Rhodopsin Molecular Evolution in Mammals Inhabiting Low Light Environments

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

Despite the divergent selection pressures reported here, we could find no clear evidence that the rhodopsin gene in mammals inhabiting low light conditions had undergone consistent spectral tuning at known critical amino acid sites (Table 2) , though these inferences need to be substantiated by mutagenesis of synthesized proteins. Previously, dim-light vision in vertebrates has been classified into deep-sea (479-486 nm), intermediate (491-496 nm), surface (500-507 nm) and red-shifted (,525 nm), based on the inferred peak sensitivity (l max ) of their rhodopsin, as well as considerations of life history and ecology [17] . Accordingly, all bats examined appeared to possess a rhodopsin that ranges in sensitivity from 497 to 501 nm, thus overlapping with some surface fishes. Similarly, rhodopsin in African mole-rats can be classified as either surface or intermediate types (496-498 nm), which are slightly blue-shifted compared to the phylogentically distinct subterranean Middle East blind mole rat and Hottentot golden mole (both 501 nm). Furthermore, we found similar predicted sensitivities for the rhodopsin gene in a range of marsupials (499 and 501 nm), murid rodents (501 nm) and primates (497-501 nm), as well as the elephant shrew (501 nm) and horse (501 nm). The largest shifts in spectral tuning appear to occur in some marine mammals, as previously reported based on electroretinogram measurements [24] . The spectral-tuning properties of cetacean rhodopsins have been linked to foraging depth [14] and, of the four cetaceans studied here, Sowerby's beaked whale has the deep-sea type of rhodopsin (l max of 484 nm), while the others possess rhodopsins with l max of 489 nm (Table 2) , which are more likely to be classified as the intermediate type. In contrast, most pinnipeds possess a surface rhodopsin with l max s ,500 nm (Table 2) , though the Northern elephant seal has a deep-sea rhodopsin with a l max of 483 nm.