A Molecular Switch for the Neuron-Glia Developmental Decision

The genetic analysis of nervous system development has revealed a· wealth of information about the molecular mechanisms that generate neural precursors and neuronal cell type diversity (Shankland and Macagno, 1992). Until recently, however, relatively little has been known about the genetic control of gliogenesis, in part because of the comparative lack of early markers specific for glial cells or their immediate precursors' in organisms amenable to genetic analysis, such as Drosophila. One general rule that has emerged from lineage analysis of neurogenesis is that neurons and glia share a common precursor. This has been demonstrated in both the CNS and the PNS of both vertebrates and invertebrates (for reviews, see Anderson, 1989; McConnell, 1991; Doe and Technau, 1993; Jan and Jan, 1994).