High Bicoid levels render the terminal system dispensable for Drosophila head development

In Drosophila, the gradient of the Bicoid (Bcd) morphogen organizes the ant eroposterior axis while the ends of the embryo are patterned by the materna l terminal system. At the posterior pole, expression of terminal gap genes Is mediated by the local activation of the Torso receptor tyrosine kinase ( Tor). At the anterior, terminal gap genes are also activated by the Tor pat hway but Bcd contributes to their activation. Here we present evidence that Tor and Bcd act independently on common target genes in an additive manner . Furthermore, we show that the terminal maternal system is not required fo r proper head development, since high levels of Bcd activity can functional ly rescue the lack of terminal system activity at the anterior pole. This o bservation is consistent with a recent evolution of an anterior morphogenet ic center consisting of Bcd and anterior Tor function.