ARGOS TRANSCRIPTION IS INDUCED BY THE DROSOPHILA EGF RECEPTOR PATHWAYTO FORM AN INHIBITORY FEEDBACK LOOP

Argos is a secreted molecule with an atypical EGF motif, It was recent ly shown to function as an inhibitor of the signaling triggered by the Drosophila EGF receptor (DER). In this work, we determine the contrib ution of Argos to the establishment of cell fates in the embryonic ven tral ectoderm, Graded activation of DER is essential for patterning th e ventral ectoderm. argos mutant embryos show expansion of ventral cel l fates suggesting hyperactivation of the DER pathway. In the embryoni c ventral ectoderm, argos is expressed in the ventralmost row of cells , We show that argos expression in the ventral ectoderm is induced by the DER pathway: argos is not expressed in DER mutant embryos, while i t is ectopically expressed in the entire ventral ectoderm following ub iquitous activation of the DER pathway. argos expression appears to be triggered directly by the DER pathway, since induction can also be ob served in cell culture, following activation of DER by its ligand, Spi tz. Argos therefore functions in a sequential manner, to restrict the duration and level of DER signaling, This type of inhibitory feedback loop may represent a general paradigm for signaling pathways inducing diverse cell fates within a population of non-committed cells.