Spatzle regulates the shape of the Dorsal gradient in the Drosophila embryo

Dorsal-ventral polarity of the Drosophila embryo is established by a nuclea r gradient of Dorsal protein, generated by successive gurken-Egfr and spatz le-Toll signaling. Overexpression of extracellular Spatzle dramatically res hapes the Dorsal gradient: the normal single peak is broadened and then ref ined to two distinct peaks of nuclear Dorsal, to produce two ventral furrow s, This partial axis duplication, which mimics the ventralized phenotype ca used by reduced gurken-Egfr signaling, arises from events in the perivitell ine fluid of the embryo and occurs at the level of Spatzle processing or To ll activation. The production of two Dorsal peaks is addressed by a model t hat invokes action of a diffusible inhibitor, which is proposed to normally regulate the slope of the Dorsal gradient.