Evolutionary conservation of patterning systems LECTURE 10: Evolutionary conservation of patterning systems 2nd Axis Claw Eye

Evolutionary Conservation of Hox Expression Patterns

Evolutionary Conservation of Neural Induction Inverted-brate Hypothesis

Patterning the Neuroectoderm in Flies and Mice Dorsal Dpp ind msh vnd dpp Drosophila Neuroectoderm Mouse Neural Tube Sonic Hh BMPs Msx Nkx2.2 Gsh

Fly Eye

Early Steps in Fly Eye Development Specification of the Furrow A P Growth and patterning of eye imaginal disc in larvae dpp hh eye disc in the embryo eyeless: A master eye gene? >1,000-X growth A P Eye Primordium 20-40 Cells

Fly Eye Development Patterning of the eye imaginal disc Sequential formation of photoreceptor clusters Furrow R 2 5 3 4 8 1 6 7 Photoreceptor Development R 8 7 Furrow dpp A P hh

Formation of the Vertebrate Eye

Comparative Eye Development Fly Vertebrate Squid

Is Eyeless/Pax6 a Master Gene for Eye Development? Arguments in Favor: 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. Wing Mis-expressing Human pax6 gene Eye Wild-type Wing

Is Eyeless/Pax6 a Master Gene for Eye Development? Arguments Against: Elimination of eyeless or pax6 gene function results in loss of more brain structures than eyes (e.g. completely headless flies). Normal Fly head Kronhamn et al., Development 2002 129: 1015-1026. 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.

Hox genes A/P Axis Abdomen Head Tail Mouth Anus/ Genitals Reconstructing the Common Ancestor of Flies and Humans Photosensitive organs Sensory Appendages? Eyespot? Sog/Chd Dpp/BMP4 Neural Ectoderm Non-neural Ectoderm D/V Axis Gills? Protrusions or appendages

The Question: Possible Answers: What Was So Great About Our the Common Ancestor? The Question: 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: 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.