DORSAL-VENTRAL PATTERNING OF THE DROSOPHILA EMBRYO DEPENDS ON A PUTATIVE NEGATIVE GROWTH-FACTOR ENCODED BY THE SHORT GASTRULATION GENE

Pattern formation in the dorsal region of the Drosophila embryo depend s on the activity of a small group of zygotically acting genes. dpp, a key gene in this group, encodes a TGF-beta-like product (Dpp) that ha s been proposed to function as a morphogen with peak levels of Dpp-spe cifying amnioserosa, the dorsal-most cell type, and lower Dpp levels s pecifying dorsal ectoderm. The short gastrulation gene also contribute s to patterning the dorsal region, but unlike the other genes involved in this process, sog activity is only required in ventral cells. Gene tic evidence indicates that sog functions to antagonize dpp activity. In this report we present further phenotypic characterization of sog m utant embryos in dorsal and lateral regions and describe the cloning o f the sog locus. sog is expressed in a broad lateral stripe of cells t hat abuts the dorsal territory of dpp-expressing cells. sog is predict ed to encode a protein with an internal signal sequence and a large ex tracellular domain containing four repeats of a novel motif defined by the spacing of 10 cysteine residues that is distantly related to doma ins present in thrombospondin and procollagen. We propose that one or more of these cysteine repeats can be liberated by proteolytic cleavag e of the primary Sog protein. These putative soluble Sog peptides may then diffuse into the dorsal region to antagonize the activity of Dpp, leading to the subdivision of the dorsal territory into amnioserosa a nd dorsal ectoderm.