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Лекция 3 (А. П. Перевозчиков). Развитие и эволюция глаз. Как эволюционировали глаза ?.

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Лекция 3 (А. П. Перевозчиков)

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  • Creationists often use the eye as a debate point to show that evolution is flawed, citing that the eye is too complex and perfect to have evolved. Darwin himself noted that To suppose that the eye could have been formed by natural selection, seems, I freely confess, absurd in the highest possible degree, (The Origin of Species). He follows, however, with the assertion that eyes could likely evolve from light sensitive neurons. This seems to be the case.


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1. (Camera Eye) () .


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  • 2.

    Found in the clam Pecten and a few ostracod crustaceans. This produces bright but reasonably hazy picture.

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    Pit or Cup eyes are found mainly in mollusks and can only resolve location of objects.


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  • Research by Dan-E Nilsson and Susanne Pelger indicates that it is in fact easier to estimate the number of generations necessary to evolve an eye than complex organs. This is because these changes can be viewed as quantitative local modifications to a pre-existing tissue.

  • In order to determine the number of generations needed to evolve an eye, Nilsson simply made calculations outlining the plausible sequence of alterations leading from a light sensitive spot to a fully developed lens eye.


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  • Nilsson assumed an organism with a light sensitive patch of cells resting on a dark pigmented background and placed in a selection for spatial recognition. The first method to create a spatial recognition is either for a depression to form in the center of the patch, or for the edges of the patch to constrict and raise. This cupping would allow for the vague correlation of light to position where the exposure of an area on the patch is dependent on its angle to the light source.


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  • This cupping evolution should first favor the formation of a depression in the patch, than the constriction of an aperture via the raising and constriction of the surrounding pigment epithelium. This results in a sunken eye cup that resemblesthat of some mollusks.


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  • This pinhole-like eye is not very good at resolving detail and creates a very dim image. Because of this, any change that improves clarity and illumination will be favored. The two routes of change for this would be the development of a lens, or the increase in the size of the eye. Increasing the size of the eye, however, presents physical problems and less acute vision than a lens would.


The nilsson and pelger theory of eye evolution

The Nilsson and Pelger Theory of Eye Evolution


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  • Nillson gauged number of 1% changes in structures in this diagram. The number of 1% steps comes out to 1829 necessary steps to progress from a light sensitive disk to a camera-eye.

  • But how many generations will that take? Prepare for math on the next slide.

  • I am so, so sorry.


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  • R=h2iVm

  • m=mean increase/decrease in a feature.

  • h2=heritability.

  • i=intensity of selection.

  • V=coefficient of variation=ratio between standard deviation and mean in a population

  • n=number of generations.

    h2=0.5 (common heritability), i and V both = .01 (low values for conservative estimation), therefore

    R=.00005m, so small variation and weak selection produce a .005% change per generation. So

    1.00005n=80129540 so n=363992 generations.


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  • So basically, it takes 363992 generations, roughly 364,000 years to evolve camera-type eyes given that reproduction occurs yearly and the brain of the animal can handle such visual processing.

  • Things to note:

  • Nilssons simulation does not take into account more specialized structures such as sclera and capillaries because they are not necessary for all types of camera eyes (gastropod mollusks lack these). This simulation also does not take into account the evolution of photoreceptors.


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  • Nilssons simulation demonstrated basic structural evolution, but what about genetic evidence? Did eyes evolve independently or is there one common ancestor for all eyes?

  • Get ready for some crazy, messed up, stuff.


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Next, the eyespot dimples inward. This increases visual acuity by allowing the eye to sense the direction the light is coming from better than a flat eyespot. Planarians (flatworms) have such dimpled eyes


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Around this point the pit begins to fill with a clear jelly-like material. It is thought that producing this jellywould be rather simple for most creatures - probably no more than one or two mutations. It is suggested that this jelly or slime helps to hold the shape of the pit, and helps to protect the light sensitive cells from chemical damage. And, the jelly might also keep mud and other debris out of the eye


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Next, a lens is needed. To get a lens, a ball-shaped mass of clear cells with a slight increase in the refractive index is needed. Once this mass is formed, it can be refined with very slight increases in the refractive index to produce greater and greater visual acuity An examples of such an eye with a "primitive" lens is found in the Roman garden snail (Helix aspersa)or slug


Pax 6

PAX-6

  • Prior to 1993 all evidence pointed to independent evolution of the eye. Then, while looking for transcription factors in fruit flies Walter Gehring and Rebecca Quiring discovered a gene nearly identical to the PAX-6 gene in mice and Aniridia in humans. All of which control the expression of eyes in a major way.

