Top-DER- and Dpp-dependent requirements for the Drosophila fos/kayak gene in follicular epithelium morphogenesis

The Drosophila fos (Dfos)/kayak,ene has been previously identified as a key regulator of epithelial cell morphogenesis during dorsal closure of the em bryo and fusion of the adult thorax. We show here that it is also required for two morphogenetic movements of the follicular epithelium during oogenes is. Firstly, it is necessary for the proper posteriorward migration of main body follicle cells during stage 9. Secondly, it controls, from stage I I onwards, the morphogenetic reorganization of the follicle cells that are co mmitted to secrete the respiratory appendages. We demonstrate that DER path way activation and a critical level of Dpp/TGF beta signalling are required to pattern a high level of transcription of Dfos at the anterior and dorsa l edges of the two groups of cells that will give rise to the respiratory a ppendages. In addition. we provide evidence that, within the dorsal-anterio r territory, the level of paracrine Dpp/TGF beta signalling controls the co mmitment of follicle cells towards either an operculum or an appendage secr etion fate. Finally, we show that Dfos is required in follicle cells for th e dumping of the nurse cell cytoplasm into the oocyte and the subsequent ap optosis of nurse cells. This suggests that in somatic follicle cells. Dfos controls the expression of one or several factors that are necessary for th ese processes in underlying germinal nurse cells. (C) 2001 Elsevier Science Ireland Ltd. All rights reserved.