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L. S. Carvalho, W. L. Davies, P. R. Robinson, J. K. Bowmaker, D. M. Hunt; Origin of the Blue-Sensitive Visual Pigment in Primates: Site 86 Revisited. Invest. Ophthalmol. Vis. Sci. 2008;49(13):3247. doi: https://doi.org/.
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© ARVO (1962-2015); The Authors (2016-present)
The primate group can be divided into two major informal groups, the Prosimians and the Simians. The Prosimian group includes the lemurs, tarsiers, bushbabies and lorises. The Simian group is divided into the Platyrrhini or New World monkeys and the Catarrhini or Old World monkeys including humans and great apes. The photopic visual system in New and Old World monkeys is based on separate M and L pigments arising from either a duplication or a polymorphism of the longwave-sensitive (LWS) opsin gene. These pigments are combined with the S pigment derived from the shortwave-sensitive type I (SWS1) opsin gene to give trichromatic colour vision. Trichromacy may also be present in some prosimian species. The loss of a functional SWS1 gene has occurred in some primate species, notably amongst the nocturnal prosimians. The peak sensitivities of SWS1 pigments range from the violet (385-445 nm) to the UV (355-365 nm) and the molecular mechanism underlying this shift is determined by the residues present in the vicinity of the retinal binding pocket, mainly at site 86. In mammals, Phe86 alone is capable of conferring UV sensitivity while violet pigments can have different residues at this site. In higher primates, the SWS1 tuning mechanism appears more complicated with up to 8 simultaneous substitutions required to generate a violet pigment. The objective of this study was to analyse the tuning mechanisms of the SWS1 pigment in Prosimians and examine the effects of the only naturally-occurring Phe86 residue in the pigment of a primate.
Coding sequences were amplified from gDNA using a recently published SPLICE technique devised in our lab and cloned into an expression vector. 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.
In vitro expression of recombinant opsins and spectral analysis of the resultant pigments confirms the violet-sensitivity of the SWS1 pigments in three prosimian species with different residues at site 86. These include the pigment in the Aye-aye, the only primate SWS1 pigment that has Phe86.
The results provide evidence that an alternative residue may play a major role in the tuning of SWS1 pigments in primates and has enabled a re-evaluation of the evolution of violet sensitivity within the primate group as a whole.
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