Phylogeny and taxonomy of the polyploid species that contain St genome (Triticeae; Poaceae) based on four nuclear DNA and three chloroplast genes

Background: The genus Pseudoroegneria (Nevski) Á.Löve contributes the St genome for more than 60% of perennial Triticeae species. However, the strong dominant character of the St genome makes it challenging to distinguish each species and/or even genus based on single or combined morphological traits. Moreover, the phylogeny and taxonomy of the St-genome containing polyploid genera remain controversial. Results: In this study, we used nuclear and chloroplast DNA-based phylogenetic analyses to reveal the systematic relationships between the St-genome containing polyploid species. The maximum likelihood (ML) tree based on nuclear ribosomal internal transcribed spacer region (nrITS) and three single-copy nuclear genes data (Acc1 + Pgk1 + DMC1) showed that polyploid species with the St genome were separated into seven genera with StStHH, StStYY, StStYYHH, StStYYPP, StStYYWW, StStPP, and StStEE genome constitutions, moreover, the polyploid species in Caucasus, America, and Australia have independent polyploidization events. The ML tree for the chloroplast DNA fragments (matK + rbcL + trnL-trnF) displayed that the P genome served as a maternal donor of Kengyilia melanthera and K. dingqinensis from the Hengduan Mountains region, while the St or StY genome served as the maternal donor of other St-genome containing species. Herein, we reported the genomic constitution of Kengyilia tibetica, K. changduensis, and K. dingqinensis with the StStYYPP genome for the first time. Conclusions: The St-genome-containing polyploid species should be treated as distinct genera according to different genome constitutions, and those species experienced independent allo-polyploidization events in different distribution regions and had two relatively independent maternal origins from the P or St/StY genomes. Besides, the Xp genome might have contributed to the unknown Y genome formation.