Animals use colour vision for a variety of behaviours including finding food or identifying a mate. Colour vision in primates, for instance, is thought to have evolved because it improved the discrimination of fruit from surrounding foliage. Although the details of visual neural circuits differ between species, the underlying principles of colour vision are similar across most species; they require at least two sets of photoreceptors, each with different spectral sensitivities. Most mammals possess two sets of cone photoreceptors, tuned to absorb light from different parts of the visible spectrum, one usually tuned for short wavelength sensitivity and the other a long wavelength sensitive opsin. In early mammals, the short wave sensitive(SWS) opsin is thought to have been tuned to ultraviolet wavelengths, but in ground squirrels indirect evidence suggested that the SWS opsin's sensitivity had shifted to the violet region of the spectrum. Livia Carvalho and her colleagues at University College London set out to find the molecular changes that occurred in the tree squirrel opsin to shift its peak sensitivity from ultraviolet to violet light.
Obtaining the complete coding sequence of the grey squirrel (Sciurus carolinensis) SWS opsin, the team expressed the opsin gene before combining the protein with the light sensitive chromophore,11-cis-retinal, to reconstruct the functional photopigment. Having purified the opsin the team measured the photopigment's absorption spectrum and found that the grey squirrel's SWS opsin did indeed have a peak absorption in the violet range of the light spectrum. When they compared the grey squirrel SWS opsin amino acid sequence with that of a typical ultraviolet sensitive pigment from the mouse, they noticed that at one position a phenylalanine (Phe) had been substituted with a tyrosine (Tyr). To determine whether this substitution is sufficient to produce the shift in the absorption spectrum, they constructed a grey squirrel opsin sequence in which the Tyr was replaced by Phe. When the absorption spectrum of this opsin was measured it was shifted back towards the ultraviolet, confirming that the Phe to Tyr substitution was responsible for the spectral shift.
Next, to see whether similar changes had occurred in other closely related species, Carvalho and her colleagues obtained partial sequences for SWS opsins from two flying squirrels: the Siberian flying squirrel (Pteromys volans) and Northern flying squirrel (Glaucomys sabrinus). The SWS sequences from both species contained a Tyr at the same position as in the Tyr in the grey squirrel's opsin, suggesting that the flying squirrels' peak sensitivities had also been shifted from the UV to violet light. However, both flying squirrels' opsin genes contained deletions that prevented the formation of functional opsins. This means both of these species lack colour vision, but this may not be a bad thing as flying squirrels are nocturnal. Many nocturnal animals lack colour vision because they must use all the available photons to get sufficient spatial and temporal resolution; by requiring multiple separate neural channels, colour vision may reduce spatial and temporal resolution.
Remarkably, many other mammals that possess a violet-sensitive opsin, such as cows, pigs and wallabies, also show a similar Tyr substitution for a Phe at the same location in the opsin gene, suggesting that the violet opsin has arisen many times during the course of evolution through convergent evolution. Loss of the SWS opsin has also occurred numerous times in nocturnal and marine mammals. Convergent changes in molecular structure have clearly played an important role in the evolution of the mammalian visual system.
