Rhodopsin Molecular Evolution in Mammals Inhabiting Low Light Environments

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

In the African mole-rats, the higher estimated v ratio along the ancestral branch, and the greater v value for the clade (see supplementary Table S3 , Supplementary Material online) suggest that the rhodopsin gene has evolved relatively rapidly in this group. Given the ecology of this group, it is tempting to afford such accelerated evolution to relaxed selection associated with living underground. Indeed, the eyes of African mole-rats are vestigial, and, in some species, the visual subsystems are severely reduced [39] . Moreover, members of this group are known to rely heavily on olfactory and tactile senses for short-distance orientation, and detect seismic signals for long-distance communication [40] [41] [42] , indicating that vision might not be essential. However, in spite of these points, it is important to note that African mole-rats have been found to possess more cones (representing ,10% of the photoreceptors) than other nocturnal rodents, the inferred adaptation to discriminate bright light has been attributed to circadian rhythm entrainment rather than scotopic vision [39] . A role in photo-entrainment could also explain the retention of the functional gene in the African bathergid mole-rats as well as the Middle East blind mole-rat and the Hottentot golden mole, both of which possess subcutaneous eyes, and have independently evolved to occupy a subterranean niche. Therefore, at this time, we cannot rule out the possibility that the elevated v ratio in the rhodopsin of mole-rats reflects a past burst of positive selection rather than relaxed selection.