Spatially restricted activation of the SAX receptor by SCW modulates DPP/TKV signaling in Drosophila dorsal-ventral patterning

Dorsal-ventral patterning within the embryonic ectoderm of Drosophila requires two TGF beta ligands, DPP and SCW, and two type I TGF beta receptors, TKV and SAX. In embryos lacking dpp signaling, increasing the level of TKV activity promotes progressively more dorsal cell types, while activation of SAX alone has no phenotypic consequences. However, SAX activity synergizes with TKV activity to promote dorsal development. Functional experiments suggest the two receptors have different ligands: DPP acts through TKV, and SCW acts through SAX. Furthermore, SOG, a negative regulator of this patterning process, preferentially blocks SCW activity. We propose that spatial regulation of the SAX pathway modulates TKV signaling to create positional information over the embryonic ectoderm.