Uncovering a gene duplication of the photoreceptive protein, opsin, in scallops (Bivalvia: Pectinidae).

Evolutionary biologists have long been interested in how expansions of the photosensory system might contribute to morphological differentiation of animals. Comparative studies in vertebrate and arthropod lineages have provided considerable insight into how the duplication of opsin, the first gene of the phototransduction pathway, have led to functional differentiation and new ecological opportunities; however, this relationship cannot be examined in many invertebrate groups as we have yet to characterize their opsin content. Scallops (Pectinidae) are a promising molluscan model to study the evolution of opsin and its potential role in speciation. Recently, we discovered a second Gq-coupled, or r-, opsin gene expressed in the eyes of two scallop species. To investigate the evolutionary origin of this opsin, we screened 12 bivalve species from 4 families, representing both mobile and sessile species, with and without eyes. Although only one ortholog was recovered from the genome of the eyeless, immobile oyster, we found both genes to have been retained in 3 families comprising the order Pectinoida. Within this clade, non-mobile species of scallops appear to have lost one gene. Phylogeny-based tests of selection indicate different degrees of purifying selection following duplication. These data, in conjunction with highly divergent gene sequences and ortholog-specific retention, suggest functional differences. Our results are congruent with a Gq-opsin gene duplication in an oyster-Pectinoida ancestor, approximately 470 Mya, and suggest the likelihood of retaining both genes is associated with either the presence of eyes and/or degree of mobility. The identification of two highly divergent Gq-opsin genes in scallops is valuable for future functional investigations and provides a foundation for further study of a morphologically and ecologically diverse clade of bivalves that has been understudied with respect to visual ecology and diversification of opsin.