CNS midline cells in Drosophila induce the differentiation of lateral neural cells.

Cells located at the midline of the developing central nervous system perform a number of conserved functions during the establishment of the lateral CNS. The midline cells of the Drosophila CNS were previously shown to be required for correct pattern formation in the ventral ectoderm and for the induction of specific mesodermal cells. Here we investigated whether the midline cells are required for the correct development of lateral CNS cells as well. Embryos that lack midline cells through genetic ablation show a 15% reduction in the number of cortical CNS cells. A similar thinning of the ventral nerve cord can be observed following mechanical ablation of the midline cells. We have identified a number of specific neuronal and glial cell markers that are reduced in CNS midline-less embryos (in single-minded embryos, in early heat-shocked Notch(ts1) embryos or in embryos where we mechanically ablated the midline cells). Genetic data suggest that both neuronal and glial midline cell lineages are required for differentiation of lateral CNS cells. We could rescue the lateral CNS phenotype of single-minded mutant embryos by transplantation of midline cells as well as by homotopic expression of single-minded, the master gene for midline development. Furthermore, ectopic midline cells are able to induce enhanced expression of some lateral CNS cell markers. We thus conclude that the CNS midline plays an important role in the differentiation or maintenance of the lateral CNS cortex.

The formation of commissures in the Drosophila CNS depends on the midline cells and on the Notch gene.

The commissures of the Drosophila central nervous system (CNS) are formed in close relation to the ventral midline cells, a morphologically distinct set of cells located at the midline of the developing CNS. To analyze the function of these cells during commissure formation, we looked for mutations that result in the absence of commissures. One example of a gene that can give rise to such a phenotype is the neurogenic gene Notch. Here we show that mutant Notch embryos are devoid of commissural connections and have an abnormal midline. The midline cells of the embryonic Drosophila CNS are specified during the blastoderm stage about two hours before the first neuroblasts start to delaminate from the neurogenic region. To analyze Notch function for commissure development further, we took advantage of the Notchts1 allele. Temperature-shift experiments demonstrated that the lack of commissures in mutant Notch embryos results from defects in the analage of the CNS midline cells. Here maternal as well as zygotic Notch function are required for the correct activation of the gene single-minded, since mutant Notch embryos derived from germ-line clones lack most of the single-minded-positive midline cells.