SIGNALING BY THE DROSOPHILA EPIDERMAL GROWTH-FACTOR RECEPTOR IS REQUIRED FOR THE SPECIFICATION AND DIVERSIFICATION OF EMBRYONIC MUSCLE PROGENITORS

Muscle development initiates in the Drosophila embryo with the segrega tion of single progenitor cells, from which a complete set of myofibre s arises. Each progenitor is assigned a unique fate, characterized by the expression of particular identity genes. We now demonstrate that t he Drosophila epidermal growth factor receptor provides an inductive s ignal for the specification of a large subset of muscle progenitors. I n the absence of the receptor or its ligand, SPITZ, specific progenito rs fail to segregate, The resulting unspecified mesodermal cells under go programmed cell death, In contrast, receptor hyperactivation genera tes supernumerary progenitors, as well as the duplication of at least one SPITZ-dependent myofibre. The development of individual muscles is differentially sensitive to variations in the level of signalling by the epidermal growth factor receptor. Such graded myogenic effects can be influenced by alterations in the functions of Star and rhomboid. I n addition, muscle patterning is dependent on the generation of a spat ially restricted, activating SPITZ signal, a process that may rely on the localized mesodermal expression of RHOMBOID. Thus, the epidermal g rowth factor receptor contributes both to muscle progenitor specificat ion and to the diversification of muscle identities.