Sequence divergence and copy number of the middle- and long-wave photopigment genes in Old World monkeys.

We have studied the sequence and organization of the genes for the middle-wave (MW) and long-wave (LW) cone photopigment genes in six species of Old World monkeys. Previous studies have shown that the MW and LW pigments of all six species exhibit peak sensitivities near 535 nm and 565 nm, respectively, and thus resemble the equivalent human pigments. In the case of man, the protein components of the MW and LW photopigments differ by 15 amino acids, although only seven of these differences involve non-homologous substitutions and are therefore candidates for a role in spectral tuning. Regions corresponding to exons 4 and 5 of these genes, and including five such candidate sites, were sequenced in the Old World monkeys. In contrast to the equivalent human genes, substitutions were found at two of these sites, position 233 and 309 of the MW gene in all six species. The role of amino acid substitutions in the spectral tuning of these photopigments is discussed. A comparison of the nucleotide sequences of the MW and LW genes provides evidence for sequence homogenization within species; the role of gene conversion in the evolution of these genes is discussed. The close juxtaposition and homology of the MW and LW genes on the X chromosome is thought to underlie the high frequency of colour vision defects in man and the presence in many individuals of extra copies of the MW gene. A study of a group of talapoin (Ceropithecus talapoin) monkeys has revealed a similar numerical polymorphism for this gene to that present in man. In contrast to the situation in man, where the MW and LW genes may contain a shortened first intron, restriction digests of genomic DNA showed that the size of this intron does not differ across the six species of Old World monkeys examined.