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I. Overview of photoreception and vision II. Photoreception in Lower Invertebrates

Photoreception and Vision. I. Overview of photoreception and vision II. Photoreception in Lower Invertebrates III. Photoreception in Protostomes A. Mollusc camera eyes B. Arthropod compound eyes IV. Evolution of Eyes. The process involves…. Transformation.

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I. Overview of photoreception and vision II. Photoreception in Lower Invertebrates

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  1. Photoreception and Vision I. Overview of photoreception and vision II. Photoreception in Lower Invertebrates III. Photoreception in Protostomes A. Mollusc camera eyes B. Arthropod compound eyes IV. Evolution of Eyes

  2. The process involves….. Transformation (biochemical energy) Reception Transduction (photosensitive molecule) (nerve impulse) Light energy Integration Light energy Rhodopsin metarhodopsin + biochemical energy

  3. Types of photoreceptor cells (contain rhodopsin) Ciliary type: photosensitive membrane derived from or associated with cilia Rhabdomeric type: photosensitive membrane derived from microvilli, not cilia

  4. There has been some debate over the phylogenetic significance of receptor cell types But some Annelid larvae have ciliary receptors while adults have rhabdomeric

  5. II. Photoreceptors in “Lower Invertebrates” A. Pigmented epithelia: detect light intensity ex. Corals, most medusae; very small # cells

  6. II. Photoreceptors in “Lower Invertebrates” B. Eye cup or ocellus: pit of pigmented cells

  7. II. Photoreceptors in “Lower Invertebrates” B. Eye cup or ocellus: pit of pigmented cells; discern direction of light light

  8. II. Photoreceptors in “Lower Invertebrates” B. Eye cup or ocellus: pit of pigmented cells flatworms Also cnidaria, nemerteans etc.

  9. Phylum Nemertina: eye cups http://www.discoverlife.org/

  10. II. Photoreceptors in “Lower Invertebrates” C. Lensed Eyes: Cubomedusae • Tiny spherical lenses 100 um wide form sharp images free of spherical aberrations (better peripheral imaging) • However the focal plane lies behind the retina thus the image perceived is blurry.

  11. II. Photoreceptors in “Lower Invertebrates” C. Lensed Eyes: Cubomedusae Why should evolution have produced such sophisticated optics that have only poor resolution? Isn’t higher resolution always better? In each rhopalium Box jellies have two types of lensed eyes one looking upward and the other horizontally as well as two other simple eyes “Reverse neurobiology” : optics are known but functions are not

  12. III. Photoreceptors in Protostomes Many species in the molluscs and arthropods have only simple eyes or no eyes. Medial dorsal eye of copepod

  13. III. Photoreceptors in Protostomes But Mollusca and Arthropoda have independently evolved eyes that rank among the best: compound eyes camera eyes

  14. III. Photoreceptors in Protostomes A. Mollusca: - most with simple eye cups 2 - some snails with lensed, image forming eyes (e.g. queen conch) - bivalves without ocular organs except for the scallop Pecten (more on this) - most cephalopods have image forming camera eyes and acute vision

  15. III. Photoreceptors in Protostomes A. Mollusca: - most with simple eye cups 2 - some snails with lensed, image forming eyes (e.g. queen conch) - bivalves without ocular organs except for the scallop Pecten (more on this) - most cephalopods have image forming camera eyes and acute vision

  16. III. Photoreceptors in Protostomes A. Mollusca: 1. scallops have as many as 60 lensed eyes lining the edge of the mantle; in some species these eyes are elaborate and functionally unusual with mirrored membranes; “ glorified shadow detectors”

  17. III. Photoreceptors in Protostomes A. Mollusca: 2. nautiloids have a large pair of pin hole eyes

  18. III. Photoreceptors in Protostomes A.2. nautiloid eyes work by the same principle as a pinhole camera. Pin hole aperture inverted, sharp, but dim image

