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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.
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