Presence and foveal enrichment of rod opsin in the "all cone" retina of the American chameleon

Abstract

The retinal photoreceptors of the eye of the American chameleon, Anolis carolinensis, have been considered to be exclusively cones. Its retina is unusual for possessing two foveas (areas associated with heightened visual acuity), with the major, central fovea deeply incised and very densely packed with photoreceptors. Immunoblotting and light- and electron microscopic-immunocytochemistry, using several opsin monoclonal antibodies previously found specific for rods, demonstrated the presence and localization of this protein in the Anolis retina. This visual pigment appears sparsely in a subpopulation of photoreceptors in the periphery but overwhelmingly in the central fovea. Complementary results with cone-specific antibody and lectin binding corroborated this spatial organization. These results, as well as those with geckos, suggest that photoreceptor morphology is not an accurate guide among the lacertilians to visual pigment content, and that this phylogenetic grouping may constitute a crossroads in vertebrate photoreceptor evolution.