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Imaginal discs of Drosophila as a model for development of large fields of cells.
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We have discussed how the mode of development in the Drosophila embryo switches
from a non-cellular, transcription factor driven process to a cell-cell signalling mode. The
gene products that are coordinating both of these processes were all discovered in
mutagenesis screens. The phenotypes of the segment polarity mutants (encoding the
signalling mode proteins) are all similar: in each and every segment a patterning defect is
seen (four classes of phenotypes). The underlying genes were cloned. Two genes,
hedgehog and wingless were shown to be expressed in a small subset of cells in the
segment, while they seemed to influence many neighbouring cells: as secreted proteins.
Most of the other known segment polarity genes were cloned and are required in a
signalling pathway downstream of either signal (i.e. same mutant phenotype: proteins
encoded by the genes work to common end).
The anterior limit of expression of hedgehog and engrailed in the anterior of the
parasegment (=posterior of segment) defines the middle of the later arising segment (pair
rule genes are responsible for this parasegment set up by precisely setting up anterior
boundaries of engrailed and hedgehog expression). The action of the hedgehog and
wingless signals is then required to set up the final segment using the parasegment
boundary as a reference point (organiser). This double “segment” system (first division of
the future segment into compartments whereafter the segment is formed) is also seen in
vertebrates.
The adult fly arises from the pupa. In the pupa, structures called imaginal discs (imago
means adult form) metamorphose into the adult structures. These imaginal discs are prepatterned by the time metamorphosis takes place (a clear fate map can be drawn up as
well as clear molecular evidence in the form of gene expression is visible in mature
discs). During larval stages, discs are therefore patterned (and grow). Discs are formed
during embryogenesis from about 5 to 10 cells, ending up with about 5000 at the end of
the third instar larval stage. In fact, initial patterning in the discs has taken place during
embryogenesis. In the mini discs of 5-10 cells, expression of hedgehog and engrailed
(posteriorly), wingless anteriorly and decapentaplegic (dpp) dorsally are already present.
The expression of hedgehog, wingless and engrailed has been discussed above and in
previous lecture (so patterning of the discs which will eventually form the legs, wings
and most of the external of the fly, is derived from the original embryonic segment!);
expression of dpp is set up by dorsal ventral mechanisms that as with anterior-posterior
systems rely initially on maternal contributions. Dpp expression is visible in a stripe of
cells running perpendicular to the wingless/hedgehog stripes. Dpp is a fly homologue of a
large family of secreted signalling proteins, Bone Morphogenetic Proteins, BMPs. The
intersection of dpp, wingless and hedgehog defines the site where the discs will form,
culminating in the expression of the distalless gene in the cells defined by the intersection
(distalless encodes for a homeobox containing transcription factor; and no surprise here,
its mutant phenotype is no distal elements, i.e. legs/wings).
How are the imaginal discs patterned? Mathematical models for patterning of simple 3D
systems have been proposed on the basis of experimental (regeneration) and molecular
data. Both aim to generate a 3-D model of development. What in reality takes place in fly
imaginal discs, we know now depends on the signalling processes by hedgehog, wingless
and dpp; the expression of these signalling proteins has been established in the discs from
the time they arise. Hedgehog signalling is required to keep the expression of dpp and
wingless on, in a narrow stripe of cells just anterior to the hedgehog expression domain
(hedgehog domain remains overlapping with the engrailed expression domain in
posterior compartment cells). So the boundary that separates hedgehog expressing cells
and the cells immediately adjacent (in embryos, parasegment boundary), expressing
wingless, hh or dpp, is maintained in the discs. The narrow band of cells expressing the
signalling molecules defines a source of morphogenetic activity (in both dorsal/ventral
and anterior/posterior directions). These signals work together to pattern the disc. Other
complex molecular mechanisms are in place to assist the system. For instance, the stable
expression of hedgehog and engrailed in the posterior cells leads to the inhibition of
hedgehog expression in the cells more anterior to the dpp/wingless stripe (through a
transcriptional repressor). Mixing of cells between all of these different areas is made
impossible by (less understood) repulsion and attraction mechanisms. As in the segments,
the narrow band of cells expressing wingless and dpp is stable in its position and will not
move despite cell division and growth, because it is maintained by hedgehog signalling
(this is all anterior-posterior). The dorsal-ventral division in the mature disc is determined
by the intersection of the wingless stripe and the dpp stripe as defined in the embryonic
disc. A third dimension is added: the cells that see all three molecules (at the intersection
of the three signalling factors (ie at the A/P and D/V boundary) define the most distal
cells, and thus the third axis, the proximal distal axis (high point of activity of all
morphogens).
Remarkably, gene homology as well as conserved gene interactions between this
patterning system and the patterning of vertebrate organs is seen in the case of the
vertebrate limb. Here we see sonic hedgehog (vertebrate hedgehog homologue) expressed
in posterior cells, where it acts as a signal to drive anterior-posterior patterning. It induces
expression of similar target genes: a dpp homologue, BMP2. In addition, dorsal-ventral
patterning driven in fly imaginal discs by the expression of wingless, is replicated in the
vertebrate limb bud (Wnt gene).
Text books:
Chapter 5, Wolpert, Principles of Development; pages 320-327 (1st edition), 351-355 (2nd edition).
Chapter 11, Wolpert, Principles of Development, pages 449-450 (1st edition), 335-339 (2nd edition).
Chapter 14, Gilbert, Developmental Biology; pages 746-753 (5th edition), 557-561 (6th edition), 584-587
(7th edition).
Chapter 14, Gilbert, Developmental Biology; pages 716-721 (5th edition), 513-514 (6th edition), 534-536
(7th edition).
Literature:
Méthot and Basler (1999) Hedgehog controls limb development by regulating the activities of distinct
trancriptional activator and repressor forms of cubitus interruptus. Cell, 96, 819-831.
Teleman and Cohen (2000). Dpp gradient formation in the Drosophila wing imaginal disc. Cell, 103, 971980.
Shubin, N., Tabin, C., Carroll, S. (1997) Fossils, genes and the evolution of animal limbs. Nature, 388, 639648.