How do photoreceptors work? Convert energy from light into nerve signals No easy feat!!
200 femtoseconds!!
Phototransduction Isomerization Opsin - transducin Transducin - PDE PDE - cgmp to GMP Low cgmp closes ion channels hyperpolarized Isomerization Opsin - transducin Transducin - PLC PLC - produces IP 3 IP 3 - release of Ca +2 Ca +2 opens ion channels depolarized
Photopigments can be characterized according to the efficiency with which they absorb light... 120 100 S Rod M L Absorbance 80 60 40 20 0 340 380 420 460 500 540 580 620 660 700 Wavelength (nm)
Principle of Univariance: Photoreceptors cannot register the wavelength of the photons they catch - i.e., the output depends on quantum catch, but not upon what quanta are caught.
Monochromat with 1 cone type Incident Photons Pigment Absorptance Number Absorbed 1000 1000 0.10.050 100 50 λ 1 λ 2 Fraction of incident light absorbed.100.075.050.025 0.00 λ 1 λ 2 400 450 500 550 600 650 700 Wavelength (nm)
Monochromat with 1 cone type Fraction of incident light absorbed.100.075.050.025 0.00 Incident Photons Pigment Absorptance Number Absorbed 1000 2000 0.10.050 100 100 λ 1 λ 2 λ 1 λ 2 A Perfect Match! 400 450 500 550 600 650 700 Wavelength (nm)
Monochromat with 1 cone type Fraction of incident light absorbed.100.075.050.025 0.00 Incident Photons Pigment Absorptance Number Absorbed 1000 1000.050.050 50 50 λ 1 λ 2 λ 1 λ 2 400 450 500 550 600 650 700 Wavelength (nm) λ 2 Unable to discriminate!
Dichromat with 2 cone types λ 1 λ 2 Fraction of incident light absorbed.100.075.050.025 0.00 λ 1 λ 2 No Match Possible 400 450 500 550 600 650 700 Wavelength (nm)
12 10 Test R & G Primaries 8 6 4 2 0 0.4 0.42 0.44 0.46 0.48 0.5 0.52 0.54 0.56 0.58 R/R + G
2 types of L pigment in normal males 70 60 50 Frequency 40 30 20 10 0 550 551 552 553 554 555 556 557 558 559 560 Peak of L pigment
Comparative color vision
Direct techniques for measuring photopigment spectra: Micorspectrophotometry (MSP) Electroretinography (ERG) Suction electrode recordings Genetic analysis (indirect??)
L UV S M L M Rh S UV 0.2 0.4 0.6 0.8
L L M M Rh S UV
L, M & S or L, L & UV??
Relative Sensitivity 15 10 5.30.38.46.54.62.70 Wavelength (microns)
1.2 1 Absorbance 0.8 0.6 0.4 0.2 0 340 380 420 460 500 540 580 620 660 700 Wavelength (nm)
Mechanisms of Spectral Tuning Changes in the opsin protein Ocular filters (lens, oil droplets) Altering the chromophore Different optical density
Helix 5 Helix 1 Helix 2 Helix 3 Helix 6 Helix 7 Helix 4 277 180 285 312 dimorphic sites: spectral tuning sites: long- vs middle-wave determining sites chromophore attachment site Neitz & Neitz (1998)
12 10 Test R & G Primaries 8 6 4 2 0 0.4 0.42 0.44 0.46 0.48 0.5 0.52 0.54 0.56 0.58 R/R + G
2 types of L pigment in normal males 70 60 50 Frequency 40 30 20 10 0 550 551 552 553 554 555 556 557 558 559 560 Peak of L pigment
Gene transcription mrna translation Protein (opsin) transduction Behavior
Exon 2 Exon 3 Exon 4 No 065 111 116 153 171 174 178 230 233 236 1 2 3 4 6 8 9 10 11 12 13 14 15 16 17 18 19 20 1 22 1 23 1 Total = 159 Amino Acid 65 111 116 153 171 174 178 230 233 236 I V Y M V V V A T S V T I S L I A I S I A M 45 32 12 10 6 5 4 4 3 2 2 2 2 1 1 1 1 1 1 Frequency Variants of the L gene in individuals with normal color vision
Adding an oil droplet in the inner segment effectively narrows the overall relative spectral sensitivity of the cone relative to the pigment...
