Abstract
Most vertebrates have two kinds of photoreceptor cells (rods and cones) in their retinas. In contrast, all the photoreceptor cells of nocturnal geckos are morphologically rods. Walls proposed an idea (Transmutation theory) that the gecko rods have evolved from cones of ancestral diurnal lizards. In order to elucidate whether the gecko rods have coneor rod-type visual pigments (photoreceptive retinal proteins), we studied two kinds of gecko visual pigments, gecko green and gecko blue. First, we isolated the cDNA clones of both the visual pigments and deduced their amino acid sequences. Gecko green and blue were highly similar in amino acid sequences to the chicken red- and green-sensitive cone visual pigments, respectively. Second, we extracted and purified the visual pigments from gecko retinas, and investigated their physiologically active form (meta II intermediate) with low temperature spectroscopy. The lifetimes of meta II intermediates of both the gecko visual pigments were much shorter than that of rhodopsin, and as short as those of cone visual pigments. From these results, we concluded that both the gecko visual pigments should be cone-type ones. The result strongly supports the Transmutation theory.
