Developmentally arrested precursors of pontine neurons establish an embryonic blueprint of theDrosophilacentral complex

Serial electron microscopic analysis shows that the Drosophila brain at hatching possesses a large fraction of developmentally arrested neurons with a small soma, heterochromatin-rich nucleus, and unbranched axon lacking synapses. We digitally reconstructed all 812 “ small undifferentiated” (SU) neurons on both hemispheres and assigned them to the known brain lineages. 54 SU neurons belonging to the DM1-4 lineages, which generate all columnar neurons of the central complex, form an embryonic nucleus of the fan-shaped body (FB). These “ FB pioneers” develop into a speci1c class of bi-columnar elements, the pontine neurons. Even though later born, unicolumnar DM1-4 neurons fasciculate with the FB pioneers, selective ablation of these cells did not result in gross abnormalities of the trajectories of unicolumnar neurons, indicating that axonal path1nding of the two systems is controlled independently. Our comprehensive spatial and developmental analysis of the SU neuron adds to our understanding of the establishment of neuronal circuitry.