ORIGINS AND ANTIQUITY OF X-LINKED TRIALLELIC COLOR-VISION SYSTEMS IN NEW-WORLD MONKEYS

It is known that the squirrel monkey, marmoset, and other related New World (NW) monkeys possess three high-frequency alleles at the single X-linked photopigment locus, and that the spectral sensitivity peaks o f these alleles are within those delimited by the human red and green pigment genes. The three alleles in the squirrel monkey and marmoset h ave been sequenced previously. In this study, the three alleles mere f ound and sequenced in the saki monkey, capuchin, and tamarin, Although the capuchin and tamarin belong to the same family as the squirrel mo nkey and marmoset, the saki monkey belongs to a different family and i s one of the species that is most divergent from the squirrel monkey a nd marmoset, suggesting the presence of the triallelic system in many NW monkeys, The nucleotide sequences of these alleles from the five sp ecies studied indicate that gene conversion occurs frequently and has partially or completely homogenized intronic and exonic regions of the alleles in each species, making it appear that a triallelic system ar ose independently in each of the five species studied. Nevertheless, a detailed analysis suggests that the triallelic system arose only once in the NW monkey lineage, from a middle wavelength (green) opsin gene , and that the amino acid differences at functionally critical sites a mong alleles have been maintained by natural selection in MV monkeys f or >20 million years. Moreover, the two X-linked opsin genes of howler monkeys (a NW monkey genus) were evidently derived from the incorpora tion of a middle (green) and a long wavelength (red) allele into one c hromosome; these two genes together with the (autosomal) blue opsin ge ne would immediately enable even a male monkey to have trichromatic vi sion.