Common chromophores used in vision Retinal - most widely used 3-dehydroretinal - freshwater fishes, amphibians and reptiles 3-hydroxyretinal - insects (moths, flies, butterflies)
1 Relative Absorbance 0.5 0.90 Increasing optical density broadens the absorbance spectrum 0.40 0.01 0 400 440 480 520 560 600 640 680 Wavelength
Evolution of Trichromacy in primates
L L M M Rh S UV
Duplication 14 KB 236 base pairs 25 KB
Divergence 100 80 60 40 20 0 Absor pti on (per cent of ma xi mu m 350 400 450 500 550 600 650 700 Wavelength (nm)
Selective expression 236 bp 14 KB Neural circuitry already in place?
One Cell-Type Model Stochastic Pigment-Gene Choice random choice mechanism??? Second Order Neurons L M L vs. M gene choice determines cell type
Evidence that the neural circuitry was in place - New World primates X 560 Y X 545 Y X 530 Y 1.2 1 0.8 0.6 0.4 0.2 0 400 440 480 520 560 600 640 680 Wavelength (nm) 1.2 1 0.8 0.6 0.4 0.2 0 400 440 480 520 560 600 640 680 Wavelength (nm) 1.2 1 0.8 0.6 0.4 0.2 0 400 440 480 520 560 600 640 680 Wavelength (nm) 1.2 1.2 1 1 0.8 0.8 X 560 X 560 0.6 0.4 0.2 0 400 440 480 520 560 600 640 680 Wavelength (nm) X 545 X 560 0.6 0.4 0.2 0 400 440 480 520 560 600 640 680 Wavelength (nm)
L M S +? Achromatic channel L M S - Red-green channel L M + - Blue-yellow channel S
* HS YY AN AP nasal AP temporal MD JP JC * * RS JW temporal BS JW nasal 5 arcmin * A. Roorda & D. R. Williams, Nature 1999
AP nasal BS
n = 62 from ERG 0.25 0.2 Frequency 0.15 0.1 0.05 0 0 10 20 30 40 50 60 70 80 90 100 L/M Proportion (%L)
How does this variation arise??
One Cell-Type Model Stochastic Pigment-Gene Choice random choice mechanism??? Second Order Neurons L M L vs. M gene choice determines cell type
Selective expression 236 bp 14 KB
Color Vision Defects in Humans X-linked inheritance pattern About 8% of Caucasian males have a R/G CVD About 15% of Caucasian females are carriers
Forms of Red-Green Color Blindness Deutan: missing functional M pigment Absorption Deuteranope 120 100 S L 80 60 40 20 0 400 450 500 550 600 650 Wavelength (nm) Absorption Deuteranomalous 120 S L L 100 80 60 40 20 0 400 450 500 550 600 650 Wavelength (nm) Protan: missing functional L pigment Protanope Protanomalous 120 100 S M 120 100 S M M Absorption 80 60 40 Absorption 80 60 40 20 20 0 400 450 500 550 600 650 Wavelength (nm) 0 400 450 500 550 600 650 Wavelength (nm)
L/M Gene Array 1) Head-to-tail tandem array on the X chromosome, 2) L and M genes are highly homologous, 3) Susceptible to unequal homologous recombination.
Unequal crossover generates genetic variation in the pigment genes
Inheritance Pattern X n X p X n Y X p Y X n X p X n X n X n Y X p Y X n X n X n X p X n Y X p X p X p Y X n Y X n X p X n X p - carrier - affected male - normal male - affected female - normal female
Sharpe & Jägle, 2001
http://www.mcw.edu/cellbio/colorvision/ http://webexhibits.org/causesofcolor/2.html http://www.vischeck.com/
Characteristics of Color Vision Defects Congenital Present at birth Type and severity is the same thru life Type can be classified precisely Both eyes equally affected Visual acuity is often unaffected Predominantly protan or deutan Higher incidence in males Acquired Onset after birth Type and severity fluctuates Type may not be easily classified. Combined defects occur Monocular differences often occur Visual acuity is often reduced Predominantly tritan Equal incidence in males and females
Points of discussion... L:M ratio and color perception Why aren t we pentachromatic?? Did R/G vision really evolve this way? Q & A