Meeting Abstract
Photoreceptive organs have evolved as many as 65 times over the course of evolutionary history. Interestingly, nearly all light sensitive structures function via the same pathway of phototransduction. Phototransduction is a two step process which causes a conformational change of the photopigment upon light absorption and then requires resetting by reuptake or recycling of the bound retinal isomer. The latter half of the phototransductive pathway is known as the retinoid visual cycle. The molluscan visual cycle functions via retinochrome which photoisomerizes all-trans to 11-cis retinal by absorbing a photon and a shuttle protein transports the retinal isomers between the photoisomerase and the opsin. In vertebrates, retinal is recycled using shuttle proteins to transport the retinal between cell lines and through a well-characterized complex of enzymes for phototransduction or storage. Insects possess bistable opsins allowing retinal recycling within the opsin; however, recent studies have shown the enzymes of insects are homologous to those found in the vertebrate visual cycle. Changes in the understanding of the insect visual cycle and lack of a described non-light dependent molluscan visual cycle leads one to challenge the current simplicity of the molluscan visual cycle. To investigate this pathway in molluscs, published transcriptomes were searched for proteins involved in vertebrate or insect visual cycles. The results show the presence of RPE65, CRALBP, RDH5, and RDH12, in molluscan species, suggesting 1) molluscs possess a more complex visual cycle and 2) the origin of the retinoid visual cycle is before the protostome-deuterostome split.
