lecture 5 anterior posterior axis formation: cell biology bicoid is a morphogen
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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
Chorion
VitellineMicropyle
D
V
A P
Cellular Blastoderm
Hartenstein 1993
Blastoderm fate map
Hartenstein 1993
Simplified Fate Map
Nusslein-Volhard et al., Science 238, 1675-1681
Remove posterior pole plasm
Classical Embryology
Remove 5-10% of the anterior cytoplasm
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
Remove posterior pole plasm
Classical Embryology
Remove 5-10% of the anterior cytoplasm
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
bcd/bcd mother
Normal egg
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-VolhardCell 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
% rescueactivity
100%
50%
% egg length100% 0%40%
Rescue activity is spatially distributed
% rescue activity
0 1/3 2/3 1
Amount of rescue is dependent on amount injected
100%
50%
Transplanted volume
Linear relationship between gene dose and rescue activity
100%
50%
0 1 2 3
% rescue activity
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
Bicoid mRNA
Bicoid protein
Hunchback mRNAexpression
Hunchback is expressed at the anterior pole
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
Bicoid mRNA
Bicoid protein
Hunchback mRNAexpression
Hunchback is expressed at the anterior pole
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
# of bicoid genes in the mother
1
2
0
4
6
hb-lacZ expression
Concentration dependence of Bicoid function
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
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