Eye

Claw

2nd Axis

Evolutionary Conservation of Hox Expression Patterns

Inverted-brateHypothesis

Evolutionary Conservation of Neural Induction

Drosophila Neuroectoderm

Mouse Neural Tube

Sonic Hh

BMPsMsx Nkx2.2Gsh

Dorsal

Dpp

indmsh

vnd

dpp

Patterning the Neuroectoderm in Flies and Mice

A P/

Hh

Dpp

Hh Hh

BMP-4

A P

Fly wing disc

A/PAxis

Vertebrate Limb Bud

D/VAxis

P/DAxis

D

V

A P

NotchSignaling A P

DV

NotchSignaling

D

V

A P

Distalless

A P

DV

Distalless

Fly Eye

A P

Specification of theeye disc in the embryo

>1,000-X growth

eyeless: A master eye gene?

Eye Primordium20-40 Cells

Furrow

A P

Growth and patterning ofeye imaginal disc in larvae

dpphh

Early Steps in Fly Eye Development

R8R7

Sequential formation ofphotoreceptor clusters

Furrow

R2R5

R3R4R8 R1

R6 R7

873 51

PhotoreceptorDevelopment

Fly Eye Development

Furrow

A P

Patterning of the eye imaginal disc

dpphh

OpticVesicle

Brain

Outgrowth of the optic vesicle

Iris Lens

Retina

Photoreceptors

Cornea

FullyformedeyeOptic Nerve

Future lens

Future retina

Brain

Connectionto brain

Induction of the lens by retina

Reciprocal lens to retina inductionFuture lens

Future retina

Induction of the cornea by lens

Future retina

Future Cornea

Future lens

Non-neural ectoderm

Neuroectoderm

Formation of the neural tube

Neural Tube Epidermis

Formation of the Vertebrate Eye

Opticnerves

Lens

Photoreceptors

Epidermis

Invaginating eyeimaginal disc

Eyedisc

Morphogenetic furrow

Ant. Post.

873 51

PhotoreceptorDevelopment

Photoreceptorsinduce lens cells

Furrow

OpticVesicle

Brain

Lens

Retina

Cornea

Opticnerve

Photoreceptors

Iris

Future lens

Future retina

Brain

Future lens

Futureretina

Iris

Lens

Retina

Cornea

Opticnerve

Photoreceptors

Iris fold

Lens

Cornealfold

Lens fold

Eye primordium

Migrating lens cells

Future Retina

Comparative Eye DevelopmentFly Vertebra

teSquid

Is Eyeless/Pax6 a Master Gene for Eye Development?

Arguments in Favor:

Wild-type Wing

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.

Wing Mis-expressing Human pax6 gene

Eye

3) Mis-expression of fly or human pax6 genes in certain fly tissues (e.g. wing) result in formation of ectopic eyes.

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).

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.

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).

Hox genes

A/P Axis

AbdomenHead

Tail

Mouth

Anus/Genitals

Sog/Chd

Dpp/BMP4

Neural Ectoderm

Non-neural Ectoderm

D/V Axis

Photosensitiveorgans

SensoryAppendages?

Eyespot?

Gills?

Protrusions orappendages

Reconstructing the Common Ancestor of Flies and Humans

What Was So Great About Our the Common Ancestor?

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:

The Question:

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.