Orthodenticle regulation during embryonic head development in Drosophila.

The homeobox gene orthodenticle (otd) specifies anterior head development in the Drosophila embryo, otd-related genes are also found in vertebrates, with expression patterns suggesting that they are important for the development of anterior regions of the head and brain. Here, we analyze the molecular mechanisms by which otd expression is activated within its normal domain in the head and repressed outside this region. We demonstrate that, contrary to early models of embryonic pattern formation, high levels of the bicoid morphogen are not required for otd activation or for the establishment of anterior head structures. We also show that the terminal system contributes to otd activation in the head primordium. Finally, we identify a novel pathway mediated by the gap gene huckebein through which three maternal systems cooperate to repress otd expression at the anterior terminus of the embryo.