水龙骨科附生蕨类Rubisco大亚基的适应性进化:正向选择位点的鉴定

Adaptive Evolution in the Rubisco Large Subunits of Epiphytic Ferns(Polypodiaceae): Identification of Positively Selected Sites

核酮糖-1,5-二磷酸羧化酶/加氧酶(Rubisco,EC 4.1.1.39)在植物光合作用中起重要作用,既负责碳同化又引发光呼吸。催化固定CO2的活性位点位于Rubisco大亚基,由叶绿体rbcL基因编码。水龙骨科是现存蕨类中最为衍生的类群,多为附生植物。为验证蕨类植物在白垩纪适应被子植物兴起而发生分化的观点,本研究以水龙骨科附生蕨类为对象,利用位点间可变ω(非同义替换率dN和同义替换率dS的比值)模型对其rbcL基因的适应性进化进行分析。通过比较模型M1a/M2a和模型M7/M8,在氨基酸水平上,共鉴定出7个正向选择位点:133L,251M,262A,265V,282Y,326I和362I。其中位点262A,265V,282Y及326I对维持Rubisco功能的重要性也得到已发表实验数据的支持。这些结果一方面显示了基于ω比值检验DNA编码序列分子适应的有效性,另一方面也提示水龙骨类可借助rbcL等功能基因的适应性进化,应对白垩纪被子植物引发的陆地生态系统改变。

Ribulose-1, 5-biphosphate carboxylase/oxgenase （ Rubisco, EC 4. 1.1.39） plays pivotal roles in plant photosynthesis, initiating both carbon assimilation and photorespiration. Rubisco large subunit, which contains the active sites involved in catalyzing CO2 fixation, is encoded by the chloroplast rbcL gene. The Polypodiaceae are likely the most derived group of extant ferns, and they mainly grow as epiphytes in tropical or subtropical climates. Recently polypod ferns have been demonstrated to undergo diversifications in the Cretaceous after the rise of angiosperms. To further test this proposal, an adaptive evolutionary analysis was performed on the rbcL gene sequences of Polypodiaceous ferns under models allowing ω （ nonsynonymous/synonymous rate ratio） to vary among sites. By comparing Models M1a/M2a and M7/M8, seven amino acid sites were identified under positive selection： 133L, 251M, 262A, 265V, 282Y, 326I and 362I. Functional significances of four sites 262A, 265V, 282Y and 326I were lent support by published experimental results. This research consolidated that evolutionary models using ω as an indicator of selective pressure provide valuable approaches for analyzing the adaption of protein-encoding sequences. Moreover, it also indicated that Polypodiaceous ferns responded to the terrestrial ecosystem changes initiated by angiosperms through molecular adaption of functional genes such as rbcL.