| Gene duplication provides raw materials for further innovative evolution in plants in response to various stressful environments and natural selection. In this study, we aimed to examine evolutionary history of the CHS-like genes in the Rheum, a genus with a radiative history with its most species occurring mainly in the Qinghai-Tibetan Plateau. The main results and conclusions are listed as following,(1)A total of 36 CHS-like sequences were obtained through sequencing the clones of the PCR CHS amplification of 16 Rheum species and eight ones from the other closely related genera. Phylogenetic analyses of these sequences suggested that all CHS-like sequences from Rheum comprised a monophyletic lineage and within the genus, two subclades with high supports were recovered. These analyses indicated that since origin of the genus Rheum, CHS had undergone an independent duplication. DNA Gel blotting analyses further confirmed this suggestion that at least two CHS-like members are present in two species.(2) Based on the branch-model, the ratio of the non-synonymous and synonymous substitutions within the subclades A (ω1=0.1454) and B (ω2=0.4532) ratios were significantly different from each other and also the background ratioω0 (0.0222). It is suggested that an increase in the nonsynonymous substitution rate occurred in the lineage ancestral to CHSB after gene duplication. Branch-site model analyses also indicated that one site (G243) within the CHSB was driven significantly by positive selection with the posterior probability (P=0.997).(3) Within each subclade, the relationships between different species were rarely recovered and the pattern is a typical "paralleling" phylogeny, suggesting a radiative diversificiation. This result is consistent with the previous phylogenetic analysis based on the chloroplast DNA sequences.
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