Long-range Dpp signaling is regulated to restrict BMP signaling to a crossvein competent zone

The sensitivity of the crossveins of the Drosophila wing to reductions in BMP signaling provides a valuable system for characterizing members of this signaling pathway. We demonstrate here two reasons for that sensitivity. First, the initial stage of posterior crossvein development depends on BMP signaling but is independent of EGF signaling. This is the opposite of the longitudinal veins, which rely of EGF signaling for their initial specification. Second, BMP signaling in the posterior crossvein depends on Decapentaplegic (Dpp) at a stage when it is being produced in the longitudinal veins. Thus, the posterior crossvein will be especially vulnerable to reductions in the levels or range of Dpp signaling. We investigated the roles of the BMP receptor Thickveins (Tkv) and the BMP inhibitor Short gastrulation (Sog) in allowing this long-range signaling. Expression of both is downregulated in the developing posterior crossvein. The Tkv downregulation depends on BMP signaling and may provide a positive feedback by allowing the spread of Dpp. The Sog downregulation is independent of BMP signaling; Sog misexpression experiments indicate that this prepattern is essential for posterior crossvein development. However, this requirement can be overridden by co-misexpression of the BMP agonist Cv-2, indicating the presence of as yet unknown cues; we discuss possible candidates.