Figure 4.

Axons of aCC/RP2 motor neurons can influence ISN formation. Figures show dorsal muscle fields of one or two consecutive hemisegments in late embryos (compare Fig. 1) (1 and 2 indicate position of terminals on DA1/DO1 and DA/DO2 muscles) with the terminals or axons of the entire ISN in magenta [visualized with FasciclinII (Fas), synapsin (Syn), or Discs large as indicated at the top left; see Results for details] and the Rn2-Gal4, U/CQ-Gal4, or MzVum-Gal4 targeted axons in green (visualized via CD8 expression). Normally, the CD8-labeled terminals of aCC/RP2 neurons (A), U neurons (B), and VUMs (C) reach most dorsal positions (asterisks). If Rdl is expressed in these neurons (A′-C′), their axons stall frequently (straight arrows); entire ISNs tend to coarrest with stalled aCC/RP2s (A′; white circles indicate noninnervated muscles; arrowheads point at tips of ISNs, open arrowhead in A′ indicates a case in which axons escaped aCC/RP2 but stalled thereafter, as in ablation experiments). ISNs mostly escape from stalled U neurons and VUMs (B′, C′, asterisks; ISN coarrestin Rn2::Rdl = 70%; U/CQ::Rdl = 10%; MzVum::Rdl = 0%). Coarrest of ISNs with stalled aCC/RP2 motor axons is severely suppressed in fasciclinII^eb112 loss-of-function mutant background as shown for the right segment in A″ where the aCC/RP2 axons (green) are stalled, whereas Dlg-labeled terminals on dorsal muscles (magenta) are visible (ISN coarrest in Rn2::Rdl; fasII^eb112 = 36%). In contrast, ISN coarrest with stalled U neurons is severely increased, if FasciclinII is coexpressed with Rdl (right axon in B″; ISN coarrest in U/CQ::Rdl::fasII = 54%). Targeted expression of activated Notch (N^ICD; D) or dominant-negative DRac1 (Rac^N17; E) in aCC/RP2s likewise leads to axon stall and coarrest of the ISN, confirming the findings with Rdl (see Fig. 1 for details). Scale bar, 16 μm.