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

2. Evolutionary Conservation of Hox Expression Patterns

3. Evolutionary Conservation of Neural Induction
Inverted-brate
Hypothesis

4. 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

6. Fly Eye

7. 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

8. 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

9. Formation of the Vertebrate Eye

10. Comparative Eye Development
Fly
Vertebrate
Squid

11. 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

12. 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 :
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.

13. 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

14. 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.