SIGNALING PATHWAYS THAT ESTABLISH THE DORSAL-VENTRAL PATTERN OF THE DROSOPHILA EMBRYO

The dorsal-ventral pattern of the Drosophila embryo is established by three sequential signaling pathways. Each pathway transmits spatial in formation by localizing the activity of an extracellular signal, which acts as a ligand for a broadly distributed transmembrane receptor. Th e components of the first two pathways are encoded by maternal effect genes, while the third pathway is specified by genes expressed in the zygote. During oogenesis, the oocyte transmits a signal to the surroun ding follicle cells by the gurken-torpedo pathway. After fertilization , the initial asymmetry of the egg chamber is used by the spatzle-Toll pathway to generate within the embryo a nuclear gradient of the trans cription factor Dorsal, which regulates the regional expression of a s et of zygotic genes. On the dorsal side of the embryo, the decapentapl egic-punt/thick veins pathway then establishes patterning of the amnio serosa and dorsal ectoderm. Each pathway uses a distinct strategy to a chieve spatial localization of signaling activity.