NEUROGENIC AND PRONEURAL GENES CONTROL CELL FATE SPECIFICATION IN THEDROSOPHILA ENDODERM

The Drosophila endoderm segregates into three non-neural cell types, t he principle midgut epithelial cells, the adult midgut precursors, and the interstitial cell precursors, early in development. We show that this process occurs in the absence of mesoderm and requires proneural and neurogenic genes. In neurogenic mutants the principle midgut epith elial cells are missing and the other two cell types develop in great excess. Consequently, the midgut epithelium does not form. In achaete- scute complex and daughterless mutants the interstitial cell precursor s do not develop and the number of adult midgut precursors is strongly reduced. Development of the principle midgut epithelial cells and for mation of the midgut epithelium is restored in neurogenic proneural do uble mutants. The neurogenic/proneural genes are, in contrast to the n euroectoderm, not expressed in small clusters of cells but initially h omogeneously in the endoderm suggesting that no prepattern exists whic h determines the position of the segregating cells. Hence, the segrega tion pattern solely depends on neurogenic/proneural gene interaction. Proneural genes are required but not sufficient to determine specific cell fates because they are required for cell type specification in bo th ectoderm and endoderm. Our data also suggest that the neurogenic/pr oneural genes are involved in the choice between epithelial versus mes enchymal cell morphologies.