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Light and the Evolution of Vision

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Light and the Evolution of Vision Eye (2016) 30, 173–178 © 2016 Macmillan Publishers Limited All rights reserved 0950-222X/16 www.nature.com/eye Light and the DL Williams CAMBRIDGE OPHTHALMOLOGICAL SYMPOSIUM evolution of vision Abstract originated; but.....as some of the lowest organ- isms.....are known to be sensitive to light, it does It might seem a little ridiculous to cover the not seem impossible that certain elements...should period over which vision evolved, perhaps 1.5 billion years, in only 3000 words. Yet, if we become aggregated and developed into nerves examine the photoreceptor molecules of the endowed with this special sensibility. most basic eukaryote protists and even before that, in those of prokaryote bacteria In a later letter to J.D. Hooker written in 1871 and cyanobacteria, we see how similar they Darwin remarks: are to those of mammalian rod and cone But if (and oh! what a big if!) we could conceive in photoreceptor opsins and the photoreceptive some warm little pond, with all sorts of ammonia molecules of light sensitive ganglion cells. This and....salts, light, heat, electricity.... present, that shows us much with regard the development a proteine [sic]..... was chemically formed ready to of vision once these proteins existed, but there undergo still more complex changes...2 is much more to discover about the evolution of even more primitive vision systems. and so perhaps a warm little pond would be a – Eye (2016) 30, 173 178; doi:10.1038/eye.2015.220; reasonable place to start as we look at the published online 6 November 2015 evolution of vision. I wonder if you remember pond dipping as a child? As far as I remember we only looked at macroscopic life-forms— water boatmen, pond skaters and the like. But if we had happened to look under a microscope no Introduction doubt we would have found two organisms, Euglena (Figure 1a) and Chlamydomonas Darwin is often quoted as seeing the (Figure 1b). development of the eye as a significant difficulty for his theory of evolution by natural selection. He writes in The Origin of Species: Euglena and Chlamydomonas To suppose that the eye, with all its inimitable Both these unicellular protists have a clear contrivances.... could have been formed by natural orange-coloured eyespot. Euglena gracilis exists selection, seems, I freely confess, absurd in the as a photosynthesising autotrophe but at low highest possible degree.1 light intensity it can survive as a heterotrophe ingesting plant material. Neither strictly plant Department of Veterinary Medicine, University of But what is omitted by many is his answer to nor animal, it occupies a third kingdom as a Cambridge, Madingley this conundrum, a few sentences further on: protist. Chlamydomonas rheinhartii might more Road, Cambridge, UK strictly be defined as a green algae in the plant Reason tells me, that if numerous gradations from kingdom. Phototaxis is essential for both Correspondence: a simple and imperfect eye to one complex and organisms; moving towards light upon which DL Williams, Department of perfect can be shown to exist.... and if such they depend for energy and nutrition, yet also Veterinary Medicine, variations should be useful to any animal.... then University of Cambridge, undergoing negative phototaxis to protect the difficulty of believing that a perfect and Madingley Road, themselves against too intense a source of complex eye could be formed by natural selection, Cambridge, CB3 0ES UK illumination. The eyespot is not the Tel: +44 (0)7 9390 7682; should not be considered as subversive of the photoreceptor itself but rather a mass of Fax: +44 (0)12 2323 2977. theory. carotenoid pigment shading the photoreceptor E-mail: [email protected] from light from one direction. This demonstrates Indeed, he notes further. Received: 21 September 2015 the essential components of any visual system; Accepted: 22 September 2015 How a nerve comes to be sensitive to light, hardly any photosensitive organism needs a Published online: concerns us more than how life itself first photoreceptor that detects the light. But that 6 November 2015 Evolution of vision DL Williams 174 Figure 1 Top left Chlamydomonas rheinhartii. Top right Euglea gracilis. Bottom Hydra vulgaris. alone would not allow the organism to determine the detection of light can be translated into a change in direction of the light source. A pigment spot reduces the flagellar movement, generally an ion flux of one kind or illumination from one direction, or changes the another. wavelength of the incident light falling on the In Euglena the photoreceptor, a tightly packed mass of photoreceptor, thus allowing the organism to move in the crystalline protein, is located next to the eyespot direction of the light or away for it. So third, a mechanism (Figure 2a). It has been estimated that the photoreceptor to promote movement is essential. To detect the light is contains around 2 × 107 molecules of a rhodopsin-like one thing but to move towards or away from it requires a protein.3 Around 108 photons impinging on this crystal motor system; the flagellae in