In both vertebrates and invertebrates, homeotic selector genes confer morphological differences along the antero-posterior axis. However, insect wing development is independent of all homeotic gene functions, reflecting the ground plan of an ancestral pterygote, which bore wings on all segments. Dipteran insects such as Drosophila are characterized by a pair of wings in the mesothoracic segment. In all other segments, wing development is essentially repressed by different homeotic genes, although in the metathorax they are modified into a pair of halteres. This necessitates that during development all homeotic genes are to be maintained in a repressed state in wing imaginai discs. In this report we show that (i) the function of the segment polarity gene engrailed (en) is critical to keep the homeotic selector gene Ultrabithorax (Ubx) repressed in wing imaginal discs, (ii) normal levels of En in the posterior compartment of haltere discs, however, are not enough to completely repress Ubx, and (iii) the repression of Ubx by en is independent of Hedgehog signalling through which the long-range signalling of en is mediated during wing development. Finally we provide evidence for a possible mechanism by which en represses Ubx. On the basis of these results we propose that en has acquired two independent functions during the evolution of dorsal appendages. In addition to its well-known function of conferring posterior fate and inducing long-range signalling to pattern the developing appendages, it maintains wing fate by keeping Ubx repressed.
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