SYNERGY BETWEEN THE HUNCHBACK AND BICOID MORPHOGENS IS REQUIRED FOR ANTERIOR PATTERNING IN DROSOPHILA

Anterior patterning of the Drosophila embryo is specified by the local ized expression of the gap genes, which is controlled by the gradient of the maternal morphogen bicoid (bcd). Another maternal component, hu nch back (hb), can substitute for bcd in the thorax and abdomen. Here we show that hb is required for bcd to execute all of its functions. R emoval of both maternal and zygotic hb produces embryos with disrupted polarity that fail to express all known bcd target genes correctly. P roper expression of hb and the head gap genes requires synergistic act ivation by hb and bcd. We propose that it is the combined activity of bcd and hb, and not bcd alone, that forms the morphogenetic gradient t hat specifies polarity along the embryonic axis and patterns the embry o. bcd may be a newly acquired Drosophila gene, which is gradually rep lacing some of the functions performed by maternal hb in other species .