Fig. 7. The genetic network involved in Drosophila heart development. In the early embryo (stage 6), both pnr and ush are induced by dpp in the early ectoderm. Twist, a bHLH factor, induces tin expression in the early mesoderm. By stage 9/10 tin expression is restricted to the dorsal mesoderm (DM) via dpp signaling from the ectoderm. By stage 11, when dpp is expressed in a thinner dorsal stripe (DS), pnr expression is initiated in the presumptive cardiac mesoderm (CM) at the dorsal mesodermal margin, presumably by dpp and wg signaling from the ectoderm in the context of tin (DM). By late stage 11, we propose that pnr (in conjunction with ectodermal dpp and wg signaling) initiates the expression of tin in the cardiac mesoderm. By late stage 11 and stage 12, ush is needed to help maintain the expression of tin in the cardiac precursors. By stage 12/13, pnr and ush are also needed to maintain ectodermal dpp expression in the dorsal stripe and are also mediating the ectodermal signal of dpp in the mesoderm. In these later stages, ush may also be needed to limit the ability of pnr to activate tin expression in the ventrolateral mesoderm. Based on the data presented here, we propose the model that during the spatial convergence of dpp, wg and tin during cardiogenesis, the crucial mediator and executioner of the dpp signal is likely to be pnr.

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