Gastrulation defective, a complement factor C2/B-like protease, interpretsa ventral prepattern in Drosophila

gastrulation defective (gd) encodes a serine protease required for specific ation of dorsal-ventral cell fates during Drosophila embryogenesis. Using R NA microinjection, I show that wildtype gd RNA can restore ventrolateral pa ttern elements with correct polarity with respect to egg shape in embryos l acking gd function. While low RNA concentrations restore ventrolateral patt ern elements, higher concentrations ventralize the embryo. Gastrulation def ective concentration has a rate-limiting effect on the domain of high Dorsa l concentration but little effect upon the slope of the gradient. In embryo s from pipe-null females, much higher RNA concentrations generate an ectopi c axis oriented with respect to the site of injection. The data suggest tha t the Dorsal gradient is not directly determined by asymmetric cues in the eggshell but arises de novo within the perivitelline space as a consequence of self-regulatory properties of the protease cascade. A homology to the m ammalian complement factors C2 and B is also described.