Roundabout signalling, cell contact and trophic support confine longitudinal glia and axons in the Drosophila CNS

Contrary to our knowledge of the genetic control of midline crossing, the m echanisms that generate and maintain the longitudinal axon pathways of the Drosophila CNS are largely unknown. The longitudinal pathways are formed by ipsilateral pioneer axons and the longitudinal glia. The longitudinal glia dictate these axonal trajectories and provide trophic support to later pro jecting follower neurons. Follower interneuron axons cross the midline once and join these pathways to form the longitudinal connectives. Once on the contralateral side, longitudinal axons are repelled from recrossing the mid line by the midline repulsive signal Slit and its axonal receptor Roundabou t. We show that Longitudinal glia also transiently express roundabout, whic h halts their ventral migration short of the midline. Once in contact with axons, glia cease to express roundabout and become dependent on neurons for their survival. Trophic support and cell-cell contact restrict glial movem ent and axonal trajectories. The significance of this relationship is revea led when neuron-glia interactions are disrupted by neuronal ablation or mut ation in the glial cells missing gene, which eliminates glia, when axons an d glia cross the midline despite continued midline repellent signalling.