Imaginal discs and what they teach us about morphogens
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Imaginal discs and what they teach us about morphogens. p [email protected] hedgehog / engrailed. time. wingless. * engrailed expression is permanent in posterior compartment cells. Wingless/engrailed.

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Imaginal discs and what they teach us about morphogens

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Imaginal discs and what they teach us about morphogens

[email protected]




*engrailed expression is permanent in posterior compartment cells


* after several hours of interdependence of wingless and hedgehog:

situation changes.

Development is a timed process; once a situation is established. Next

one is set up…

Where do adult fly structures come from?

Each part of the adult fly is derived

from an imaginal(imago=adult) disc.

Imaginal discs metamorphose into

adult structures in pupal stage

Each part of body grows and

develops independently

Each disc is derived from

set of ectodermal cells in

early embryo.

During metamorphosis

The flat sheets of cells, shoot out to from

3-D structure.

Where do imaginal (derived from the imago=adult) discs come from?

wingless and hedgehog/engrailed and intersection with:

in dorsal ventral axis: dpp



wingless expression



decapentaplegic expression

dorsal-ventral restricted stripe

Step 1: what gene activities “control” where discs are going to form?







Step 2: molecules that set up the patterning within disc


Disc derived from intersection of

wingless/hedgehog and dpp

expression (these activate


Expression of these genes is

maintained and drives patterning




Patterning in anterior-posterior axis:

Hedgehog, Wingless, decapentaplegic

Their signallingdrives cells into particular

differentiation pathways.


leg disc

Expression of dpp and wingless is maintained by

hedgehog signalling

Stable boundary from embryos (compartment

boundary): forms the source of morphogenetic


Intersection of dpp/wingless:


leg disc

In fact, dorsal ventral boundary was already

there in embryos (cells see either green or

pink or combinations of these).

In a body part sticking out: proximal-distal axis…?

Intersection between dorsal-ventral

and anterior-posterior defines proximal-distal axis!

High point is most distal!

Distalless expression defines limb extremities


What is a morphogen I: The “classic” definition by Lewis Wolpert

  • A morphogen is a diffusible molecule that elicits direct long-range concentration-dependent changes in gene expression and (in the end) in cellular behaviour.

  • Seductive idea because: it provides a simple correlation between specific substances (morphogens), an input (their concentration) and an output (the response of the cells, which is expressed as a developmental pattern).

What is a morphogen IIThe key attributes of morphogens — on the basis of the classical definition and on present perceptions — are:

  • They are secreted, diffusible molecules that come to be distributed in a concentration gradient from a fixed spatial source.

  • They generate several (at least three) discrete cellular states in response to different thresholds of the concentration gradient. These cell states are usually associated with differential gene expression.

  • They are instructive (that is, they function as a determinant of the cell state) in a direct manner (that is, without intermediates; the response of the cells does not depend on the cell changing states first).

  • Examples of molecules and situations that fulfil the criteria above are: Spatzle, specifying the dorsoventral axis of the Drosophila melanogaster embryo. Bicoid,specifying the anterior-posterior axis of the Drosophila embryo

  • Activins, specifying and patterning the mesoderm during the early development of amphibians and other vertebrates. 

  • Sonic Hedgehog, in vertebrate limb development

Development of vertebrate


naïve group of cells

What is a morphogen III: Sonic Hedgehog in vertebrates

A patterning activity present

in posterior cells

Works as morphogen

in anterior-posterior axis






Wingless as a “classic” case of a morphogen (or?)

Set up by pair rule

  • Uniform overexpression of wingless: modest expansion of engrailed



Ubiquitous wingless

Engrailedis expressed only in competent cells

Fate map

In pupae, discs turn into structures (by extension mainly), cells have a “molecular definition”.

* Can we use development of fly wings/legs as a model for

“making” an extremity (understand the underlying genes)?

* Study molecular basis of disc patterning….

* Study morphogens….

Zecca M, Basler K & Struhl G Direct and Long-Range Action of a Wingless MorphogenGradient, Cell, Volume 87, Issue 5, 1996, 833 - 844

THE FLP-out technique





CD2, y+



CD2, y+




Clone induction at 1st instar

Observation at 3rd instar

Zecca M, Basler K & Struhl G Direct and Long-Range Action of a Wingless MorphogenGradient, Cell, Volume 87, Issue 5, 1996, 833 - 844

Characterization of membrane-tethered Wingless expressed

from the wingless locus.

C Alexandreet al. Nature 505, 180-185(2014) doi:10.1038/nature12879

Gene expression in wingless-null mutant patches surrounded

by wild-type or Neurotactin–Wingless-expressing cells.

C Alexandreet al. Nature 505, 180-185(2014) doi:10.1038/nature12879

Activity of the wingless promoter in the prospective wing.

C Alexandreet al. Nature 505, 180-185(2014) doi:10.1038/nature12879

Tissue-specific allele switching to assess the contribution of Wingless release to organ-autonomous growth rate and organismal developmental timing.

C Alexandreet al. Nature 505, 180-185(2014) doi:10.1038/nature12879

No long range function for Wingless

  • Since Wg does not act at a distance then it is not a morphogen with the classical sense of the definition. An idea that held gospel status in developmental biology. Time to revisit Wnts? Tear up the textbooks?

  • Or as a famous victorian lady commented on Darwinism: “Let’s hope that it is not true but, if it is true let us make sure that it is not widely known”

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