Evolution of colour vision
After J Neitz, J Arroll, M NeitzOptics & Photonics News, pp. 26-33, Jan 2001.
Neural mechanisms of seeing colour
Light sensitive receptorsneural components for processing
extracting relative responses from neighbouring receptors
wavelength sensitive encoding output to labelled lines
Black and white perception
Small cluster of receptors illuminated by a small spot of light
information gathered from illuminated receptors from their immediate neighbours
Brain nerve fibres receive output from cluster of receptors from the “white” labelled lines cluster of receptors from the “black” labelled lines
one of the two outputs is inverted compared to the other
Hue perception
Encoding in two components, each of them responsible for a pair of sensations, sensations in each pair are opposed to one another, blue-yellow hue system red-green hue system
each draws from a common set of photoreceptors: L, M, S; outputs via different neural components:
different labelled lines.
Cone photoreceptors
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350 450 550 650 750
wavelength, nm
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L-cone
M-cone
S-cone
Hue systems
blue-yellow(B-Y): output from the S cones, comparing it to L + M cone responses
red-green(R-G): output from the L cones, comparing it to M cone responses
only blue-yellow system draws from S cones, S cones differ from M and L in physiology and retinal distribution
B-Y more vulnerable: toxic exposure, eye diseases, trauma
Different evolutionary history
Blue-Yellow colour vision system
Trichromatic colour vision in mammals: only in man and some subset of primates
Some mammals are monochromatsMost mammals are dichromats, e.g.
dog, system is homologous to the “blue-yellow” system
Cone photopigment sensitivity of dogs
Dogs have two types of cone-pigments most similar to human S and L pigments. The bar at the bottom approximates how a dog can distinguish among colours
Tomatoes: which one is ripe, seen by a dog
Tomatoes: which one is ripe, seen by a trichromat
Photopigments and their genes
Composition of the photopigments chromophore: 11-cis-retinal protein component, covalently bound: opsin
In terrestrial animals the chromophore is the same, the opsin varies the opsin tunes the absorption maximum the opsins belong to a comon family
Photopigments and their genes
Molecular genetic methods can deduce the amino acid sequencees of photopigment opsins
The two classes of dichromatic pigments have strikingly different amino acid sequences (50 %):
Indication for early differentiation of the S and L photopigments in evolutionary terms
Photopigments and their genes -evolution of colour vision
S and L pigments amino acid sequences different
Seven amino acid changes produce the 30 nm difference between the M and L pigments
Extrapolation and speculation: 6 % difference in amino acid sequence required for the 100 nm shift between S and L cones
Speculation on evolution
Comparison: differences in rod pigments of species as clock, constant rate genetic drift
S and L/M cone differenti-ation about 1000 million years ago (MYA)
Oldest fossils: 6000MYA
Speculation on evolution
Dichromacy almost as old as visionDistinction among colours, humans see
200 grey levels Dichromacy: 50 discernible chromatic
steps, provides 10.000 stepsWavelength sensing is as
fundamental to vision as is light detection
Red - Green colour vision system
L and M photopigments individually polymorchic, on average difference: 15 amino acids
Genetic clock estimate: L and M difference 50 MYA (Old and New World primates split about 60 MYA)
Three neuronal line pairs: (Black-White, Y-B, R-G)
100 steps in R-G direction: 106 distinguishable colours
Beyond trichromacy
Non-mammal diurnal vertebrates (birds, fish, etc.) have four photopigments: also UV
Mammals were nocturnal when appeared at the time of the dominance of dinosaurs
Nocturnal ancestors of modern primates were reduced to dichromacy
Primates invented trichromacy separately
Neural circuits for red-green colour vision
Diurnal primates: acute spatial vision: small receptive fields (midgets), contacting single cones
Opponent signals from surrounding neighbours: new receptor (L or M) compares also colour, no new wiring needed
Mammalian visual cortex molded by experience
Directions of colour vision research
L and M photopigment genes might misalign during meiosis and recombine: mixed sequences might occur
Variants common in L gene, females have two X chromosomes, the two might have different L pigments
X-chromosome inactivation can produce two L cones in females: four spectrally different receptors.
Directions of colour vision research
The two L cones are very similar: few steps of colour discrimination
Females found who showed increased colour discrimination ability
L/M cone ration can change from 1:1 to 4:1, with no measurable colour vision difference: plasticity of nervous system?
Chromatically altered visual environment has long term influence on colour vision
Further speculation
If neural circuits for colour vision are sufficiently plastic gene therapy could replace missing photopigments could add a fourth cone type