Chapter 18: Developmental Genetics

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Chapter 18 Developmental Genetics Lecture
Goals 1. Understand basics of Drosophila
Development 2. Understand how the Anterior/
Posterior Axis is established. 3.
Understand how the Dorsal/Ventral Axis is
established. 4. Understand the early steps
in establishing polarity in the oocyte.
5. Understand vertebrate homologies of
Drosophila genes.
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All higher organisms begin life as a single
cell (fertilized egg or zygote). This cell is
largely self sufficient in becoming a complex
organism with integrated tissue and organ
systems, and in some cases complex neural
pathways that allow complex behavioral responses.
The beauty of studying developmental systems is
the challenge of unraveling this impossibly
complex process.
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All higher organisms begin life as a single
cell (fertilized egg or zygote). This cell is
largely self sufficient in becoming a complex
organism with integrated tissue and organ
systems, and in some cases complex neural
pathways that allow complex behavioral responses.
The beauty of studying developmental systems is
the challenge of unraveling this impossibly
complex process.
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Most of the early clues to understanding
development were from model systems.
a). Transplantation of the dorsal lip of the
frog embryo produced a twinned
tadpole. b). Transplantation of the limb bud ZPA
of the chick embryo produced a twinned
limb.
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Most of the early clues to understanding
development were from model systems.
a). Wild type fly. b). Mutations in the
Ultrabithorax (Ubx) gene gives rise to double
wings (actually double 2nd thorax). c).
Mutations in the Antennapedia (Ant) gene changes
the antenna into legs.
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Antennapedia mutation.
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Drosophila Development
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Developing Egg Chamber

• The germ line cell divides mitotically to produce
  an oocyte and 15 nurse cells.
• The nurse cells are interconnected via ring
  canals and synthesize the ooplasm.
• These 16 cells surrounded by about 1000 somatic
  follicle cells, which secrete the egg shell and
  help establish positional information within the
  oocyte.

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Formation of the Blastoderm

• Following fertilization, nuclear divisions are
  rapid producing over 500 nuclei in the first 2
  hours,
• The first cells to form are at the posterior end
  (pole cells). These cells, containing polar
  granules, will form the germ line cells.
• The nuclei continue to divide and migrate to the
  plasma membrane to form the syncytial blastoderm.
• After about 5 hours (and more than 5000 nuclei)
  the plasma membrane invaginates around each
  nuclei to form the cellular blastoderm stage
  embryo

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Gastrulation Furrows

• Shortly following the formation of the cellular
  blastoderm, two prominent furrows form
  (gastrulation).
• The ventral furrow (along the ventral midline)
  and the cephalic furrow about one sixth the way
  back from the anterior end.

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Larva Formation

• These events, morphologically establish the A/P,
  D/V and R/L axis of the embryo.
• By 10 hours, clear organizational pattern is
  established.
• By 24 hours a fully form and functional larva
  hatches out of the egg shell.

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How does the embryo obtain the positional
information that it needs to form anterior
structures at one end and posterior
structures at the other end? Dorsal/ventral?
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Establishing the Anterior/Posterior Axis

• Three proteins are initial important in helping
  to establish the anterior/posterior axis (bicoid,
  nanos, and hunchback-maternal).
• Bicoid is a transcription factor,
• localized to the anterior end
• of the egg and embryo in a
• steep gradient.
• Hunchback-maternal is also
• a transcription factor, which
• localizes to the anterior end
• of the egg and embryo but in
• a more shallow gradient.

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Proteins involved in the A/P Axis

• In contrast to bicoid and hunchback-maternal,
• nanos (nos) is localized to the posterior end
  of the
• egg and embryo.
• Nanos is a translational repressor protein.
• Mutants lacking nanos, are missing their
  posterior end.

NANOS
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Role of microtubules in mRNA distribution

• The mRNAs encoding both bicoid and nanos are
• tethered to the anterior end (bicoid) or the
  posterior end
• (nanos) via microtubules.
• Once translated the proteins are free to
  diffuse.

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Role of 3 UTR in mRNA localization

• The transport and anchoring of the Bicoid
  (anterior) and nanos (posterior) mRNAs is
  mediated by sequences in the 3 UTRs of their
  mRNAs.

If the 3 UTRs of the bcd mRNA is
attached to the nos mRNA, it causes nos
to be localized to the anterior end
where it inhibits bicoid and causes the
embryo have two tails.
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Bicoid is required for anterior structures

• Mutants that are lack bicoid (no anterior
  structures) can be rescued by injection of
  anterior cytoplasm from a normal egg, or by
  injection of bicoid mRNA.

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The effect of bicoid expression levels

• Mutants that contain excessive bicoid, increase
  the percentage of the embryo that develops into
  head structures. An early indication of this is
  the location of the cephalic furrow.

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Nanos translationally inhibits hb-m.

