Flashcards in Colour vision Deck (50):
where has colour vision evolved?
twice, in vertebrates and in Ecdysozoa - insects and crustaceans.
It is now the dominant form of visual information
evolved at the same time as coloration and visual beh.
define colour vision
using only spectral composition of light to distinguish between two fields of view of the same shape and size.
what forms a visual pigment
Opsin protein and chromophore
(the pigment, not the opsin, has spectral sensitivity)
what are 3 mechanisms of colour vision
1. changes to the opsin
2. change of chromophore
describe vertebrate and invertebrate visual pigments
Vertebrate: C-type opsin, 7 transmembrane helical structure. chromophore buried inside
Invertebrate: R-type. Similar, longer helices, C terminus longer, rhabdomeric photoreceptors.
5 steps of vertebrate transmission from the photoreceptor
1.Light hits rhodopsin
2.chromophore absorbs photon and changes shape verrrrry rapidly as it expels chromophore.
3.opsin becomes active metarhodopsin
4. metarhodopsin coupled with G protein transducin (Gt), causes closing of ion channels in the cell membrane, and cell hyperpolarises.
5. accumulation of closed channels gives signal from photoreceptor.
3 steps of invert transduction
1. light absorbed by chromophore, bistable chromophore in inverts.
2. Opsin couplesto a different G protein (Gq).
3. opens trp channels, causing depolarisation, signal out of cell.
describe how multiple visual pigments allow colour vision
1st mechanism of colour vision
different opsins formed when different AA seqs shift the spectral sensitivity, possibly evolved trough gene duplication events.
Insects commonly have 3 visual pigments - B, G, UV. (G receptor also has a beta band in UV. pigments can absorb all light but peak at a specific WL).
each pigment expressed in a different cell around the rhabdom.
What family are opsins part of, and how many opsin types are there?
C type - vertebrates
R type - inverts
group 4 - light sensitive pigments exist in brain
some species have more genes for opsins than actually expressed.
describe a tuning site which changes UV sensitive pigment to blue, in some mammals.
Changes in the AA sequence at site 86 in the 2nd transmembrane helix,
change from Phe86 to Tyr86 gives violet sensitivity.
changing the AA affects the shape and axis of helix.
which chromophores do vertebrates and inverts use?
chromophores are derivatives of vitamin A.
verts - A1 and A2 (A2 longer wl shifted).
retinol = A1.
Can change between A1 and A2
inverts - A1 and A3,4, 9 etc
Invert chromophore is bistable - ability to change between A1 and A3.
what allows variation in spectral sensitivity of pigments?
1. variation in the AA sequences of opsins
2. variation in the chromophores
species beh and enviro influences how mechanisms have evolved.
how have animals changed use of chromophore
convert A1 to A2,
difference is a c=c double bond.
A2 have a greater sensitivity to longer WL, so see better reds.
eg pacificsalmon, out in ocean are silvery and use A1. return to FW, more pink coloured, change to A2 chromophore.
describe an adaptation in bullfrogs.
depending on light environment, can change A1 to A2.
A1 - expressed in part of retina which looks up into the air,
A2 expressed where retina is looking down into the water.
Can see diff concs of A1 and A2 in samples of dorsal and ventral retinas.
how can you measure amount of A1 and A2?
measure conc of specific chemical.
use spectroscopy and see where peaks are. early smaller peak indicated A2. present after zebrafish treated with Thyroid hormone.
How was the enzyme responsible for chromophore conversion found in zebra fish (same in bullfrogs)?
compare transcripts of gene expression, in WT and TH treated fish. Presence of cyp27c1 enzyme correlates w presence of vit A2. A1 to A2 is a TH mediated switch,
seen increase in cyp27c1 transcript when TH treated.
however, no A2 if no enzyme, even is TH.
same in bullfrogs.
transfected cells in vitro with cyp27c1 and they converted a good amount of A1 into A2.
