Karyotypes Of Rheum(Polygonaceae), A Species-rich Genus In The Qinghai-Tibetan Plateau And Adjacent Regions

Rheum, a highly diversified genus of Polygonaceae, comprising about 60 species, is mainly distributed in the mountainous and desert regions in Asia and Europe. Its high diversification in southwest China and adjacent areas, where approximately 40 species have been described, was suggested to result from the adaptive radiation triggered by the recent uplifts of the Qinghai-Tibetan Plateau. In this study, we attempted to examine whether the karyotypes had differentiated distinctly between 12 morphologically diversified species belonging to five sections of the genus Rheum native to the Qinghai-Tibetan Plateau and adjacent regions, aiming to find the cytological factors giving rise to the species diversification. The basic chromosome number x=11 and two ploidy level (diploid and tetraploid) previously reported are confirmed here. Nine species of twelve we studied are diploid with 2n=22 while three are tetraploid with 2n=44. The chromosome numbers and karyotypes are reported here for the first time for seven species:R. nobile, R. alexandrae, R. rhomboideum, R. reticulatum, R. nanum, R. likiangense, and R. pumilum, the chromosome number of the rest species is consistent with previous reports. Karyotypic analyses suggested that all of the sampled 12 species have metacentric chromosomes with a few submetacentric ones that are small size, and the karytypes are highly uniform belonging to the 2A type of the STEBBINS'karyotype asymmetry classification. These results suggest that karyotypic differentiation had contributed little to interspecific morphological divergence within this genus. However, along with the former studies, we inferred that polyploidy may have played an important role in driving its speciation and diversification.