Uniformity of karyotypes inRheum(Polygonaceae), a species-rich genus in the Qinghai-Tibetan Plateau and adjacent regions

In this study, we aimed to examine whether the karyotypes had differentiated distinctly between 12 morphologically diversifi ed species belonging to fi ve sections of the genus Rheum. The basic chromosome number x = 11 and two ploidy level (diploid and tetraploid) previously reported are confi rmed here. Nine species are diploid with 2n = 22 while three are tetraploid with 2n = 44. The chromosome numbers and karyotypes are reported here for the fi rst time for seven species: R. nobile, R. alexandrae, R. rhomboideum, R. reticulatum, R. nanum, R. likiangense, and R. pumilum. Karyotypic analyses suggested that all of the sampled 12 species have metacentric chromosomes with a few submetacentric ones, belonging to the 2A type of the Stebbins’ karyotype asymmetry classifi cation. These results suggest that kayotypic differentiation had contributed little to interspe-cifi c morphological divergence within this genus. However, polyploidy seems to have played an important role in driving its speciation and diversifi cation.