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Single-copy nuclear PolA1 gene sheds light on the origin of S genome with relationships to B and G genomes of polyploid wheat species
Authors:Bhuwan Rai  Hiroko Takahashi  Kenji Kato  Yo-ichiro Sato  Ikuo Nakamura
Institution:1. Graduate School of Horticulture, Chiba University, 648 Matsudo, Matsudo, Chiba, 271-8510, Japan
2. Faculty of Agriculture, Okayama University, 1-1-1 Tsushima-Naka, Okayama, 700-8530, Japan
3. Research Institute of Humanity and Nature, Kamigyo, Kyoto, 602-0878, Japan
Abstract:PolA1, a single-copy nuclear gene encoding the largest subunit of RNA polymerase I, comprises highly polymorphic intron 19 and nucleotide tag (Ntag) sequences. We analyzed these sequences in 42 accessions, which differed in ploidy, of TriticumAegilops and Hordeum species. The lengths of the intron 19 sequences were ca. 110?bp long in TriticumAegilops species, except in four Sitopsis species, Ae. longissima, Ae. searsii, Ae. sharonensis, Ae. speltoides, which had introns similar in length to those of Hordeum species, i.e., ca. 240?bp long. Phylogenetic analyses of the Ntag sequences showed that the four Sitopsis and remaining TriticumAegilops species were classified into two discrete Hordeum and Triticum clades, respectively. The A and D genome-specific Ntag sequences of polyploid wheats were highly homologous with those of T. urartu and Ae. tauschii, respectively. In Ae. bicornis, another Sitopsis species, two accessions had the short intron 19 and Triticum–type Ntag sequence, which were highly homologous with those of the B genome in polyploid wheats, whereas one accession contained the long intron 19 and Hordeum–type Ntag sequences. In contrast, partial sequence analyses revealed that the three accessions of Ae. bicornis shared highly homology to single-copy DMC1 and EF-G genes. The discrepancy between these results indicates that the Sitopsis species were probably established by hybrid speciation including ancient introgressive hybridization between progenitors of TriticumAegilops and Hordeum. Although many researchers have proposed Ae. speltoides as a candidate for the B genome donor, our data suggest the existence of diploid B genome species in the past that were responsible for the origin of both polyploid wheats and Sitopsis species, including Ae. speltoides.
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