Commissure formation in the embryonic CNS of Drosophila II. Function of the different midline cells

Most of the neurons of the ventral nerve cord send out long projecting axon s which cross the midline. In the Drosophila central nervous system (CNS) c ells of the midline give rise to neuronal and glial lineages with different functions during the establishment of the commissural pattern. Here we pre sent evidence that beside the previously known NETRIN/FRAZZLED (DCC) signal ling system an additional attractive system(s) is operating In the developi ng embryonic nervous system of Drosophila. Attractive cues appear to be pro vided by the midline neurons. We show that the glial cells present repulsiv e signals to the previously described ROUNDABOUT receptor in addition to a permissive contact-dependent signal helping commissural growth cones across the midline. A novel repulsive component is encoded by the karussell gene. Furthermore the midline glial cells separate anterior and posterior commis sures. By genetic criteria we demonstrate that some of the genes we have id entified are acting in the midline glia whereas other genes are required in the midline neurons. The results lead to a detailed model relating differe nt cellular functions to axonal patterning at the midline.