Department of Ecology and Evolutionary Biology, University of Toronto, Toronto, Ontario, M5S 3G5, Canada.
Current Address: Department of Vertebrate Zoology, National Museum of Natural History, Smithsonian Institution, 10th and Constitution Ave NW, Washington, DC, 20560-0162.
Evolution. 2019 Sep;73(9):1958-1971. doi: 10.1111/evo.13810. Epub 2019 Aug 28.
Most vertebrates use a combination of rod and cone photoreceptors to enable vision in conditions ranging from starlight to direct sunlight. Nocturnal geckos, however, have simplex retinas that contain only rods in terms of morphology and physiology, but these rods are thought to be derived from cones through an evolutionary process known as photoreceptor transmutation. To investigate this, we generated eye transcriptomes and analyzed patterns of phototransduction gene evolution in geckos in comparison to other reptiles. We confirm that geckos have lost several major components of the rod phototransduction pathway, including rod opsin (RH1), which we identified as a pseudogene in multiple genomes. We also identified a partial rod transducin transcript, but found no evidence of the protein in retinal sections. However, we find that geckos express several complete rod phototransduction transcripts in the eye, which may contribute to the rod-like physiology of nocturnal gecko photoreceptors. Finally, we found surprising evidence that even though photoreceptor transmutation evolved independently in geckos and snakes, they have experienced parallel shifts in selective constraint on phototransduction genes. These results implicate adaptive change in the underlying molecular machinery of visual transduction, in addition to the convergent changes in cellular morphology, during photoreceptor transmutation.
大多数脊椎动物使用棒状和锥状光感受器的组合来实现从星光到阳光直射等各种条件下的视觉。然而，夜行动物壁虎的视网膜结构简单，形态和生理上只包含棒状细胞，但这些棒状细胞被认为是通过一种被称为光感受器转导的进化过程从锥状细胞衍生而来的。为了研究这一点，我们生成了壁虎的眼部转录组，并分析了与其他爬行动物相比，光感受器转导基因进化的模式。我们证实，壁虎已经失去了棒状光感受器转导途径的几个主要组成部分，包括棒状视蛋白（RH1），我们在多个基因组中鉴定出它是一个假基因。我们还鉴定出了部分棒状转导蛋白的转录本，但在视网膜切片中没有发现该蛋白的证据。然而，我们发现壁虎在眼睛中表达了几种完整的棒状光感受器转导转录本，这可能有助于夜间壁虎光感受器的棒状生理学。最后，我们发现了令人惊讶的证据，即使光感受器转导在壁虎和蛇中独立进化，它们在光感受器转导基因的选择约束方面也经历了平行的变化。这些结果表明，除了细胞形态的趋同变化外，在光感受器转导过程中，视觉转导的基础分子机制也发生了适应性变化。
