Molecular mechanism underlying color discrimination ability of frogs and geckos in the dark

Abstract

Most vertebrates including humans have monochromatic vision at night and color vision during the day. This is explained by the repertoire of retinal photoreceptor cells, that is, a single type of rod cell for scotopic vision and multiple types of cone cells for photopic vision. Rods and cones contain different types of visual pigments, rhodopsin and cone pigments, respectively. Noteworthily, frogs and nocturnal geckos can discriminate colors under scotopic conditions, which is thought to be attributed to multiple types of rods in their retinas. Frogs have two types of rods, one of which contains green-sensitive rhodopsin and the other of which contains blue-sensitive cone pigment. And nocturnal geckos have three type of rods which contain red-, green- and UV-sensitive cone pigments. It is well known that scotopic vision is underlain by a much lower spontaneous activation rate of rhodopsin than of cone pigments. Therefore, it is of interest to investigate whether or not frogs and nocturnal geckos changed the molecular properties of cone pigments to adjust to scotopic vision. Our biochemical analysis showed that frogs and geckos suppressed the spontaneous activation rate of cone pigments to mimic rhodopsin. This shows that frogs and nocturnal geckos uniquely acquired rhodopsin-like cone pigments by convergent evolution for scotopic color vision, which is advantageous for their nocturnality.