The evolution of photoreceptors and visual photopigments in vertebrates

Abstract

All classical vertebrate retinal photoreceptors have an evolutionary origin from ciliary cells and retain a modified cilium that links the inner and outer segments. Vertebrate visual pigments fall into five classes, a rod pigment that is found in rod photoreceptors that are responsible for dim-light vision and four spectral classes of cone photoreceptors that are responsible for vision in daylight and provide for the sensation of color if two or more classes of cone are present. The underlying mechanisms that determine the overall sensitivity of individual photoreceptors to different light levels and hence whether they are functional in dim or bright light however extend beyond the type of visual pigment present into the processes of phototransduction. The kinetics of phototransduction is determined by the different rod and cone isoforms that comprise many of its component processes, combined with the concentration of certain key components. The spectral tuning of visual pigment is determined by the type of chromophore present and its interaction with the opsin protein. The amino acid present at key sites within the opsin protein determines spectral tuning; these sites are limited in number and tend to be shared across different pigment classes. Other changes include the protonation of the Schiff base for short wavelength-sensitive type 1 (SWS1) pigments and the binding of chloride ions for long wavelengthsensitive (LWS) pigments. Opsin gene loss and/or duplication is common across the different vertebrate taxa, with certain classes of cone opsins absent in certain lineages. These changes can be related in many cases to differential expression during development or to evolutionary changes in lifestyle and the light environment.