparison with the homologous regions from the hexaploid wheat cv. Chinese Spring indicated considerable sequence divergence of the two A genomes at the genomic level. In comparison, conserved regions between the two B genomes were identified that included α-gliadin pseudogenes containing shared nested TE insertions. Analyses of the genomic organization and phylogenetic tree reconstruction indicate that although orthologous gene pairs derived from speciation were present, large portions of α-gliadin genes were likely derived from differential gene duplications or deletions after the separation of the homologous wheat genomes ~ 0.5 MYA. The higher number of full-length intact α-gliadin genes in hexaploid wheat than that in wild emmer suggests that human selection through domestication might have an impact on α-gliadin evolution. Our study provides insights into the rapid and dynamic evolution of genomic regions harboring the α-gliadin genes in wheat.
Highlights
Bread wheat is one of the most important food crops in the world, providing about 20% of the calories in the human diet
To reconstruct the α-gliadin gene genomic regions in wild emmer wheat, we first searched wild emmer sequence scaffolds using BLASTn with α-gliadin and glutamate-like receptor (GLR) gene sequences identified in the hexaploid wheat cv
The GLR genes were included to define locus boundaries since the three homoeologous α-gliadin loci in Chinese Spring were all flanked by GLR genes (Huo et al 2018b)
Summary
Bread wheat is one of the most important food crops in the world, providing about 20% of the calories in the human diet. The adaptability and high yields of wheat contribute to its success as an important food crop, there is no doubt that the unique properties of wheat flour that allow it to be processed into a range of food products provide advantages over other cereals (Shewry 2009). These unique properties are determined by the structures and interactions of gluten proteins which comprise. The third genomic region located on the short arms of wheat group 6 chromosomes harbors the Gli-2 loci encoding α-gliadins
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