The evolution and function of melanopsin in craniates

In addition to well-characterised visual systems, many organisms, including the craniates, possess a complex sensory system of non-visual photoreceptors that detect light for a diverse array of non-image-forming tasks. Like the photoreceptors of image-forming systems, the pigments contained within nonvisual photoreceptive cells comprise a protein component (opsin) linked to a lightsensitive retinal chromophore derived from vitamin A. In mammals, one of the most important of these non-visual pigments is melanopsin (encoded by the OPN4 gene, specifically that of the "mammal-like" or "m-class"), which is restricted in expression to a subset of retinal ganglion cells and has been shown to be the conduit through which light regulates many physiological activities, including the photoentrainment of circadian systems (e.g. the sleep cycle) and the pupillary reflex response. In non-mammals, melanopsin exists as two distinct gene lineages, namely the m-class and x-class (" Xenopus - like"), and both are expressed in many different tissues, including the eyes, skin, fins, gills, brain and pineal gland, however, the functional roles mediated by melanopsin in these "lower" vertebrates remain to be fully elucidated. In this review, we discuss the evolutionary history of the melanopsin gene, its diverse patterns of expression and transcriptional output, the functional roles so far determined, and the clinical significance of this critical and phylogenetically most ancient opsin-based system of irradiance detection.