Evolution and Diversity of Visual Pigments in Connection with Their Functional Differences

Living organisms have generated and optimized their photoreceptor cells to acquire information from the outer environment. Among the photoreceptor cells, rod and cone photoreceptor cells (rods and cones) present in vertebrate retinas have evolved to mediate vision. Rods are responsible for dim light vision whereas cones act for bright light and color vision. Corresponding to their difference in physiological role, both cells contain functional proteins constituting signal transduction cascades with different properties. Visual pigment is the upstream protein that absorbs light and triggers the subsequent signal transduction cascade. It has diversified into five groups: the L (LWS/MWS), S (SWS1), M1 (SWS2), M2 (RH2), and rhodopsin (RH1) groups. The visual pigments contained in the first four groups are collectively called cone pigments, and they have similar molecular properties, although they have a wide range of absorption maxima (from 359 to 584 nm) that work for color discrimination. Cone pigments exhibit molecular properties distinct from rod pigment rhodopsins, such as their thermal stability and kinetics of retinal incorporation and release, which contribute to the difference in cellular response between cones and rods. Because rod and cone pigments have a common ancestor, they must have acquired diversified molecular properties during the course of molecular evolution. In this chapter, the molecular basis of the evolution and diversification of vertebrate visual pigments as related to their physiological functions is described.