Crucially, functional variation of OPN1SW is widespread among mammals ( Emerling et al. The peak spectral absorbance (λ max) of opsins is sensitive to natural selection and varies in response to environmental conditions and natural histories ( Parry et al. It is therefore challenging to infer the ancestral state of the primate visual system. Limited sampling, however, has precluded a formal comparative analysis of opsin genes in the grandorder Euarchonta, that is, the orders Scandentia (treeshrews), Dermoptera (colugos), and Primates. The M/LWS opsin variation that causes allelic trichromacy is widespread among primates ( Tan et al. Among primates, OPN1LW has differentiated into multiple alleles (lemurs, most New World monkeys) or paralogs (howler monkeys, Old World primates), resulting in spectrally shifted photopigments that confer allelic or routine trichromatic vision, respectively ( Jacobs 2009 Kawamura et al. 2008) and every successive lineage, such as primates, that subsequently lost or gained opsin genes ( Jacobs 2013 Meredith et al. Some variant of this dichromatic phenotype ( Peichl 2005) is the probable ancestral state of therian mammals ( Wakefield et al. The color vision of mammals is based on the expression of two opsin genes ( OPN1SW and OPN1LW) that encode short- (SWS1) and middle-to-long-wavelength sensitive (M/LWS) photopigments.
These findings suggest functional variation in the color vision of nocturnal mammals and a distinctive visual ecology of early primates, perhaps one that demanded greater spatial resolution under light levels that could support cone-mediated color discrimination. derbianus and, 3) the absence of OPN1LW polymorphism among diurnal treeshrews. lowii, but a signature of purifying selection in those of C. derbianus and a shift from UV to blue spectral sensitivities at the base of Euarchonta 2) ancient pseudogenization of OPN1SW in the ancestors of P. Our results indicate: 1) retention of ultraviolet (UV) visual sensitivity in C.
In addition, we examined the opsin genes of the Central American woolly opossum ( Caluromys derbianus), an enduring ecological analogue in the debate on primate origins. Here, we report on the genes ( OPN1SW and OPN1LW) that encode SWS1 and M/LWS opsins in seven species of treeshrew, including the sole nocturnal scandentian Ptilocercus lowii. The ancestral state of primate color vision is therefore uncertain.
A problem with this approach is that opsin gene variation is incompletely known in the grandorder Euarchonta, that is, the orders Scandentia (treeshrews), Dermoptera (colugos), and Primates. For example, it is hypothesized that variable expression of short- (SWS1) and middle-to-long-wavelength sensitive (M/LWS) opsins, which confer color vision, can be used to infer ancestral activity patterns and therefore selective ecological pressures. Debate on the adaptive origins of primates has long focused on the functional ecology of the primate visual system.