Fig. 3. SNa motor axon pathfinding defects in plexB mutants resemble plexA mutants and display novel guidance errors. (A-F) Filleted preparations of late stage 16 embryos stained with the anti-Fasciclin II monoclonal antibody to reveal motor axons of the SNa. Anterior, left; dorsal, up. (A) In wild-type embryos, the dorsal branch of the SNa projects correctly between muscles 22 and 23 before defasciculating and innervating muscle 24 (arrowhead). (B) In plexA^Df(4)C3 mutants, the dorsal-most projecting axon of the SNa fails to reach its proper target, muscle 24 (open arrowhead). (C') plexB^KG00878 mutant SNa pathways show defects in pathfinding choice. This mutant SNa axon bundle projects incorrectly between muscles 21 and 22 (open arrow), and then extends toward its proper target, muscle 24. The dashed line indicates the path that this bundle of SNa axons normally follows. (C'') plexB^KG00878 mutants, like plexA^Df(4)C3 mutants, also show a lack of muscle 24 innervation (open arrowhead). (D) Neuronal expression of plexB in a plexB^KG00878 mutant background restores proper neuromuscular connectivity (arrowheads). (E) Neuronal plexB expression in a plexA^Df(4)C3 mutant background fails to rescue the plexA^Df(4)C3 mutant phenotype (open arrowheads). (F) Neuronal expression of plexA in a plexB^KG00878 mutant background fails to rescue the plexB^KG00878 mutant pathfinding and innervation defects (open arrow and open arrowhead, respectively). (G) Schematics of two adjacent hemisegments illustrating SNa phenotypes observed in wild type (left), plexA mutants (middle) and plexB mutants (right). Scale bar in A: 10 µm for A-F.