Fig. 2. Rescue of tracheal cell migration and patterning defects by hybrid receptors. The lumen of the tracheal system of stage 15 embryos was visualized with the 2A12 antibody (green, A,B,D,E,G,H,J,K,M,N; red, C,F,I,L,O) and terminal cells were visualized with the anti-DSRF antibody (green, C,F,I,L,O). Embryos obtained from the crosses as described in the Materials and Methods were collected at 18° C and 25°C. (A) Tracheal system of a wild-type embryo. (B) In homozygous btl mutant embryos, tracheal cells failed to migrate completely. (C) Tracheal system of a wild-type embryo outlining the DSRF/bs-expressing terminal cells. (D,E) All aspects of tracheal cell migration were fully rescued in btl mutant embryos expressing a btl transgene under the control of btl-Gal4. (F) The correct number of terminal cells differentiated at the proper positions as in wild-type embryos. (G,H) The Btl-Htl chimeric receptor construct was able to fully substitute for Btl and rescued tracheal cell migration. (I) The Btl-Htl chimeric receptor construct was able to induce proper terminal cell fates. In F,I, DSRF/bs expression in visceral branches is out of the focal plane. (J,K) The Btl-Tor fusion protein was able to rescue tracheal cell migration in btl mutant embryos. Some dorsal branches were missing (arrowheads) and the lateral trunk did not fuse in each segment (arrow). Apart from these defects, dorsal trunk fusion, visceral branch and ganglionic branch formation was not affected and the general pattern of the tracheal system was restored. Note the felted phenotype of the tracheal system, owing to the fine ectopic terminal branches (see K). (L) The Btl-Tor construct led to the ectopic formation of terminal cells (arrow). (M,N) The Btl-EGFR fusion protein was able to rescue tracheal cell migration in btl mutant embryos but less efficiently than Btl-Tor. Some dorsal branches were missing (dorsal arrowheads), the lateral trunk did not fuse (ventral arrow, N), ganglionic branches often failed to migrate (ventral arrowhead, M) and in some embryos, the visceral branches were misguided (arrow, M). (O) DSRF/bs expression and terminal cell formation was strongly reduced (arrow) when compared with wild type (C).