Abstract
Pattern formation in the dorsal region of the Drosophila embryo depends 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 has been proposed to function as a morphogen with peak levels of Dpp-specifying amnioserosa, the dorsal-most cell type, and lower Dpp levels specifying dorsal ectoderm. The short gastrulation gene also contributes to patterning the dorsal region, but unlike the other genes involved in this process, sog activity is only required in ventral cells. Genetic evidence indicates that sog functions to antagonize dpp activity. In this report we present further phenotypic characterization of sog mutant embryos in dorsal and lateral regions and describe the cloning of the sog locus. sog is expressed in a broad lateral stripe of cells that abuts the dorsal territory of dpp-expressing cells. sog is predicted to encode a protein with an internal signal sequence and a large extracellular domain containing four repeats of a novel motif defined by the spacing of 10 cysteine residues that is distantly related to domains present in thrombospondin and procollagen. We propose that one or more of these cysteine repeats can be liberated by proteolytic cleavage 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 and dorsal ectoderm.