Chlamydomonas and saturate the protein with a maximum absorption at Euglena. But also a mechanism is required by which around 500–525 nm.4,5 Photostimulation leads to positive Eye Evolution of vision DL Williams 175 Figure 2 Photoreceptor of Euglena gracilis, left: measuring the photoelectric effect after Nichols and Rikmenspoel (1977) right: photoreceptor rhodopsin crystal positioned next to emergent flagellum after Barsanti et al (2012). phototaxis with the ion flux giving what appears the organism away from light is modulated through surprisingly like an electroretinogram (Figure 2b). increased levels of cyclic AMP produced by a blue-light- In Chlamydomonas (Figure 1b) the eyespot apparatus activated adenylyl cyclase (PAC).10,11 Similarly, whereas consists of thylakoid membranes with layers of negative phototaxis in Chlamydomonasis is modulated carotenoid rich globules and photoreceptor molecules in by PAC, photoattraction occurs through the action of the membranes between these globules (Figure 3a). two sensory rhodopsins CSRA and CSRB discovered Chlamydomonas has a flavin-associated blue-sensitive by Sineshchekov et al in 2002,12 also called channel- chromophore,6 highly sensitive to blue light at a rhodopsins, ChR1 and ChR2 by Nagel’s group,13 and wavelength of 440 nm, intimately linked to its circadian termed Acop1 by Susuki et al.14 The two photoreceptor clock. Although this algal species is essentially a plant, proteins have different abosrption profiles with CSRA genetic analysis of the chromophore shows what has absorbing predominantly at blue–green wavelengths and previously been considered an animal chromophore promoting a photophobic response in response to strong CRY2.7 Indeed CRY1 and CRY2 have been detected in illumination while CSRB absorbs at shorter wavelengths light-sensitive ganglion cells in the human retina and leads to phototaxis at weaker light intensties.15 There responsible for pupillary light reactivity and setting of are around 9 × 104 molecules of CSRA and 1.5 × 104 photocycles.8 molecules of CSRB in each cell.16 ChR2 is a How nice it would be to see these two organisms as photoactivated cation channel but in addition acts as a fore-runners of on the one hand the rhodopsin-based light-driven proton pump. Although this double action photodetection systems of mammalian rod and cone might seem somewhat perverse it is not unique—other photoreceptors and on the other precursors of the more chloride channels also act as proton pumps, recently discovered blue-wavelength detection systems in demonstrating their origin as Cl−/H+ exchangers. But the light-sensitive ganglion cells. Sadly, evolution is never as ChRs do not exhibit homology with other ion channels simple as that. For a start each of these organisms has but rather with rhodopsins. Indeed, these photoactive ion multiple light detecting molecules. Euglena orientates channels did not start their evolutionary existence in the itself towards light using a rhodopsin photopigment, but Euglenoids and Chlamydomonads we have been discussing also has a blue-light receptor.9 The photo-reorientation of above. Such eukaryotes originated probably between 800 Eye Evolution of vision DL Williams 176 Figure 3 Photoresonses in Chamydomnas reinherdtii. The photoreceptive proteins reside in between thylakoid membranes (arrows) with carotenoid-rish granules (asterisks) between them. A photorelectic discharge is seen after a light impulse and this is maximal when correlating with the natural helical motion of the alga. and 1200 million years ago,17 but prokaryotes have been existing in much higher copy number than the other present in the fossil record around 1500 million years proteins, can act on its own as shown by producing blind earlier.18 Lynn Margulis presented the theory by which mutants and recovering photosensitivity by reconstituting eukaryotes form through endophagy of prokaryote BR alone.23 BR is a purple molecule, with an absorption algae to produce chloroplasts and bacteria to yield maximum in green wavelengths, hyperpolarising the cell mitochondria, both examples of endosymbiosis.19 membrane at around 570 nm, whereas HR absorbs at So we should look for the origin of these eukaryote green–yellow wavelengths, depolarising the membrane. photopigments in prokaryote bacteria.20 The BR gene has limited sequence homology with other photoreceptive rhodopsins such as those in mammalian rod photoreceptors,20 but does have the structural Photopigment origins similarities of the seven transmembrane domains24 and The photoresponsive prokaryotes to investigate are both a protonated Lys-216 Schiff base where the prosthetic those that are photosynthetic such as Synechocystis and group of retinal binds. The big difference is what while those like Halobacterium that are not. Halobacterium eukaryote rhodopsins are associated with a G protein,25 species21,22 have four photosensory proteins; prokaryote rhodopsins are not. It had been considered Bacteriorhodopsin (BR) a proton pump, Halorhodopsin that eukaryote and prokaryote rhodopsins were a prime (HR), a light-gated chloride pump, and two sensory example of evolutionary convergence,26 but structural rhodopsins.
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