09-P036 Orthodenticle controls timely photoreceptor development in Drosophila

right ventricle and the outflow tract of the heart. Here we have investigated the role of tailup (tup), the Drosophila homologue of vertebrate islet-1 in the development of the dorsal vessel. In Drosophila, cardiac progenitor cells are specified bilaterally in the dorsal mesoderm, which requires complex interactions between the transcription factors Tinman/Nkx2.5, Pannier/GATA factor, and Dorsocross/Tbx factor. Additionally, the growth factors Wingless and Decapentaplegic provide crucial signals for the induction and maintenance of the cardiac regulatory network. Tup mutants are characterized by reduced expression of the essential transcription factors Tin, Pnr and Doc in early cardiogenesis. Conversely, Tup expression depends on each of these cardiac factors, as well as on the early inductive signals Dpp and Wg. Genetic interactions show that tup cooperates with tin, pnr and Doc in cardiogenesis. Lossand gain-of-function analyses for Tup and Pnr suggest that a proper balance of these factors is essential for cardiac specification. Inhibition of Tup in differentiated cardiac cells shows, that later in development, Tup is required to maintain cardiac identities. In summary, our data add tup as a critical factor to the early cardiac regulatory network that determines a cardiac fate.