鲸类视觉退化分子机制的研究：基于GNAT2和CNGB3基因的进化分析

The molecular mechanism of visual degradation in cetaceans based on GNAT2 and CNGB3 gene

鲸类是一类次生性的水生哺乳动物,其陆生祖先大约53～56 Ma从陆地返回海洋。为了适应水下的弱光环境,鲸类的光感受器以视杆细胞为主,视锥细胞功能大多退化,缺乏辨别颜色的能力,然而鲸类视觉退化的分子机制尚不清楚。本文选择在视锥细胞中表达且对光传导级联反应起重要作用的GNAT2和CNGB3基因,通过PCR扩增、测序以及在数据库中下载已有的基因序列,共获得8个鲸类代表性物种的同源序列。MEGA6. 0软件比对发现侏儒抹香鲸和抹香鲸的GNAT2基因分别在148位和1012位插入了1个碱基A,而抹香鲸的CNGB3分别在554位和1407位有1个碱基的缺失,导致提前终止密码子出现;另外,小须鲸CNGB3基因在1525～1527位出现了提前终止密码子,提示鲸类的这两个基因可能为假基因。通过I-TASSER在线预测GNAT2和CNGB3蛋白的三维结构,发现出现移码突变终止密码子的位置均位于这两个基因的重要功能域。另外,运用PAML软件的Branch model分析表明发生假基因的鲸类物种GNAT2和CNGB3基因出现选择压力放松,且假基因化可能是一个近期事件。侏儒抹香鲸、抹香鲸以及小须鲸的GNAT2和CNGB3基因出现假基因可能与其深潜习性以及完全的水生生境导致其色觉功能丢失相关。此外,Free-ratio model分析发现其他鲸类的ω值接近于1,说明GNAT2和CNGB3基因出现了选择压力放松,这可能是长期适应水生生境视觉退化的结果。

Cetaceans are a mysterious group of secondarily-adapted marine mammals that “returned” from the land to the sea approximately 53–56 million years ago （Ma）. In order to adapt to dim aquatic environment, cetaceans have mostly rod photoreceptors and few cone photoreceptors, resulting in a loss of color vision. However, the genetic basis of visual degradation in cetaceans remains poorly explored. The GNAT2 and CNGB3 genes, expressed in cone photoreceptors play the key role in the phototransduction cascade. GNAT2 and CNGB3 genes of eight representative species of cetaceans were obtained by PCR amplified and gene downloaded from NCBI. Protein sequences of GNAT2 and CNGB3 were aligned by MEGA6.0. We found GNAT2 gene might become a pseudogene due to a 1-bp insertions at sites 148 and 1012 in Kogia sima and Physeter catodon, respectively. Similar results were indentified at the CNGB3, with 1-bp deletions identified at sites 554 and 1407 of CNGB3 in P. catodon and apremature stop codon at site 1525-1527 in Balaenoptera acutorostrata. We predicted the three-dimensional （3D） structures of GNAT2 and CNGB3 using I-TASSER, and mapped the pseudogenized position onto the 3D structures. 3D structureal analysis showed that the insertion, deletion and premature stop codon were located in an important domain. Furthermore, branch model analysis revealed that relaxed selective pressure acted on cetacean lineages with pseudogenized GNAT2 and CNGB3. The pseudogenized GNAT2 and CNGB3 in cetacean lineages are suggested to be related to its deep diving and visual degradation to adapt completely aquatic environment. In addition, relaxed selection pressure of GNAT2 and CNGB3 was also detected in other cetacean lineages with increased ω values by free-ratio model, consistent with the morphological changes of eye for adapting to aquatic habitats.