Axis Specification in Drosophila - University of...
Transcript of Axis Specification in Drosophila - University of...
Axis Specification
in Drosophila
Developmental Biology – Biology 4361
July 7, 2009
Drosophila Development - Overview
Cleavage
Fertilization
Gastrulation
Drosophila body plan
Oocyte formation
Genetic control of axis specification
Anterior-posterior
Dorsal-ventral
Segmentation genes
Homeotic genes
Drosophila Fertilization
Eggs are activated prior to fertilization.
- oocyte nucleus has resumed meiotic division
- stored mRNAs begin translation
Eggs have begun to specify axes by the point of fertilization.
Sperm compete with each other! Romano Dallai
Drosophila bifurca
Sperm enter at the micropyle.
- probably prevents polyspermy
Superficial Cleavage
Syncytial blastoderm stage
- zygotic nuclei undergo 8 divisions
- nuclei migrate to periphery
- karyokinesis continues
Cellular blastoderm stage
- following division 13, oocyte
plasma membrane folds inward
- partitions off each nucleus and
associated cytoplasm
- constricts at basal end
Superficial Cleavage in Drosophila
cellular blastoderm
Gastrulation
germ band
movementventral furrow
(primordial
germ cells)
anterior midgut
invagination
posterior midgut
invagination
Late Gastrulation / Segmentation
Establishing the Drosophila Body Plan
Segments form along the anterior-posterior axis, then become specialized.
Specification of tissues depends on their position along the primary axes.
A/P and D/V axes established by interactions between the developing
oocyte and its surrounding follicle cells
Head
segment
Thoracic
segments T1 – legs
T2 – legs & wings
T3 – legs & halteres
Drosophila Body Plan - Egg Stage
Translation leads to formations of patterning protein
(e.g. morphogen) gradient within the embryo.
Body axes are determined in the egg by distribution of
maternal mRNAs and proteins.
How are asymmetric distributions of messages and proteins
established in the egg?
Drosophila Development - Overview
Cleavage
Fertilization
Gastrulation
Drosophila body plan
Oocyte formation
Genetic control of axis specification
Anterior-posterior
Dorsal-ventral
Segmentation genes
Homeotic genes
Oocyte Formation (A-P, D-V Axes)
modified from Kalthoff, K. 2001.
Analysis of Biological Development.
Drosophila ovariole- oogonium divides into
16 cells
- 1 oocyte
- 15 nurse cells
- all interconnected
nurse cells contribute
mRNA, proteins
cytoplasm
Anterior-Posterior Axis Formation
Torpedo (RTK Gurken receptor) present on follicular cells
Gurkin binding results in “posteriorization” of follicles
- posteriorized follicles re-organize egg microtubules; (-) = anterior
Gurkin protein localized between nucleus and cell membrane
- Note – Gurken diffuses only a short distance
Nurse cells synthesize gurken (TGF-β family)
- gurken mRNA transported toward oocyte nucleus (in posterior region)
Microtubules
Tubulin
dimer
protofilament
(+)
(-)
microtubule
(+)
(-)
Andreas Merdes
disassemblygrowth
(+)
(-)
(+) (-)
kinesindynein
A-P Axis: bicoid / Oskar / nanos
Nurse cells manufacture
bicoid and nanos mRNA
- deliver cytoplasm
into oocyte
bicoid binds to dynein
- moves to non-growing
(-) end of microtubules
oscar mRNA forms
complex with kinesin I
- moves toward growing
(+) end of microtubules
Oskar binds nanos mRNA
- retains nanos in
posterior end
“posteriorized” follicles
produce organized (+/-)
microtubules
Dorsal - Ventral Axis Formation: Further Gurken Effects
The oocyte nucleus (with associated gurken) moves
anteriorly along the dorsal margin
Gurkin/Torpedo interactions “dorsalize” follicle cells
- results in Dorsalactivation
Oocyte
Pipe (ventral cells) eventually triggersnuclear Tollreceptor activity;
Dorsal determines ventral fates
Syncitial blastoderm
Gurken/Torpedoinhibits Pipesynthesis in dorsal cells
D-VPolarity
Distribution of Dorsal
Dorsal activates genes that create
mesodermal phenotype
- transcribed only in cells with highest
Dorsal concentrations
Dorsal also inhibits dorsalizing genes
- these genes have low affinity enhancers
(lots of Dorsal necessary)
Dorsal:
- large amount = mesoderm
- lesser amount = glial/ectodermal
ventral cells
form medoderm
Dorsal
Zygotic Patterning Genes
Dorsal (TF) – expressed ventrally; establishes diffusion gradient dorsally
decapentaplaegic (dpp),
zerknüllt (zen), tolloid are
dorsal patterning genes
- repressed by Dorsal
High Dorsal – activates twist and snail (low affinity enhancers)
- mesoderm determinants
Intermediate dorsal activates
rhomboid (no Twist or snail)
- determines neural ectoderm
- rhomboid + twist = glial cells
Intermediate dorsal activates fgf8
- fgf8 repressed by snail
- promotes mesodermal ingressiondorsal
~ 30 genes directly affected by Dorsal
Anterior-Posterior Body Plan
Drosophila use a hierarchy of gene expression to
establish the anterior-posterior body plan.
