A morphogenetic gradient of hunchback protein organizes the expression of the gap genes Krüppel and knirps in the early Drosophila embryo.

Segmentation of the Drosophila embryo depends on a hierarchy of interactions among the maternal and zygotic genes in the early embryo. The anterior region is organized maternally by the bicoid (bcd) gene product, which forms a concentration gradient in the anterior half of the embryo. The gap genes are also involved in establishing the body plan, with hunchback (hb) being expressed both maternally and zygotically. Zygotic expression of hb is directly activated by the bcd gene product, leading to a subdivision of the embryo into an anterior half expressing zygotically provided hb protein and a posterior half that does not. A similar effect on maternally provided hb protein is caused by the gene nanos, which represses the translation of maternally provided transcripts in the posterior half. This regulation of hb protein is a prerequisite for abdomen development, because the presence of hb protein in the posterior half represses posterior segmentation. This repression mechanism suggests that posterior segmentation might not directly depend on maternal positional cues, but be solely organized at the zygotic level. Here we report further evidence to support this hypothesis and show that the hb protein itself is crucially involved in organizing abdominal segmentation. Differential concentrations of hb protein determine the anterior and posterior borders of expression of the gap gene Krüppel (Kr) and the anterior border of the gap gene knirps (kni), thus defining three positional values. These regulatory pathways are controlled in a redundant way, in part by bcd and in part by the maternal hb gene product.