Phylogenetic analysis of Elymus (Poaceae) in western China.

Elymus L. is often planted in temperate and subtropical regions as forage. Species in the genus have 5 allopolyploid genomes that are found in the grass tribe Triticeae. To determine the phylogenetic relationships in Elymus species from western China, we estimated phylogenetic trees using sequences from the nuclear ribosomal internal transcribed spacer and non-coding chloroplast DNA sequences from 56 accessions (871 samples) of 9 polyploid Elymus species and 42 accessions from GenBank. Tetraploid and hexaploid Elymus species from western China had independent origins, and Elymus species from the same area or neighboring geographic regions were the most closely related. Based on the phylogenetic tree topology, the St- and Y-genomes were not derived from the same donor and Y-genome likely originated from the H-genome of Hordeum species, or they shared the same origin or underwent introgression. The maternal genome of tetraploid and hexaploid Elymus species originated from species of Hordeum or Pseudoroegneria. Additionally, Elymus species in western China began diverging 17-8.5 million years ago, during a period of increased aridification as a consequence of the Messinian salinity crisis. Elymus species adapted to drought and high salinity may have developed based on the environmental conditions during this period. Elymus evolution in western China may have been affected by the uplift of the Qinghai-Tibetan Plateau (5 million years ago), when Elymus seeds were dispersed by gravity or wind into a newly heterogeneous habitat, resulting in isolation.