Regeneration and transdetermination: the role of wingless and its regulation.

Imaginal discs of Drosophila have the remarkable ability to regenerate. After fragmentation wound healing occurs, ectopic wg is induced and a blastema is formed. In some, but not all fragments, the blastema will replace missing structures and a few cells can become more plastic and transdetermine to structures of other discs. A series of systematic cuts through the first leg disc revealed that a cut must transect the dorsal-proximal disc area and that the fragment must also include wg-competent cells. Fragments that fail to both transdetermine and regenerate missing structures will do both when provided with exogenous Wg, demonstrating the necessity of Wg in regenerative processes. In intact leg discs ubiquitously expressed low levels of Wg also leads to blastema formation, regeneration and transdetermination. Two days after exogenous wg induction the endogenous gene is activated, leading to elevated levels of Wg in the dorsal aspect of the leg disc. We identified a wg enhancer that regulates ectopic wg expression. Deletion of this enhancer increases transdetermination, but lowers the amount of ectopic Wg. We speculate that this lessens repression of dpp dorsally, and thus creates a permissive condition under which the balance of ectopic Wg and Dpp is favorable for transdetermination.