MOLECULAR EVOLUTION OF TRICHROMACY IN PRIMATES

Although trichromacy in Old and New World primates is based on three v isual pigments with spectral peaks in the violet (SW, shortwave), gree n (MW, middlewave) and yellow-green (LW, longwave) regions of the spec trum, the underlying genetic mechanisms differ. The SW pigment is enco ded in both cases by an autosomal gene and, in Old World primates, the MW and LW pigments by separate genes on the X chromosome. In contrast , there is a single polymorphic X-linked gene in most New World primat es with three alleles coding for spectrally distinct pigments. The one reported exception to this rule is the New World howler monkey that f ollows the Old World system of separate LW and MW genes. A comparison of gene sequences in these different genetic systems indicates that th e duplication that gave rise to the separate MW and LW genes of Old Wo rld primates is more ancient than that in the howler monkey. In additi on, the amino acid sequences of the two howler monkey pigments show si milarities to the pigments encoded by the polymorphic gene of other Ne w World primates. It would appear therefore that the howler monkey gen e duplication arose after the split between New and Old World primates and was generated by an unequal crossover that placed two different f orms of the New World polymorphic gene on to a single chromosome. In c ontrast, the lack of identity at variable sites within the New and Old World systems argues for the origin of the separate genes in Old Worl d primates by the duplication of a single form of the gene followed by divergence to give spectrally distinct LW and MW pigments. In contras t, the similarity in amino acid variation across the tri-allelic syste m of New World primates indicates that this polymorphism had a single origin in New World primates. A striking feature of all these pigments is the use of a common set of substitutions at three amino acid sites to achieve the spectral shift from MW at around 530 nm to LW at aroun d 560 nm. The separate origin of the trichromacy in New and Old World primates would indicate that the selection of these three sites is the result of convergent evolution, perhaps as a consequence of visual ad aptation in both cases to foraging for yellow and orange fruits agains t a green foliage. (C) 1998 Elsevier Science Ltd. All rights reserved.