Abstract
Purpose: :
In mammals, colour vision is achieved by two classes of cone photoreceptors that contain either a shortwave– (SWS1) or longwave–sensitive (LWS) visual pigment. Only in primates this is increased to three classes by either a LWS gene duplication or polymorphism. The SWS1 pigments have peak sensitivities (max) ranging from 355nm in the ultraviolet to >430nm in the violet region of the spectrum. The order Rodentia is subdivided into two suborders, the Sciurognathi and the Hystricognathi. Within the Sciurognathi, the SWS1 class of visual pigments is universally ultraviolet–sensitive (UVS) amongst the largely nocturnal murine species, whereas violet–sensitive (VS) pigments are thought to be present in diurnal ground and tree squirrels. Since UVS pigments are most likely ancestral amongst the vertebrates, the evolution of VS pigments in a branch of the Sciurognathi must have been a separate event to that in other vertebrate taxa. To establish whether squirrels possess a VS pigment, we have obtained the full coding sequence of the SWS1 gene in the grey squirrel, a species that is strictly diurnal, and expressed it in vitro. In addition, we have examined the SWS1 gene sequence in two species of nocturnal flying squirrels.
Methods: :
Sequences were obtained using RT–PCR and RACE, cloned and sequenced. Mutant opsins were generated by site–directed mutagenesis. Wild type and mutant SWS1 coding sequences were transiently transfected into HEK 293T cells, the resulting opsins were purified by immunoaffinity chromatography and reconstituted with 11–cis–retinal.
Results: :
We have confirmed that the grey tree squirrel has a VS SWS1 pigment with a max around 440nm, and established that the underlying evolutionary event in the longwave shift was a single Phe86Tyr substitution. The flying squirrel SWS1 gene also has Tyr86, making it potentially VS. The gene however also has deletions and we have shown that it does not regenerate to form a functional pigment in vitro.
Conclusions: :
The data suggest that the evolution of the VS pigment in rodents was achieved by different mechanisms in the Sciurognathi and Hystricognathi suborders. Flying squirrels also have Tyr86, but the presence of deletions implies that the gene is non–functional. Flying squirrels have dispensed therefore with colour vision and this may be associated with a nocturnal life style.
Keywords: color pigments and opsins • color vision • photoreceptors