THE SCREW GENE ENCODES A UBIQUITOUSLY EXPRESSED MEMBER OF THE TGF-BETA FAMILY REQUIRED FOR SPECIFICATION OF DORSAL CELL FATES IN THE DROSOPHILA EMBRYO

The decapentaplegic (dpp) gene product, a TGF-beta related ligand, act s as an extracellular morphogen to establish at least two cellular res ponse thresholds within the dorsal half of the Drosophila embryo. Null mutations in the screw (scw) gene are phenotypically similar to moder ate dpp mutants and cause dorsal cells to adopt ventral fates. We show that sew encodes a novel TGF-beta protein and is an integral part of the signal that specifies dorsal pattern. Although sew is expressed un iformly during blastoderm stages, its effect on development appears gr aded and is restricted to the dorsal side of the embryo. Our results i ndicate that DPP activity alone is insufficient to specify different d orsal cell fates. We propose that SCW and DPP act together to establis h distinct response boundaries within the dorsal half of the embryo, p erhaps by forming heterodimers that have higher activity than homodime rs of either molecule alone.