  • Mutations in these analogues can truncate the development of eyes in mice and cause serious defects in the human eye.

  • Could this be evidence against independent evolution of the eye? Asked Gehring. In order to find out he created


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Kumar moses 2001

, Kumar, Moses, 2001


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  • In situ hybridisation, knockouts, expression of dominant-negative transgenes, and application of growth factors in tissue culture all suggest that fibroblast growth factors, FGFs, form part of the distalising signal from the surface ectoderm.

  • FGFs upregulate neural retina genes and downregulate RPE genes.

  • Extra-ocular mesenchyme (the cells surrounding the optic cup), upregulate genes specific for the RPE.

  • E10.5 mouse embryo - neural retina is composed of a field of undifferentiated retinal progenitor cells (RPCs).

  • All RPCs express a common suite of transcription factors:

  • Pax6, Rx1, Six3, Six6, Lhx2, Hes1.

  • They are multipotent and can differentiate into ganglion cells, bipolar, amacrine, horizontal cells, photoreceptors and Mller glia


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  • Marquardt et al. (2001) Cell 105, 43-55.

  • (Retina-specific KO of Pax6, showed Pax6 is required for maintaining this multipotency. Pax6-/- cells can only become amacrine neurons.

  • Regionally restricted patterns of expression of transcription factors imposes dorso-ventral and naso-temporal specificity in cells within the developing optic cup.

  • Nicole Baumer et al. (2002) Pax6 is required for establishing the naso-temporal and dorsal characteristics of the optic vesicle. Development 129, 4535-4545.

  • Maureen A. Peters and Constance L. Cepko (2002) The dorsal-ventral axis of the neural retina is divided into multiple domains of restricted gene expression which exhibit features of lineage compartments. Dev. Biol. 251, 59-73.


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  • Pax6 the master regulatory gene?

  • Required in many tissues throughout eye development from very early stages.

  • Loss of function leads to loss of eyes in mice and flies.

  • Expression is conserved in eyes in many different metazoan phyla with many different designs of eye, incl. octopus, clams, photosensitive ocellus of Ascidians and sense organs of nematodes.

  • Ectopic expression in leg/wing/halteres/antennae imaginal discs of Drosophila leads to formation of ectopic eyes (i.e. Pax6 is sufficient to override the genetic programming of imaginal discs and make them form eyes). These eyes are functional in some cases.

  • Get similar dramatic effects in vertebrates, ascidians, squid (ectopic expression gives ectopic eye structures)..


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, . PAX6: eyeless, twin of eyeless, eyegone = mouse/human Pax6.EYA: eyes absent = mouse/human Eya1, Eya2, Eya3, Eya4.SIX:sine oculis/D-Six4 = mouse/human Six1, Six2 / Six4, Six5.Optix = mouse/human Six3, Six6.DACH:dachshund = mouse/human Dach1, Dach2.


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  • In vertebrates, although homozygous mutations in Pax6 lead to failure of eye development, loss of function of any single member of the EYA, SIX and DACH families does not (may get milder eye abnormalities). Redundancy?

  • Even in Drosophila not all tissues that normally express the PAX6, EYA, SIX, DACH genes go on to form eyes and when these genes are ectopically expressed in leg or wing imaginal discs, only a subset of the cells go on to form eyes - requires other signals e.g expression of decapentaplegic (= BMP2/4).

  • The PAX6, EYA, SIX, DACH interaction might be a conserved regulatory network that can drive differentiation of many tissues, with specificity depending on other extrinsic or intrinsic signals.


W gehring 2002

W.Gehring2002


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. . (Photograph courtesy of T. Venkatesh.)


Pax6 w gehring 2002

( Pax6)( W. Gehring2002)


Flytato

Flytato: ( ), .

  • By turning on this gene, dubbed eyeless in developing cells that do not normally express it, it caused the fly to develop EXTRA EYES IN ODD PLACES. AS DID THE ADDITION OF THE PAX-6 AND ANIDIRIA GENES!


Flytato1

Flytato:

  • Extra eyes ARE light sensitive, ARE NOT wired into the brain like normal eyes.

  • Is this evidence for a single origin of the eye? MAYBE.

  • Ernst Mayr contests many eyeless organisms have similar genes.

  • Mayr believes that this gene was originally part of a group of genes that shape the nervous system. As different organisms evolved, its role shifted.

  • PAX-6 also regulates expression of the nose in mice and the production of tentacles in naughty children squid.


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  • The Apposition Eye

  • Ommatidia function independently.

