Pattern formation under the control of the terminal system in the Drosophila embryo.

The specification of the most anterior and posterior domains of the Drosophila embryo depends on the activity of the torso protein, a putative tyrosine kinase receptor. Localized torso activity at the poles of the embryo generates graded information that specifies distinct portions of the body. The primary response to the terminal signal in the posterior end of the embryo is likely to be the activation of the gap genes huckebein and tailless. Here I address the question of how the graded maternal signal generates different elements of the pattern at the posterior end of the embryo and what role huckebein and tailless activities may play in this process. These experiments show that distinctly localized activities of huckebein and tailless are responsible for the appropriate expression of other genes known to be under the control of the terminal system. Moreover, they suggest that different elements of the terminal pattern can be specified in response to distinct levels of graded tailless activity.