L7 - Drosophila Body Plan Flashcards
What is the Drosophila adult segmentation pattern?
Pattern of pigmentation on the abdomen and thorax
Each thoracic segment has a pair of legs associated with it
What is the Drosophila embryo segmentation pattern?
Denticle belts in embryo
- Small hairs that emerge from the otherwise naked cuticle
- Help larva crawl too food
What is the history of segmentation?
Segmentation is an ancient and conserved way of building bodies
Go from an unpatterned egg to a patterned egg in 24 hours
What were the results of the 1980 mutagenesis screen carried out by Nusslein-Volhard and Wischaus?
Total lines established – 27,000
Lethal mutations – 18,000
Mutations causing embryonic lethality - 4,000
Mutations causing embryonic phenotypes – 600
Complementation groups – 139
Complementation analysis -If AxA = mutant phenotype
If BxB = mutant phenotype
AxB if in same gene = mutant phenotype
Fail to complement
Complementation analysis -If BxC = no phenotype
If AxC = no phenotype
Complement - if different genes then
B mutant is over a wildtype on the opposite chromosome
C mutant is over a wildtype on the opposite chromosome
What were the results of the complementation analysis?
A and B are mutations of same gene
C is different from A and B
What is saturation?
Hitting the genome often enough to ensure we are not finding any new mutations
What were the 3 different genes discovered in the mutagenesis experiments?
Didn’t find consistent defects – range of phenotypes
Gap genes
Pair rule gene
Segment polarity genes
What are gap genes?
E.g. knirps
What are pair rule genes?
E.g. paired
Missing the paired segments
What are segment polarity genes?
E.g. gooseberry
Lose the naked cuticle in between the hairs
What are maternal genes?
Bicoid – anterior
Nanos – posterior
What is the order of the hierarchy of genes that progressively define the egg?
Maternal genes define gap genes
Gap genes define pair rule genes
Segment polarity genes control the fine details
What is Bicoid?
A DNA binding transcriptional activator
What are the two bicoid binding sites?
High affinity binding sites – activated at lower concentrations of bicoid
Low affinity binding sites – activated at high concentrations of bicoid
How does Bicoid enter the embryo?
Maternally loaded into developing oocyte
What are the characteristics of the bicoid mutant?
Lose head structures
Lose thoracic structures
What was the first experiment used to identify bicoid mutant?
Transplantation of cytoplasm from anterior end into bicoid mutant
- Partial rescue
What was the second experiment used to identify bicoid mutant?
Took donor cytoplasm and transplanted into middle of bicoid mutant
- Head like structures in the middle
- Mirror image thoracic segments
What is the expression pattern of pair rule genes in egg with 1 gene copy of bicoid?
7 stripes
Equal gaps at posterior and anterior end
What is the expression pattern of pair rule genes in egg with 0 gene copies of bicoid?
5 stripes
Fairly equal spacing
What is the expression pattern of pair rule genes in egg with 4 gene copies of bicoid?
7 stripes
Large gap at anterior end
How do gap genes show a readout of bicoid gradient? - e.g. Kruppel
Bicoid above threshold – Kruppel not on
Bicoid below threshold – Kruppel not on
Bicoid at standard – Kruppel on
You get a stripe effect of gap genes
Pair rule genes - transcriptional repressors example
Anterior and posterior ends
E.g. Kruppel and giant
Switch off gene expression of target gene
Pair rule genes - transcriptional activators example
In the middle
E.g. bicoid and hunchback
Switch on gene expression of target gene
Overview of pair rule gene control
Expression of pair rule genes is controlled stripe by stripe
Dependent on the interaction of positively and negatively acting transcriptional regulators
Many of which are gap genes
What are segment polarity genes?
Parasegments and segments are half a segment out of place
Parasegments are where the gene expression barriers are
Where are Hh and Wg expressed?
Hh maintains Wg expression
Wg is expressed in a cell which directly neighbours a cell expressing Hh
What is the role of Wg?
Forms a morphogen gradient which is asymmetric
- Further anterior than posterior
Wg suppresses denticle development
What are selector genes?
Hox genes
Where are selector genes expressed?
Expression of hox genes along A/P body axis occurs in the same order as the genes are within the genome
What controls the expression of selector genes?
A combination of gap and pair-rule genes
What does the homeobox contain?
DNA binding transcription factors
What is an example of a long germ band insect?
Drosophila
- All 14 segments are defined at once
- Quick - embryogenesis complete in just 24 hours
- Complicated - maternal, gap and pair rule genes interact for every segment
How do short and intermediate germ band insects develop?
Start with head and thoracic segments
- Via an ancestral version of the system Drosophila uses
Abdominal segments added sequentially
- Posterior disc (proctodeum) bud off segments as it gets smaller
Moderate complexity
Not too slow
Segment addition in Strigamia maritima
Adjacent stripes of Delta and Hes4 set up feedback loop necessary for oscillation
Segmentation clock - feedback loops
Notch activation causes down regulation of Notch ligand
Time lag in response causes oscillation between strong and weak signalling levels
Propagation of signal between cells causes wave of activation
How do Strigamia get their segmentation?
Use segmentation clock
How do Tribloium get their segmentation?
Use segmentation clock and a few gap genes
How do Drosophila get their segmentation?
Segmentation all occurs at once using many gap genes
Segmentation in vertebrates
The majority of known candidate pacemaker genes lie in the Notch pathway