1. Maternal effect genes (e.g. bicoid, nanos)
- mRNAs differentially placed in eggs
- transcriptional or translational
regulatory proteins
- diffuse through syncytial cytoplasm
- activate or repress zygotic genes
3. Pair-rule genes
4. Segment polarity genes
2. Gap genes
embryonic segmentation genes
Anterior Specification - 1
Bicoid binds to caudal
3’UTR; prevents
translation
Caudal specifies
posterior domain
Anterior Specification - 2
Bicoid controls gap
gene Hunchback*
- anterior patterning
*hunchback is also
maternally expressed
Bicoid Mutants
Martin Klingler
Bicoid – homeodomain
transcription factor;
morphogen
Manipulating Bicoid
Posterior Specification - 1
Nanos prevents
hunchback translation
Oskar binds nanos;
remains in posterior
At least 9 maternal genes make up the posterior patterning group; including
nanos trap:
Staufen allows
oskar translation
staufen, oskar, nanos
Posterior Specification - 2
Model of Anterior-Posterior Patterning
mRNA in oocytes
(maternal messages)
Early cleavage
embryo proteins
hunchback translation
repressed by Nanos
caudal translation
repressed by Bicoid
Terminal Specification - 1
Torso – transmembrane RTK
Torso uniformly distributed
Torso activated by
Torso-like protein
- located only at
ends of egg
Terminal Specification - 2
Distinction between
anterior and posterior
= Bicoid
Bicoid = acron
formation
Torso kinases inactivate
an inhibitor of tailless
and huckebein
Tailless and Huckebein
specify termini
Drosophila Development - Overview
Cleavage
Fertilization
Gastrulation
Drosophila body plan
Oocyte formation
Genetic control of axis specification
Anterior-posterior
Dorsal-ventral
Segmentation genes
Homeotic genes
Segmentation Genes
Cell fate commitment:
Phase 1 – specification
Phase 2 – determination
- early in development cell fate depends on interactions
among protein gradients
- specification is flexible; it can alter in response to signals
from other cells
- eventually cells undergo transition from loose commitment
to irreversible determination
The transition from specification to determination in Drosophila is
mediated by the segmentation genes.
- these divide the early embryo into a repeating series of segmental
primordia along the anterior-posterior axis
Anterior-Posterior Body PlanDrosophila use a hierarchy of gene expression to
establish the anterior-posterior body plan.
1. Maternal effect genes (e.g. bicoid, nanos)
Establish polarity:
- mRNAs differentially placed in eggs
- transcriptional or translational
regulatory proteins; transient
- diffuse through syncytial cytoplasm
- activate or repress zygotic genes
2. Gap genes: first zygotic genes expressed
Divide embryo into regions
- expressed in broad, partially
overlapping domains (~ 3 segments wide)
- code for transcription factors; transient
- activated or repressed by
maternal effect genesHunchback Krüppel
overlap
bicoid gradient
Krüppel Specification by Hunchback
Wolpert, 2007
Threshold activation/repression
Anterior-Posterior Body Plan
3. Pair-rule genes;
Establish segmental plan
- regulated by combinations of gap genes
- code for transcription factors; transient
- divide the embryo into periodic units
- pattern of seven transverse bands
- delimit parasegmentseven-skipped (red),
fuschi tarazu (black)
Pair-Rule Gene Regulation
e.g. – even-skipped
- each stripe regulated
by a different set
of enhancers
- expression patterns are
stabilized by interactions
among other gene products
e.g. even-skipped
expression limited
by Giant
Regulation of even-skipped Expression
Wolpert, 2007
Anterior-Posterior Body Plan
3. Pair-rule genes;
Establish segmental plan
- regulated by combinations of gap genes
- code for transcription factors; transient
- divide the embryo into periodic units
- pattern of seven transverse bands
- delimit parasegmentseven-skipped (red),
fuschi tarazu (black)
4. Segment polarity genes;
Set boundaries of segments
(i.e. establish A-P for each segment)
- activated by pair-rule genes
- code for variety of proteins; stable
- divide embryo into 14 segmental unitsengrailed
Maternal effect genes
Gap genes
hunchback
huckebein
giant
even-skipped
fushi tarazu
Pair-rule genes
Segment polarity genes
engrailed
wingless
hedgehog
patched
bicoid
nanos
fushi tarazu – pair-rule gene
Segments and Parasegments
Segments and parasegments
organized from
A/P compartments
out of phase
Cells of adjacent
compartments
do not mix
Expression patterns in early embryos are not delineated
by segmental boundaries, but by
- parasegments:fundamental units of embryonic gene expression
P A P A P A P A P A P A P
Segments
Compartments
Parasegments
muscles/hinge
nerves
Segments and Parasegments - Adult
5. Homeotic selector genes;
Provide segmental identity
- interactions of gap, pair-rule,
and segment polarity proteins
- determines developmental fate
of each segment
Homeotic Selector Genes
Sean Carroll
Homeotic Selector Genes
Homeotic genes specify:
- head segments
- labial palps
- antennae
- thoracic segments
- wings
- halteres
- legs
- abdominal segments
Homeotic Gene Expression
distal-less – jaws, limbs
Antennapeida –
thoracic
Ultrabithorax –
abdomen
Ultrabithorax MutantUbx-/- transforms 3rd thoracic segment (halteres)…
into duplicate 2nd thoracic segment (wings).
Antennapedia Mutant