The Molecular Evolution of Cetacean Dim-Light Vision: In Vitro Studies of Rhodopsin Over a Macroevolutionary Transition

Cetaceans (whales and dolphins) are fully aquatic mammals that have captured the imagination of biologists due to their iconic evolutionary transformation from terrestrial ancestors. Nevertheless, much about how this extreme macroevolutionary transition occurred at the molecular level remains unknown. The vertebrate visual system is ideal for studying molecular adaptation due to the reliance organisms place on it for fitness-related behaviours, and because key molecular components of the visual transduction cascade in photoreceptors are relatively well understood. Light activation of the dim-light pigment, rhodopsin, is the first step of the visual transduction cascade, and is thought to have been the target of selection across a wide variety of vertebrate lineages. Cetaceans, due to their rich evolutionary history, are model organisms for understanding how changing environments put selective pressure on sensory processes. In this dissertation, I investigate the molecular basis for adaptive evolution in cetacean rhodopsin using a combination of in vitro expression experiments and computational analyses. First, I evaluate evidence for adaptation in cetacean rhodopsin through characterization of its spectral properties in the killer whale (Orcinus orca), and a statistical assessment of selection patterns in a representative dataset of cetacean rhodopsin genes. Next, I focus on the evolutionary significance of key amino acid substitutions in killer whale rhodopsin that confer both spectral and non-spectral (kinetic) functional shifts, specifically through comparison with rhodopsin pigments from two outgroup species (bovine and hippopotamus). Finally, I use an ancestral sequence reconstruction and protein resurrection approach to establish functional and corresponding molecular shifts across the terrestrial-aquatic transition at the root of Cetacea. Together, these projects provide a more complete investigation of cetacean rhodopsin, not only addressing questions regarding dim-light (aquatic) adaptation in cetaceans, but also generating new hypotheses about how the ecology of living and ancient cetaceans has shaped the visual system.Ph.D