Widespread Variation in Retinochrome Spectral Absorbance Across Scallop Phylogeny

Vision is a two-step process, including phototransduction initiated by a retinal-bound opsin photopigment, and subsequent photopigment regeneration wherein the inactive state is restored. Because retinal isomerization, or changing of the molecular structural conformation, drives the activation of opsin photopigments, the reverse process is a necessary part of regeneration. In some animal lineages, a second opsin functions as a photoisomerase that converts retinal back to the isomer needed for the visual opsin, but very little is known about their spectral properties. We investigated spectral variation and spatial expression of scallop retinochrome, an opsin used for light-driven regeneration of molluscan visual photopigments. We show that retinochrome is expressed in scallop retina, along with visual opsins, indicating a conserved role in supporting vision. We compared retinochrome spectral absorption across six species with different ecologies, revealing considerable functional variation possibly associated with photic environments. Finally, we characterized multiple amino acid sites contributing to spectral tuning-shifts in the specific maximally sensitive wavelength of light-of this opsin. These results reveal retinochrome as a functionally dynamic component of the scallop visual cycle that may be adapted to different photic environments across species and highlights spectral tuning in an ocular but otherwise non-visual photopigment, which has been largely overlooked in studying the evolution of visual cycles.