Fig. 2. Expression of Slit and its receptors during tracheal development. Confocal analysis of wild-type embryos carrying the UAS-EGFPF marker expressed under the control of the tracheal-specific driver btl-GAL4, double stained with anti-GFP to visualize the tracheal cells in green and anti-Slit (A,B), anti-Robo (C) or anti-Robo2 (D,E) in red. (A) Slit expression at the ventral midline close to the migrating tracheal tip cells of the ganglionic branch (top) and at the dorsal side, in cells just underneath the migrating dorsal branches (bottom, dorsal view). Dotted lines indicate the part of the tracheal dorsal branches that is out of focus. (B) Slit expression at the contacts of the gut epithelium and the tracheal visceral branches (anterior is upwards). (C-E) Lateral views showing the expression of Robo and Robo2 in the invaginating tracheal cells at stage 11 (C,D, top), the dorsal trunk and dorsal branch at stage 13 (C,D, bottom). (E) Robo2 expression in the growing visceral branches. (F) Summary drawings of Robo and Robo2 expression in different branches (DB, dorsal branch; DT, dorsal trunk; VB, visceral branch; GB, ganglionic branch) at stages 12 and 16. At stage 12 (top) both Robo (black) and Robo2 (gray) are expressed in cells of the DT, but Robo2 is also expressed in the DB and the VB. After stage 13, Robo is no longer detectable in the trachea, whereas Robo2 is expressed in DT and DB until the end of embryogenesis (bottom). Scale bars: 8 µm in A-E.

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