Generating neuronal diversity in the Drosophila central nervous system:: A view from the ganglion mother cells

The generation of cellular diversity in the developing embryonic central nervous system of Drosophila melanogaster requires the precise orchestration of several convergent molecular and cellular mechanisms. Most reviews have focused on the formation and specification of neuroblasts (NBs), the putative neural stem cell in the Drosophila central nervous system. NBs divide asymmetrically to regenerate themselves and produce a secondary precursor cell called a ganglion mother cell (GMC), which divides to produce neurons and glia. Historically, our understanding of GMC specification has arisen from work involving asymmetric localization of intrinsic factors in the NB and GMC. However, recent information on NB lineages has revealed additional intrinsic factors that specify general and specific GMC fates. This review addresses what has been revealed about these intrinsic cues with regard to GMC specification. For example, Prospero, an asymmetrically localized determinant, plays a general role to enable GMC development and to distinguish GMCs from NBs. In contrast, the temporal gene cascade functions within NB lineages to ensure that each GMC in a lineage acquires a different fate. Two different mechanisms used to make the progeny of GMCs different will also be discussed. One is a generic mechanism, regulated by Notch and Numb, that allows sibling cells to adopt different fates. The other mechanism involves genes, such as even-skipped and klumpfuss that specify the fate of individual GMCs. All of these mechanisms converge within a GMC to bestow upon it a unique fate. (C) 2005 Wiley-Liss, Inc.