Rhodopsin Molecular Evolution in Mammals Inhabiting Low Light Environments

Zhao, Huabin; Ru, Binghua; Teeling, Emma C.; Faulkes, Christopher G.; Zhang, Shuyi; Rossiter, Stephen J.

By comparison, elevated v ratios detected in clades of both cetaceans and pinnipeds (see supplementary Table S3 , Supplementary Material online) are more likely to have some adaptive significance in vision. Consistent with living in low light conditions (rather than complete darkness), the retinae of both groups have been found to be highly rod-dominated with only 0.4%-2% of photoreceptors represented by cones [43] . Moreover, behavioural studies of members of these groups suggest they are functionally dichromatic [44] [45] [46] despite the fact that recent genetic [20, 24] and immuonocytochemical [43] evidence reveals that cetaceans and pinnipid species have typically lost their blue cones (reviewed by [47] . It has been suggested that without SWS1 opsins, these animals discriminate color by comparing the signals from the green cones and rods (see [48] . Consequently, the rhodopsin gene in these clades might have undergone molecular adaptation to confer dichromatic vision in low light. In constrast, the sea otter and West Indian manatee had a similar and lower v value than other mammals, respectively. Since both species spend considerable time near the water surface and also possess a functional SWS1 opsin, it seems probable that their rhodopsin genes have predominantly been subject to purifying selection, as appears to be the case for most mammals. Indeed, the visual system of the manatee appears to be morphologically similar to terrestrial mammals [7] .