Origin And Reticulate Evolutionary Process Of Wheatgrass Elymus Trachycaulus(Triticeae:Poaceae)

The tribe Triticeae,including about 350-450 species widely distributed around the world,is precious germplasm resources in crop improvement and forage breeding.Triticeae contains hundreds of species which are annual and perennial types,self-pollination and cross-pollination,diploid and polyploid.Therefore,the tribe Triticeae is the excellent material for studies of the origin of species,evolutionary biology,polyploidy origin mechanism and genetic variation.Elymus trachycaulus with StH genomes,which originated from hybridization between the species from Pseudoroegneria(St genome)and Hordeum(H genome).Understanding the origin and evolution of the species will have an important guiding significance on the study of molecular phylogeny and evolution of the tribe Triticeae.To study evolutionary mechanism of tetraploid Elymus trachycaulus,thirteen accessions of tetraploid E.trachycaulus were analyzed using low-copy nuclear genes encoding phosphoenolpyruvate carboxylase(Pepc)and the largest subunit of RNA polymerase II(Rpb2)and one non-coding chloroplast region,TrnL/F.Previous studies showed that two different genomes,St genome and H genome,were the genome donors to E.trachycaulus.Our chloroplast data indicated that Pseudoroegneria was the maternal donor.Rpb2 data here indicated that the St genome in E.trachycaulus was originated from either P.strigosa,P.stipifolia,P.spicata or P.geniculate.The Hordeum-like sequences of E.trachycaulus are polyphyletic in the Pepc tree,suggesting that the H genome in E.trachycaulus were also contributed by multiple sources,whether due to multiple origins or to introgression resulting from subsequent hybridization.Failure to recovery St copy of Pepc sequences in most accessions of tetraploid E.trachycaulus might be caused by genome convergent evolution in allopolyploids.Multiple copies of H-like Pepc sequences from each accession with relative large deletions and insertions might be caused by either instability of Pepc sequences in H-genome or incomplete concerted evolution.Our results highlighted complex evolutionary history of E.trachycaulus.