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 Morgan
and 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
wildtype
Wieschaus and
Nüsslein-Volhard
looked for mutants
that affect the
fly body plan
The fruit fly body plan is self-assembled
in 24 hours: how is it specified?
Anterior-Posterior Pattern Formation in Flies
Maternal effect genes
Figure 6.17
Maternal effect genes establish the
anterior/posterior axis of the embryo
nurse
cells
Oocyte
Anterior
Determinant
bicoid mRNA
Posterior
Determinant
nanos mRNA
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)
Fig. 9.1
Syncytial specification: specification by interactions between
cytoplasmic regions rather than cells
A gradient of the bicoid transcription factor 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
bicoid mutants have no head!!
wildtype larva
bicoid mutant
Figure 6.24
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
The gap genes depend on each other to
form boundaries and provide identity to
unique regions where they overlap
Hunchback
Krüppel
Fig. 6.17
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
Pair-rule genes, such as Even-skipped,
refine the segments
See Fig. 6.17D for beautiful localization of another pair-rule gene, Fushi tarazu
The segment-polarity gene Engrailed is activated
by the Even-skipped and Fushi tarazu pair-rule
transcription factors
Figure 6.33
Anterior-Posterior Pattern Formation in Flies
Maternal effect genes
Figure 6.17
wildtype
Antennapedia mutant
See Fig. 6.37 for a close-up
of an Antennapedia mutant
Ed Lewis was far ahead of his time …
Fig. 6.35
Wildtype
Ultrabithorax mutant
Figure 6.36
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
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
T1 specific gene
T3 specific gene
ON
A1 specific gene
OFF
A5 specific gene
The Homeotic genes in Drosophila
ANT-C
BX-C
Fig 6.35
Antennapedia expression is negatively regulated
by the Bithorax complex homeotic proteins
ANT-C
BX-C
Fig 6.35
All abdominal segments take on a T2
identity if the bithorax complex is deleted
Wildtype
Ubx abdA AbdB
triple mutant
T2
T3
A1
T2
T2
T2
A8
T2
Bithorax complex homeotic proteins
ANT-C
BX-C
Fig 6.35
Ultrabithorax, abdA, and AbdB
normally repress expression
of the thorax-specific “leg gene” Distalless
in the abdominal segments
wild-type
T1 T2 T3
Ubx abdA AbdB triple mutant
abdomen
Lewis hypothesized
that the duplication
and diversification
of homeotic
master regulators
underlies the evolution of
an increasingly complex
body plan
The human body
is also
built up from
reiterated units
(segments)
with different
identities along
the A/P axis
Mammals also have homeotic genes
expressed 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
ON
OFF
4th rib specific
gene
12th rib specific
gene
ON
13th rib specific
gene
OFF
15th rib specific
gene
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!
then one of their number got some big ideas
and started to ingress inside…
you guys stay here and
keep up the good work!
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)
Some cells become neuroblasts
and signal their neighbors to
remain epidermis
If signal is missing...
all cells eventually ingress
and become neuroblasts
Nervous system
Epidermis
Extra nervous
system
No epidermis!
Cells lacking signal behave differently than
cells lacking receptor
Thanks, I
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If mutant cells lack signal,
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mutant
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neighbors which make signal.
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If mutant cells lack receptor,
they cannot be rescued by wild type
neighbors which make signal.
Cells lacking signal behave differently than
cells lacking receptor
Thanks, I
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DELTA mutant cells can be rescued
by wild type neighbors.
Therefore, DELTA must be the SIGNAL.
What? I can't
hear you!
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NOTCH mutant cells cannot be rescued
by wild type neighbors.
Therefore, NOTCH must be the
Receptor.
neuroblast
epidermis
After binding Delta,
the cytoplasmic
domain of Notch
undergoes proteolytic
cleavage
Figure 3.33
neuroblast
epidermal-specific genes