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Anterior-posterior patterning in Drosophila
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The fly body plan: Each segment has a unique identity and produces distinctstructures
3 head
3 thorax
8 abdomen
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fate map larva
wild-type
anterior bicoid
posterior oskar
terminal torso
Mutations affecting the antero-posterior axis 3 independent maternal systems: anterior, posterior, terminal
single mutants double mutants
triple mutants
additive phenotypes
active systems
A P T
- P T
A - T
A P -
- - -
- P -
- - T
A - -
active systems
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Maternal effect mutations
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Zygotic effect mutations
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Embryo from wild-type mother
Embryo from bicoid mother
bicoid mutant phenotype
Wild-type Bicoid promotes anterior fates and inhibits posterior fates.
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Wild type
blastoderm fate map
bicoid mutant
Anterior: bicoid is required for head and thorax
abdomen abdomenhead
+ thorax
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Bicoid mRNA localization in embryo(tethered to microtubules)
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Nuclei divide without cell division in Drosophila to produce a syncytial blastoderm
embryo
Fig. 9.1
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Bicoid protein gradient in syncytial blastoderm embryo- diffuses after translation from localized mRNA- protein unstable
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Transplantation of egg cytoplasm An organizer of the anterior-posterior pattern is located at the anterior pole
wt
rescue of
pattern
wt
head in the center
polarity reversal
wt
thorax at posterior pole
polarity reversal abdomen only
polarity normal
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bicoid mRNA
bicoid mRNA induces head and thorax
bicoid (bcd) gene encodes a homeo-domain transcription factor
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Injection of bicoid mRNA:anterior (head) structures at site of injection
no head
& reorganization of polarity
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Bicoid protein: transcriptional and translational regulator
zygotic target genes maternal target mRNA
(promotes anterior fates) (inhibits posterior fates)
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Transplantation of egg cytoplasm Posterior cytoplasm also has polarizing activity
wt
rescue of
pattern
wt
head in the center
polarity reversal
wt
thorax at posterior pole
polarity reversal
wt
double abdomen
polarity reversal abdomen only
polarity normal
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fate map larva
wild-type
anterior bicoid
posterior oskar
terminal torso
Mutations affecting the antero-posterior axis 3 independent maternal systems: anterior, posterior, terminal
single mutants double mutants
triple mutants
additive phenotypes
active systems
A P T
- P T
A - T
A P -
- - -
- P -
- - T
A - -
active systems
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Nanos is the maternal effector of the posterior system
rescue of all posterior-system mutants
by injection of nanos mRNA
mutant rescued rescued
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Embryonic polarity genes
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Anterior-Posterior pattern formation in flies
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The Bcd gradient is converted into domains of gene expression
Bcd protein binds differentially to enhancers of target genesDifferent thresholds of Bcd concentration are required to turn on different genes
target genes are zygotically expressed Gap genes
low affinity
high affinity
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bcd mRNA
Bcd protein
target genes
Bcd gradient and expression domains of target genes
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Expression patterns of proteins encoded by Gap genes
Bicoid and Nanos regulate Gap gene expression
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Gap gene mutants lack different body regions
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Wild type Krüppel hunchback knirps
Gap gene mutants lack different body regions
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Hunchback Krüppel
The gap genes regulate each other and form domains with distinct combinations of gene expression.
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Anterior-Posterior pattern formation in flies
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Wild type
fushi tarazu mutant
Pair-rule mutants
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Even-skipped expression pattern
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08_18_reporter.gene.jpg
Modularity of the Drosophila even-skipped promoter
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parasegment
Krüppelhunchback giant eve stripe #2
repressorrepressor
activator
1 2 3 4 5
Regulation of expression stripe no. 2 of Even-skipped (eve)
multiple binding sites in enhancer of eve repressorsactivators
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Regulation of the Second Stripe of Transcription from the even-skipped Gene
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Regulation of the even-skipped gene
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Fushi tarazu expression
early
late
Eve, Ftz expression
Refinement of expression domains over time
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Refined expression domains in distinct cell rows
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Anterior-Posterior pattern formation in flies
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Segment polarity mutants
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Segment polarity mutants
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Wingless signaling specifies cell fates in the ventral epidermis
arm mutantWild type
AnteriorcellsmakeHair
PosteriorcellsmakeNakedcuticle
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Expression of segment polarity gene wingless
02 4 6
8
1012
LA
fghg
ap
1 3 57
9
1113
Segment polarity genes – 14 stripes
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Segments and Parasegments
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The Even-skipped and Fushi tarazu pair-rule transcription factors activate the segment-polarity gene Engrailed
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Intercellular feedback maintains pair-rule gene expression states
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Intercellular feedback maintains pair-rule gene expression states
=Wnt
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Wnt signaling pathway
-
+ -
-
-
-
-
+
+
+
+
+
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Gradients of Wingless and Hedgehog pattern each segment
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Anterior-Posterior pattern formation in flies