SEQUENCE DIVERGENCE OF THE RED AND GREEN VISUAL PIGMENTS IN GREAT APES AND HUMANS

We have determined the coding sequences of red and green visual pigmen t genes of the chimpanzee, gorilla, and orangutan. The deduced amino a cid sequences of these pigments are highly homologous to the equivalen t human pigments. None of the amino acid differences occurred at sites that were previously shown to influence pigment absorption characteri stics. Therefore, we predict the spectra of red and green pigments of the apes to have wavelengths of maximum absorption that differ by <2 n m from the equivalent human pigments and that color vision in these no nhuman primates will be very similar, if not identical, to that in hum ans. A total of 14 within-species polymorphisms (6 involving silent su bstitutions) were observed in the coding sequences of the red and gree n pigment genes of the great apes. Remarkably, the polymorphisms at 6 of these sites had been observed in human populations, suggesting that they predated the evolution of higher primates. Alleles at polymorphi c sites were often shared between the red and green pigment genes. The average synonymous rate of divergence of red from green sequences was approximate to 1/10th that estimated for other proteins of higher pri mates, indicating the involvement of gene conversion in generating the se polymorphisms. The high degree of homology and juxtaposition of the se two genes on the X chromosome has promoted unequal recombination an d/or gene conversion that led to sequence homogenization, However, nat ural selection operated to maintain the degree of separation in peak a bsorbance between the red and green pigments that resulted in optimal chromatic discrimination. This represents a unique case of molecular c oevolution between two homologous genes that functionally interact at the behavioral level.