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Fig. 4. Functional networking of Wnt and Notch signalling during
Drosophila wing development. During wing primordium patterning
and sensory organ precursor (SOP) specification, the Wingless and Notch
pathways act in a dynamic signalling landscape that endows cells with identity
and orientation. The images to the left and right show wingless
(wglacZ) expression in the epidermis of two wing discs at very early
(left) and mid-third (right) instar. The white line straddling the middle of
the Wingless stain represents the primordium of the wing margin (wm).
Throughout the figure, ventral (V) is orientated down, dorsal (D) up,
posterior (P) to the left and anterior (A) to the right. (A) Schematic
of a wing disc between the first and second larval instars. Wing development
is initiated at the intersection of the AP and DV boundaries by the joint
activity of Notch and Wingless signalling. (B) During the transition
from second to third instar, the DV boundary is established through the
activity of Notch signalling triggered, initially, by the asymmetrically
localized Notch ligands Serrate (Ser, dorsal, yellow) and Delta (Dl, ventral,
red) that lead to the activation of wingless expression (blue) in a
wide domain with a peak in the middle. This domain becomes progressively
restricted to the DV boundary through an autoinihibitory effect of Wingless on
its own expression. (C) Serrate and Delta are targets of Notch
signalling, and Wingless signalling can contribute to this activation
(arrows). As a result, a pattern emerges (D) of symmetric expression of
Delta and Serrate (orange) and of Wingless (Wg, blue) at the DV boundary. This
combination of events (B-D) leads to the formation and definition of the DV
compartment boundary and its maintenance through a feedback loop in which
Wingless maintains the expression of Serrate and Delta, which in turn maintain
the expression of Wg. (E-G) As the third instar develops, the
peripheral nervous system emerges (E). At the wing margin, it appears around
the DV boundary within the domain of Delta and Serrate expression. In the
second half of the third instar, high levels of Wingless signalling (blue)
lead to the expression of proneural genes, such as senseless and
members of the Achaete/Scute family (pink in F), which promote the appearance
of SOPs (red in G). Notch-mediated lateral inhibition generates the spaced
precursors that express high levels of AS-C and that will develop into sensory
organs.
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