FIGURE 6-1C Dorsoventral development in Drosophila (Nüsslein-Volhard 1991). The egg shell contains Pipe protein on the future ventral side, deposited there during oogenesis. After fertilization, the egg secretes several proteins into the space between the egg shell and plasma membrane. Pipe activates one of the proteins, which then sets off others in a protease cascade, the last member of which cleaves the Spätzle protein, releasing a ligand that binds to the Toll transmembrane receptor, which is uniformly distributed over the egg surface but ligand-activated only on one side. The activated receptor, via several intracellular steps, activates the Dorsal protein, a transcription factor, which enters local nuclei and activates two genes, Twist and Snail, which also encode transcription factors. Those activate other genes for gastrulation and for mesoderm formation on the ventral side. Thus, the Pipe protein is involved in a kind of mesoderm induction. Active Dorsal protein also represses the Zen and Dpp genes on the ventral side. On the dorsal side, Dorsal protein remains inactive and the Zen and Dpp genes are expressed. Laterally, there is enough active Dorsal protein to repress Zen and Dpp but not enough to activate Twist and Snail. Here, the Sog gene is permissively expressed and not repressed, preparatory to neurogenic ectoderm formation. Thus, the dorsoventral dimension of the egg is divided into three domains of gene expression. Later, the Sog protein is secreted and diffuses to the Zen, Dpp region, inhibiting Dpp signaling and allowing the division of that region into two subregions the prospective amnioserosa and prospective dorsal ectoderm.