Drosophila - setting up the body axis

Question 1

Creating of body axes in flies

Answer 1

Egg quite symmetrical
How the axes set up and break symmetry:
- A/P axis - form head, tail, thorax and abdominal region. Th and Ab are segmented
- D/V axis - ventral most mesoderm, ventral ectoderm, dorsal ectoderm, amnioserosa
amnioserosa - an extraembryonic tissue
Screens by Nüs-Vol and Wie identified genes control this

Question 2

Antero-Posterior patterning

Answer 2

Many AP patterning genes in drosophila can be grouped into hierarchy
Initial maternal gradients result expression GAP genes define regions in embryo
GAP genes lead to periodic expression of Pair-Rule genes, these specify “para-segments” and foreshadow larva segmentation
Segmentation genes elaborate patterning in each para-segment
All happens when egg in a syncytium
Patterning of segments happens when embryo has cellularised
Cell-to-cell signalling essential to coordinate patterning process
Final group genes are Homeotic selector genes, determine segment identity

Question 3

Setting up body axes

Answer 3

Mother provides initial information set up two main perpendicular axes
Requires action of maternal genes - genes that have to be present in genome of mother
Identified by specially adapted genetic screens where homozygous mutant mothers screened for defects in all progeny
In screening identified around 50 genes essential for development of AP

Question 4

AP axis set up by 3 classes of maternal genes

Answer 4

Bicoid
Nanos and Caudal
Torsal signal

Question 5

Bicoid - the first example of Morphogen

Answer 5

Existence of morphogens been postulated by theoretical biologists to explain patterning in embryos
Morphogen is molecule that emanates from specific region and present in a concentration gradient
M should be able to induce more than one fate
First protein gradient across AP axis of syncytial embryo, its RNA localised at anterior of egg
Bicoid - transcription factor, switches on different genes at different threshold concentrations
Bicoid can only function like morphogen, because egg is syncytium

Question 6

Nanos and Caudal

Answer 6

Under influence of Oskar
Happens at posterior section of egg, can also diffuse to anterior end and can form a morphogenic concentration gradient
Only role of Nanos appears to be preventing Hunchback translation in posterior of egg
Caudal also important in posterior patterning
Bicoid protein has second role in addition to transcription - preventing anterior caudal mRNA translation

Question 7

Torso signal (need to update once gone back over the lecture)

Answer 7

Torso receptor is present everywhere
“Trunk protein” which can form the ligand is present everywhere
However the protease “Torso-like”, required to release the ligand, only present at the poles

Question 8

DV polarity

Answer 8

Similar to “terminal signal”
Receptor “Toll” and ligand “Spätzle” is everywhere around the embryo
Localised enzyme “Pipe” creates active ligand on ventral side
Nuclear localisation of “Dorsal” protein on ventral side
Dorsal protein acts on ventral side, the genes were names after their phenotype

Question 9

Polarisation of the oocyte AP

Answer 9

Polarity of Ovarile is transmitted to egg
Signals from older egg chamber induce stalk cells, oocyte adheres to these, places oocyte at posterior of egg chamber
Stalk signal to follicle cells - if signal coincides with “Gurken” signal from oocyte they will become posterior follicle cells
If no Gurken signal will become anterior follicle cells
Gurken related to vertebrate Transforming Growth Factor alpha (TGF-α), member of Epidermal Growth Factor (EGF) family and is important to setting up AP axis
As result signal from posterior follicle cells microtubules rearrange with positive ends towards posterior end of oocyte and negative end to anterior
Microtubules form cellular skeleton
Also function as transport highway, guide transport motors and cargo:
- Kinesin to positive end
- Dynein to negative end

Question 10

Polarisation of the oocyte DV

Answer 10

Signal from posterior follicle cells and rearrangement of cytoskeleton has consequences:
- Nucleus also repositions itself to anterior corner, it then locally expresses same Gurken signal that earlier induced AP axis formation
That signal make dorsal follicle cells different from ventral ones, leads to ventral specific deposition of “pipe”

Question 11

Summary

Answer 11

Anteroposterior patterning occurs as a gradual process
Initial signals define Anterior, Posterior and Terminal, these cues are layed down by the mother using maternal genes.
These cues are interpreted by a hierarchy of zygotic genes : Gap, Pair-Rule, and Segmentation genes, to create a basic pattern.
Bicoid which defines anterior, was the first molecular example of a MORPHOGEN
Dorsoventral patterning is driven by nuclear gradient or Dorsal protein, a transcription factor: Nuclear ventral, cytoplasmic dorsal
The original asymmetries of the egg originate in the ovary of the mother, in a complex sequence of reciprocal inductive events, between oocyte and nearby follicle cells.
In this process, the GURKEN signal sets up both A/P and D/V axes in the egg