Download Early cleavage stages

Drosophila and Syncitial
Specification
Early Cleavage Stages
• Syncytial blastoderm
– Superficial cleavage
– 13 cycles of rapid nuclear divisions (8 min)
– Each nucleus surrounded by microtubules and
microfilaments
– Future germ cells cellularize first (pole cells)
• Cellular blastoderm
– 6000 cells
– 4 hrs. post fertilization
Nuclear Migration
Segmentation of Embryo and Adult
What Specifies A-P Segmentation?
• How does this pattern arise?
• How are anterior-posterior and dorsal-ventral axes
established?
• How is a segmented pattern generated along the
anterior-posterior axis?
• How is each segment differentiated from the
others?
What is the Evidence for Early
Polarity
• Anterior half and posterior half of embryo have different
potentials shortly after fertilization
– ligation experiments
• Cut in half gives ends but no middles
– “organizing centers” at anterior and posterior ends?
– interference with anterior organizing center
• UV or RNase
• embryos lack head and thorax
• develop mirror symmetric abdomen and ends
• Suggests “morphogens” that exist in concentration
gradients
• What are the morphogens?
How Do You Find the
Morphogens Genetically?
• Christiane Nusslein-Volhard began a genetic approach
(1980s)
• Identify (screen for) mutations that cause defects in
embryonic polarity and classify
– Maternal-effect
• bicoid, hunchback (embryo with no head and thorax region)
• nanos, caudal (embryo with no posterior abdominal region)
• torso, torso-like (embryo with no ends)
– Zygotic-effect
• Regulated by above
A Maternal Anterior
Determinant: Bicoid
• bicoid mutant
phenotype:
• bicoid mRNA is
placed into the
oocyte
asymmetrically
• mRNA attaches
to microtubules
in anterior
Oogenesis Polarity
• Adult ovariole:
Mechanism of Bcd
mRNA
Localization
How Does Bicoid Get To
Anterior?
• Oocyte gerkin protein tells terminal follicle cells to be
posterior
• Posterior follicle cells signal to activate protein kinase A in
oocyte membrane
• Controls MT orientation (+ end toward posterior)
• Bicoid mRNA 3’ UTR binds to exuperantia and swallow
that bind to dynein
– go to anterior
• (Other mRNAs bind to kinesin)
– to go to posterior)
Where is Bicoid Protein Found?
Protein gradient is more extensive than mRNA
Translation occurs after fertilization
Comparison of Gradients
What Happens if mRNA Is Misplaced?
• Evidence that BCD is sufficient to initiate anterior
development:
What Happens If Gradient Shape
Is Altered?
• Evidence that BCD
concentration is important
for placement of segments
• swallow and exuperantia
mutants:
– BCD gradient is not as
sharp
• Or overexpress bcd
ant
post
wt
A H T Ab Te
exu
H
T
Ab Te
A Maternal Posterior
Determinant: Nanos
• nanos phenotype is opposite
of bicoid:
– Required for posterior
development
• mRNA is
asymmetrically
localized
– At posterior end
– Kinesin bound oskar
mRNA and staufen protein
– Staufen allows translation
of oskar which binds
nanos mRNA at posterior
– Nanos protein gradient
What Do Morphogens Do?
• bicoid and nanos regulate gene expression of other
maternal RNAs:
– hunchback
– caudal
What Do Morphogens Do?
• Bicoid inhibits
translation of caudal
mRNA
• Nanos inhibits
translation of
hunchback
Setting Up AP Gradients
What Do Morphogens Do?
• 2 ant -> post
gradients
• 2 post -> ant
gradients of
proteins
• bicoid, caudal,
hunchback act as
transcriptional
regulators of
zygotic genes
A Third Set of Maternal mRNAs:
Terminal Genes
• Terminal group: torso and torso-like
– mutants lack extreme anterior and posterior
structures (acron and telson)
What Type of Protein is Torso?
• Made in ovarian cells and mRNA placed
into oocyte
– torso RNA and protein are evenly distributed
– torso = RTK, a transmembrane protein
• Constitutive mutant (always on) converts
whole anterior to acron and posterior to
telson
– So must be spatially activated
What Activates Torso?
• Torso-like
• Not expressed in
oocyte, but in follicle
cells
• Only in anterior
and posterior
• Protein secreted and
activates torso locally
What Does Torso Signal?
• Torso RTK causes phosphorylations that
ultimately inactivate groucho protein
• Groucho is transcriptional inactivator of
some zygotic genes needed for specification
of ends
• Inhibiting an inhibitor activates