Adult brain compartment formation requires proper scaffolding by secondary neuronal lineages. Jennifer K Lovick, Volker Hartenstein. MCDB, UCLA, Los Angeles, CA.

   The Drosophila brain is a highly ordered, compartmentalized structure. Central brain lineages (~100 paired stereotyped neural elements), mapped at the larval stage on the basis of the characteristic axon bundle formed by each lineage, constitute the fundamental units of brain circuitry. Each lineage is formed by a stem cell (neuroblast) that undergoes two phases of proliferation. An early, embryonic phase produces 10-20 primary neurons. After that neuroblasts become quiescent and subsequently reactivate at different time points during the second larval instar to initiate a second, longer phase of proliferation that produces secondary neurons (sNs); these differentiate during metamorphosis and form the vast majority of neurons of the adult brain. In the current study, we undertook systematic application of hydroxyurea pulses (chemical ablation of mitotic cells) to staged larvae, which resulted in selective loss of sN lineages (visualized in late larvae and adults with lineage-specific GAL4 reporter lines and markers for sNs). Ablation in discrete temporal windows in early larvae reveals that NBs exit quiescence at very specific times to produce sNs during larval development, thus allowing us to assign each sN lineage a birth date. Furthermore, selective ablation of different sets of lineages, resulting from HU pulses at different time intervals, showed specific effects on the volume of brain compartments at the adult stage. We see that many compartments are reduced in size or missing when a lineage or sets of lineages are ablated. Combining our knowledge of secondary lineage and compartment morphologies in the adult brain allowed us to determine the primary or scaffolding lineage for compartments. Our goal is to show the precise order in which post-embryonic lineages are born in the larvae and how this may influence the manner in which compartments develop during metamorphosis.