Transcript Evolution of colour vision

Evolution of colour vision
After
J Neitz, J Arroll, M Neitz
Optics & Photonics News,
pp. 26-33, Jan 2001.
Neural mechanisms of seeing
colour
Light sensitive receptors
neural 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
log cone action sensitivity
1
0
-1
-2
L-cone
-3
M-cone
-4
S-cone
-5
-6
-7
-8
350
450
550
wavelength, nm
650
750
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 monochromats
Most mammals are dichromats, e.g. dog,
system is homologous to the “blue-yellow”
system
Cone photopigment
sensitivity of dogs
Dogs have two
types of conepigments 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
differentiation about
1000 million
years ago
(MYA)
Oldest
fossils:
6000MYA
Speculation on evolution
Dichromacy almost as old as vision
Distinction among colours, humans see
200 grey levels
Dichromacy: 50 discernible chromatic steps,
provides 10.000 steps
Wavelength 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 redgreen 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