Molecular evolution of vision and visual pigments

点击翻译按钮获取中文摘要

Visual pigment activation by light triggers the first step of the sensory transduction cascade in photoreceptors of the eye that ultimately results in a neural signal to the brain. Typical vertebrate retinas have two types of photoreceptors, cones for vision during the day, and rods for dim light. These photoreceptors are known to contain different components of the visual transduction cascade, including rod and cone visual pigments, which are thought to underlie some of the physiological differences between the two. Among vertebrates, reptiles possess some of the most diverse visual systems in known in nature, especially in terms of the photoreceptor composition of their retinas, with frequent losses of either of the two photoreceptor types. A longstanding theory postulated that this might be the result of unusual evolutionary transformations from one photoreceptor cell type into another, a process rare outside of reptiles. This was first discovered in nocturnal geckos, in which the rods are thought to have evolved from ancestral cones, retaining cone visual transduction genes. Our group has demonstrated that a similar evolutionary transformation has taken place, but in the opposite direction, in a strongly diurnal species of garter snake. Reptilian retinas appear to be capable of striking evolutionary transformations in terms of retinal composition, but the consequence of these differences in cellular context for the visual transduction machinery of the photoreceptor cell, including the visual pigments, is not well understood, and its origins remain shrouded in mystery. Reptilian visual systems represent a rare opportunity to study the origins and molecular consequences of these fascinating evolutionary photoreceptor transformations. My research program uses innovative evolutionary approaches, both experimental and computational, to understand fundamental mechanisms of visual pigment function by studying adaptive variation found in reptiles and other animals living in diverse environments where light intensities and available spectrum can be dramatically different. In this research proposal, we have three objectives. (1) We will experimentally investigate visual pigments from evolutionary transformed photoreceptors, comparing the functional shifts in snakes and geckos, and use mutagenesis approaches to determine if similar mechanisms underlie shifts in function in the two groups. (2) We will use a combination of sequencing, evolutionary analysis, and experimental assays of reconstructed ancestral squamate visual pigments to investigate the evolutionary origins of photoreceptor transformations in reptilian retinas. (3) Finally, my group has recently developed the first high-throughput assay of visual pigment activation, which allows us to rapidly screen thousands of mutations efficiently. We will use this to screen mutations in visual pigments, and better understand tradeoffs in different aspects of function.