Heart development in both vertebrates and Drosophila is initiated by bilaterally symmetrical primordia that may be of equivalent embryological origin: the anterior lateral plate mesoderm in vertebrates and the dorsal-most mesoderm in arthropods. These mesodermal progenitors then merge into a heart tube at the ventral midline (vertebrates) or the dorsal midline (Drosophila). These observations suggest that there may be similarities between vertebrate and invertebrate heart development. The homeobox gene, tinman, is required for heart as well as visceral mesoderm formation in Drosophila, and at least one of several vertebrate genes with similarities in protein sequence and cardiac expression to tinman is crucial for heart development in vertebrates. Inductive signals are also required for Drosophila heart development: The secreted gene product of wingless (wg) is critical for heart development during a time period distinct from its function in segmentation and neurogenesis. Here, we show that wg is epistatic to hedgehog (hh), another secreted segmentation gene product, in its requirement for heart formation. We also provide evidence show that downstream of wg in the signal transduction cascade, dishevelled (dsh, a pioneer protein) and armadillo (arm, beta-catenin homolog) are mediating the cardiogenic Wg signal. In particular, overexpression of dsh can restore heart formation in the absence of wg function. We discuss the possibility that Wg signaling is part of a combinatorial mechanism to specify the cardiac mesoderm.
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