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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L-cone

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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