Species Distribution
Melanopsin genes have been described in all vertebrate classes, and an extra melanopsin ortholog has been discovered in fish, bird, and amphibian genomes. Within the mammals studied thus far (which includes rodents, primates, and humans), the melanopsin protein has a similar pattern of tissue distribution; the protein is expressed only in the retina, and only in 1-2% of retinal ganglion cells. In non-mammalian vertebrates, melanopsin is found in a wider subset of retinal cells, as well as in photosensitive structures outside the retina such as the iris muscle of the eye, deep brain regions, the pineal gland, and the skin.
Mammals have orthologous melanopsin genes named Opn4m, which are derived from one branch of the Opn4 family. However, non-mammalian vertebrates have two versions of the melanopsin gene, Opn4m and Opn4x. Chicken Opn4m appears capable of triggering a light-induced and retinaldehyde cofactor dependent G-protein coupled receptor cascade, much like Opn4m studied in mammals. Opn4x appears to have a much weaker light-induced response.
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