Novel segment polarity gene interactions during embryonic head development in Drosophila.

In the trunk of the Drosophila embryo, the segment polarity genes are initially activated by the pair-rule genes, and later maintain each other's expression through a complex network of cross-regulatory interactions. These interactions, which are critical to cell fate specification, are similar in each of the trunk segments. To determine whether segment polarity gene expression is established differently outside the trunk, we studied the regulation of the genes hedgehog (hh), wingless (wg), and engrailed (en) in each of the segments of the developing head. We show that the cross-regulatory relationships among these genes, as well as their initial mode of activation, in the anterior head are significantly different from those in the trunk. In addition, each head segment exhibits a unique network of segment polarity gene interactions. We propose that these segment-specific interactions evolved to specify the high degree of structural diversity required for head morphogenesis.