Geographical distribution, origin, and dispersal of Leymus Hochst. (Poaceae)

The geographical distribution,origin,and dispersal of Leymus Hochst (Triticeae,Poaceae) were studied through field investigation,specimen collection,literature review,and comprehensive analysis of geological,climatic,and evolutionary relationships between groups.Results showed there were 53 species (including varieties) of Leymus belonging to three sections,mainly distributed in Eurasia and North America.In China,there were 40 species (including varieties) of Leymus belonging to three sections,mainly distributed in the northwest,north,northeast,and southwest regions of China. Leymus species mostly concentrated in these areas.The most abundant region for sections (three) and species (22) was in the Altai area of northern Xinjiang and the Tangut region of the northeastern Qinghai-Tibet Plateau,which included the most primitive and the most advanced in the genus,suggesting that this area was probably the distribution center of Leymus.The Altai area also possessed many primitive taxa and outgroups of Leymus,and hence was likely the region of origin sometime during the Oligocene.The climate and environment changed dramatically with the uplifting of the Qinghai-Tibet Plateau at the end of Oligocene,and thus the genus further developed and differentiated into other areas of China undergoing significant geological activities.Therefore,the geographical distribution pattern today formed mainly through two stages and three path diffusions.