Then in vivo experiment. treat line of zebra fish with TH, so dont express gene at all. cyp27c1 is not ever seen, and no conversion of A1. shows that this enzyme is the only thing which convverts chromophore.
How does beh of zebra fish change in response to chromophore conversion?
phototactic, associate more w brighter light.
Illuminate tank with different colours from different sides. treat cyop27c1 and WT fish with TH treatment and vehicle treatment.
upon exposure to 590nm light, all fish showed phototactic response. under 770nm, only WT, TH treated fish showed a response, the others could not see the light.
what acts as a colour filter in bird retinas?
bird cones contain an oil droplet before the outer segment, acts as a colour filter.
Not in rods
The UV sensitive cone has clear oil droplet.
describe avian double cones and their evolution.
look like 2 single cones joined.
either same or diff (true) visual pigments in each.
describe the evolution of oil dropets
widely distributed across vertebrates, no pattern seen in evolutionary history. perhaps secondary loss in amphibians. perhaps evolved independently.
How do oil droplets function?
1. filters light. eg red droplets absorb B and G.
2. acts like a small lens, couple more light in the outer segment by refraction.
with these filters, it is easier to discriminate between different colours.
what determines the sensitivity of an avian cone cell
product of oil droplet transmission and visual pigments.
multiply the transmission WLs of oil droplet by the curve of spectral pigment absorption. reulting curve is sensitivity. filters out beta band and peaks narrowed.
what is a colour space?
graphical relationship between colours.
used to show how a visual system may combine input from different combinations of cone WL specific receptors.
trichromatic visual system gives 2D colour space. tetra chromatic system gives pyramid shaped space. a point in the space gives relative stimulation of receptors.
if all stimulated, gives white.
which group has the most complex colour visual system?
fossils recognisable by features inc raptorial appendages, which must need good vision to use effectively.
two stomatopod hunting strategies
smashers - hit so fast that water drops in pressure and boils so creates impact from boiling water
describe the structure of stomatopod eyes
dorsal and ventral hemisphere - apposition compound, 2 visual pigments each - UV and green sensitive.
midband - distinctive section, either 6 or 2 rows of cells.
pseudopupil - all light at this point is. this part absorbed, looking directly at you.
gives 3 fields of view, but in two eyes, so hexascopic. fields overlap so can judge distance effectively.
describe stomatopod spectral sensitivity.
12 consistent visual pigments
4 UV, 8 colour, >700nm.
much more evolved than any other known animal.
- D and V hemispheres: UV, R8, and Green, R1-7, pigments. forspatial and polarisation vision.
- Midband - for colour vision. rows 1-4, 2 tiers of photoreceptors, rows 5-6, UV and green, and circular polarization.
- overall, >20 different channels.
where do stomatopods have colour filters?
- intrarhabdominal filters and receptor tuning, similar to avian oil droplets, narrows spectral sensitivity.
- also in crystalline cones of midband rows. fluoresce differently under UV, showing they filter different WL. modify WL, narrowing the sensitivities.
describe the open rhabdom structure of drosophila ommatidia.
8 cells, R7 is on top of R8. 1-6 have the same spectral sensitivity, green, and then B and UV in 7, 8, in centre.
Found 2 different types, differing in pigment expressed in R7 (Rh3 and 4) and R8 (Rh5 and 6). random distribution across eye.
describe the closed photoreceptor structure of nymphalid butterfly ommatidia.
3 different ommatidial types.
1. cells 1 and 2 are UV sensitive
2. 1 and 2 are blue sensitive
3. 1 is UV and 2 is blue sensitive.
other 6 cells are LWS, green, pigmented.
How are other butterfly visual systems more complex?
other families more complex, using coexpression of different opsins in same cell, and more pigments. dorsal and ventral retina differ.
how have heat maps been used to study photoreceptors
to show coexpression of visual pigments in cichlid, Metraclima zebra. varies between individuals showing extreme phenotypic variation.
