connectivity in the brain. In this grant we will address the question of neurite branching in Drosophila. Neurite branching is one of the key processes that shape connectivity in the CNS of all animals. Understanding the molecular mechanisms of neurite branching will be important, not only for the neuroscientist interested in understanding brain development, but also for the clinician following the rapidly evolving therapeutic approaches that utilize transplants of undifferentiated neurons for neurodegenerative diseases, such as Parkinson's or Alzheimer's disease. For such transplants to be effective, it would greatly help if one could manipulate the manner in which the donor neurons interact with the host's microenvironment and, in response, form branched neurites that become integrated within the host's circuitry. Drosophila provides an excellent model system to address neurite branching at a high level of resolution because the wealth of molecular-genetic tools and the fact that neurons of the brain fall into lineages that are generated from a relatively small number of stem cell-like neuroblasts. In work of the previous funding period we have generated a comprehesive map of the larval brain lineages and their initial path of projections in the larva. The nerve fiber tracts of each lineage undergo a characteristic pattern of branching, which makes it possible to recognize and work with them. The proposed studies address the following hypotheses: (i) interaction between axons and glia at the cortex-neuropile boundary set branchpoints; (ii) within axons, the setting of branchpoints depends on the Par complex, which according to (now extended) preliminary results is localized at future branchpoints; (iii) the interaction between neurites and glia and the ensuing concentration of Par3 at the branchpoint involves cadherin-mediated interaction. Our main support for this set of hypotheses is the observation that reducing each of the three factors, glia, Par, and DEcadherin results in a similar phenotype in larval lineages, which consists in defects in proximal branching. In aim #1 we will investigate structurally and experimentally neurite-glia interactions. In aims #2 and #3 we propose an experimental and genetic analysis of two molecular pathways, the Par complex and the Cadherin adhesion complex, in PIB formation. Aim#4 entails genetic screens for novel genes: a gain-offunction screen to identify genes that affect interstitial branching in the central larval brain, and a second screen to identify genes/enhancers expressed in specific lineages.