Transcript Model Genetic Organisms

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.