Fig. 4. Loss of DRI results in axon defasciculation in the Drosophila embryonic CNS. Dorsal view of dissected ventral nerve cord preparations. Anterior is towards the left. (A) The first axon bundle in control dri heterozygotes is revealed by anti-GFP staining of ftz-GAL4-driven UAS-tauGFP. (B) ftz-GAL4-driven UAS-tauGFP staining of a dri homozygous mutant embryo shows that the first axon bundle is correctly established. (C) Immunochemical staining with mAB 1D4, which recognizes the Fasciclin 2 protein, reveals the three axon bundles either side of the midline in a wild-type stage 16 embryo. (D) In dri¹/dri² mutant embryos, the first axon tract occupies the correct position relative to the midline, while the spacing between other tracts is slightly increased. Moreover, a moderate axon defasciculation is evident. In addition, axons occasionally cross between fascicles (arrow). (E) Another example of a dri¹/dri² mutant embryos, showing defasciculation of the three axon bundles. (F) pnt^rm254 mutants exhibit a mild defasciculation of axons, similar to those observed in dri mutant embryos. (G) Strong pathfinding defects are observed in gcmP1 mutant embryos.