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Lecture 5
• Anterior Posterior axis formation: Cell
Biology
• Bicoid is a morphogen
Lab 5 schedule
• On Monday and Tuesday come in at the
regular time.
• On Tuesday and Wednesday come to
B&G 3015 to mount your embryos.
Elyse will discuss the time of day to
come.
Exams
• Midterm Tuesday Feb. 28, 2012
UCC 63, 7-10PM
• Tentative Final exam date Thursday
April 19, 2012 at 9:00AM
Structure of the egg
Dorsal appendage
A
D
P
Chorion
V
Micropyle
Vitelline
Cellular Blastoderm
Hartenstein 1993
Blastoderm fate map
Hartenstein 1993
Simplified Fate Map
Nusslein-Volhard et al., Science 238, 1675-1681
Classical Embryology
Remove 510% of the
anterior
cytoplasm
Remove posterior
pole plasm
Nusslein-Volhard et al., Science 238, 1675-1681
Transplant anterior cytoplasm to posterior
Nusslein-Volhard et al., Science 238, 1675-1681
Transplant posterior cytoplasm to anterior
Nusslein-Volhard et al., Science 238, 1675-1681
Transplant posterior cytoplasm to anterior
Nusslein-Volhard et al., Science 238, 1675-1681
Transplant posterior cytoplasm to an
anterior that lacks the anterior signal
Nusslein-Volhard et al., Science 238, 1675-1681
Properties of the polar center signals
• Both exert long range effects
• The anterior and posterior activities inhibit
each other
• Both activities have inductive properties
Properties of the polar center signals
• Both exert long range effects
Classical Embryology
Remove 510% of the
anterior
cytoplasm
Remove posterior
pole plasm
Nusslein-Volhard et al., Science 238, 1675-1681
Properties of the polar center signals
• The anterior and posterior activities inhibit
each other
Transplant posterior cytoplasm to anterior
Nusslein-Volhard et al., Science 238, 1675-1681
Transplant anterior cytoplasm to posterior
Nusslein-Volhard et al., Science 238, 1675-1681
Properties of the polar center signals
• Both activities have inductive properties
Transplant anterior cytoplasm to posterior
Nusslein-Volhard et al., Science 238, 1675-1681
Transplant posterior cytoplasm to an
anterior that lacks the anterior signal
Nusslein-Volhard et al., Science 238, 1675-1681
What you may know already
Bicoid: Anterior signal
Nanos: Posterior signal
How do we know this?
Maternal effect screens
Normal egg
bcd/bcd mother
Larva missing a head
Nusslein-Volhard et al., Science 238, 1675-1681
Three classes of maternal effect phenotypes
Nusslein-Volhard et al., Science 238, 1675-1681
The real cuticles
wt
bcd
oskar
torso
Nusslein-Volhard et al., Science 238, 1675-1681
Mutant combinations
St Johnston and Nusslein-Volhard
Cell 1992
Transplantation of cytoplasm into bcd mutants
Nusslein-Volhard et al., Science 238, 1675-1681
Controlling anterior posterior polarity
Nusslein-Volhard et al., Science 238, 1675-1681
The difference between the posterior and
anterior signals
Nusslein-Volhard et al., Science 238, 1675-1681
Distinct Properties of the signals
• Anterior signal-instructive. Induces
polarity at site of injection.
• Posterior signal-permissive. Polarity
independent of injection site.
Evidence for Bicoid
• Rescue activity spatially distributed
• Amount of rescue dependent on amount
injected
• Linear relationship between gene dose and
rescue activity
• mRNA asymmetrically localized and protein
expressed in a gradient
Bicoid rescue assay
Nusslein-Volhard et al., Science 238, 1675-1681
Rescue activity is spatially distributed
100%
% rescue
activity
50%
100%
40%
% egg length
0%
Amount of rescue is dependent on
amount injected
100%
% rescue
activity
50%
0
1/3
2/3
Transplanted volume
1
Linear relationship between gene dose and
rescue activity
100%
% rescue
activity
50%
0
1
2
3
Dose of bicoid gene in mothers
mRNA assymetrically localized and protein
expressed in a gradient
mRNA
Protein
Driever and Nusslein-Volhard Cell 54, 83-93
Bicoid
• Bicoid has the properties of the anterior
signal
• Does Bicoid need to interact with any
other anteriorly localized signals to
work?
Bicoid is necessary and sufficient
Bicoid mRNA
made in vitro
mRNA injected
at posterior pole
Bicoid protein
expression
Larva with
two heads
Driever et al., 1990
Development 109, 811-820
Bicoid
• Bicoid is the anterior signal
• What does Bicoid encode?
• How does Bicoid work?
Bicoid encodes a homeodomain containing
protein.
Hunchback lacks anterior segments
Nusslein-Volhard
and Wieschaus
1980 Nature 287,
795
Hunchback is expressed at the anterior pole
Bicoid mRNA
Bicoid protein
Hunchback mRNA
expression
Driever 1993
Hunchback gene
Driever and Nusslein-Volhard
Nature 337, 138-143
Bicoid is a transcriptional activator
Driever and Nusslein-Volhard
Nature 337, 138-143
mRNA assymetrically localized and protein
expressed in a gradient
mRNA
Protein
Driever and Nusslein-Volhard Cell 54, 83-93
Morphogen
• Long range action
• Affect polarity
• Concentration dependent
• Different fates at different
concentrations
Morphogen
• Long range action
Hunchback is expressed at the anterior pole
Bicoid mRNA
Bicoid protein
Hunchback mRNA
expression
Driever 1993
Morphogen
• Affect polarity
Bicoid is necessary and sufficient
Bicoid mRNA
made in vitro
mRNA injected
at posterior pole
Bicoid protein
expression
Larva with
two heads
Driever et al., 1990
Development 109, 811-820
Morphogen
• Concentration dependent
Concentration dependence of Bicoid function
Concentration dependence of Bicoid function
hb-lacZ expression
# of bicoid
genes in
the mother
0
1
2
4
6
Struhl et al., 1989 Cell 57,
1259-1273
Morphogen
• Different fates at different
concentrations
Distinct Bicoid binding sites are activated
at different concentrations of Bicoid
Morphogen
• Long range action
• Affect polarity
• Concentration dependent
• Different fates at different
concentrations
How is bicoid mRNA localized to the
anterior pole?
Other anterior class mutants
Driever and Nusslein-Volhard Cell 54, 94-104
Bicoid gradient affected in the mutants
Driever and Nusslein-Volhard Cell
54, 94-104
The mutants affect different stages of bicoid
mRNA deposition in the developing oocyte
St. Johnston et al., 1989 Development sup 13-19
Localization of bicoid mRNA: recent paper
Irion and St Johnston 2007 Nature 445, 554-558