Lecture 15 - Drosophila development Flashcards
(20 cards)
How are genetics involved in patterning organs & organisms?
asymmetric division & lateral inhibition. Morphogens are components that work in a concentration dependent manner - however it is morphogen activity that is important
What are mutagenesis screens used for?
Mutagenesis screens were used to identify which genes were used as part of the patterning of the abdominal dorsal segments.
- mutagenesis male flies
- establish inbred lines from single sons
- inbreed siblings
What genes are used for axis - formation?
- Egg-polarity genes (maternal effect)
What genes are used for segment identity?
- Gap genes
- Pair-rule genes
- Segment polarity
What did the first mutant lead to (in terms of body plan)?
This led to a lethal phenotype.
These mutants become known as Gap genes.
What did the second set of mutants lead to (in terms of body plan)?
Deletions of alternative stripes - known as pair-rule genes
What did the third set of mutants lead to (in terms of body plan)?
Segment polarity genes - rather than having deletions of part of the body plan they would have phenotypic issues affecting the polarity of a segment.
What did the final set of mutants lead to (in terms of body plan)?
Homeotic mutants - first mutants identified in Drosophila
What did mutagenesis screens establish?
This implied that there were genes regulating large area of the body plan.
Analysis of the mutants showed that these mutants coundn’t explain everything that occured during development.
What is seen in maternal effect mutants?
- This lead to the hypothesis that maternal genes are involved in regulating early developmental processes.
- When talking about maternal effects, it is the genotype of the mother that affects the phenotype of progeny.
Maternal genes were researched by taking female flies from F3 generation & look at phenotype of their progeny.
Mutations maternal effect genes result in large deletions of the body plan.
What are the 3 classes of mutations within A/P domains?
Anterior (bicoid mutant) - loss of head & thoracic segments
Posterior (nanos mutant) - loss of abdominal segments
Terminal (torso mutant)
These are progeny of F3 generation. If mated with wild-type male, then it will be heterozygous.
When is axis formation determined?
BEFORE FERTILIZATION
Egg polarity genes are Maternal-effect genes.
The maternal genotype determines the embryonic phenotype.
A/P polarity is determined in the oocyte.
Where is the oocyte located?
Oocyte located in posterior of the egg chambers.
Nurse cells everywhere else
Where are maternal determinants?
Found that examination of oocyte (unfertilized) shows that, within the egg chamber:
- Bicoid mRNA is found tethered to anterior pole.
- Nanos mRNA is associated at the posterior pole.
Polarisation functions through microtubules. Prior to fertilization, there is already polarity.
Describe how protein gradients in oocyte are seen
In-situ hybridisation can be used to located mRNA or protein. Shading shows detection of target.
Bicoid has a concentration gradient from anterior to posterior.
Nanos has a concentration gradient from posterior to anterior
Translation of mRNA occurs after fertilization.
What does Bicoid determine?
Bicoid does determine head structure of development
- Bicoid mutants have anterior deletions.
- Bicoid is required for the formation of anterior regions.
What affect does BICOID & NANOS have on CAUDAL & HUNCHBACK translation?
Bicoid inhibits caudal translation - therefore areas of high levels of bicoid - low levels of caudal
Nanos inhibits hunchback translation - high levels of nanos = low levels of hunchback.
How is Bicoid an unusual regulator?
Bicoid binds DNA & RNA - both require Homeodomain & Arginine 54 determines RNA binding
Bicoid regulates transcription - HUNCHBACK & Gap genes.
What is the early Drosophila embryo?
A syncytium - doesn’t contain cells but many nuclei - free within a system. This means these proteins (bicoid & nanos) have free access to the DNA in the nuclei to regulate the expression of genes. Soon after this, rapid nuclear division occurs. The nuclei starts to move to the periphery of embryo. This cellularizes to make a cellular blastoderm.
What will be the phenotype of the progeny of a cross between NANOS/nanos female & nanos/nanos male?
Wild-type - mother has a wild-type genotype.
