Polymorphic Variations in Long- and Middle-Wavelength-Sensitive Opsin Gene Loci in Crab-Eating Monkeys
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Old World primates have trichromatic color vision as a result of three types of cone opsin, each of which is maximally sensitive to either short (S), middle (M), or long (L) wavelengths. Most color vision defects in humans are L/M abnormal that are caused by unequal crossing over between L and M visual pigment genes. We previously reported that only three crab-eating monkeys of the 2788 macaque monkeys are protanopic (L pigment absent) and that the frequency of dichromats in this species is lower than that in humans . To understand the mechanism leading to the difference in L and M genes between humans and crab-eating monkeys more profoundly, we investigated different genotype variants of the L and M genes in the crab-eating monkeys using 758 available DNA samples of this species. We first identified polymorphisms in the copy number of L and M genes. In humans, about 60% of males have a gene array consisting of a single L gene and more than one M opsin gene, but only 5% exhibit this genotype in crab-eating monkeys. We next investigated anomalous trichromats by detecting amino acid substitutions on exon 5 that are responsible for L/M spectral tuning (sites Y277F and T285A). As a result, there were no anomalous trichromats, while about 6% of humans have such color vision defects. In addition, we estimated amino acid substitution at S180A in exon 3, because the substitution causes spectral shift but does not result in color vision defects. In humans, about 40% of males have such a substitution but only 1% of crab-eating monkeys do. The percentages of each variant were quite different between the two species, and the variants were found in locally restricted areas in crab-eating monkeys.