Biphasic activation of the BMP pathway patterns the Drosophila embryonic dorsal region

The BMP pathway patterns the dorsal region of the Drosophila embryo. Using an antibody recognizing phosphorylated Mad (pMad), we followed signaling directly. In wild-type embryos, a biphasic activation pattern is observed. At the cellular blastoderm stage high pMad levels are detected only in the dorsal-most cell rows that give rise to amnioserosa. This accumulation of pMad requires the ligand Screw (Scw), the Short gastrulation (Sog) protein, …