Transcript Anterior-posterior patterning in Drosophila

Anterior-posterior patterning in Drosophila
The fly body plan:
each segment has a
unique identity
and produces
distinctive
structures
3 head
3 thorax
8 abdomen
Mutations affecting the antero-posterior axis
3 independent maternal systems: anterior, posterior, terminal
fate map
larva
active
systems
triple mutants
active
systems
A P T
wild-type
single mutants
anterior
bicoid
posterior
oskar
terminal
torso
- - -
double mutants
- P T
- P -
A - T
- - T
A P -
A - additive phenotypes
Maternal effect mutations
Zygotic effect mutations
bicoid mutant phenotype
Embryo from
wild-type
mother
Embryo from
bicoid mother
Anterior: bicoid is required for head and thorax
Wild type
bicoid mutant
blastoderm fate map
head
+ thorax
abdomen
abdomen
Bicoid mRNA localization in embryo
(tethered to microtubules)
Bicoid protein
gradient in
syncytial
blastoderm
embryo
- diffuses after
translation from
localized mRNA
- protein unstable
Nuclei divide without cell division in Drosophila
to produce a syncytial blastoderm embryo
Fig. 9.1
Transplantation of egg cytoplasm
An organizer of the anterior-posterior pattern is located at the anterior pole
wt
abdomen only
polarity normal
wt
rescue of
head in the center
pattern
polarity reversal
wt
thorax at posterior pole
polarity reversal
bicoid mRNA induces head and thorax
bicoid
mRNA
bicoid (bcd) gene encodes a homeo-domain transcription factor
Injection of bicoid mRNA:
anterior (head) structures at site of injection
& reorganization of polarity
no head
Transplantation of egg cytoplasm
Posterior cytoplasm also has polarizing activity
wt
abdomen only
polarity normal
wt
rescue of
head in the center
pattern
polarity reversal
wt
thorax at posterior pole
polarity reversal
wt
double abdomen
polarity reversal
Nanos is the maternal effector of the posterior system
mutant
rescued
rescued
rescue of all posterior-system mutants
by injection of nanos mRNA
Embryonic polarity genes
Anterior-Posterior Pattern Formation in Flies
Bcd gradient and expression domains of target genes
bcd mRNA
Bcd protein
target genes
The Bcd gradient is converted into domains of gene expression
Bcd protein binds differentially to enhancers of target genes
Different thresholds of Bcd concentration are required to turn on different genes
low affinity
high affinity
target genes are zygotically expressed gap genes
Bicoid and Nanos regulate Gap genes
Hunchback
Krüppel
Knirps
Expression patterns of proteins encoded by gap genes
Bicoid protein: transcriptional and translational regulator
zygotic target genes
maternal target mRNA
Maternal anterior and posterior systems
head + thorax
nurse cells
bcd mRNA
abdomen
follicle cells
(soma)
oocyte
oskar mRNA
Stau protein
nanos mRNA
Hbmat protein
Bcd protein
Nanos protein
knirps
Hbzyg protein
giant
gene X
Hunchback (Hb), knirps and giant are zygotic gap genes
Gap gene mutants lack different body regions
Wild type
Krüppel hunchback
knirps
The gap genes regulate each other and
form domains with unique combinations of
gene expression
Hunchback
Krüppel
Anterior-Posterior Pattern Formation in Flies
Pair-rule mutants
Wild type
fushi tarazu
mutant
early
Fushi tarazu
expression
late
Eve, Ftz expression
Even-skipped expression pattern
Modularity of the Drosophila even-skipped promoter
08_18_reporter.gene.jpg
Regulation of expression stripe no. 2 of Even-skipped (eve)
hunchback
giant
Krüppel
eve stripe #2
activator
repressor
parasegment
1
repressor
2
3
4
5
multiple binding sites in enhancer of eve
repressors
activators
Regulation of the Second Stripe of
Transcription from the even-skipped Gene
Regulation of the even-skipped gene
Segments and Parasegments
Segment polarity mutants
Segment
polarity
mutants
Wingless signaling specifies cell fates in
the ventral epidermis
Anterior
cells
make
Hair
Posterior
cells
make
Naked
cuticle
Wild type
arm mutant
Segment polarity genes – 14 stripes
A
L
ap
fg
13 12
11
10
9
hg
0
1
2
3
4
5
6
Expression of segment polarity gene wingless
7
8
The Even-skipped and Fushi tarazu pair-rule
transcription factors activate the segmentpolarity gene Engrailed
Intercellular feedback maintains pair-rule gene expression states
Intercellular feedback maintains pair-rule gene expression states
Wnt signaling pathway
-
+
-
+
-
+
+
-
-
+
-
+
Anterior-Posterior Pattern Formation in Flies
Drosophila homeotic mutations
Wild type
Antennapedia mutant
Wild type
Ultrabithorax mutant
Homeotic Gene Expression in Drosophila