  • The Superposition Eye

  • Ommatidia cooperate to produce a brighter, superimposed image on the retina.


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Differentiation of photoreceptors inthe Drosophila compoundeye. The morphogenetic furrow (arrow) crosses the disc from posterior (left) to anterior (right). (A) Confocal micrograph of a triple-labeled late larval eye/antennal imaginal disc,showing hairy expression in green ahead of the morphogenetic furrow (arrow). Within the furrow, the Ci protein (red) is expressed as a consequence of the Hedgehog signal. (It will activate the decapentaplegic gene.) The neural specific protein, 22C10, is stained blue in the differentiating photoreceptors behind the morphogenetic furrow. (The bluehorizontal line of staining is Bolwegs nerve.) (B) Behind the furrow, the photoreceptor cells differentiate in a defined sequence. The first photoreceptor cell to differentiate (shown in blue) is R8. R8 appears to induce the differentiation of R2 and R5, and a cascade of induction continues until the R7 photoreceptor is differentiated. (A, photograph courtesy of N. Brown, S. Paddock, and S. Carroll; B after Tomlinson 1988.)


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Wolff, T. and Ready, D. F. (1993). . In: The Development of Drosophila melanogaster. Cold Spring Harbor Laboratory Press. Vol. 2 Pp. 1277-1325


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Summary of the major genes known to be involved in the induction of Drosophila photoreceptors. For development to continue beyond the differentiation of the R8, R2, and R5 photoreceptors, the rough gene (ro) must be present in both the R2 and R5 cells. For the differentiation of the R7 photoreceptor, the sevenless gene (sev) has to be active in the R7 precursor cell, while the bride of sevenless gene (boss) must be active in the R8 photoreceptor. From Gilbert, 2003; After Rubin 1989.)


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  • There are two basic types of eyes, the Simple and Compound eyes.

  • Simple eyes include the Pinhole Eye, the Concave Mirror Eye and the Positive Lens Eye.

  • Compound eyes are composed of multiple Ommatidia and have Apposition types and Superposition types.

  • Two types of photoreceptors are believed to have evolved from a proto-receptor, Rhabdomeric and Ciliary.

  • Nillson demonstrated how the structure of the eye could evolve from a light sensitive region to a Camera Eye structure in less than half a million years.

  • PAX-6, Aniridia and eyeless are relatively analogues genes that control the expression of eyes.

  • Flytato?


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Ciliary photoreceptors require transducin, a member of the Gi/o-family of G-proteins, whereas rhabdomeric photoreceptors use a member of the Gq/11-family of G-proteins


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  • Melanopsin's strong homology with invertebrate opsins and the depolarizing light response of ipRGCs suggests they may use a rhabdomeric phototransduction cascade. However, early patch clamp and pharmacological studies of ipRGCs could not directly confirm this hypothesis.

  • This was most likely due to the whole-mount retina recording configuration often used in studying ganglion cell function. The combination of photosensitive ipRGC dendrites buried deep within the IPL, and a membrane sheath covering the ganglion cell bodies, can create a significant diffusion barrier for pharmacological agents, especially hydrophobic agents commonly used to study transduction mechanisms. To overcome this hurdle, Graham et al. recorded from dissociated ipRGC cell bodies in culture to study the intracellular phototransduction cascade.

  • Isolated ipRGCs survive remarkably well in culture, generating robust light responses for up to 6 days and allowing for excellent pharmacological manipulation. Using this system, they showed that ipRGC phototransduction follows a rhabdomeric-like phosphoinositide cascade, requiring a member (or possibly members) of the Gq/11 family of G-proteins and the effector enzyme phospholipase C (PLC) ( (Graham, Wong et al. 2008). I

  • n addition, the presence of specific Gq/11 and PLC isoforms was confirmed in ipRGCs using single-cell RT-PCR and immunocytochemistry, consistent with the pharmacological findings (Fig. 8) (Graham, Wong et al. 2008).


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Shubin tabin carroll 2009

Shubin, Tabin, Carroll, 2009

  • Do new anatomical structures arise de novo, or do they evolve from pre-existing structures? Advances indevelopmental genetics, palaeontology and evolutionary developmental biology have recently shed lighton the origins of some of the structures that most intrigued Charles Darwin, including animal eyes, tetrapodlimbs and giant beetle horns. In each case, structures arose by the modification of pre-existing geneticregulatory circuits established in early metazoans. The deep homology of generative processes and cell-typespecification mechanisms in animal development has provided the foundation for the independent evolutionof a great variety of structures.


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