Fig. 4. plexB mutants display a novel CNS axon bundling defect. (A-F) Filleted preparations of late stage 16 embryos stained with the anti-Fasciclin II monoclonal antibody to reveal three fascicles of longitudinally projecting axon bundles on both sides of the midline (dashed line in A) within the ventral nerve cord. Anterior, left. (A) In a wild-type embryo the three bundles of axons on either side of the midline are tightly fasciculated (arrowheads). (B) The outermost bundle of axons in plexA^Df(4)C3 mutants is disrupted at several places along the embryo (open arrows). (C) The medial bundle of Fasciclin II-positive axons in plexB^KG00878 mutants is split apart and appears as two separate fascicles (open arrowheads). (D) Neuronal expression of plexB in a plexB^KG00878 mutant background restores proper bundling of the medial fascicle (arrowheads). (E) Neuronal plexB expression in a plexA^Df(4)C3 mutant background fails to rescue the plexA^Df(4)C3 mutant phenotype (open arrows). (F) Neuronal plexA expression in a plexB^KG00878 mutant background fails to rescue the plexB^KG00878 mutant phenotype (open arrowheads). (G) Schematics illustrating CNS phenotypes observed in wild type (left), plexA mutants (middle) and plexB mutants (right). Scale bar in A: 10 µm for A-F.