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COLLOQUIUM ON VISION: FROM PHOTON TO PERCEPTION
Table 1. Summary of primate opsin genes, cone photopigments, and color vision
Chromosome 7 genes
Photopigments (λMAX, nm)
Some nocturnal primates
Defective S-opsin gene
Some diurnal prosimians
≈430 + 543
Many New World monkeys
420−435 + (535, 543, 550, 556, 562)
All (?) Old World monkeys; apes
M + L (multiple copies)
430 + 530 + 562
M + L (multiple copies/polymorphisms)
410−430 + 530/535 + 556/562
Uniformly trichromatic; significant polymorphisms
The λMAX values for the M and L cones represent averages obtained from electrophysiological measurements (see Fig. 1). Those for the S cones are taken from a variety of different types of measurement; a range indicates that there is uncertainty or that there may be alternative pigments in different species in the group. The alternative possibilities suggested for the human M and L cones reflect a polymorphism of the cone opsins.
mammals is two classes of cone pigment and dichromatic color vision (2). Some contemporary primates conform to this norm; others would be this way except that mutational changes have rendered their S-cone opsin gene nonfunctional. Molecular comparisons of cone opsin genes suggest that the divergence that led to two separate M and L cone pigments occurred about 30 million years ago (17,55). Presumably this was subsequent to the separation of New and Old World primate lineages but prior to the separation of cercopithecoid and hominoid primates (26). This divergence event yields the photopigment basis for routine trichromatic color vision. It can be argued that the arrangement of genes and cone pigments in the New World monkeys provides a blueprint as to how this may have happened. In a routinely dichromatic species, only a single nucleotide substitution in an opsin gene is required to yield a novel M/L pigment. When the novel gene appears in a heterozygous female she will produce two spectrally discrete M/L pigments and, if her visual nervous system is arranged like that of many New World monkeys, trichromatic color vision emerges. Additional altered genes can increase the frequency of female trichromacy; for instance, with three alleles two-thirds of all female New World monkeys can achieve that status. To make trichromacy routine requires a second gene locus. This could have come either from an unequal crossover between chromosomes having different alleles or through gene duplication and subsequent gene conversion.
Although these ideas provide the mechanics for the evolution of trichromatic color vision, they fail to reveal the selective pressures that conditioned these changes. A standard idea is that trichromatic color vision substantially enhances one's ability to detect, identify, and evaluate objects in the environment (56). In the case of primates, the objects of concern were probably colored fruits. For instance, the trichromatic color vision of primates will allow a rapid and accurate detection of yellow and orange fruit hidden among the abundant green foliage of tropical forests. In turn, the primate harvester then serves as an agent to disperse seeds to new locations. Although definitive proof is lacking, it seems likely that this contractual interaction may have provided the setting for the evolution of primate trichromacy.
Two things are missing from the current picture. First, there remains a dearth of information about cone opsin genes and color vision for many species of primate. Second, although informed discussions about the functional utility of color vision now appear with increasing frequency (e.g., refs.57–61), we still lack a detailed understanding of the many ways in which primates use spectral information in their successful dealings with the environment.
Over the years I have received indispensable help on this project from the following collaborators: B. Blakeslee, J. K. Bowmaker, M. A. Crognale, J. F. Deegan II, J. D. Mollon, M. Neitz, and, especially, J. Neitz. My work summarized here was funded by grants from the National Science Foundation and the National Eye Institute.
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