Download in follicle cells

Drosophila melanogaster development
How do Drosophila embryos develop?
How can one use genetics to find genes that
regulate embryo development?
Life cycle of Drosophila
4 days
egg
female
embryogenesis
pupa
1 day
DROSOPHILA
LIFE CYCLE
larva
1st instar
1 day
4 days
larva
3rd instar
2 days
larva
2nd instar
OOGENESIS IN DROSOPHILA
Germarium
Germline Cyst Formation
Cystoblast
germline
stem cells
follicle
stem cells
ring canal
Pro-Oocyte
(undergoes
meiosis)
germline: stem cell > cystoblast > 1 oocyte + 15 nurse cells
OOGENESIS IN DROSOPHILA
Germarium
Vitellarium
follicle cells nurse cell
germline
stem cells
follicle stalk
stem cells
oocyte
oocyte
border cells
oocyte + nurse cells surrounded by (somatic) follicle cells
Drosophila oocyte and supporting cells
Nurse cells
Ring canals
Oocyte
Follicle cells
(from Gonzalez-Reyes and St Johnston (1994) Science 266: 639-642.)
Drosophila oocyte and supporting cells
Nurse cell nuclei
Follicle cell nuclei
(from Gonzalez-Reyes and St Johnston (1994) Science 266: 639-642.)
Nuclear divisions start without cell division in
Drosophila (superficial cleavage)
Fig. 9.1
Zygotic gene expression begins
Larvae already have substantial patterning
T1
anterior
T2
head
thorax
ventral
T
dorsal
T3
A1
A2
A3
abdomen
A4
A
A5
A6
posterior
egg
epithelium
(6,000 cells)
A7
“stripy” expression
of segmentation gene
fushi tarazu (ftz)
blastoderm
A8
fate map
larva
(cuticle)
The fruit fly body plan is assembled
in 24 hours: How?
Christiane Nüsslein-Volhard and Eric Wieschaus
used genetics to identify proteins that
set up the embryonic body plan
wildtype
Wieschaus and
Nüsslein-Volhard
looked for mutants
that affect the
fly body plan
Genes identified in a famous screen for Drosophila
mutants with embryo patterning defects
Screen for developmental mutants (Drosophila)
Lethal hits = 100% (essential genes - ca. 5.000)
(efficiency of mutagenesis = number of hits per gene)
zygotic mutants
embryonal-lethal mutants
with morphological defects
- segmentation defects (AP)
- tissue types defective (DV)
female-sterile mutants
with effects on embryogenesis
(= maternal-effect mutants)
- antero-posterior pattern
- dorso-ventral pattern
100 %
25 %
3%
0.5 %
0.5 %
8%
2%
0.4 %
0.3 %
(male-sterile mutants)
ca. 2% of all genes involved in embryo pattern formation
(ca. 100 of >15.000 protein-encoding genes, only 5.000 essential genes)
Maternal-effect mutations
Genes expressed during oogenesis (= before fertilization)
or genes expressed in maternal cells (follicle)
All progeny of heterozygous mother are normal.
All progeny are affected only if mother is homozygous mutant
Zygotic mutations
Genes expressed during embryogenesis (= after fertilization)
Only genetically mutant embryos are affected.
(25% of progeny of heterozygous mother are affected.)
Drosophila axis detemination;
dorsal/ventral polarity
How does the embryonic dorsal-ventral axis get
translated into differentiation of different tissue
types?
Cell fate specification at the blastoderm stage
amnioserosa
dorsal
dorsal
ectoderm
neuroectoderm
mesoderm
ventral
fate map
mesoderm formation
Dorsal-Ventral fate map
Gurken protein specifies the Anterior-Posterior
axis of the Drosophila embryo during oogenesis
(Similar to EGF)
Localized
maternal
mRNA sets
up anterior
and posterior
poles
Gurken also signals dorsal pole formation during
oogenesis
oocyte
nucleus
1-6
A
+
7
8
D
migration of
nucleus
10A
D
-
P
anterior
posterior
V
V
follicle cells
microtubules
8
10A
1-6
+
gurken expression
in the oocyte
gurken expression in the oocyte
Expression of the
Gurken Message
and Protein
Between the
Oocyte Nucleus
and the Dorsal
Anterior Cell
Membrane
DORSO-VENTRAL PATTERN
FORMATION
follicle cells
Torpedo = EGF receptor
(in follicle cells)
Gurken = Epidermal
Growth Factor (EGF)
Oocyte
pipe
expression
X
Ventral follicle
cell
Pipe (Golgi?)
X
Wind (ER?)
Nucleus
X
mod. from van Eeden & St.Johnston
Dorso-ventral pattern formation
Maternal effect mutations
ndl, pipe, wbl
nudel, pipe, wbl
gd, snk, ea - serine proteases
dorsal RNA
spz - ligand
Toll protein
Toll
Tl - membrane receptor
Spätzle protein
Dorsal protein
dorsal
Zygotic mutations
Dorsal protein
tube - cytoplasmic protein
pelle - ser/thr protein kinase
cactus
amnio serosa
cact - cytoplasmic inhibitor
dorsal
ectoderm
of Dorsal nuclear
translocation
dorsal
neuroectoderm
dl - transcription factor
(morphogen)
mesoderm
Dl
nuclear protein
Dorso-ventral pattern formation
dorsal
Wild type
dorsal
cactus
mutant
mutant
A7
A8
A6
A5
A4
A3
A2
T3
A1
T2
T1
ventral
dorsalized
ventralized
Translocation of Dorsal protein into ventral
nuclei but not lateral or dorsal nuclei
Wild type
toll mutant
cactus mutant
Generation of Dorsal-Ventral Polarity in Drosophila
Generation of Dorsal-Ventral Polarity in Drosophila
Dorso-ventral pattern formation
pivotal role of Toll pathway
Inject wild-type cytoplasm
into toll mutant eggs
toll mutant
ventral
dorsalized
local
rescue
Wild type
dorsal
ectoderm
dorsal
neuro-ectoderm
(denticle belts)
mesoderm
polarity reversal
Conserved pathway for
regulating nuclear
transport of
transcription factors in
Drosophila and
mammals
Cells with highest nuclear Dorsal levels become mesoderm
Zygotically expressed genes
Action of Dorsal protein in ventral cells
Action of Dorsal protein in ventral cells
High affinity for promoter,
Not much Dorsal needed to activate
Action of Dorsal protein in ventral cells
Lower affinity for promoter,
More Dorsal needed to activate
Dorso-ventral pattern formation: summary
nudel, pipe,
windbeutel
dorsal RNA
Toll protein
Spätzle
protein
Dorsal protein
Dorsal protein
dpp
twist
oocyte nucleus dorsal
> repression of ventral fate
in dorsal follicle cells
ventral production of ligand
> activation of Toll receptor
> graded nuclear uptake
of Dorsal morphogen
> regulation of zygotic
target gene expression
> cell fates along DV axis
Use of a similar regulatory system to pattern insects and vertebrates
Patterns ectoderm in Drosophila
Patterns mesoderm in vertebrates
Gastrulation in Drosophila
Schematic
representation of
gastrulation in
Drosophila