During organ morphogenesis, cells coordinate their behaviours in both time and space, but how do they achieve this complex feat? On p. , Michael Boyle and Celeste Berg reveal that, during Drosophila egg chamber development, the transcription factor Tramtrack69 (TTK69) interacts with Notch and the insect steroid hormone Ecdysone to coordinate cellular behaviours temporally and spatially. Fly egg chambers, which mature through 14 stages, contain a single oocyte covered with columnar follicle cells, which later form dorsal appendages (DAs; protrusions that facilitate gas exchange). The researchers show that TTK69 and Notch form a mutually repressive feedback loop, and that an Ecdysone-mediated switch from Notch to TTK69 expression regulates the fates and shapes of the columnar follicle cells at stage 10B. Later in development, TTK69 controls DA tube volume. These and other results, suggest the authors, support a model for the regulation of egg chamber development in which spatially restricted co-factors work with TTK69 to define appropriate responses to a globally available temporal signal.
