Flies are quick!

The fly body plan: each segment has a unique identity and produces distinctive structures

3 head

3 thorax

8 abdomen

Model Organisms: Drosophila• small (adult < 5 mm long). Can keep hundreds in a small vial.• short generation time - 8 days• embryo develops outside the body in a short time - so can easily study development• history - scientists have been doing genetics and collecting mutations for many years (since 1910)• very cheap to keep

• reproducible anatomy• segmentation visible• many anatomical, developmental, & behavioral similarities to vertebrates

Small Genome = 120 Mb

Thomas Hunt Morganand the white eye mutant

wildtype fly

white mutant

Christiane Nüsslein-Volhard and Eric Wieschaus used genetics to identify proteins that

set up the embryonic body plan

Wieschaus and Nüsslein-Volhard

looked for mutants that affect the fly body plan

wildtype

The fruit fly body plan is self-assembled in 24 hours: how is it specified?

Anterior-Posterior Pattern Formation in Flies

Figure 9.17

Maternal effect genes

Maternal effect genes establish theanterior/posterior axis of the embryo

Posterior Determinantnanos mRNA

Anterior Determinantbicoid mRNA

Oocyte

nurse cells

bicoid protein accumulates in a gradient

100

0

Level of bicoid

A Phead tail

Remember that cleavage starts without cell division in

Drosophila (superficial cleavage)

Fig. 9.1Syncytial specification: specification by interactions between cytoplasmic regions rather than cells

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" bicoid

wildtype larva

bicoid mutant

bicoid mutants have no head!!

Figure 9.23

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

Hunchback

Krüppel

Knirps

Expression pattern of proteins encoded by gap genes

Gap gene mutants are missing different regions of the body

Hunchback Krüppel

The gap genes depend on each other to form boundaries and provide identity to unique regions where they overlap

Fig. 9.17

The transcription factors encoded by gap genes cooperate to create even more complex patterns of gene expression

Expression domain of Hunchback

The expression domains of Hunchback and Krüppel overlap

Some genes require

both Hunchback and Krüppel

present to be turned on

Expression domain of Krüppel

Alberts Chapter 8 (p. 266)

Pair-rule genes, such as Even-skipped, refine the segments

The segment-polarity gene Engrailed is activated by the Even-skipped and Fushi tarazu pair-rule transcription factors

Figure 9.33

Anterior-Posterior Pattern Formation in Flies

Figure 9.17

Maternal effect genes

wildtype

Antennapedia mutant

Ed Lewis was far ahead of his time …

Ultrabithorax mutant

Figure 9.36

Wildtype

Fig. 9.28

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

different from T2?

Yes! Ubx is expressed in T3 and A1

Experiment #1

Ultrabithorax is expressed in the region of the embryo that

will become the 3rd thoracic and 1st

abdominal segmentsIn these segments, the Ultrabithorax

protein acts as a transcription factor, turning on genes specific for the 3rd

thoracic and 1st abdominal segments

T3 specific gene

A1 specific geneT1 specific geneA5 specific gene

OFFOFF

ON

ON

Experiment #2

Does Ultrabithorax bind DNA and regulate genes specific for

T3 and A1?

The Homeotic genes in Drosophila

Fig 9.35

ANT-C BX-C

Antennapedia expression is negatively regulated by the

Bithorax complex homeotic proteins

Fig 9.35

ANT-C BX-C

Wildtype Ubx abdA AbdB triple mutant

T2T3A1

A8

T2T2T2

T2

All abdominal segments take on a T2 identity if the bithorax

complex is deleted

Bithorax complex homeotic proteins

Fig 9.35

ANT-C BX-C

Ultrabithorax, abdA, and AbdBnormally repress expression

of the thorax-specific “leg gene” Distalless in the abdominal segments

wild-type Ubx abdA AbdB 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 units (segments)with different identities alongthe A/P axis

Mammals also have homeotic genesexpressed at different places along the A/P axis

Mouse homeotic genes also encode homeodomain transcription factors that act as master regulators of segment 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

gene4th rib specific gene

15th rib specific gene

OFF

OFF

ONON

Notch and the competition to be a neuron

The story of the epidermal vs. neural cell fate decision in

Drosophila They started as one big happy ectodermal epithelium…

I feel the 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 ingress inside…

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

When the story takes a turn for the worse … the fly neurogenic mutants (mastermind, big brain, notch, delta)

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 ingress 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 behave differently than cells lacking receptor

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 behave differently than cells lacking receptor

+

++

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

epidermis

neuroblast

After binding Delta, the

cytoplasmic domain of Notch

undergoes proteolytic

cleavage

Figure 6.26 epidermal-specific genes

neuroblast