Origin and molecular evolution of the X-linked duplicate color vision genes in howler monkeys

Like humans and Old World monkeys (OWMs), the howler monkeys, a genus of New World monkeys (NWMs), have trichromatic vision because they possess 1 autosomal (blue pigment) and 2 X-linked (red and green pigments) color vision genes. In contrast, the other NWM species investigated in detail have only 1 autosomal and 1 X-linked color vision gene, though the X-linked locus is polymorphic with 3 alleles. To understand the origin of trichromacy in howler monkeys, several NWM species were examined for the number of X-linked pigment loci, and intron 4, and exons 3, 4, and 5 of the red and green pigment genes of a male howler monkey were sequenced. The spider monkey, the woolly monkey, the saki monkey, and the bearded saki monkey were shown by the technique of single strand conformational polymorphism (SSCP) and by Southern blotting to have only 1 X-linked color vision gene, suggesting that within NWMs, the howler monkeys are the only genus with 2 X-linked pigment loci. The sequences of exons 3, 4, and 5 and intron 4 reveal that the gene duplication in the howler monkey was independent of that in the human-ape-OWM lineage. In addition, the amino acids at 4 critical sites for spectral tuning suggest that the duplication in the common ancestor of howler monkeys was derived from the incorporation of 2 alleles that were, respectively, very similar to the P535 (green) and P562 (red) pigment alleles currently existing in the squirrel monkey and capuchin (2 NWM genera). This hypothesis implies that the P535-P562 polymorphism existed before the platyrrhini (NWM) radiation, which took place about 20 million years ago. Furthermore, the distribution of sequence differences in intron 4 between the 2 howler monkey genes suggests that the 2 intron 4 sequences have been homogenized by recent gene conversion events, providing further evidence for the frequent occurrence of gene conversion between X-linked pigment genes.