The evolution of trichromatic color vision by opsin gene duplication in New World and Old World primates

Trichromacy in all Old World primates is dependent on separate X-linked MW and LW opsin genes that are organized into a head-to-tail tandem array flan ked on the upstream side by a locus control region (LCR). The 5' regions of these two genes show homology For only the first 236 bp, although within t his region, the differences are conserved in humans, chimpanzees, and two s pecies of cercopithecoid monkeys. In contrast, most New World primates have only a single polymorphic X-linked opsin gene; all males are dichromats an d trichromacy is achieved only in those females that possess a different Fo rm of this gene on each X chromosome. By sequencing the upstream region of this gene in a New World monkey, the marmoset, we have been able to demonst rate the presence of an LCR in an equivalent position to that in Old World primates. Moreover, the marmoset sequence shows extensive homology from the coding region to the LCR with the upstream sequence of the human LW gene, a distance of >3 kb, whereas homology with the human MW gene is again limit ed to the first 236 bp, indicating that the divergent MW sequence identifie s the site of insertion of the duplicated gene. This is further supported b y the presence of an incomplete Alu element on the upstream side of this in sertion point in the MW gene of both humans and a cercopithecoid monkey, wi th additional Alu elements present further upstream. Therefore, these Alu e lements may have been involved in the initial gene duplication and may also be responsible for the high frequency of gene loss and gene duplication wi thin the opsin gene array. Full trichromacy is present in one species of Ne w World monkey, the howler monkey, in which separate MW and LW genes are ag ain present. In contrast to the separate genes in humans, however, the upst ream sequences of the two howler genes show homology with the marmoset for at least 600 bp, which is well beyond the point of divergence of the human MW and LW genes, and each sequence is associated with a different LCR, indi cating that the duplication in the howler monkey involved the entire upstre am region.