AN ANTEROPOSTERIOR DORSAL GRADIENT IN THE DROSOPHILA EMBRYO

Dorsoventral (DV) patterning of the Drosophila embryo is initiated by a broad Dorsal (Dl) nuclear gradient, which is regulated by a conserve d signaling pathway that includes the Toll receptor and Pelle kinase. We investigate the consequences of expressing a constitutively activat ed form of the Toll receptor, Toll(10b), in anterior regions of the ea rly embryo using the bicoid 3' UTR. Localized Toll(10b) products resul t in the formation of an ectopic, anteroposterior (AP) Dl nuclear grad ient along the length of the embryo. The analysis of both authentic do rsal target genes and defined synthetic promoters suggests that the ec topic gradient is sufficient to generate the full repertory of DV patt erning responses along the AP axis of the embryo. For example, mesoder m determinants are activated in the anterior third of the embryo, wher eas neurogenic genes are expressed in central regions. These results r aise the possibility that Toll signaling components diffuse in the pla sma membrane or syncytial cytoplasm of the early embryo. This study al so provides evidence that neurogenic repressors may be important for t he establishment of the sharp mesoderm/neuroectoderm boundary in the e arly embryo.