Abstract
In the late nineteenth century, the examination of visual pigments led to the discovery of a protein—the opsin—covalently bound to a chromophore. Opsins are G protein-coupled receptors (GPCR) used for both visual and non-visual photoreception, and these proteins evolutionarily date back to the base of the bilaterians. In the current sequencing age, phylogenomic analysis has proven to be a powerful tool, facilitating the increase in knowledge about diversity within the opsin subclasses and so far, nine paralogs have been identified. While phylogeny may help infer function, direct functional studies of opsins in vertebrates, cephalopod mollusks, and fruit flies have shown that there are multiple pathways involving various opsins for visual photoreception along with several other processes. Within echinoderms, opsins have been studied in Echinoidea and Ophiuroidea, but these two groups do not possess proper image forming eyes, but rather widely dispersed dermal photoreceptors. However, most species of Asteroidea, the starfish, possess true eyes and studying them will shed light on the diversity of opsin usage within echinoderms and help resolve the evolutionary history of opsins. Using high-throughput RNA sequencing, we have sequenced and analyzed the transcriptomes of different Acanthaster planci tissue samples: eyes, radial nerve, tube feet and a mixture of other organelle tissue. At least 9 opsins belonging to 7 potential opsin paralogs were identified, and seven of them were found significantly differentially expressed in both eyes and radial nerve, providing new important insight into the involvement of opsins in visual and nonvisual photoreception in echinoderms. Of relevance, we found the first evidence of an r-opsin photopigment expressed in a well developed visual eye in a deuterostome animal.