Lecture 14- Drosophila appendage development
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What can be observed and concluded when a region of posterior limb bud expressing Shh is transplanted to the anterior?
Mirror image duplicated lib is produced indicating Shh acts as a morphogen in patterning structures
What can cause defects and limbs duplications?
Defect in Hh signalling
What is the name of the tissues in which many adult drosophila structures derive from?
Imaginal discs
Which imaginal disc gives rise to the wings?
Haltere disc
At what stage of Drosophila development do imaginal discs become patterned?
During metamorphosis
Give 3 example of how gene expression can be visualised through staining in the dissected imaginal disc
• Antibody staining • Reporter lines (lacz) • Molecular markers
What imaginal discs are observed in T1/2/3 segments?
T1 = leg disc T2 = leg and wing disc T3 = leg and haltere disc
What are the 3 fates for wing imaginal discs?
• Wing blade • Hinge • Notum
How many cells does a wing disc contain and what is the thickness by the end of the third larval instar and
50,000 columnar epithelial cells 2 cell thick
What is the process called in which imaginal discs unfold into their adult shape?
Eversion
What 4 compartments does the wing imaginal disc divide into?
Dorsal, ventral, anterior and posterior
What cellular properties help maintain compartment boundaries?
Different cell-cell affinities promoting local cell-cell interactions and inhibiting cell movement and intermixing
What type of gene is engrailed and where is it expressed?
Engrailed is a segment polarity gene expressed exclusively in the posterior compartment
Outline how engrailed expression in the posterior compartments
• Use Engrailed-lacz reporter gene • lacZ is expressed in place of engrailed but under the control of engrailed enhancers • lacZ converts soluble colourless X-gal into insoluble blue precipitate allowing expression to be visualised
How does Hh expression compare to Engrailed in the wing imaginal disc and why is this?
Both expressed at high levels posteriorly but not anteriorly as engrailed drives Hh expression
Why is Ci expressed anteriorly in the wing imaginal disc?
Because engrailed is expressed posteriorly and surpresses Ci expression
What is the movement of Hh in the wing imaginal disc?
Diffuses away from the posterior compartment and slightly into the anterior
Describe the expression of Ptc in the ing imaginal disc
Low levels anteriorly and posteriorly but high levels along the Hh pathway boundary
Why is there low levels of Ptc posteriorly despite high levels of Hh?
Because there are low levels of Ci (due to engrailed suppression) and Hh pathway requires Ci
Describe the Hh pathway when no Hh is present
When there is no Hh present, Ptc suppresses Smo which causes Ci to become phosphorylated and cleaved Ci acts as a repressor of gene expression
Describe the Hh pathway when Hh is present
When Hh is present, Hh binds to Ptc so no longer supresses Smo which eventually leads to the switching on of target genes
Given 3 examples of target genes of Hh signalling involved in the wing imaginal disc
Dpp, Wg and En
Where is Dpp expressed in the wing imaginal disc?
Along the A/P border
What causes Dpp expression to be high in the wing imaginal disc?
High levels of Hh lead to the Hh pathway stimulation and leads to high levels of Dpp expression as Dpp is a Hh pathway target gene