Evolution of the Eye

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Evolution of the Eye

• Erin Broadus
• April 28, 2004
• BIOL 430

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Introduction

• Eye is a very complex organ.
• Many different types of specialized cells.
• Many Different Aspects
• Structure
• Development
• Cellular Level
• Molecular Level

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Structure of the Eye
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Lenses

• Lenses are made of cells from the ectoderm
  surface
• Vertebrates have lenses made up of high
  concentrations of proteins called crystallins a,
  ?, and ?.
• Invertebrates have lenses that are made up almost
  entirely of enzymes, specifically glutathione
  S-transferase.

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The Development of Eyes

• Vertebrates
• Retina made of multiple layers of ganglionic,
  bipolar, and photoreceptor cells.
• Evaginated the neural ectoderm
• Formed from transparent lens fiber cells

• Invertebrates
• The retina has only one layer of photoreceptor
  cells.
• Evaginated from the ectoderm
• Formed by cellular fusions

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The Development of Eyes
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Types of Eyes

• There are two main types of eye
• The Simple Eye The Compound
  Eye

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The Mysid Shrimp

• Could be the missing link between simple eyes
  and compound eyes
• Characteristics of both types of eye
• Simple part single lens which can focus
• Compounds part Consists of many facets which can
  magnify
• Both types of eye may be needed to provide the
  shrimp with vision that is straightforward and
  vision that can be magnified.

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How Eyes Work

• Sensitive to the wavelengths in the energy
  spectrum
• Wavelengths are used to gather sensory
  information

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Photon Capture
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Phototransduction

• Vertebrates use transducin which causes the cell
  to depolarize.
• Invertebrates, on the other hand, use inositol
  triphosphate which causes the cell to
  hyperpolarize.
• After the chromophore molecule gathers a photon,
  a transformation in the actual chromophore
  occurs.
• This causes the opsin molecule to be activated.
• Opsin then causes there to be an electrical flow
  within the eye.
• This electrical energy can then be used by the
  nervous system and brain.

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Phototransduction
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How Eyes Evolved

• Biologists claim that the eyes have evolved at
  least 40 different times
• 1st Stage Eyespots with very few photoreceptors.
  
• 2nd Stage The system that produces an image was
  added.

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The First Eye

• Simplest organism had eye spots made up of a few
  photoreceptor cells
• First organisms lived in very slimy water
• Only two wavelengths could penetrate the water
• Natural selection acted so that the eyes of these
  animals became sensitive to only these
  wavelengths
• Only useful for light detection

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Current Form of Eye

• Developed in the Cambrian period during the
  Cambrian explosion.
• Many animal species emerged from the water onto
  land.
• All of the wavelengths can be seen on land.
• Eyes improved at great deal because animals had
  to be able to see to avoid predators and to catch
  prey.

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The Human Eye
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Color Vision

• Natural selection played a role in the selecting
  color vision for many animals
• Allowed for better mating, increased prey
  detection, and increased scanning for enemies
• By changing the opsin molecule through amino acid
  changes, color vision is possible

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Molecular Genetics

• It was found that genes for eye development are
  found on every chromosome.
• Pax-6 has been isolated from humans, mice, rats,
  fowl birds, drosophilae and sea urchins.
• Pax-6 gene is a transcription factor that is
  attached to a homeobox.
• The first eye was most likely developed by the
  Pax-6 gene.

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Mutations in Pax-6

• Mutations in this gene cause the eye to lose
  certain structures.
• The mutation causes the eye to lose the function
  of detecting and absorbing wavelengths.
• Though the name of each condition is different
  for each species, the disease causes the same
  effects.

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Regulation of the Lens by Pax-6

• Pax-6 gene is also involved with the regulation
  and expression of crystallin genes, which form
  the lens.
• Vertebrate crystallins came from lactate
  dehydrogenase and some heat shock proteins.
• Pax-6 also involved in invertebrate lens
  formation but through different proteins.
• Suggests that the eye evolved from a common
  ancestor.

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Conclusion

• Photon Capture ? Different Ancestors
• Structure ? Different Ancestors
• Phototransduction ? Common Ancestor
• Eye Development ? Common Ancestor
• Pax-6 Gene ? Common Ancestor
• Overall, most evidence points to one common
  ancestor that was most likely an organism with
  simple eyespots or a few photoreceptor cells.

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References

• www.eyedesignbook.com/ ch2/fig2-01aBG.jpg
• http//mednews.stanford.edu/stanmed/2002summer/sto
  ry-images/eye-message.jpg
• http//cougar.slvhs.slv.k12.ca.us/pboomer/physics
  lectures/secondsemester/light/lenses/eye.jpg

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