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Y. Inoue, S. Hayashi. Center for Developmental
Biology, RIKEN, Kobe, Japan.
The wing and leg imaginal discs are derived from the embryonic limb
primordium, and their cells are specified by the differential
activation of signaling such as Wg, Dpp and EGFR in the ectodermal
tissue. It has been suggested that early disc formation occurs in a
tissue-autonomous fashion, without significant involvement by other
tissues.
Here we show details of wing primordial cell movement revealed by
GFP imaging of living embryos, and present evidence for extensive
interaction between trachea and wing primordia. The wing primordium
appears at stage 13 as a subset of the limb primordial cells, and
its cells invaginate to form a sac-like structure. We found that
during the process of invagination microtubules in the wing
primordial cells became polarized along the anterior-posterior axis,
and that numerous actin-rich filopodia extended posteriorly from the
basal surfaces of the cells. Surprisingly, invaginated wing disc
cells were found to associate extensively with tracheal cells.
Although it is known that tracheal branches attach to the wing disc
in the 3rd larval instar, there are no reports on the early
interactions between these two structures.
To study the developmental significance of these attachments, we
examined early wing disc formation in embryos defective in tracheal
development. In FGF mutant embryos tracheal branching was severely
defected, however, the attachment between wing and tracheal cell was
maintained. We overexpressed Reaper and Hid to activate apoptotic
signaling in tracheal cells, and also activated Rac to cause
epithelial-mesenchymal transformation. Although these treatments
resulted in marked changes in tracheal cell morphology, the tracheal
cells continued to adhere to the wing disc cells. The robust nature
of this adhesion suggests that interaction between the wing disc and
tracheal branches might play an important role in early wing disc
formation. |
