The Evolution of Trichromatic Color Vision: Part 3 Lecture Notes
Key Words and Terms
evolution, color vision, New World primate, Old World primate, color opponent processing, spectral tuning, X-chromosome inactivation, blue cone monochromacy, locus control region, genetically engineered mice
Primate evolution: the plate tectonic separation of South America and Africa led to a division into New World and Old World primate lineages. Humans and our great ape relatives are Old World primates.
What is the selective value of trichromatic color vision relative to dichromatic color vision? Possible answers include an improved ability to identify ripe fruit, especially among dappled foliage, and an ability to assess individual differences among members of one’s own species.
Non-primate mammals (dogs, cats, mice, cows, etc.) have an S-pigment and a single longer wavelength pigment (encoded on the X-chromosome). The X-chromosome-encoded pigment is the ancestor of the M and L pigments of Old World primates.
The neural circuitry that supported an ancient system of dichromatic color system among non-primate mammals still exists essentially unchanged in the primate retina. Current evidence suggests that the comparison between M and L cones uses a neural circuit that evolved to measure spatial variation in light intensity by comparing the activities among a homogeneous set of photoreceptors at different locations in the retina. These photoreceptors contained a visual pigment that was the ancestor of the present-day primate L and M pigments.
The difference in absorbance spectra between the primate L and M pigments is principally due to amino acid differences at only three locations (out of 364 amino acids total).
The requirement for expression of L and M pigments in distinctive cone types has been solved in two different ways by New and Old World primates.
In most New World primates, the X-chromosome carries only a single cone pigment gene, but sequence variation within this gene in the population creates a series of cone pigments (typically three) that differ in their absorbance spectra. These species use X-chromosome inactivation – a phenomenon common to all female mammals that randomly silences most of the genes on one of the two X-chromosomes – to create a mosaic of spectrally distinctive cone photoreceptors. Of course, this mechanism is only useful for those females who inherited two different alleles of the X-linked visual pigment gene. A striking aspect of this mechanism is that it generates a random mosaic of spectrally distinct cones, since the decision to inactivate one or the other X-chromosome is stochastic.
In Old World primates, the choice between expressing L or M visual pigment genes also appears to be governed by a random process. In this case, current evidence suggests that there is a random choice to activate either the L or the M promoter, but not both, in each cone that is destined to express one of the X-linked visual pigment genes. This mechanism succeeds because the X-chromosomal location of these genes means that this random choice is made on only one chromosome in each cone cell. (Recall that males have only one X-chromosome, and female cells express the L and M pigment genes only from the one active X-chromosome.)
These primate mechanisms of gene choice have been examined by genetic engineering experiments in mice. First, gene expression by random gene choice of either the L or M pigment genes from an human gene array has been reproduced in mice, showing that this phenomenon does not require any primate-specific proteins. Second, by replacing the mouse M pigment gene (located on the X-chromosome) with sequences coding for a human L pigment, a mouse line has been engineered to carry X-chromosome visual pigment gene variation of the type that characterizes New World primates. By color vision testing, heterozygous female mice were found to have acquired the ability to distinguish lights of different wavelength that normal mice could not distinguish. These experiments show that the mammalian brain has an inherent plasticity that permits it to take advantage of new sensory inputs so that evolutionary variation in sensory receptor cells can be immediately useful for the species – without a need to evolve more complex neural circuits to extract or compare this new information. This type of plasticity is likely to be used in the evolution of other sensory systems.