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Fig. 2. Motor axons are misguided in piranha^D427/piranha^K788 mutant larvae. (A-F) Confocal micrographs of the ISNb innervating ventral muscles 12, 13, 6 and 7 in dissected wild-type (A-C) and piranha mutant (D-F) larvae. Muscles and postsynaptic terminals are visualized with CD8-GFP-Sh (A,D), motor axons and presynaptic endings are stained with anti-Fasciclin II (B,E). The ISNb normally innervates muscle 12 on its ventral side via a ventrodorsal projection (arrowheads in B,C). In a piranha mutant hemisegment, muscles 12, 13 and 6 are innervated at ectopic sites by a dorsoventral nerve projection (arrowheads in E,F). Muscles 6 and 7 remain uninnervated (arrows in D-F). Arrows mark the cleft between muscles 6 and 7. Asterisks indicate type II boutons. (G-L) Transgenic labeling of motor axons reveals guidance defects in living piranha^D427/piranha^K788 mutant larvae (exterior views). In addition to CD8-GFP-Sh, larvae express dsRed2 in all motoneurons using OK371-Gal4. (G,J) Schematic diagram (G) and confocal micrograph (J) of the hemisegmental nerve defasciculating into five nerve branches at the ventral choice point of a wild-type larva. (H,K) Schematic diagram (H) and confocal micrograph (K) of the defasciculation defects in a piranha mutant larva. The ISNb failed to branch into the ventral region and migrated in a parallel pathway along the ISN. The ISN, ISNb and SNa pathways are visible as separate nerve bundles (arrow). An axon branching out of the misguided ISNb innervates muscle 13 at an ectopic position (arrowhead). (I,L) Schematic diagram (I) and confocal micrograph (L) of the ventral muscle field of a piranha mutant larva. The SNa stays attached to the SNc and defasciculates too late (arrow). Asterisks in (J-L) indicate nerves innervating neighboring hemisegments.

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