Fig. 1. Multiple signaling pathways are required for tracheal branching. The top half of the figure depicts tracheal placodes (blue) at embryonic stage 10 and the sources of activation (magenta) of four signaling pathways known to function during tracheogenesis (EGFR, FGF, DPP, WG). The lower portion of the figure depicts tracheal metameres at stage 13/14. By this stage, wild-type trachea have formed the five primary branches: dorsal branch (DB), dorsal trunk anterior and posterior (DTa, DTp), visceral branch (VB), lateral trunk (LT), and ganglionic branch (GB). TC, transverse connective. Defects in the components of the pathways are schematized (representative genotypes of the mutants are noted below each metamere). White circles represent absence of appropriately migrating tracheal cells in mutant metameres; blue circles represent the observed positions of the tracheal cells. EGFR signaling is initiated by a localized source of RHO within the placode (magenta; Bier et al., 1990). In the absence of EGFR signaling, many cells fail to invaginate and remain clustered on the surface of the embryo, and cells are missing from every branch (see also Fig. 3G-H'). The BNL/FGF ligand is expressed in patches outside the trachea (magenta) and signals to the BTL/FGFR, which is expressed in the tracheal placode. In the absence of FGF signaling, all branches fail to migrate. The DPP ligand has localized sources dorsal and ventral to the placode (magenta) and signals through the receptors TKV (expressed in the placode) and PUT (expressed ubiquitously; Affolter et al., 1994; Ruberte et al., 1995). kni expression is DPP-dependent in the DB, LT, and GB. In the absence of DPP signaling, kni expression is lost in the DB, LT and GB, and these branches do not form. WG is expressed adjacent to the tracheal placodes (magenta) and signals to the tracheal placode to direct the transcriptional activities of ARM and dTCF. One downstream target of WG signaling is sal. In the absence of WG signaling, sal expression is lost, and DT cells fail to migrate away from the transverse connective (TC). It should be noted that other WNT molecules may be required to activate signaling through ARM/dTCF since arm and dTCF mutant phenotypes are stronger than the wg phenotype alone (Llimargas, 2000). In embryos lacking rib function, no DT is formed and the LT and GB are stunted in their migration. Stages are according to Campos-Ortega and Hartenstein (Campos-Ortega and Hartenstein, 1985).