L7 - Drosophila Body Plan Flashcards
What organisms can segmentation be seen in?
Many even humans just is slighlty more obscure
How quickly is patterning established in drosophila
Very quciky - within 24 hours
Describe what can be seen of a scanning EM of the early drosophila embryo
Can see the first folds are the basis for the first segments
What did Volhard and Wischauus perform in 1979/1980
Undertook a saturation mutagenesis to identify genes involved in the development and patterning of the naked cuticle
How many lines did Volhard and Wischauus establish
26,978
How many complementation groups (genes) established
139
What are complementation tests needed for
To test whether different genes had been hit or if it was the same gene which had been hit many times
How many copies of the gene are required usually for normal function
1 - haplosufficent
Describe the results for A/B where a phenotype was produced
Shows that they are the same gene ==> fail to complement
Describe the results for A/C where no phenotypes are produced
Shows they are in different genes ==> complement
What is saturation mutagenesis
Hitting the genes many times on average - so can be statisitcally confident that there are no more genes - as these would have also have been hit
Desribe gap genes when mutated
Leads to gaps (e.g. loss of the middle section of the abdomen)
What are gap genes required for
To specify large regions of the body
Example of a gap gene
Knirps
Describe the effect of a mutation in a par rule gene
Pairs are missing - e.g. every other segement is missing
Example of a paired rule gene (2)
Even/odd skipped
What is the effect of a mutation in a segment polarity gene
Loss of polarity of the segment
EG –> loss of the naked cuticle part of the sgement
Hieracrchy for gene interaction during development
Maternal genes –> Gap genes –> Pair rule genes –> Segment polairity genes
Describe the action of maternal genes
Progressively divide up the mebryo
Where is bicoid locaised
What is bicoid
How does it act
At the anterior
DNA binding transcriptional activator
Acts a morphogen
When are high afifnity bicoid binding sites activated
At lower threshold concentrations
When are low affinity bicoid binding sites activated
At higher thresholds concs
Where is nanos localised
At the posterior
Describe the pheno of a bicoid mutant egg
Lacks all of the anterior structures
Describe the pheno of a bicoid mutant egg which has had some anterior wt egg cytoplasm transplanted into the anterior
Some anterior structures develop
Partial rescue of the phenotype
Describe the pheno of a bicoid mutant egg which has had some anterior wt egg cytoplasm transplanted into the MIDDLE
Ectopic head in the middle
Mirror image of the thoracic segements going outwards
Describe the effect on striping of low levels of bicoid
5 stripes instead of 7
Describe the effect on striping of normal bicoid levels
7 stripes
Describe the effect on striping of overexpressing bicoid
Stripes still form but pushed much more anteriorly = large gap between anterior tip and first pair rule gene
BECAUSE bicoid is now too high at the extreme anterior to allow the development of the head structures
Bicoid gradient converted to
Hunchback gradient
Gap genes are a read out of
Bicoid gradient
Different gap genes are activated at different th concs, this is a balance between
Activation and repression
Where is Krupple expressed in the wild type
Discrete band in the middle of the embryo
Where is Krupple expressed in a bicoid mutant
Expressed throughout the entire anterior domain of the embryo
Pair rule expression is controlled …
Bit by bit
Pair rule expression dependent on
The interaction of postively and negatively acting transcriptional regulators
Describe the expression of engrailed
In the anterior of each parasegment
In the posterior of each segement
What is the difference between segements and parasegements
They are 1/2 a segement out of phase
Where are Hh and Wg expressed
In adjacent cells
What is Hh expression dep. on
The expression of Wg
What is Wg expression depend on
The expression of Hh
Hh and Wg are …
MUTUALLY DEPENDENT PATHWAYS
What does Wg expression repress
How is this seen in a mutant phenotype
Supresses formation of the denticles
Wg (-/-) - hedgehog phenotype - lawn of denticles as there is no supression of hair development
Wg morphogen gradient is
Asyymetric
What do Hox genes provide
Positional information to each of the segments
Expression of Hox genes is in the same order as
Genes are laid out in the genome
What do HOX genes code for
DNA binding TFs
What is the same about each HOX gene
Binds to the same region of DNA but has different effects
What are the two hox clusters in drosophila
Antennapedia complex
Bithorax complex
What 3 hox genes are in the bithroax complex
UBX ABD-A ABD-B
Is drosophila a long or short germ band insect - why?
Long Since all of the segments are formed at once
Describe development in short and intermeidate germ band insects
Start with the head and thoracic segments
Abdominal segments are then added sequentially
Describe the segment addition in Strigamia martima
When a number of genes cloned and an in-situ performed notch and delta were easily defined
What is involved with the notch pathway
Her/Hes4
Describe the cycling of the notch pathway
When notch activated in the cell the levels of delta become downreg
When one cell is active the adjacent one becomes down regualted
Why is there a time delay in the notch clock
Time delay to allow for the changes in gene transcription and translation
What does notch activation cause
Downregulation of the delta ligand
What does the Notch-delta time delay cause
An oscillation between strong and weak signalling levels - propagation of this signals between cells then causes a wave of activation
THe common ancestor of vert and invert likely used …
The notch clock as a method for segmentation
____________ (in fish) and ___________ (in chick) cycle to give rise to the somites
Presomatic mesoderm - fish
Primitive streak - chick
Where to the majority of the known candidate pacemaker genes lie
In the notch pathway
