Visual Pigment Evolution in Reptiles

Abstract

AbstractThe long history and great ecological and morphological diversity of reptiles (all amniotes except mammals and birds) is matched by their visual system diversity. Although less known than in other amniotes, visual pigments have been studied in all extant reptile orders except Sphenodontia. There have been no additions to the five visual pigments present in the ancestral vertebrate, although there have been multiple independent losses. Crocodylians retain three visual pigments, many lizards as well as Testudines four or five and snakes one to three. Adaptive pigment evolution includes tuning site amino acid substitutions and switches between chromophore types that together generate ultraviolet to infrared spectral sensitivity. Reptiles present some of the best evidences of evolutionary rod–cone and cone–rod transmutation with, for example typically cone visual pigments expressed in rod‐like photoreceptors. Reptile visual pigments show evidence of substantial adaptive evolution, at least some of which is associated with major ecological shifts.Key ConceptsReptiles have no more than five visual pigments, as few as one and typically at least three.Up to four of the ancestral vertebrate visual pigments have been lost independently in different reptile lineages, and no visual opsin gene duplications have been identified so far.Testudines have between four and five visual pigments and a wide range of photoreceptor oil droplets, representing one of the most complex visual pigment systems in tetrapod vertebrates.Despite many of them being nocturnal, no rhodopsin 1 has been detected in geckos.Nosws1orrh2opsin genes can be detected in genomic sequence data for crocodylians.Among squamates, some lizards have a mixture of A1and A2chromophores incorporated in their visual pigments, allowing a wide colour sensitivity, including infrared vision.In the green anole (Anolis carolinensis), only cone opsins have been reported (SWS1, SWS2, RH2 and LWS) in studies of the eye. However, a 515‐nm tunedrh1rhodopsin gene occurs in the genome of this species.Extant snakes have lost two visual pigments (SWS2 and RH2), likely as an adaptation to a low light environment inhabited by a snake ancestor.Extreme burrowing in scolecophidians (blind, worm and thread snakes) is correlated with the loss of the SWS1 and LWS visual pigments and cones, rendering these snakes rod (RH1) monochromats.The expression of RH1, SWS1 and LWS pigments in snake lineages with seemingly all‐cone and all‐rod retinae suggests multiple transmutations from cone to rod andvice versain snakes. Similar transmutations likely occurred in some lizards and perhaps crocodylians.Visual pigment spectral sensitivity in reptiles has been found to correlate with the light transmission properties of the ocular media (e.g. snakes) and photoreceptor oil droplets (e.g. testudines), that is the pigments are not sensitive to wavelengths of light filtered out by these structures.