The Xenopus Dorsalizing Factor noggin Ventralizes Drosophila Embryos by Preventing DPP from Activating Its Receptor

noggin is expressed in the Spemann organizer region of the Xenopus embryo and can promote dorsal cell fates within the mesoderm and neural development within the overlying ectoderm. Here, we show that noggin promotes ventral development in Drosophila, specifying ventral ectoderm and CNS in the absence of all endogenous ventral-specific zygotic gene expression. We utilize constitutively active forms of the dpp receptors to demonstrate that noggin blocks dpp signaling upstream of dpp receptor activation. These results suggest a mechanistic basis for the action of Spemann's organizer during Xenopus development and provide further support for the conservation of dorsal-ventral patterning mechanisms between arthropods and chordates.