The role of different tissue interactions in the formation of the early wing disc primordia in Drosophila

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.

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