Vision in the ultraviolet

Sensitivity to ultraviolet light (UV) is achieved by photoreceptors in the eye that contain a class of visual pigments maximally sensitive to light at wavelengths < 400 nm. It is widespread in the animal kingdom where it is u sed for mate choice, communication and foraging for food. UV sensitivity is not, however, a constant feature of the visual system, and in many vertebr ate species, the UV-sensitive (UVS) pigment is replaced by a violet-sensiti ve (VS) pigment with maximal sensitivity between 410 and 435 nm. The role o f protonation of the Schiff base-chromophore linkage and the mechanism for tuning of pigments into the UV is discussed in detail. Amino acid sequence analysis of vertebrate VS UVS pigments indicates that the ancestral pigment was UVS, with loss of UV sensitivity occurring separately in mammals, amph ibia and birds, and subsequently regained by a single amino acid substituti on in certain bird species. In contrast, no loss of UV sensitivity has occu rred in the UVS pigments of insects.