DPP controls tracheal cell migration along the dorsoventral body axis of the Drosophila embryo

We report that DPP signaling is required for directed tracheal cell migration during Drosophila embryogenesis. The failure of tracheal cells to receive the DPP signal from adjacent dorsal and ventral cells results in the absence of dorsal and ventral migrations. Ectopic DPP signaling can reprogram cells in the center of the placode to adopt a dorsoventral migration behavior. The effects observed in response to ectopic DPP signaling are also observed upon the tracheal-specific expression of a constitutive active DPP type I receptor (TKV(Q253D)), indicating that the DPP signal is received and transmitted in tracheal cells to control their migration behavior. DPP signaling determines localized gene expression patterns in the developing tracheal placode, and is also required for the dorsal expression of the recently identified BRANCHLESS (BNL) guidance molecule, the ligand of the BREATHLESS (BTL) receptor. Thus, DPP plays a dual role during tracheal cell migration. It is required to control the dorsal expression of the BNL ligand; in addition, the DPP signal recruits groups of dorsal and ventral tracheal cells and programs them to migrate in dorsal and ventral directions.