Abstract
BackgroundPrevious cytological and single copy nuclear genes data suggested the St and Y genome in the StY-genomic Elymus species originated from different donors: the St from a diploid species in Pseudoroegneria and the Y from an unknown diploid species, which are now extinct or undiscovered. However, ITS data suggested that the Y and St genome shared the same progenitor although rather few St genome species were studied. In a recent analysis of many samples of St genome species Pseudoroegneria spicata (Pursh) À. Löve suggested that one accession of P. spicata species was the most likely donor of the Y genome. The present study tested whether intraspecific variation during sampling could affect the outcome of analyses to determining the origin of Y genome in allotetraploid StY species. We also explored the evolutionary dynamics of these species.Methodology/Principal FindingsTwo single copy nuclear genes, the second largest subunit of RNA polymerase II (RPB2) and the translation elongation factor G (EF-G) sequences from 58 accessions of Pseudoroegneria and Elymus species, together with those from Hordeum (H), Agropyron (P), Australopyrum (W), Lophopyrum (Ee), Thinopyrum (Ea), Thinopyrum (Eb), and Dasypyrum (V) were analyzed using maximum parsimony, maximum likelihood and Bayesian methods. Sequence comparisons among all these genomes revealed that the St and Y genomes are relatively dissimilar. Extensive sequence variations have been detected not only between the sequences from St and Y genome, but also among the sequences from diploid St genome species. Phylogenetic analyses separated the Y sequences from the St sequences.Conclusions/SignificanceOur results confirmed that St and Y genome in Elymus species have originated from different donors, and demonstrated that intraspecific variation does not affect the identification of genome origin in polyploids. Moreover, sequence data showed evidence to support the suggestion of the genome convergent evolution in allopolyploid StY genome species.
Highlights
Hybridization and polyploidization have played an important role in the history of plant evolution, and contribute greatly to speciation [1]
Sequence variation The amplified patterns from each diploid species show a single band for both the RPB2 and elongation factor G (EF-G) sequences with a size of approximately 800,1000 bp, which corresponds well to previous findings [21,22]
The phylogenetic analyses of Elymus and as many as 31 accessions from 8 St genome diploid species, in the present study, provide support for the distinct origin of the Y genome in polyploid StY species. Both RPB2 and EF-G phylogenetic trees have well separated the Y genome from the St genome. These results are in accordance with the previous findings by MasonGamer et al [19,20] and Sun et al [21], and support Dewey’s hypothesis that there is a Y diploid species from which the Y genome originated
Summary
Hybridization and polyploidization have played an important role in the history of plant evolution, and contribute greatly to speciation [1]. Previous studies have reported that hybridization and chromosome doubling would create genetic shocks and the newly formed allopolyploids would need to undergo major intraand inter-genomic changes Many of these genome-wide alterations in allopolyploids could arise from rapid loss and recombination of low-copy DNA, retrotransposon activation, DNA methylation pattern changes and epigenetic gene silencing during or following polyploidization [2,3,4]. These rapid genomic changes may lead to genetic asymmetry evolution resulting in conformity and convergent effects caused by the inter-genome invasion of chromatin segments in either only one pair of chromosomes (chromosome-specific sequences) or in several chromosome pairs of one genome (genome-specific sequences) in the alloploids [5,6,7,8,9,10].
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