串珠藻目植物rbcL基因的适应性进化分析

ADAPTIVE EVOLUTIONARY ANALYSIS ON rbc L GENE OF BATRACHOSPERMALES

淡水红藻是藻类植物进化中的一个重要类群,其中最主要的类群是串珠藻目。核酮糖1,5二磷酸羧化酶/加氧酶(Rubisco)是一种既负责碳同化又引发光呼吸,在植物光合作用中起重要作用的酶,其催化固定CO_2的活性中心位于Rubisco大亚基,由叶绿体rbcL基因编码。为了探究串珠藻目植物适应特殊生存环境的分子水平机制,本文选取了串珠藻目及一些相近类群淡水红藻的rbcL基因39条,利用PAML4.8软件,运行位点模型、分支模型及分支-位点模型,对选取类群的rbcL基因进行了适应性进化分析。结果表明:(1)通过最大似然法构建的系统发育树显示,所有内类群聚集为6个分支,其中,分支A为暗紫红毛菜,分支B为胶串珠藻,分支C为扁圆串珠藻,后验概率同样为100%,分支D为熊野藻属,分支E为连珠藻属,分支F为弧形西斯藻;(2)分支-位点模型中,在3个分支中分别鉴定出350S、277L和280L为正选择位点;(3)在构建出的Rubisco大亚基的参考三维模型中,277L和280L位于Rubisco大亚基羧基末端保守的8个α螺旋和8个β片层构成的α/β桶状结构域中第7个α螺旋和第7个β片层之间的loop结构上,350S位于羧基末端邻近α/β桶状结构域的一个α螺旋上。研究结果一方面显示了基于ω比值检验基因适应性进化的准确性和有效性,另一方面也揭示了串珠藻目植物rbcL基因确实发生了适应性进化,对串珠藻目适应特殊生存环境产生了有益的作用。

Freshwater red algae are an important lineage in algal evolution. Batrachospermales is an order with the most species. Ribulose-1,5 bisphosphate carboxylase/oxygenase（Rubisco） is an enzyme responsible for both carbon assimilation and photorespiration, and plays an important role in plant photosynthesis. The Rubisco large subunit containing active sites for catalyzing CO_2 fixation, is encoded by chloroplast rbc L gene. To explore the molecular mechanism of algal adaption to special living environment, 39 rbc L genes of Batrachospermales and other relative taxa were selected to analyze the adaptive evolution. Using software PAML4.8, the site model, branch model, and branch-site model were tested. The results show that（1） in the phylogenetic tree constructed by the maximum likelihood method, all the internal groups cluster into six branches, i.e., Cluster A is Bangia atropurpurea, B is Batrachospermum gelatinosum, C is B. helminthosum, D is the genus Kumanoa, E is the genus Sirodotia, and F is Sheathia arcuata;（2） 350 S, 277 L, and 280 L are identified as positive selection sites on three branches in the branch-site model;（3） in the reference three-dimensional model of Rubisco large subunit, 277 L and 280 L are located in a loop between the 7th α helix and the 7th β sheet of the conserved α/β-barrel domain that consists of 8 α-helices and 8 β-sheets at the C-terminus, and 350 S is located on an α helix in the C-terminus adjacent to the α/β-barrel domain. Therefore, the results demonstrated the accuracy and effectiveness of examining the adaptive evolution of gene based on ω ratio, and revealed that the rbc L gene has undergone adaptive evolution, which has been beneficial to Batrachospermales for the adaption to the special living environment.