At what stage does segmentation begin?
Stage 11
What is twitching?
When muscles start to form
In what 3 phyla is segmentation found in?
Arthropods (spiders, insects)
Annelids (earthworms, leeches)
Vertebrates (fish, frogs, mammals)
What is pupation?
To develop into a pupa. Vaginal discs expand.
What would’ve happened by 180mins in Drosophila life cycle?
All 3 germ layers would have been produced (via gastrulation) and most of segmentation.
What do most adult structures in Drosophila arise form?
Imaginal discs
What are parasegments and segments?
Parasegments are the gene expression
Segments is how it actually looks.
How many parasegments is there and what happens to them?
- They combine in a staggered fashion to form the 14 true segments of the adult.
What are the 3 classes of segmentation defects that can occur in Drosophila?
- General A-P problems, unusual inheritance= maternal determinants.
- Contiguous segments affected= Gap genes (chunk of animal gone)
- Alternate segments affected=Pair rule genes
What is a bicoid mutant?
Mutant with two posteriors ie. 2 tails, no head.
What does bicoid do?
Generates an anterior end.
In what stage is the embryo when bicoid is produced? Why is that good?
Embryo is a syncytium (no PM separate the nuclei). This allows bicoid to form a gradient.
Cellularisation occurs after bicoid gradient is established.
What happens in areas where there is high conc. of bicoid.
That’s where the head forms.
How does bicoid regulate target genes?
If bicoid conc is high then the gene is turned on. If bicoid conc is low then the gene is turned off. This is due to the gradient that bicoid achieves which produces different segments.
What is the target gene for bicoid and what does it do?
Hunchback. Hunchback is everywhere but it is activated by bicoid in the anterior region to make anterior structures.
What does Nanos do?
Repress hunchback translation, keeping it out of the posterior end of the embryo.
How is gap gene expression controlled?
By the maternal effect genes and by cross-regulation among the gap genes themselves.
What does expression of gap genes mark?
The conversion of the initial gradients into broad stripes.
Properties of the even-skipped gene.
- Expressed in odd numbered parasegments.
- Mutant lacks each odd numbered parasegment.
- Encodes a transcriptional repressor.
Properties of the fushi tarazu gene
- Expressed in even numbered parasegments.
- Mutant lacks even numbered parasegments.
- Encodes a transcriptional activator.
What are the activators and repressors of the even skipped gene?
Activators: bicoid, hunchback
Repressors: giant, kruppel
