Fig. 6. DPP signaling inhibits terminal branch extension. (A) When a high level of Dpp signaling was maintained by expressing dpp, elongating terminal branches were misoriented and were stalled (white arrowheads). Compare this with the control terminal branch shown in Fig. 5C. Overexpression of Dpp promoted excess recruitment of tracheal cells into DB (Vincent et al., 1997), but the number of terminal cells expressing SRF remained one per DB (data not shown). (B-F) Premature reduction of Dpp signaling by overexpression of Dad causes misdirection of the terminal branch. (B) Time-lapse images of a terminal branch that crossed the dorsal midline (arrowhead). (C-F) Phenotypes of Dad expression in terminal branches were classified as either class 1 (C, normal), class 2 (D, drifting along the AP axis) or class 3 (E, midline crossing). Class 2 and 3 phenotypes included another subcategory, the class 4 phenotype (F, bipolar elongation). The asterisk indicates that this phenotype is a subset included within the class 2 and 3 phenotypes. A total of 103 terminal branches that reached dorsal midline (broken line) were scored.