GENE-EXPRESSION DURING IMAGINAL DISC REGENERATION DETECTED USING ENHANCER-SENSITIVE P-ELEMENTS

When imaginal disc fragments from Drosophila are cultured in adult fem ale hosts, they either duplicate the part of the pattern specified by the fate map, or regenerate to replace the missing part. The new tissu e is added by proliferation of a small number of cells from the cut ed ge, brought together when the wound heals to form a regeneration blast ema. Specification of the new pattern has been explained by assuming i nteractions among cells of different positional value in the regenerat ion blastema. In order to identify genes which might mediate these eve nts, we screened over eight hundred independently isolated autosomal i nsertions of an enhancer-sensitive P-element, for altered lac-z expres sion in regenerating discs following cell death induced by a temperatu re-sensitive cell-lethal mutation. Two further screens divided the pos itive lines into four groups based on appropriate timing of the lac-z response in the cell-lethal mutant background and the expected respons e to an alternate source of cell death. Expression in wing disc fragme nts cultured in vivo was most frequent in the target class defined by the screens. In this direct test, lac-z expression was found in 23 tin es and in most cases was spatially and temporally correlated with the formation of the regeneration blastema. Our results suggest a very sub stantial transcriptional response during the early stages of imaginal disc regeneration. lac-z expression in control imaginal discs, embryos and adult ovaries of the positive tines was also assayed. The selecte d insertions included: a small class expressed only in discs undergoin g regeneration and apparently not at any other stage, possibly represe nting genes active exclusively in regeneration; a larger class express ed in the embryo or during oogenesis, but not normally in imaginal dis cs, as expected for functions recruited from earlier stages of the dev elopmental program; and finally a class with spatially patterned expre ssion in normal discs. This class included several insertions with exp ression associated with compartment boundaries, including one at the d ecapentaplegic (dpp), and one at the crumbs (crb) locus, a growth fact or homologue, and an EGF-repeat gene respectively. Some of the express ion patterns observed in cultured disc fragments provide evidence for cell communication in the regeneration blastema.