Aegilops section Sitopsis species contains the introgressive PolA1 gene with a closer relationship to that of Hordeum than Triticum-Aegilops species

The section Sitopsis in the genus Aegilops includes five species, Ae. speltoides, Ae. longissima, Ae. sharonensis, Ae. searsii, and Ae. bicornis, which share the SS genome. Although extensive molecular studies have indicated Ae. speltoides as a donor of BB or GG genome to polyploid wheat species, the precise relationships among SS, BB, and GG genomes remain unclear. PolA1 is a single-copy nuclear gene encoding the largest subunit of RNA polymerase I. Highly polymorphic PolA1 exon 20 sequences were analyzed for 11 Triticum–Aegilops, 13 Hordeum and three related species. Phylogenetic analyses of the PolA1 gene showed that Triticum–Aegilops and Hordeum species were distinctly separated into two clades. Two related species, Secale cereale and Dasypyrum villosum, were grouped into Triticum and Hordeum clades, respectively. Interestingly, seven accessions of the Sitopsis species were clustered into the Hordeum clade whereas two accessions belonged to the Triticum clade. In contrast, all accessions of Sitopsis species shared the same haplotype of plastid PSID sequences with Triticum–Aegilops species. This inconsistency in phylogeny between nuclear and cytoplasmic sequences suggested that the Sitopsis species probably originated through introgressive hybridization between ancestral species of Triticum–Aegilops and Hordeum.