In both humans and insects, hormones play a significant role in muscle development. Conditions which perturb hormone levels in mammals, such as hypo- or hyperthyroidism, disrupts muscle development and differentiation; yet, the molecular mechanisms by which steroids act to control myoblast proliferation and differentiation are not well understood. The tobacco hawkmoth M. sexta, the insect model system for endocrine control of development, will be used to explore the hormonal regulation of cell proliferation and apoptosis by ecdysteroids during myogenesis. Specifically, several ecdysteroid inducible early gene transcription factors have been identified and cloned and likely play a role in this hormonal control of cell fate. This proposal outlines experiments aimed at determining the role of the E74, Broad,and E75 early genes in muscle development during the pupal-adult transformation. Using physiology-based techniques, such as in vivo hormonal manipulations as well as a primary myocyte culture system, the role of ecdysone versus its metabolite 20-hydoxyecdysone in the control of cell proliferation, apoptosis and differentiation will be established. The identity and the timing of early gene expression will be determined in vivo and correlated with cell fate. Finally, using both anti-sense technology and transfection assays to deplete or express specific early genes, we will look at the ability of specific early genes to induce the proliferative or apoptotic cell fate.