The vast arrays of different neural cell types that characterize the complex circuits of the brain are generated by neural stem cells.  During brain development, these neural stem cells produce defined sets of neural progeny composed of specific neural cell types which interconnect to form functional circuitry.  Understanding the molecular mechanisms that underlie this process and give rise to the astonishing number and diversity of precisely defined cell types in the brain is one of the most challenging problems in biology.

     In the past two decades, significant progress has been made in understanding the mechanisms underlying specification, polarization and division control in neural stem cells, notably in the neurogenetic model system Drosophila.  In Drosophila, the neural stem cells, called neuroblasts, are similar to vertebrate neural stem cells in their ability to self-renew and to produce many different types of neurons and glial cells.  Recent work has shown that the numerous cell types that make up the central brain of Drosophila derive from a set of approximately 100 neuroblast pairs, each of which can be individually identified, and each of which generates its own lineage-specific unit of neural progeny.  However, how this limited number of neuroblasts can generate the enormous number of neural cell types that make up adult brain circuitry is currently poorly understood. 

     The overall goal of the research outlined here is to analyse the developmental mechanisms by which neuroblasts generate the lineage-specific units of the adult brain and specify the number and diversity of cell types in each of these units through a lineage-based molecular genetic dissection.  This proposal consists of three research projects all of which incorporate the powerful genetic methods made possible by GAL4/UAS and MARCM technologies.  The first project is an analysis of the role of neuroblast-derived intermediate progenitors in generating neuronal and glial cell type diversity in specific lineages.  For this, we will capitalize on our discovery of a novel mode of neurogenesis by which identified protocerebral neuroblasts increase their progeny number and diversity through transit amplifying progenitors.  We will analyze the roles of these intermediate progenitors in postembryonic gliogenesis, in the transition from gliogenesis to neurogenesis, and in the generation of neural cell types in the central complex, a major neuropil center in the brain. The second project is an investigation of the role of early patterning genes that are re-expressed in postembryonic brain development and control neuroblast lineage-specific specification of neural cell types.  Here we will focus on the role of the cephalic gap gene empty spiracles in controlling the neuroanatomical features of interneurons in a set of deutocerebral neuroblast lineages.  Moreover, we will investigate the expression and function of the Hox gene labial in postembryonic development of the adult-specific neurons in the tritocerebrum.  The third project is a characterization of the role of programmed cell death in sculpting lineage- and sublineage-specific neural cell types and in determining final neuronal numbers in the adult brain.  In this part we will study the role of programmed cell death in regulation of cell type diversity through differential elimination of specific neuroblast hemilineages.  Moreover, we will analyze the cellular and molecular basis of a remarkably massive cell death process that affects the lineages which generate the columnar cell system of the central complex.

     The planned studies should result in a number of significant advances in our understanding of the generation and specification of neural cell types in the developing brain.  First, we should attain a better understanding of how primary and intermediate progenitors amplify number and diversity of neural cells in the fly brain.  This will be important for understanding similar neural stem cell-dependent processes in mammalian brain development.  Second, this work should advance our understanding of how lineage-dependent neuronal ensembles contribute to the formation of complex brain circuitry, and also identify and characterize key developmental control genes that act in this process.  Third, we should obtain insight into the mechanisms by which programmed cell death influences both the number and the types of adult-specific neural cells that constitute the circuits of the mature brain.