Fig. 5. The F-actin/microtubule crosslinking activity and Ca²⁺ binding domains of Shot L(A) are required in neurons for motor axon extension. (A-H) The pattern of muscle innervation in late stage 16 Drosophila embryos. (A) Wild type. In each abdominal hemisegment, the ISN motor axons innervate dorsal muscles, and form three characteristic neuromuscular junctions (1-3) in each abdominal hemisegment (short arrows). The dorsal trunk of the trachea (T) runs approximately between the dorsalmost two connections. The SNa contacts lateral muscles and bifurcates above muscle 12 into a dorsal (d) and a lateral (l) branch (concave arrows). The ISNb (long arrows) innervates ventral muscles 12, 13, and the cleft between muscles 6 and 7. (B) shot³ null mutant. The ISN stalls (s) at the approximate position where it would normally make the second dorsalmost muscle contact (short arrow). The SNa stalls (s) at the point where it would normally bifurcate (concave arrow). The ISNb stalls (s) in the ventral muscle field (long arrows), and fails to reach muscle 12. (C-H) The indicated transgenes are expressed specifically in all neurons in shot³ mutant embryos. (C) Shot L(A)-GFP expression restores normal muscle innervation. All Shot L(A)-GFP derivatives produced green fluorescence and were present in axons when expressed in Drosophila embryonic neurons (data not shown). (D) Shot L(C)-GFP expression does not rescue normal muscle innervation. (E) Shot L(A)-rod1-GFP (Table 1) expression restores normal muscle innervation. (F) Shot L(A)-GAS2-GFP expression does not rescue normal muscle innervation. (G) Shot L(C)-GFP and Shot L(A)-GAS2-GFP co-expression does not rescue normal muscle innervation. (H) Shot L(A)-EF-hand-GFP expression does not rescue normal muscle innervation. Scale bar: 15 µm. Anterior, left; dorsal, top.

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