Dual role of the Drosophila pattern gene tailless in embryonic termini.

One of the first zygotically active genes required for formation of the terminal domains of the Drosophila embryo is tailless (tll). Expression of the tll gene is activated ectopically in gain-of-function mutants of the maternal terminal gene torso (tor); this suggests that tor normally activates the tll gene in the termini. Ectopic expression of tll under the control of an inducible promoter results in differentiation of ectopic terminal-specific structures, the Filzkörper, and leads to the activation of at least one gene, hunchback, that is required to form these structures. Ectopic expression of the tll gene also represses segmentation by repressing the gap genes Krüppel and knirps and probably also pair rule genes.