The Absence of the Cone-Specific Visual Cycle in Sea Lamprey and Its Evolutionary Implications
Unlike rods which mediate low-light/night vision, cones which mediate day-vision never saturate despite exposure to intense, ambient light. A contributing explanation is a M�ller cell-mediated visual cycle, found in mammalian models, that continually regenerates photopigment in cones (Wang et al., 2011). Given our knowledge of the “camera” eye (Lamb et al., 2007), we wanted to know if this cone-specific visual cycle was also highly conserved in vertebrates prompting the use of sea lamprey, an ideal model for evolutionary study. Using ERG recordings, we found no significant regaining of response sensitivity in isolated retinas, indicative of pigment regeneration, after a substantial bleaching light exposure. These results confirm the absence of a cone-specific visual cycle in sea lamprey and suggest that this pathway had arisen after the divergence of jawed and jawless vertebrates ~500 mya.