| Summary: | 博士 === 國立成功大學 === 生命科學系碩博士班 === 95 === Rice, one of the staple crops in the world, is cultivated widely in Asia. Oryza, a genus of the Poaceae, consists of three sections and 24 species. Since last century, the industrial pollution has been seriously contaminating environments. Among various pollutants, heavy metals in the soil influences the crop growth to a great extent. Of them, cadmium is toxic to most animals and flowering plants. To vascular plants, cadmium is a nonessential metal; only species or populations that are tolerant to cadmium can survive in the wild. Phytochelatin (PC) proteins, which are synthesized from glutathione via a transpeptidation reaction mediated by the phytochelatin synthase (PCS), enable plants to tolerate heavy-metal ions. This study focuses on molecular evolution and selection effects of the PCS gene family in Oryza. Phylogenetic analysis revealed that monocots, dicots, and pteridophytes each formed a monophyletic group. Reciprocal monophyly of PCS1 and PCS2 paralogs in the Oryza was supported. As directional selection as an evolutionary force shaping the PCS1 gene was detected in other species, PCS1 gene mediated by maintaining the ancestreal lineages in cultivated rice, resulting positive Tajima’s D values. In contrast, PCS2 genes evolved under a negative selection in the genus. Linkage disequilibrium (LD) decayed slowly. Compared to the PCS2 gene, a lower level of LD in PCS1 gene was detected. The possible explanations are that the balancing selection and high-frequency recombination events of PCS1 have led to such lower LD.
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