Abstract
SummaryMost alpha‐gliadin genes of the Gli‐D2 locus on the D genome of hexaploid bread wheat (Triticum aestivum) encode for proteins with epitopes that can trigger coeliac disease (CD), and several contain a 33‐mer peptide with six partly overlapping copies of three epitopes, which is regarded as a remarkably potent T‐cell stimulator. To increase genetic diversity in the D genome, synthetic hexaploid wheat lines are being made by hybridising accessions of Triticum turgidum (AB genome) and Aegilops tauschii (the progenitor of the D genome). The diversity of alpha‐gliadins in A. tauschii has not been studied extensively. We analysed the alpha‐gliadin transcriptome of 51 A. tauschii accessions representative of the diversity in A. tauschii. We extracted RNA from developing seeds and performed 454 amplicon sequencing of the first part of the alpha‐gliadin genes. The expression profile of allelic variants of the alpha‐gliadins was different between accessions, and also between accessions of the Western and Eastern clades of A. tauschii. Generally, both clades expressed many allelic variants not found in bread wheat. In contrast to earlier studies, we detected the 33‐mer peptide in some A. tauschii accessions, indicating that it was introduced along with the D genome into bread wheat. In these accessions, transcripts with the 33‐mer peptide were present at lower frequencies than in bread wheat varieties. In most A. tauschii accessions, however, the alpha‐gliadins do not contain the epitope, and this may be exploited, through synthetic hexaploid wheats, to breed bread wheat varieties with fewer or no coeliac disease epitopes.
Highlights
The genome of allohexaploid (2n = 6x = 42) bread wheat (Triticum aestivum) is composed of three subgenomes (A, B and D)
The notion of a hybridisation bottleneck is supported by four recent population genetics studies that show high levels of genetic diversity among genebank accessions of wild A. tauschii accessions sampled across the species range, based on the 10K Infinium single nucleotide polymorphism (SNP) array
Sequencing of amplified transcript fragments, representing variants of the first variable domain of alpha-gliadin that were expressed in the wheat grain endosperm, was performed from the 30-end, directly entering the first repetitive domain, instead of the 50-end as done in Salentijn et al (2013) in tetraploid durum wheat
Summary
The genome of allohexaploid (2n = 6x = 42) bread wheat (Triticum aestivum) is composed of three subgenomes (A, B and D) It originated from hybridisation between allotetraploid Triticum turgidum (AB) and the diploid species Aegilops tauschii (D) around 8000 years ago (Nesbitt and Samuel, 1996). This hybridisation probably took place in agricultural fields, as bread wheat does not exist as a wild species. The genetic variation in the D genome of bread wheat is much lower than that present in the A and B genomes (Dubcovsky and Dvorak, 2007) This suggests that the hybridisation event involved only a small subset of A. tauschii genotypes, resulting in a strong genetic bottleneck (Dvorak et al, 1998).
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