Genomes and

Proteomes Conn

ection

The Study of t

he Pax 6 & Eye

less

Gene

In this Chapter,

We talked about how specific genes influence the development of species with novel shapes and forms.

BUT HOW ABOUT HOW COMPLEX ORGANS COME TO EXISTENCE?

This is still a major challenge for evolutionary biologists.

The Complexity of the Eye in Vertebrates

• Ability to adjust focus.

• Let in different amounts of light.

• Detection of a Spectrum of colors.

Where do Complex Eyes Come From?

Darwin suggests:

Eyes have evolved from a simple structure through the process of descent through modification.

Components of a Simple Eye

Photoreceptor Cell

Pigment cell

Simple Eye Structure

The Simple Eye Cont.

Photoreceptor Cell

Type of nerve cell

Able to absorb light and respond to it

Pigment cell

Stops the light from going to the other side of the the Photoreceptor cell.

Allows the organism to sense light and the direction it comes from.

Primitive eyes give an organism an additional way to sense the environment.

Might help avoid predators or locate food.

Eye Evolution

Over time, eyes could be more complex by enhancing the ability to absorb different amounts of wavelengths.

Since Darwin, evolutionary biologists have wrestled with the concept of eye evolution.

This is because anatomically, researchers have discovered different types of eyes.

Evolutionary Biologists Luitfried von Salvini-Plawen and Ernst Myer proposed that eyes might have independently arisen many times during evolution.

UNTIL…!

This Notion Was

Accepted…

The Discovery of Pax6 Gene

The protein encoded by Pax6 is a transcription factor that controls the expression of genes that are involved in the development of the eyes of rodents and humans.

Mutation of Pax6 in Mice Small Eyes

Mutation of Pax6 in Humans Aniridia

Similarly the eyeless gene in flies (Drosophilia) control genes for its eye development.

These genes are homologous

Walter Gehrig•He and his colleagues were able to show that the expression of the eyeless gene in parts of the fly where it was normally inactive.

•Using genetic engineering techniques they were able to express the eyeless gene where the antennae should form.

•So What Happened?

• When the eyeless gene was expressed in the antennae region, a small eye formed there.

•Also, when the Pax6 gene was expressed in the leg region, an eye was formed there also.

Experiment Results

The Controversy

Since the discovery of the eyeless and Pax6 gene, many other homologs of this gene have been discovered in many other species.

Gehrig and his colleagues hypothesize with Darwin that all eyes do come from a common ancestral form .

BUT, others argue that Pax6 only control certain features of eye development and different types of eyes may have evolved independently.

More research still needs to be done.