Figure 3. pebble and RhoGAPp190 are Required for Motor Axon Defasciculation and Target Recognition.

(A–L) Filleted preparations of late stage 16 embryos stained with anti-FasII (the MAb 1D4) to visualize motor axon projection patterns. Anterior is left and dorsal is up. Schematic diagrams showing motor axon guidance in wild type and indicated mutant backgrounds are presented in each panel. Scale bar, 15 µm (A–C and J–L); 20 µm (D–I).

(A) In wild-type embryos, ISNb axons undergo sequential defasciculation when they arrive between muscles 7 and 6, at the proximal edge of muscle 13, and at muscle 12 to innervate appropriate muscle targets (arrows). Mild premature branching is occasionally observed (arrowhead).

(B) In pbl⁰⁹⁶⁴⁵ mutants, ISNb axons fail to defasciculate from one another and the ISNb displays an abnormally thick morphology (closed arrowheads).

(C) ISNb axons in pbl⁰⁹⁶⁴⁵ mutants follow aberrant pathways and fasciculate ectopically with adjacent axons (open arrowheads), including the transverse nerve (TN) (closed arrowhead).

(D) In wild-type embryos, SNa axons defasciculate and give rise to two branches. The dorsal branch projects to innervate muscles 22–24 with two characteristic turns, while the lateral branch extends posteriorally and innervates its target muscles.

(E) In pbl⁰⁹⁶⁴⁵ mutants, SNa axons follow the ISN trajectory (detour phenotype, open arrowhead) and the dorsal branch of the SNa is often missing (closed arrowhead).

(F) In pbl⁰⁹⁶⁴⁵ mutants, lateral SNa branches are missing (closed arrowhead).

(G) In wild-type embryos, ISN axons project to the dorsal-most muscle fields and elaborate three distinct branches: first (FB), second (SB), and third (TB).

(H) In pbl⁰⁹⁶⁴⁵ mutants, ISN axons defasciculate prematurely (open arrowheads).

(I) In pbl⁰⁹⁶⁴⁵ mutants, ISN axons often cross the segment boundary and ectopically contact the adjacent ISN (open arrowhead).

(J) RNAi–mediated knockdown of p190 causes premature defasciculation (arrow) and probably a defect in muscle target recognition (arrowhead) since two ISNb axons are thought to target the proximal edge of muscle 12.

(K) Premature defasciculation rarely initiates before ISNb axons reach muscle 6 (arrow); an innervation defect is also shown (arrowhead).

(L) RNAi–mediated knockdown of p190 in a heterozygous pbl² background leads to a characteristic defasciculation defect at the edge of muscle 12 (arrowhead).