- also much variation in distribution of cone and ganglion cell densities across retina. centre has highest density of ganglion cells.
describe intraretinal variability on archer fish
pigments in dorsal retina match dominant WL of light from below, Ventro nasal match from above. beh has clearly driven selection of pigments.
ventro-temporal match general environmental WLs of vegetation.
How are variations in the reflectivity of the tapetum adaptive?
in midwater fish, 500-4000m,
Dorsal tapetum reflects yellow, optimal for seeing upwelling bioluminescence. ventral reflects blue, optimal for spotting objects against downwelling blue light .
what are selection pressures on colouration?
predators, prey and mate choice,
coevolution of color vision and colouration
What is the most common pigment?
Melanin - 2 forms: Differ by aspect ratio.
pheomelanin: brown/red, longer
Eumelanin: black, spherical.
MC1R protein is a GPCR, if active, eumelanin is produced, or pheomelanin if it is blocked.
which pigments are usually obtained through diet?
ratios of different forms control colour. Aphids can use symbiotic bacteria to produce
Zeaxanthin, red, cut by enzymes to produce yellow galloxanthin.
Oil droplets in retina contain carotenoids.
only zeaxanthin is ingested, but enzymes used to modify it into different colours.
what is structural colour?
constructive interference of light. non pigmented colour
in beetles, butterflies and fish.
Less costly, easier access to all of colour space as easy to modify.
what causes blue colour on butterfly scales?
blues produced as structural colour. eg by ridges on scales - 'christmas tree' structures on scale surface. lamellae = 'branches', and there are gaps of air inbetween trees, smaller than light WL. different spacing of gaps allows different colours, blue to orange.
in general, structures and 'gaps' must be made of materials with different refractive indicies.
define constructive/destructive interference.
constructive - 2 waves in the same direction and peaks and troughs align o enhance wave.
if out of phase, cancel, = destructive. this is used to make reflection films on windows.
what is a quarter wave reflector?
eg the christmas tree structures on butterfly scales.
if a light wave is reflected off a more dense medium, there is a half WL shift.
If chitin is a quarter WL thick, then it is constructive: wave enters chitin (loses 1/4 WL), and is reflected back out (losing another 1/4), and going to a less dense medium so shift of 1/2. therefore overall shift of 1, so in phase.
describe beetle cuticle structure`
layers of cuticle form directional planes, layered so that they rotate. spaces between them are 1/2 WL, and layers are 1/4 thick.
allows max 50% reflectivity.
how have some beetles evolved control over polarisation of reflected light?
Plusiotis resplendens = 2 cuticle layers with chiral substructure to control polarization.
1. Incident light hits first chiral substructure, gets circularaly pol, split into R and L handed light. 50% incident light reflected, R.
2. Travels through 1/2 WL gap and changed to R handed.
3. Hits chiral substructure, R Reflected, 100%.
4. Travels through 1/2 WL gap, handedness hanged back to L.
5. Hits first layer again, all L transmitted, so 100% light reflected.
Has a gold shine, from gradient in the size of gaps.
2D periodic structure
in cats eyes, periodicity in horizontal and vertical directions, so light entering at any angle can be constructive interference.
3D periodic structure
gives rise to non irridescent saturated colours eg blue damselflies or birds feathers.
also in stomatopods for communication - bright blue formed by oval discs stacked up.
how is Plusiotis boucadi beetle coloured?
matt green colour made of hexagonal array of pin points, 10 microns wide. bowl shaped points, orange in centre and green around edge. combined to form matt green.
how can white be formed structurally?
complete disorder in structures causes multiple scattering and v reflective white.
what can be seen in the fossil record of colour?
well preserved melanosomes in beetles, can see colouration of early birds, irridescence preserved - repeat system can be seen, seen that nanostructures are the same in modern birds.
describe an experiment to test if patterns were beneficial by providing disruptive colouration or not.
innes cuthill, 2005.
colured small triangles differently and placed with mealworm bait in Leigh woods. edge patterns had the highest survival.