THE FORMATION OF COMMISSURES IN THE DROSOPHILA CNS DEPENDS 017 THE MIDLINE CELLS AND ON THE NOTCH GENE

The commissures of the Drosophila central nervous system (CNS) are for med 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 l ooked for mutations that result in the absence of commissures. One exa mple of a gene that can give rise to such a phenotype is the neurogeni c gene Notch. Here we show that mutant Notch embryos are devoid of com missural connections and have an abnormal midline. The midline cells o f the embryonic Drosophila CNS are specified during the blastoderm sta ge about two hours before the first neuroblasts start to delaminate fr om the neurogenic region. To analyze Notch function for commissure dev elopment further, we took advantage of the Notch(ts1) allele. Temperat ure-shift experiments demonstrated that the lack of commissures in mut ant Notch embryos results from defects in the anlage of the CNS midlin e cells. Here maternal as well as zygotic Notch function are required for the correct activation of the gene single-minded, since mutant Not ch embryos derived from germ-line clones lack most of the singleminded -positive midline cells.