Fig. 8. Ectopic Engrailed expression in all midline cells affects the differentiation of all midline lineages. Midline precursors in wild type (A-E) and embryos expressing Engrailed in all midline cells (sim-GAL4/UAS-En, F-K) were labelled with the fluorescent dye, DiI. After differentiation of the progeny, DiI was photoconverted to generate a permanent brown stain. The subset of lineages are indicated: MP1, midline precursor 1 neuron; UMI, unpaired median interneurons; VUM, ventral unpaired median neurons; MNB, median neuroblast neurons. Clones in F,J are also stained with anti-Engrailed (En, black). (A) Midline glia tightly enwrap the commissures. (E) Midline glia ectopically expressing Engrailed are found on the dorsal surface of the CNS. (B) The bifurcated axons of MP1 neurons project along the longitudinal tracts. We never obtained a clone resembling MP1 neurons in embryos expressing Engrailed in all midline cells. (C) The projection of one UMI (arrow) differs from the second UMI axon (arrowhead). (G) Expression of ectopic Engrailed in UMI only slightly affects their projections. (D) The axons of VUM motoneurons (arrow) bifurcate in the anterior commissure, whereas the VUM interneuronal axons (arrowhead) bifurcate in the posterior commissure. The cell bodies of VUM neurons are out of focus, at the ventral surface of the CNS. (H) When Engrailed is expressed ectopically in midline cells, all of the axons of the VUM motoneurons (arrow) exit the CNS on the same side. (I) Interneuronal axons (arrowhead) can show the same phenotype. (E) The axons of MNB motoneurons (arrows) and interneurons (arrowhead) bifurcate in the anterior commissure. (J,K) Ectopic Engrailed in MNB neurons prevents or delays the bifurcation of their axons. Ectopic Engrailed expression on the midline causes all subsets to be mispositioned along the dorsal surface of the CNS. Ventral views, anterior to the left.