Rhodopsin evolution in the dark

The gene for the visual pigment rhodopsin has been extensively studied from biochemical, molecular and evolutionary perspectives. This makes it ideal for investigating the relationship between protein structure and function1,2, and the effects of loss of functional constraint on the evolution of a gene. We specifically addressed the question of what happens when there is no light available by studying rhodopsin genes from three pairs of cave-dwelling and surface-dwelling freshwater crayfish species. Contrary to predictions, we found no differences in the rate of evolution between the cave and surface species or between the conserved and variable structural motifs of the rhodopsin protein. This suggests that rhodopsin might have a previously unknown function in the absence of light.