ECTOPIC ACTIVATION OF TORPEDO EGFR, A DROSOPHILA RECEPTOR TYROSINE KINASE, DORSALIZES BOTH THE EGGSHELL AND THE EMBRYO/

The Drosophila gene torpeno/Egfr (top/Egfr) encodes a homolog of the v ertebrate Epidermal Growth Factor receptor, This receptor is required several times during the life cycle of the fly for the transmisson of developmental cues, During oogenesis, Top/Egfr activation is required for the establishment of the dorsal/ventral axis of the egg and the em bryo, To examine how ectopic Top/Egfr activation affects cell fate det ermination, we constructed an activated version of the protein. Expres sion of this activated form (lambda top) in the follicle cells of the ovary induces dorsal cell fates in both the follicular epithelium and the embryo, Different levels of expression resulted in different dorsa l follicle cell fates, These dorsal cell fates were expanded in the an terior, but not the posterior, of the egg, even in cases where all the follicle cells covering the oocyte expressed lambda top, The expressi on of genes known to respond to top/Egfr activation, argos (aos), kekk on1 (kek 1) and rhomboid (rho), was also expanded in the presence of t he lambda top construct. When lambda top was expressed in all the foll icle cells covering the oocyte, kek 1 and argos expression was induced in follicle cells all along the anterior/posterior axis of the egg ch amber, In contrast, rho RNA expression was only activated in the anter ior of the egg chamber, These data indicate that the response to Top/E gfr signaling is regulated by an anterior/posterior prepattern in the follicle cells, Expression of lambda top in the entire follicular epit helium resulted in an embryo dorsalized along the entire anterior/post erior axis. Expression of lambda top in anterior or posterior subpopul ations of follicle cells resulted in regionally autonomous dorsalizati on of the embryos. This result indicates that subpopulations of follic le cells along the anterior/posterior axis can respond to Top/Egfr act ivation independently of one another.