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Flies are quick!. 3 head. The fly body plan: each segment has a unique identity and produces distinctive structures. 3 thorax. 8 abdomen. Model Organisms: Drosophila. small (adult < 5 mm long). Can keep hundreds in a small vial. short generation time - 8 days

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Slide2 l.jpg

3 head

The fly body plan:each segment has a unique identity

and produces distinctive

structures

3 thorax

8 abdomen


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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


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Thomas Hunt Morgan

and the white eye mutant

wildtype fly

white mutant


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Christiane Nüsslein-Volhard and Eric Wieschaus

used genetics to identify proteins that

set up the embryonic body plan


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wildtype

Wieschaus and

Nüsslein-Volhard

looked for mutants

that affect the

fly body plan


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The fruit fly body plan is self-assembled

in 24 hours: how is it specified?


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Anterior-Posterior Pattern Formation in Flies

Maternal effect genes

Figure 9.17


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Oocyte

Anterior

Posterior

Determinant

bicoid mRNA

Determinant

nanos mRNA

Maternal effect genes establish the

anterior/posterior axis of the embryo

nurse cells


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bicoid protein accumulates

in a gradient

head

tail

A

P

100

Level of

bicoid

0


Remember that cleavage starts without cell division in drosophila superficial cleavage l.jpg
Remember that cleavage starts without cell division in Drosophila (superficial cleavage)

Fig. 9.1

Syncytial specification: specification by interactions between

cytoplasmic regions rather than cells


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A gradient of the bicoid transcriptionfactor turns on different genes at different "thresholds"

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


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bicoid mutants have no head!!

wildtype larva

bicoid mutant

Figure 9.23


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

Hunchback

Krüppel

Knirps

Expression pattern of proteins encoded by gap genes


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Gap gene mutants are missing

different regions of the body


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

Krüppel

Hunchback

Fig. 9.17


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The transcription factors encoded by gap genes cooperate

to create even more complex patterns of gene expression

Expression domain of

Hunchback

Expression domain of

Krüppel

The expression domains

of Hunchback and Krüppel overlap

Some genes require

both Hunchback and Krüppel

present to be turned on


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

Alberts Chapter 8 (p. 266)


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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 flies20 l.jpg
Anterior-Posterior Pattern Formation in Flies

Maternal effect genes

Figure 9.17


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wildtype

Antennapedia mutant



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Fig. 9.28

Wildtype

Ultrabithorax mutant

Figure 9.36


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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


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Does Ultrabithorax bind DNA

and regulate genes specific for T3 and A1?

Experiment #2

Ultrabithorax is expressed in the region of the embryo that will become the

3rd thoracic and 1st abdominal segments

In these segments, the Ultrabithorax protein acts as a transcription factor, turning on genes specific for the 3rd thoracic and 1st abdominal segments

ON

OFF

T3 specific gene

ON

OFF

A5 specific gene

T1 specific gene

A1 specific gene



Antennapedia expression is negatively regulated by the bithorax complex homeotic proteins l.jpg
Antennapedia expression is negatively regulated by the Bithorax complex homeotic proteins

ANT-C

BX-C

Fig 9.35


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All abdominal segments take on a T2 identity if the bithorax complex is deleted

UbxabdA AbdB

triple mutant

Wildtype

T2

T3

A1

T2

T2

T2

A8

T2


Bithorax complex homeotic proteins l.jpg
Bithorax complex homeotic proteins complex is deleted

ANT-C

BX-C

Fig 9.35


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Ultrabithorax, abdA, and AbdB complex is deleted

normally repress expression

of the thorax-specific “leg gene” Distalless

in the abdominal segments

wild-type

UbxabdA AbdB triple mutant

abdomen

T1 T2 T3


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Lewis hypothesized complex is deleted

that the duplication

and diversification

of homeotic

master regulators

underlies the evolution of

an increasingly complex

body plan


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The human body complex is deleted

is also

built up from

reiterated units (segments)

with different

identities along

the A/P axis


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Mammals also have complex is deletedhomeotic genes

expressed at different places along the A/P axis


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Mouse homeotic genes complex is deletedalso 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

OFF

15th rib specific

ON

gene

ON

12th rib specific

OFF

gene

13th rib specific

4th rib specific

gene

gene


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Notch and the complex is deleted

competition to be a neuron


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I feel the need to be complex is deleted

a neuroblast!

you guys stay here and keep up the good work!

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

They started as one big happy ectodermal epithelium…

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


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When the story takes a turn for the worse … complex is deleted

the fly neurogenic mutants

(mastermind, big brain, notch, delta)

If signal is missing...

Some cells become neuroblasts

and signal their neighbors to remain epidermis

all cells eventually ingress and become neuroblasts

Nervous system

Extra nervous

system

Epidermis

No epidermis!


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+ complex is deleted

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Cells lacking signal behave differently than

cells lacking receptor

Thanks, I

needed that!

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If mutant cells lack signal,

they can be rescued by wild type

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neighbors which make signal.

What? I can't

hear you!

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If mutant cells lack receptor,

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they cannot be rescued by wild type

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neighbors which make signal.


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+ complex is deleted

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Cells lacking signal behave differently than

cells lacking receptor

Thanks, I

needed that!

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DELTA mutant cellscan be rescued

by wild type neighbors.

Therefore, DELTA must be theSIGNAL.

+

What? I can't

hear you!

+

NOTCH mutant cellscannot be rescued

by wild type neighbors.

Therefore, NOTCH must be theReceptor.

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+


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neuroblast complex is deleted

epidermis


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neuroblast complex is deleted

After binding Delta, the cytoplasmic domain of Notch undergoes proteolytic cleavage

epidermal-specific genes

Figure 6.26


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