Abstract
BackgroundThe gamma-gliadins are considered to be the oldest of the gliadin family of storage proteins in Aegilops/Triticum. However, the expansion of this multigene family has not been studied in an evolutionary perspective.ResultsWe have cloned 59 gamma-gliadin genes from Aegilops and Triticum species (Aegilops caudata L., Aegilops comosa Sm. in Sibth. & Sm., Aegilops mutica Boiss., Aegilops speltoides Tausch, Aegilops tauschii Coss., Aegilops umbellulata Zhuk., Aegilops uniaristata Vis., and Triticum monococcum L.) representing eight different genomes: Am, B/S, C, D, M, N, T and U. Overall, 15% of the sequences contained internal stop codons resulting in pseudogenes, but this percentage was variable among genomes, up to over 50% in Ae. umbellulata. The most common length of the deduced protein, including the signal peptide, was 302 amino acids, but the length varied from 215 to 362 amino acids, both obtained from Ae. speltoides. Most genes encoded proteins with eight cysteines. However, all Aegilops species had genes that encoded a gamma-gliadin protein of 302 amino acids with an additional cysteine. These conserved nine-cysteine gamma-gliadins may perform a specific function, possibly as chain terminators in gluten network formation in protein bodies during endosperm development. A phylogenetic analysis of gamma-gliadins derived from Aegilops and Triticum species and the related genera Lophopyrum, Crithopsis, and Dasypyrum showed six groups of genes. Most Aegilops species contained gamma-gliadin genes from several of these groups, which also included sequences from the genera Lophopyrum, Crithopsis, and Dasypyrum. Hordein and secalin sequences formed separate groups.ConclusionsWe present a model for the evolution of the gamma-gliadins from which we deduce that the most recent common ancestor (MRCA) of Aegilops/Triticum-Dasypyrum-Lophopyrum-Crithopsis already had four groups of gamma-gliadin sequences, presumably the result of two rounds of duplication of the locus.
Highlights
The gamma-gliadins are considered to be the oldest of the gliadin family of storage proteins in Aegilops/Triticum
In the variety Chinese Spring the number of gamma-gliadins was preliminary estimated at 15–40 [8,9] and, in contrast to the situation in alpha-gliadins, only a small fraction (~14%) of the gamma-gliadin genes in hexaploid bread wheat consisted of pseudogenes [10]
The fraction pseudogene sequences differed among the eight Aegilops/Triticum species analysed
Summary
The gamma-gliadins are considered to be the oldest of the gliadin family of storage proteins in Aegilops/Triticum. Prolamin storage proteins are produced in large amounts in the developing endosperm of Triticeae species. These storage proteins are a complex mixture of alpha/beta-, gamma- and omega-gliadins and high- and low molecular weight glutenins, collectively called ‘gluten’ in wheat. They are encoded by medium to large multigene families. The gamma-gliadins are considered to be the most ancient of the gliadins and LMW-glutenins [5] In bread wheat they are encoded by the homoeologous Gli-1 loci (Gli-A1, Gli-B1 and Gli-D1), located on the short arms of the homoeologous chromosomes 1 [6,7]. Sequence analysis showed that the gamma-gliadins form a highly diverse gene family [9,10]
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