ESTABLISHMENT OF VENTRAL CELL FATES IN THE DROSOPHILA EMBRYONIC ECTODERM REQUIRES DER, THE EGF RECEPTOR HOMOLOG

The embryonic ectoderm in Drosophila displays a highly organized arran gement of specific structures along the dorsal-ventral axis. To establ ish this characteristic design, cells must receive instructive cues re garding their position. We present evidence that during stages 8-9 of embryonic development, the Drosophila EGF receptor homolog (DER) is es sential for determining the identity of cells within the ventral ectod erm. In the absence of DER activity at this phase, alterations in cell fate are observed: Ventral cells acquire more dorsal fates, as visual ized by the expression profile of specific markers. The ventralizing e ffect of DER appears to function later than that of the dorsalizing dp p pathway, and the spatial overlap between them is minimal. A model fo r the determination of cell fates along the dorsal-ventral axis involv ing the two pathways is presented. Some aspects of the mutant ectoderm al and CNS phenotypes of the DER locus (faint little ball, flb) resemb le the phenotype of mutations from the spitz group. Synergistic intera ctions between flb and spitz or Star mutations suggest that these gene s participate in a common signaling pathway.