In higher eukaryotic organisms such as humans, certain tissues consist of a special type of cells known as stem cells. Stem cells divide to self-renew and at the same time to generate a progeny that is committed to a differentiation pathway. Although of much importance, very little is known of how a stem cell acquires its identity or how it functions. The long term goal is to understand the mechanisms that govern the development and functioning of one of the stem cell lineages, the central nervous system (CNS), of the fruit fly Drosophila. During neurogenesis, a neuroblast (NB) functions as a stem cell and divides by asymmetric mitosis to self-renew and to produce a chain of ganglion mother cells (GMCs). A GMC, though pluripotential, does not self-renew; instead it divides asymmetrically to generate two distinct neurons. At the end of neurogenesis, each half segment is thought to contain ~320 distinct and highly specialized neurons. The principal investigator has selected a typical neuroblast stem cell lineage, the NB4-2 lineage, that generates the motoneuron, RP2, and its sibling cell (RP2-sib), in the ventral nerve chord (the NB4-2-->GMC-1->RP2/sib lineage), to address the problems of how neuronal precursor cells assume their identity and how they undergo asymmetric cell division during neurogenesis. Using the power of Drosophila genetics, the applicant has identified by genetic screens well over 50 new point and deletion mutations that perturb the development of the NB4-2-->GMC-1->RP2/sib lineage. These mutations can be categorized into those which cause: loss of RP2s, gain of RP2s, loss and gain of RP2s, division defects, migration defects and special types of defects (e.g., missing RP2s from only the right side of the CNS). These reveal many novel aspects of the elaboration of a neuroblast stem cell lineage. In this proposal, the principal investigator describes experiments to characterize further a subset of these mutant lines. These experiments address: A) the asymmetric division of the GMC-1, B) the specification of the GMC-1 progeny, RP2 and RP2-sib, and C) the left-right asymmetry during the specification of the GMC-1 identity. The proposed studies include: 1) many young ones 1, many young ones 2 and a previously identified GMC-1 specification gene, mitimere, during the asymmetric division of GMC-1, 2)the signaling molecule Drosophila Wnt3, the miti gene and failed GMC division in the GMC-1 division and RP2/sib identity specification, and 3) one sided and the left-right asymmetry during the specification of the RP2/sib lineage. These studies will help understand pathways that govern the lineage elaboration in the Drosophila CNS as well as in other eukaryotic organisms including humans.
