Genetic evidence for the ancestral loss of short-wavelength-sensitive cone pigments in mysticete and odontocete cetaceans.

Abstract

All mammals ancestrally possessed two types of cone pigments, an arrangement that persists in nearly all contemporary species. However, the absence of one of these cone types, the short-wavelength-sensitive (SWS) cone, has recently been established in several delphinoid cetacean species, indicating that the loss of this pigment type may be widespread among cetaceans. To evaluate the functional condition of SWS cones in cetaceans, partial SWS cone-opsin gene sequences were obtained from nuclear DNA for 16 species representing 12 out of the 14 extant mysticete (baleen) and odontocete (toothed) families. For all these species one or more mutations were identified that indicate that their SWS cone-opsin genes are pseudogenes and thus do not code for functional visual pigment proteins. Parsimonious interpretation of the distribution of some of these mis-sense mutations indicates that the conversion of cetacean SWS coneopsin genes to pseudogenes probably occurred before the divergences of the mysticete and odontocete suborders. Thus, in the absence of dramatic homoplasy, all modern cetaceans lack functional SWS cone visual pigments and, by extension, the visual capacities that such pigments typically support.