A positive role for short gastrulation in modulating BMP signaling during dorsoventral patterning in the Drosophila embryo

Positional information in the dorsoventral axis of the Drosophila embryo is encoded by a BMP activity gradient formed by synergistic signaling between the BMP family members Decapentaplegic (DPP) and Screw (SCW). short gastru lation (sog), which is functionally homologous to Xenopus Chordin, is expre ssed in the ventrolateral regions of the embryo and has been shown to act a s a local antagonist of BMP signaling. Here we demonstrate that SOG has a s econd function, which is to promote BMP signaling on the dorsal side of the embryo. We show that a weak, homozygous-viable sog mutant is enhanced to l ethality by reduction in the activities of the Smad family members Had or M edea, and that the lethality is caused by defects in the molecular specific ation and subsequent cellular differentiation of the dorsal-most cell type, the amnioserosa. While previous data had suggested that the negative funct ion of SOG is directed against SCW, we present data that suggests that the positive activity of SOG is directed towards DPP. We demonstrate that Chord in shares the same apparent ligand specificity as does SOG, preferentially inhibiting SCW but not DPP activity. However, in Drosophila assays Chordin does not have the same capacity to elevate BMP signaling as does SOG, ident ifying a functional difference in the otherwise well conserved process of d orsoventral pattern formation in arthropods and chordates.