• Like bicoid, hunchback-maternal protein is
  also a
• transcription factor, which becomes localized
  to the anterior
• end of the egg and embryo.
• However, the hunchback-maternal mRNA is not
  tethered to the
• anterior pole but is distributed equally
  throughout the egg and
• embryo.
• Its translation however is
• inhibited the nanos
• protein at the posterior
• end the egg and embryo,
• thus eliminating hb-m at
• the posterior end of the egg.

NANOS
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Refining the A/P patterning The Gap Genes

• Kruppel (kr), is repressed by high levels of
  bcd but induced by low levels of bcd and hb-m.
• Knirps (kni), is repressed by any amount of
  bcd but requires low levels of hb-m.
• Hunchback-zygotic (hb-z), is expressed anywhere
  kr and kni are not.

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Refining the patterning

• These Gap genes in turn help specify the even
  more refined pattern of expression of the pair
  rule, segment polarity
• and segment identity (homeotic) genes.

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Even skipped stripe 2
The even skipped gene is activated in certain
areas (stripes) by enhancers and repressors that
read the amount of other proteins present. Low
levels of giant and kruppel, plus the presence of
hunchback and bicoid induce stripe 2.
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Even skipped stripe 2
Giant and kruppel repress expression.
Hunchback and bicoid enhance expression.
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Establishing the Dorsal/Ventral Axis

• Active Spaetzle (spz) protein
• is produce on the ventral side
• of the embryo (in follicle cells).
• It acts as a ligand and binds
• Toll (membrane receptor).
• This binding transduces a
• signal through the ooplasm
• that phosphorylates Dorsal.
• Dorsal is a transcription factor
• that specifies ventral fate.
• In absence of Dorsal there are
• no ventral structures.

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The Phosphorylation of Dorsal

• Dorsal is expressed in all areas but held
  inactive by cactus.
• The phosphorylation of Dorsal, releases it from
  cactus and allows it to move to the nucleus.
• Dorsal is a transcription factor that specifies
  dorsal/ventral patterning genes.

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Establishing the Dorsal/Ventral Axis
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Establishment of Polarity in the Oocyte

• How does the egg become polarized in the first
  place, how is it that bcd is anterior, nos is
  posterior and spz is ventral?
• A/P and D/V polarity seems to form very early in
  oocyte development.

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Establishment of Polarity in the Oocyte

• Initially the nucleus is positioned close to
  one end of the oocyte near the follicle.
• The oocyte nucleus has near it gurken mRNA.
• Gurken is secreted locally (membrane protein)
  and binds to the EGF receptor on the nearby
  follicle cells causing them to adopt a posterior
  fate.

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Establishment of Polarity in the Oocyte

• The establishment of posterior follicle cells
  induces a reorganization of the microtubules,
  pushing the oocyte nucleus dorsally.
• As it moves it exposes the dorsal follicle
  cells to gurken.
• This interaction inhibits the dorsal cells form
  releasing activated spaetzle limiting spz to the
  ventral side.

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Vertebrate homologies The vertebrate Hox
genes are similar in sequence and organizational
structure to the Drosophila HOM genes.
Likewise they are expressed in a anterior
to posterior specific manner.
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(No Transcript)
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Vertebrate homologies Drosophila Dorsal
regulation uses the same components as the
immunoglobulin gene regulation.
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• Sample Questions Chapter 18
• 2. Hunchback-maternal, hb-m, is
• A. a translational repressor.
• B. initially concentrated at the posterior
  end, but
• later becomes localized at the anterior
  end.
• C. is a transmembrane receptor that
  transduces a
• signal from nanos.
• D. is localized with the aid of microtubules.
• E. None of the above.

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• Sample Questions Chapter 18
• 2. Hunchback-maternal, hb-m, is
• A. a translational repressor.
• B. initially concentrated at the posterior
  end, but
• later becomes localized at the anterior
  end.
• C. is a transmembrane receptor that
  transduces a
• signal from nanos.
• D. is localized with the aid of microtubules.
• E. None of the above.

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Sample Questions Chapter 18 3. What happens to
the Drosophila larva if no bicoid protein is
present during development? A. It forms larva
with 2 heads. B. It forms larva with no head. C.
It forms larva with no ventral structures. D. It
forms normal larva but the adults are
deformed. E. It has no effect on the larva since
hb-m can serve the same genetic function.
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Sample Questions Chapter 18 3. What happens to
the Drosophila larva if no bicoid protein is
present during development? A. It forms larva
with 2 heads. B. It forms larva with no head. C.
It forms larva with no ventral structures. D. It
forms normal larva but the adults are
deformed. E. It has no effect on the larva since
hb-m can serve the same genetic function.
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Sample Questions Chapter 18 4. TRUE or FALSE
In the early Drosophila embryo, the
gurken protein is first present at the anterior
end then later on the dorsal side of the embryo.
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Sample Questions Chapter 18 4. TRUE or FALSE
In the early Drosophila embryo, the
gurken protein is first present at the anterior
end then later on the dorsal side of the embryo.