I can’t waitto grow up!

Laughnow

The fruit fly body plan is self-assembled in 24 hours

How is the body plan specified?

Egg ---> animalin 24 hours!

Eric Wieschaus and Christiane Nüsslein-Volhardused genetics to identify proteins that

set up the embryonic body plan

The molecular genetics of pattern formation in candy corn

The wild-type pattern

Anterior

Posterior

We collected a series of mutants affecting the body plan

Wild-type

Anterior group

Gap gene

Posterior group

Morphogenesisdefects

The wild-type body is segmented

and each segment has a unique identityand thus produces

distinctive structures

Wieschaus and Nüsslein-Volhard

removed single genes and looked for effects

on the body plan

Wildtype larva

bicoid mutant

bicoid mutants have no head!!

Gilbert Fig. 9.13

Remember that cleavage starts without cell division in Drosophila

Nurse cells synthesize macromolecules and pump them into the oocytethrough the cytoplasmic bridges

oocyte

Molecules synthesized by the nurse cells and specifically localized in the oocyte determine

the anterior/posterior axis of the embryo

Posterior determinant

Anterior determinant

Oocyte

= bicoid mRNA = nanos mRNA

bicoid protein becomes distributed in a graded fashion

100

0

Level of bicoid

Increasing Bicoid concentration

Head Tail

Bicoid accumulates in a gradient

A gradient of the bicoid transcription factor turns on different genes

at different "thresholds"

100

0

Level of bicoid

Gene A- turned on only by high level

of bicoid

Gene B- turned on only by intermediate

level of bicoid

Gene C- turned off by bicoid and thus only on where bicoid is absent

A gradient of the bicoid transcription factor turns on different genes at

different "thresholds"

These “bicoid target genes”are known as the gap genes

Hunchback

Kruppel

Knirps

Expression patternof proteins ecoded

by gap genes

Hunchback Kruppel

Gap gene mutants are missing different regions of the body

Wildtype Krüppelhunchback knirps

The transcription factors encoded by gap genes cooperate to create even more

complex patterns of gene expression

Expression domain of gap gene transcription

factor A

The expression patterns of gap

gene transcription factors A and B

overlap

Some genes require both gap gene

transcription factors A and B present to be

turned on

Expression domain of gap gene transcription

factor B

Hunchback Kruppel

Alberts Chapter 8 title page

So, in summary so far…

Maternal Determinants

Zygotic Gap Genes

Zygotic Pair-rule Genes

Figure 9.8(1) Model of Drosophila Anterior-Posterior Pattern Formation

Isn’t life wonderful, beautiful and incredible!

Ed Lewis was far ahead of his time

who’s

Wildtype

Antennapediamutant

Wildtype Ubx

mutant

T2

A2

T3 and A1 take on a 2nd thoracic identity

T2

T2

WildtypeUbx abdAAbdB triplemutantT2

T3A1

A8

T2T2T2

T2

All segments take on a 2nd thoracic identity

Is Ubx is expressed at the right time and place to make in T3

different from T2?

Yes! Ubx is expressed in T3 and A1

Figure 9.28 Homeotic Gene Expression in Drosophila

The homeotic genes encode transcription factors that act as

master regulators of segmental identity

Ultrabithorax is expressed in the region of the embryo that will

become the 3rd thoracic and 1st abdominal segments

In these segments, Ultrabithorax protein acts as a transcription

factor that turns on genes specific for the 3rd thoracic and 1st

abdominal segments

A1 specific gene

A1 specific gene

A5 specific gene

T1 specific gene

OFF

OFFON

ON

Ultrabithorax, abdA, and AbdBnormally repress expression of the“leg gene” Distalless in the abdominal segments

Distalless in wild-type

Distalless in triple mutant

T1 T2 T3 abdomen

Lewis hypothesizedthat the duplicationand diversificationof homeotic master regulatorsunderlies the evolution ofan increasingly complexbody plan

The human body is alsobuilt up fromreiterated unitswith different identities alongthe A/P axis

Mouse homeotic genes also encode transcription factors that act as

master regulators of segmental identity

Hox 3.1 is expressed in the region of the embryo that will become the 12th and 13th ribs

In these segments, Hox 3.1 protein acts as a transcription factor that

turns on genes specific for the 12th and 13th ribs.

12th rib specific gene

13th rib specific gene

4th rib specific gene

15th rib specific gene

OFF

OFFON

ON

As we’ll see later, mammals also have homeotic genesexpressed at different places along the A/P axis

Peer group pressure: The story of the epidermal versus neural decision in the fruit fly

They started as one big happy ectodermal

epithelium

I feel a need to be a neuroblast

You guys stay here and keep up

the good work

Then one of their number got some big ideas

and started to migrate inside

As it left, it sent a message to its neighbors, telling them to stick with the epidermal fate

The failure of peer group pressure: Neurogenic mutants in the fruit fly

Nervous system

Epidermis

Extra nervous system

No epidermis

Some cells become neuroblasts and signal their neighbors to remain

epidermis

If signal is missing...

All cells eventually invaginate and become neuroblasts

Notch and Delta encode related transmembrane proteins but Delta lacks

a large intracellular domain

Delta

Lipid bilayer

Inside

Outside

Notch

+

+

+mutant

++

++

++

+mutant

mutant +

++

+

+

+

++

++

++

++

Cells lacking signal or lacking receptor behave differently

If mutant cells

lack signal, they can be rescued by

wild type neighbors which make signal.

If mutant cells lack receptor,

they cannot be rescued by wild type neighbors

which make signal.

+

+

+mutant

++

++

++

+mutant

mutant +

++

+

+

+

++

++

++

++

Thanks, I needed that!

What? I can't hear you!

+

+

+mutant

++

++

++

+mutant

mutant +

++

+

+

+

++

++

++

++

Cells lacking signal or lacking receptor behave differently

If mutant cells

lack signal, they can be rescued by

wild type neighbors which make signal.

If mutant cells lack receptor,

they cannot be rescued by wild type neighbors

which make signal.

+

+

+mutant

++

++

++

+mutant

mutant +

++

+

+

+

++

++

++

++

Thanks, I needed that!

What? I can't hear you!

DELTA mutant cells can be rescued by wild type neighbors. Therefore, DELTA must be the SIGNAL.

NOTCH mutant cells cannot be rescued by wild type neighbors. Therefore, NOTCH must be the Receptor.

A model for the cellular roles of Notch and Delta

Lipid bilayer

Outside

Inside

Delta = membrane-bound

signal"Don't do it-don't be a

neuroblast!"

Lipid bilayer

Inside

Outside

Neuroblast

Notch= receptor

Signaling pathway leading to epidermal

cell fate