The evolution of longevity

Why look for more photosensory and photoisomerase opsins? Photoreception is not limited to the rods and cones, but can occur in cells of the inner retina, pineal, brain or skin. Action spectra for these photoresponses have implicated opsin/vitamin A based photopigments, but the photosensory genes and proteins remain unknown. It is also unclear how photopigment chromophore is regenerated in photoreceptors that lack an retinal pigment epithelium-like structure. The assumption has been that there will be a ‘local’ photoisomerase to perform this task, but none has been identified. What is melanopsin and where is it expressed? Melanopsin was first isolated from the photosensitive melanophores of Xenopus. Orthologues have been found in most vertebrate classes, including mammals. All show little homology to the photosensory opsins and to each other. In mammals, melanopsin is expressed in a subset of photosensitive retinal ganglion cells, and in non-mammals, in photoreceptive structures such as the pineal and hypothalamus.