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2nd Axis Eye Claw Evolutionary Conservation of Hox Expression Patterns Evolutionary Conservation of Neural Induction Inverted-brate Hypothesis Patterning the Neuroectoderm in Flies and Mice Drosophila Neuroectoderm Dpp dpp msh ind vnd Dorsal Msx Gsh Nkx2.2 BMPs Mouse Neural Tube Sonic Hh Fly wing disc Vertebrate Limb Bud A/P A/P Axis P A Hh Hh Hh BMP-4 Dpp A P Notch Signaling D/V Axis A D V P D V Notch Signaling A P A P/D Axis D V Distalless P D V Distalless Fly Eye Early Steps in Fly Eye Development Specification of the eye disc in the embryo Growth and patterning of eye imaginal disc in larvae eyeless: A master eye gene? >1,000-X growth A P Eye Primordium 20-40 Cells Furrow A dpp hh P Fly Eye Development Patterning of the eye imaginal disc Sequential formation of photoreceptor clusters Furrow Photoreceptor Development R8 Furrow A dpp hh R2 R5 R3 R4 R1 R6 R7 R8 P 1 3 5 7 8 R7 Formation of the Vertebrate Eye Formation of the neural tube Reciprocal lens to retina induction Future lens Neuroectoderm Non-neural ectoderm Epidermis Neural Tube Future retina Induction of the cornea by lens Outgrowth of the optic vesicle Future Cornea Optic Vesicle Future lens Future retina Brain Cornea Induction of the lens by retina Iris Future lens Lens F u tu re retin a Retina Brain Connection to brain Photoreceptors Optic Nerve Fully formed eye Comparative Eye Development Fly Vertebrate Squid Is Eyeless/Pax6 a Master Gene for Eye Development? Arguments in Favor: 1) Decreased activity of pax6 genes results in reduced eye size in flies, mice, and humans. 2) pax6 genes are expressed in the early eye primordia of flies, humans, and squid, in which eyes were thought to have evolved independently. 3) Mis-expression of fly or human pax6 genes in certain fly tissues (e.g. wing) result in formation of ectopic eyes. Eye Wild-type Wing Wing Mis-expressing Human pax6 gene Is Eyeless/Pax6 a Master Gene for Eye Development? Arguments Against: 1) Elimination of eyeless or pax6 gene function results in loss of more brain structures than eyes (e.g. completely headless flies). Kronhamn et al., Development 2002 129: 1015-1026. Normal Fly head Fly Lacking eyeless Function 2) eyeless expression only induces eyes in certain tissues (e.g., wing). 3) Several other genes (sine oculus, eyes absent,daschund) play roles similar to pax6 in eye development. 4) The regulatory relationships between eye determining genes are different in flies versus vertebrates. Reconstructing the Common Ancestor of Flies and Humans Eyespot? Sensory Appendages? Head Abdomen Tail Anus/ Genitals Mouth A/P Axis Gills? D/V Axis Dpp/BMP4 Non-neural Ectoderm Hox genes Sog/Chd Neural Ectoderm Photosensitive organs Protrusions or appendages What Was So Great About Our the Common Ancestor? The Question: 1) The common ancestor of humans and flies must have lived in a complex eco-system with many other species, some of which ate it and others of which it ate. 2) The creature and its decedents somehow displaced all other animal forms. 3) Why?? What was so great about this animal?? Possible Answers: 1) The ancestor evolved a mechanism for extracting low levels of oxygen from the atmosphere and delivering them to internal tissues. 2) The ancestor evolved HOX genes, which allowed for the subsequent diversification of individual body parts.