Lecture 12 - Drosophila - setting up the body axis Flashcards
What parts of the body are formed through the development of the A/P axis?
- a head, a tail, a thorax and abdominal region are formed
- the thorax & abdomen are SEGMENTED
What 4 regions are formed through the development of D/V axis?
- the ventral most mesoderm
- the ventral ectoderm
- dorsal ectoderm
- aminoserosa (an extra embryonic tissue)
What is the initial expression of genes that result in the expression of GAP genes that define different regions in the embryo?
initial maternal gradient (maternal genes)
Where does the expression of maternal genes occur?
occurs in syncytium - multiple nucleus in one cytoplasm
What does the expression of GAP genes lead to?
GAP gene expression leads to the periodic expression of the Pair-Rule gene.
What do the GAP genes do?
divide the embryo
What does a mutation in GAP genes lead to?
loss of an area in embryo
What does the Pair-Rule gene do?
- continue to refine the body - these specify the so called ‘para-segments’ and foreshadow segmentation of the larva
What follows the Pair-Rule gene?
the segmentation genes
What does the segmentation genes do?
- elaborate patterning within each para-segment
- patterning of these segments happens when the embryo has cellularised.
- so cell-to-cell signaling is essential to coordinate this patterning process
What follows segmentation genes?
homeotic seletion genes
What do homeotic selector genes do?
determine segment identity
Where does the cascade (initiated by maternal genes) occur?
Occurs in syncytium - multiple nuclei in one cytoplasm (NOT IN THE ZYGOTE ITSELF)
Where are the maternal genes located?
mother - genes that need to be present in the genome of the mother to provide the initial information
How were initial mother genes identified?
identified by specially adapted genetic screens where homozygous mutant mothers were screened for defects in all their progeny
What are the 3 relatively independent classes of maternal genes that set up the A/P axis?
- anterior
- posterior
- terminal system
How were the 3 classes of maternal genes defined?
these were defined by mutations that led to the corresponding defects either in anterior, posterior or terminal structures in the larva
A mutation in what gene leads to a defect in anterior?
Bicoid - an example of a MORPHOGEN
What is a morphogen?
a molecule that emanates (originates) from a specific region and is present in a concentration gradient.
- a morphogen should be able to induce more than one fate
What type of gradient does Bicoid form?
a protein gradient across the A/P axis of the syncytial embryo
Where is RNA of Bicoid localised in the egg?
at the anterior
Where is Bicoid transcribed?
transcribed at the anterior and diffuses towards the posterior
What is Bicoid?
GENE - transcription factor - switches on different genes at different THRESHOLD CONCENTRATIONS
- Bicoid can only function like a morphogen, because the egg is a syncytium
- UNUSUAL as a morphogen, as it is a transcription factor
What gene acts on the anterior?
Bicoid
What gene acts on the posterior?
Nanos
What is the role of Nanos?
- preventing Hunchback translation in the posterior of the egg
Where does Nanos travel from?
- from posterior to anterior (creating posterior gradient)
- this create a lot of the anterior & posterior pattern
What does the Torso signal do?
specifies the extreme ends of the egg
Where is the Torso receptor present?
everywhere on the outside of the egg
What binds to the Torso receptor?
Trunk protein
What is required to release the ligand ‘Trunk Protein’, which is attached to the ‘Torso receptors’?
the protease ‘Torso-like’
Where is the protease ‘Torso-like’ located?
only present at the poles - leading to torso activation only at the poles
Describe the basic principles of cell-cell signalling
- ligand can’t cross membrane
- attaches to receptor
- ligand changes structure (depending on signalling pathway)
- activation of transcription factor
- leads to action in cell
What is the receptor on the outside of the egg that determines D/V polarity?
Toll
What is the ligand that attaches to Toll, which is involved in creating D/V polarity?
Spatzle
What is the localised enzyme which creates an active ligand on ventral side?
Pipe
What is Dorsal?
a protein and transcription factor that moves into the nuclei on the ventral side (nuclear localisation)
What side does ‘Dorsal’ the protein act on?
ventral
- high levels of Dorsal on ventral side
- low levels of Dorsal on dorsal side
What does differing levels of Dorsal on the ventral & dorsal side create?
creates a gradient of the Dorsal protein in nuclei
How is Dorsal activated?
via nuclear localisation
Why is toll activated only on the ventral side?
Toll is activated on ventral side cos PIPE is only found on that side (pipe only on ventral side of egg)
What is a summary of the creation of D/V polarity?
PIPE (enzyme) - only ventral side
Spatzle (ligand) - everywhere
Toll (receptor) - everywhere
Dorsal (protein) - nuclei on ventral side
Pipe activates spatzle which activates toll which activates Dorsal
How does the polarity in the oocyte needed to form A/P axis come about?
polarity of the ovaries is somehow transmitted to the egg - this occurs through nuclear positioning.
the side where the nucleus is closer to, is the side where the dorsal side develops
What is the one cell-cell singal that is required to set up BOTH anterio-posterior and dorso-ventral axes?
Gurken
Where does Gurken come from?
nucleus
