PRONEURAL AND NEUROGENIC GENES CONTROL SPECIFICATION AND MORPHOGENESIS OF STOMATOGASTRIC NERVE-CELL PRECURSORS IN DROSOPHILA

The stomatogastric nervous system (SNS) of the Drosophila embryo devel ops from a placode which appears in the stomodeal epithelium. Most cel ls of this placode invaginate as three pouches (the iSNSPs) into the i nterior of the embryo. After separating from the stomodeum, the SNS po uches transiently form epithelial vesicles and eventually dissociate i nto solid clusters of cells which migrate on the foregut epithelium an d differentiate into the neurons of the SNS. Prior and during iSNSP in vagination, two small subpopulations of SNSPs (dSNSPs and tSNSPs) dela minate as individual cells from the SNS placode (Hartenstein et al., 1 994). The results presented in this paper show that the neurogenic and proneural genes are expressed and required during all phases of SNS d evelopment to control the number, pattern, and structural characterist ics of the SNSP subpopulations. First, loss-of-function mutations of t he proneural and gain-of-function mutations of the neurogenic genes re sult in the absence or reduction of delaminating SNSPs; loss of functi on of neurogenic genes leads to the overproduction of d/tSNSPs and a l oss of iSNSPs. Second, both proneural and neurogenic genes are involve d in the invagination and dissociation of iSNSPs. Reduction of neuroge nic gene function leads to a premature dissociation of iSNSPs; gain of neurogenic gene function blocks invagination and dissociation of thes e cells. Since all iSNSPs form a homogenous population with regard to their differentiative fate as SNS neurons, these results indicate that lateral inhibition is not a necessary aspect of the developmental pro cess controlled by neurogenic and proneural gene function. (C) 1996 Ac ademic Press, Inc.