The evolution of primate cone pigments raises interesting general questions in evolutionary genetics, and is reviewed extensively elsewhere (Jacobs, 1996, Nathans, 1999 and Surridge et al., 2003). Most mammals are dichromatic with L (long wavelength) and S (short wavelength) cone pigments, but Old-World monkeys (Catarrhini) and howler monkeys (Alouatta spp.) independently duplicated the single ancestral LWS/MWS gene, which is on the X-chromosome, to give separate LWS and MWS pigments. Pigment sensitivity maxima of howlers and all known Old-World species are at about 430 nm (SWS), 530 nm (MWS) and 560 nm (LWS). Other primates retain a single LWS/MWS gene, but in most New-World species (Platyrrhini) and several lemurs this gene is polymorphic so that heterozygotes (i.e. a proportion of females) are trichromats. The majority of individuals are
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These show that there is selection against the shorter wavelength (535 nm) alleles in favour of 560 nm alleles or intermediate wavelength pigments (e.g. 545 nm; Osorio et al., 2004). Assuming that trichromacy favours the 535 nm/560 nm combination with equal allele frequencies, it is likely that the bias against the 535 nm allele arises in dichromats. This could be due either to the consequences for their dichromatic colour vision, which favours a wide separation of the two pigments (Lewis and Zhaoping, 2006 and Osorio and Vorobyev, 1996), or in luminance vision (Osorio and Nilsson, 2004, Osorio and Vorobyev, 2005 and Osorio and Nilsson, 2004).
Because there is no evidence for any anatomical or molecular distinction between M/L cones in primates other than their photopigment an important problem for primates is in how information from these cones is segregated to allow chromatic opponent receptive fields in the visual system (e.g. primary visual cortex; Wachtler,
These show that there is selection against the shorter wavelength (535 nm) alleles in favour of 560 nm alleles or intermediate wavelength pigments (e.g. 545 nm; Osorio et al., 2004). Assuming that trichromacy favours the 535 nm/560 nm combination with equal allele frequencies, it is likely that the bias against the 535 nm allele arises in dichromats. This could be due either to the consequences for their dichromatic colour vision, which favours a wide separation of the two pigments (Lewis and Zhaoping, 2006 and Osorio and Vorobyev, 1996), or in luminance vision (Osorio and Nilsson, 2004, Osorio and Vorobyev, 2005 and Osorio and Nilsson, 2004).
Because there is no evidence for any anatomical or molecular distinction between M/L cones in primates other than their photopigment an important problem for primates is in how information from these cones is segregated to allow chromatic opponent receptive fields in the visual system (e.g. primary visual cortex; Wachtler,