The bicoid and dorsal morphogens use a similar strategy to make stripes in the Drosophila embryo.

The anterior-posterior (A-P) and dorsal-ventral (D-V) axes of the early Drosophila embryo are established by two key maternal morphogens: bicoid (bcd) and dorsal (dl), respectively. The bcd protein is expressed in a broad concentration gradient along the A-P axis, with peak levels present at the anterior pole, while dl is expressed in a gradient along the D-V axis with peak levels along the ventral surface. The two morphogens are unrelated and their gradients are formed by distinct processes. Nonetheless, we have obtained evidence that they generate sharp on/off stripes of target gene expression through a similar mechanism. Both morphogens establish overlapping patterns of transcriptional activators and repressors in the early embryo. The activators and repressors bind to closely linked sites within short (300 to 500 bp) target promoter elements that have the properties of on/off switches. The activators act in concert with the morphogen to define a broad region where target genes can be initiated. Borders of target gene expression are established by the repressors, resulting in the formation of stripes.