Fig. 3. foi mutants show defects in tracheal branch fusion. (A) Schematic of the developing tracheal system highlighting lateral trunk (LT) fusion during stages 14 and 15. Drawing courtesy of Mark Krasnow and is modified from Manning and Krasnow (Manning and Krasnow, 1993). GB, ganglionic branch. (B-D) Confocal images of stage 16 embryos (breathless Gal4 x UAS-mCD8-GFP) immunostained to label the tracheal cells (anti-GFP, red) and the tracheal lumen (Mab 2A12, green). (B) Wild-type embryo. (C,D) foi^20.71 mutant embryos. Short arrows indicate segments where tracheal branch fusion has occurred and long arrows indicate segments where fusion has failed to occur. Note that the ganglionic branches still extend ventrally (down) from segments with failed fusion. Inset in D highlights protrusions between fusion tip cells in a segment where fusion failed to occur. (E) Confocal image of a stage 15 foi^20.71 mutant embryo containing the escargot^G66B enhancer trap, immunostained to reveal enhancer trap expression (anti-ß-gal, red) and the tracheal lumen (Mab 2A12, green). Asterisks indicate fusion tip cells expressing the esg enhancer trap. Note that the fusion tip cells from the segment that has failed to fuse still express the enhancer trap. Short arrows indicate segments where tracheal branch fusion has occurred and long arrows indicate segments where fusion has failed to occur.