ZYGOTIC CAUDAL REGULATION BY HUNCHBACK AND ITS ROLE IN ABDOMINAL SEGMENT FORMATION OF THE DROSOPHILA EMBRYO

caudal (cad) is a maternally and zygotically expressed gene in Drosoph ila whereby the two phases of expression can functionally replace each other. The zygotic expression forms an abdominal and a posterior doma in, whereby only the posterior domain has so far been studied with res pect to its regulation and function. We show here that the abdominal c ad domain is regulated by the hunchback (hb) gradient through repressi on at high concentrations and activation at low concentrations of HB p rotein. To study the function of the abdominal cad domain in the absen ce of redundant interactions, we have utilized an experimental system in which the embryo lacks the normal bicoid (bcd) and hb expression. A n artificial hb gradient is then introduced into such embryos, which r esults in an induction of an ectopic zygotic cad domain in the more an terior region. Employing this system, we show that the cad domain func tions by activating the expression of the abdominal gap genes knirps ( kni) and giant (gt). We conclude that cad is the so far missing region -specific activator of abdominal segmentation genes.