Molecular evolution of visual pigments

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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. Variation in this first critical step of the visual transduction cascade, visual pigment activation, is known to contribute to important aspects of the vertebrate visual system, such as its enormous dynamic range in terms of variation in light intensities, and in wavelengths of light perceived. However, aside from a few model systems, the remarkable diversity of visual pigments in nature has not been investigated in detail. How have animals adapted to vision under extreme dim light conditions? My research program uses an innovative approach to understanding fundamental mechanisms of visual pigment function by studying adaptive variation found in animals living in environments where the light intensities and available spectrum are dramatically reduced: 1) Nocturnal snakes. Among terrestrial vertebrates, the visual systems of snakes are known to possess many peculiar features that have been attributed to their subterranean origins. Colubrid snakes are fascinating in that many are diurnal species adapted to daylight vision with an all-cone retina, but there are also crepuscular and nocturnal species adapted for dim-light vision. The molecular basis of these adaptations are poorly understood. We will use heterologous expression to investigate visual pigment function in a diversity of snakes adapted for dim-light vision. This will be combined with eye transcriptome sequencing, followed by evolutionary computational analyses in order to identify visual genes under positive selection. This research will not only result in the first eye transcriptomes reported for any reptile, it will also be combined with experimental investigations into the underlying molecular mechanisms resulting in dim light adaptations in snakes. 2) Diving whales. Cetaceans such as whales and dolphins are known to possess remarkable adaptations to their fully aquatic lifestyle. The cetacean retina is dominated by rod photoreceptors, which would contribute to increased visual sensitivity at low light levels. There are hints that this specialization for dim light vision also extends to molecular mechanisms of visual transduction in cetaceans, but this has yet to be studied in detail. Taking advantage of a number of recent genome sequencing studies in cetaceans, we will functionally characterize visual pigments from a variety of diving cetaceans using spectroscopic assays, in order to investigate aspects of visual pigment function that may be related to their aquatic lifestyle. This research is exciting as it will be one of the few experimental studies of molecular adaptation in cetaceans that enabled their fascinating transition from terrestrial to fully aquatic lifestyles, one of the most dramatic transformations in evolutionary history.