INTERACTION BETWEEN DROSOPHILA EGF RECEPTOR AND VND DETERMINES 3 DORSOVENTRAL DOMAINS OF THE NEUROECTODERM

Neurogenesis in Drosophila melanogaster starts by an ordered appearanc e of neuroblasts arranged in three columns (medial, intermediate and l ateral) in each side of the neuroectoderm. Here we show that, in the i ntermediate column, the receptor tyrosine kinase DER represses express ion of proneural genes, achaete and scute, and is required for the for mation of neuroblasts, Most of the early function of DER is likely to be mediated by the Ras-MAP kinase signaling pathway, which is activate d in the intermediate column, since a loss of a component of this path way leads to a phenotype identical to that in DER mutants, MAP-kinase activation was also observed in the medial column where esg and proneu ral gene expression is unaffected by DER. We found that the homeobox g ene vnd is required for the expression of esg and scute in the medial column, and show that vnd acts through the negative regulatory region of the esg enhancer that mediates the DER signal, suggesting the role of vnd is to counteract DER-dependent repression. Thus nested expressi on of vnd and the DER activator rhomboid is crucial to subdivide the n euroectoderm into the three dorsoventral domains.