  19. III. Photoreceptors in Protostomes A.2. nautiloid eyes work by the same principle as a pinhole camera.

  20. III. Photoreceptors in Protostomes A. Mollusca: Lensed camera eyes of cephalopods form very sharp images cornea ‘pupil’ ‘sensory Retina ’ Optic lobe

  21. III. Photoreceptors in Protostomes Some differences between the cephalopod eye and the human (chordate) eye: Direct v. indirect eye arrangement Rhabdomeric v. ciliary chromophores Photoreceptors from nervous system in chordates but from epidermis in cephalopods cornea Focusing ability of cornea ( I.e. the refractive index of water and air) -- A more dense lens is needed to focus light originating in a fluid medium; complications in focusing

  22. Aquatic lens eye Cornea lens eye

  23. III. Photoreceptors in Protostomes B. Arthropod Compound eyes Amphipod crustacean with simple eye and compound eye

  24. Made up of hundreds to thousands independent optical units: ommatidia facets

  25. Single ommatidium Lens and cone help direct the light Retinula cells produce the photosensitive rhabdomes Pigment cells can keep light from moving into adjacent ommatidia “pupils”

  26. The performance of compound eyes • Erect, compound images • Wide field of vision • The higher the # of ommatidea the sharper the image • But no system to adjust focus; image is grainy • Whole eyes are sensitive to motion • In some species, lateral inhibition improves sensitivity

  27. Each facet must Represent 10,000 ommatidea for this compound eye to reach the visual acuity of the human eye

  28. Among Compound Eyes, the House fly eyes achieve best focus

  29. Among Crustaceans, mantis shrimp have the most elaborate eyes Sharp image Trinocular vision Use polarized light Infrared and UV sensitivity Can be pivoted on eye stalk, increasing the visual field to nearly 360 degrees

  30. IV. How many times did eyes evolve? 1977- Ernst Mayr and colleagues estimate at least 40 times, although 5 phyla have developed image-forming eyes (Chordates, Molluscs, Arthropods, Annelids, Cnidarians) : convergent evolution to vision (image formation)

  31. Eye cups (pits) • Basic compound • Aquatic lens • Corneal lens • Apposition compd • Superposition • Mirrored eyes • Reflecting super-position eyes

  32. IV. How many times did eyes evolve? 1977- Ernst Mayr and colleagues estimate at least 40 times, although 5 phyla have developed image-forming eyes (Chordates, Molluscs, Arthropods, Annelids, Cnidarians) : convergent evolution to vision (image formation) 1993 - Developmental geneticists discover “eye opening” gene called eyeless in fruit flies 1994 - similar gene found in mice and human eyes more recently even in squid eyes (pax-6 etc.) (90% similarity) 1995 - experiments in which eyeless wasactivated in other parts of fly body by human and mouse genes implanted in fly embryos

  33. When eyeless is turned on in parts of the body where it is inactive it could initiate development of topical eyes in unusual places... and in other species! Work of Walter Gehring and colleagues

  34. How many times did eyes evolve? Gehring calls eyeless a “master control gene” for eye development. Argues that its occurrence in many phyla indicates eyes evolved only once perhaps from a proto-eye: eyes are homologous Mayr counters that species of worms without eyes also have eyeless-like genes.Argues that these are genes that evolved long ago in other functions ( i.e. Nervous system ) then got co-opted into eye development several times as eyes evolved. Co-Option

  35. Dickinson: 1. same molecule can assume different functions 2. duplication produces paralogous genes whose members can encompass a wide number of roles 3. domain shuffling generates molecules with clear homologies but potentially different functions

  36. What Dan Nilsson (2004) calls “genetic promiscuity” in eye evolution (Current Opinion in Neurobiology) Kosmik et al. Expression of c-opsin in eye melanin is the shielding pigment of cubomedusa

  37. Compromise view proposed, but at present there is no consensus among researchers: Parallel evolution Photosensitivity is monophyletic at some level; pax, eyeless used in cell, eye development But image forming eyes evolved independently, each time incorporating developmental pax genes in the developmental process

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