Drosophila Appendage Development Flashcards
1
Q
What are imaginal discs and how are they relevant to drosophila wing development?
A
- specialized groups of epithelial cells in the larvae that undergo significant transformation during metamorphosis to form the adult structures of the fly
- Wing imaginal discs are located in the second thoracic segment (T2) and each larvae has two pairs of wing imaginal discs, one on each side of the body
2
Q
Describe the structure of wing imaginal discs
A
- Contains about 50,000 columnar epithelial cells by the end of the third larval instar (larvae development stage)
- Stereotypical pattern of neurons, Hairs on back and edge that detect movement
- stereotypical location means the cells must know where they are on the epithelium
3
Q
How does the 2D wing become 3D?
A
- In a process called ‘eversion’
- Means the flat disc turns inside out to begin forming the 3D wing structure
- Disc elongates and expands outward from the body
- Wing blade formation
4
Q
How do the cells in the wing know where they are?
A
- Patched runs A-P
- Wingless runs from D-V
- ‘grid coordinates’ based on the mix of concentrations from morphogen gradients
- shows patterning starts with axis formation
5
Q
What is engrailed?
A
- a segment polarity gene, located in the posterior part of each thoracic segment (lower half of the wing)
6
Q
How is engrailed visualised?
A
- Using an enhancer trap
- en-lacz reporter gene expressed in the place of egrailed
- lacz converts soluble colourless X-gal into insoluble blue precipitate (visualised)
7
Q
Explain the expression of Hh in the wing in A-P patterning
A
- Hh receptor, patched, usually inhibits smoothened
- When Hh is present, smo isnt inhibited and can activate TF called Ci for gene expression
- Ci is not present in the posterior as engrailed inhibits it
- It IS present in anterior
- This means cells in the posterior CANNOT RESPOND to Hh but they can still make the Hh protein- Engrailed actually acts as a Tf for Hh promoting its transcription
- Hh protein diffuses to anterior region where it activates downstream cascade and causes patched genes in anterior to be expressed, fine tuning the Hh signalling
- Means where the pathway is activated there is a boost in patched levels.
8
Q
Explain the role of Dpp in A-P patterning
A
- Member of TGF-b family, acts as a morphogen forming a GRADIENT from the A-P boundary
- Ci from Hh signalling induces the expression of Dpp at the A-P boundary
- Once expressed, Dpp is secreted and forms a gradient extending anteriorly and posteriorly
- Cells respond to different concentrations of Dpp, activation of distinct sets of target genes depending on location
- Phosphorylated Mad is the activated TF downstream of Dpp
- omb and sal are activated at high Dpp conc and brinker is activated at low Dpp conc.
9
Q
What are experimental manipulations you can carry out to observe effects of A-P signalling?
A
- anterior-like cells on posterior side:
- anterior expresses Ci, makes Dpp as there are cells around making Hh so there is a ring of Dpp around the anterior like cells - posterior-like cells in the anterior:
- posterior cells express engrailed, no Ci but Hh expression, Hh diffuse out in to anterior causing Dpp ring again - patched mutant cells in anterior
- patched inhibits smo, no smo = active
- smo activates Hh signalling
- Hh signalling +Ci from anterior means they resemble boundary cells and so form Dpp (again) but all over this time
10
Q
Explain the D-V patterning of the wing
A
- No wingless = no signalling, wingless = signalling
- horizontal stripes of wingless at D-V boundary which diffuses out to form diffusion gradient
- AC gene activated at highest Wg levels
- DII activated at intermediate Wg levels
- Vg activated at lowest Wg levels
11
Q
So how are the 4 sectors in the 2D matrix characterised?
A
- So the horizontal stripe of Wg and the vertical stripe of Dpp causes a 2D matrix
- Determinants for A/P such as engrailed solidify the 2D coordinate
