In the world of Drosophila embryogenesis, Spätzle is the extracellular ligand of the receptor Toll. Dorsoventral axis formation during embryogenesis is established as the ubiquitously expressed Toll recognizes the gradient of Spätzle expression.
Some of the other genes involved in dorsoventral axis formation include easter, nudel, gastrulation defective, tube, pelle, cactus, and dorsal. Easter, nudel, and gastrulation defective are involved in the proteolytic cleavage of the pro-Spätzle protein into the active form. The other genes are involved in the signal transduction downstream of the receptor Toll.
A lot of this was worked out by Kathryn V. Andersen and Christiane Nusslein-Volhard. Important stuff, in the field of embryogenesis, and as it turns out, immunology, as the receptor for Spätzle, Toll, are similar to genes in mammals that are involved in recognizing microbes.