Evolution of visual pigments

Abstract

Rod visual pigments exhibit a variety of absorption maxima, determined partly by the nature of the prosthetic group, retinal or 3-dehydroretinal, and partly by the apoprotein opsin. Some animal taxa exhibit little diversity. On the other hand, selective pressures arising from the different light habitats of aquatic environments are believed to have produced the correspondingly wide range of λ max in fish visual pigments, which often extends over 80 nm. Opsins appear to have similar molecular weights. Latest estimates of molecular weight suggest that they have approximately 315 amino acid residues. Some species, notably members of the salmonidae, have visual pigments with different λ max yet have diverged only during the past several million years. This would indicate a very high rate of protein evolution in circumstances where we should expect that amino acid substitution would be severely restricted. Certain fish species (e.g. the deepwater sculpin, Myoxocephalus) have mixtures of visual pigments with λ max separated by only 6 nm. The physiological usefulness of such a pair is dubious, but it may indicate polymorphism at the gene locus coding for opsin. The use of retinol and 3-dehydroretinol, the basis of rhodopsin and porphyropsin, may vary between quite closely related species. Extraneous factors such as light and hormones are known to be important in amphibian larvae and fishes, but there is also clear evidence for genetic influences, as revealed by the divergent evolution of the system in various populations of originally anadromous smelt after only 10 000 years of isolation.