A novel rod‐like opsin isolated from the extra‐retinal photoreceptors of teleost fish

We have isolated a novel opsin from the pineal complex of Atlantic salmon (Salmo salar) and from the brain of the puffer fish (Fugu rubripes). These extra‐retinal opsins share approximately 74% identity at the nucleotide and amino acid level with rod‐opsins from the retina of these species. By PCR, we have determined that the novel rod‐like opsin is not expressed in the salmon retina, and the retinal rod‐opsin is not expressed in the salmon pineal. Phylogenetic analysis suggests that the rod‐like opsins arose from a gene duplication event approximately 205 million years ago, a time of considerable adaptive radiation of the bony fish. In view of the large differences in the coding sequences of the pineal/brain rod‐like opsins, their extra‐retinal sites of expression, and phylogenetic position we have termed these novel opsins ‘extra‐retinal rod‐like opsins’ (ERrod‐like opsins). We speculate that the differences between retinal rod‐opsins and ERrod‐like opsins have arisen from their differing photosensory roles and/or genetic drift after the gene duplication event in the Triassic.