Fig. 1. Ablations of glial cells affect tracheal cell guidance. (A-C) Confocal projections showing the morphology of the ganglionic branch tip cell (GB1) visualized in green by the 1-eve-1 lacZ cytoplasmic marker. Robo staining, in red, was used to mark the longitudinal connectives. (A) At stage 15, GB1 has reached the ventral side of the neuropil and has extended several cytoplasmic processes. (B) At stage 16, GB1 turns at the CNS midline and extends one cytoplasmic protrusion dorsally and posteriorly along the dorsoventral channel glia to reach the dorsal side of teh neuropil. The GB1 nucleus has reached its position just ventral of the longitudinal connective. (C) By late stage 16, GB1 has extended laterally and posteriorly on the dorsal side of the neuropil. (D) Schematic drawing showing a ventral view of three pairs of GBs and their path in the wild-type embryonic stage 16 nerve cord. Inside the CNS, GB1 (dark blue) contacts first the ventral longitudinal glia (pale green). It turns before reaching the ventral midline and the midline glia (red), and migrates dorsally at the midline in close contact with channel glia (yellow). When GB1 reaches the dorsal side of the neuropil, it associates with the dorsal longitudinal glia (dark green) and extends laterally and posteriorly. The stalk cell of the ganglionic branch, GB2 is shown in pale blue, longitudinal axon tracts in brown and the borders of the CNS with broken lines. (E-I) GBs are misrouted when longitudinal glia are ablated by the expression of UAS-ricinA. The tracheal lumen of stage 16 embryos is shown in blue and the longitudinal and midline glia cells in E,F or the longitudinal connectives in G-I in brown. (E) Longitudinal and (F) midline glial cells are shown by the expression of UAS-lacZ driven by C321c-GAL4 and by Mz520-GAL4, respectively. (H) When longitudinal glia are ablated GBs stall outside CNS (arrow) or turn to migrate posteriorly before crossing the longitudinal tracts (arrowhead). Ablation of midline glial cells leads to midline crosses (I) of the GBs (arrows) and the longitudinal axon tracts (arrowhead) (compare with G). All panels show ventral views, anterior is towards the left. Scale bars: 4 µm in A-C; 8 µm in E-I.