INTRODUCTION The adult cuticular structures of Drosophila differentiate at metamorphosis from segmentally derived single-layered epithelial invaginations called imaginal discs. Each disc normally forms a fixed portion of the pattern of cuticular elements of the corresponding segment, but the develop- mental fates of individual cells are not irreversibly deter- mined, and the evidence suggests that the fate of a cell can be influenced both by its lineage history or compartmental cell-state (Garcia-Bellido et al., 1973) and by positional information (Wolpert, 1969). When complementary imaginal disc fragments are cul- tured in vivo, one fragment can regenerate the part of the pattern that was deleted, while the other can only duplicate the part it normally forms in situ (Schubiger, 1971). Thus the new partial pattern added to each fragment is identical, but its extent differs depending on the location of the cut. This suggested that the new pattern might be specified by positional information at the cut edge common to both frag- ments (Bryant, 1971; Postlethwaite and Schneiderman, 1973). The way the new pattern is generated in each disc fragment involves folding of the cut edge and healing to restore continuity of the epithelial surface across the wound. The patterns regenerated in a wide variety of experiments have been explained by postulating strictly local interac- tions between the cells brought together in this way (French et al., 1976; Bryant et al., 1981). According to this model, positional disparities across the wound stimulate local cell proliferation, resulting in the interpolation of the missing intermediate positional values. Clonal labelling of disc cells at the time these events are initiated (Girton and Russell, 1980; Abbott et al., 1981), showed that the new pattern is derived from a very small 1287 Development 117, 1287-1297 (1993) Printed in Great Britain © The Company of Biologists Limited 1993 When imaginal disc fragments from Drosophila are cul- tured in adult female hosts, they either duplicate the part of the pattern specified by the fate map, or regen- erate to replace the missing part. The new tissue is added by proliferation of a small number of cells from the cut edge, brought together when the wound heals to form a regeneration blastema. Specification of the new pattern has been explained by assuming interactions among cells of different positional value in the regener- ation blastema. In order to identify genes which might mediate these events, we screened over eight hundred independently isolated autosomal insertions of an enhancer-sensitive P-element, for altered lac-z expres- sion in regenerating discs following cell death induced by a temperature-sensitive cell-lethal mutation. Two further screens divided the positive lines into four groups based on appropriate timing of the lac-z response in the cell-lethal mutant background and the expected response to an alternate source of cell death. Expression in wing disc fragments cultured in vivo was most fre- quent in the target class defined by the screens. In this direct test, lac-z expression was found in 23 lines and in most cases was spatially and temporally correlated with the formation of the regeneration blastema. Our results suggest a very substantial transcriptional response during the early stages of imaginal disc regeneration. lac-z expression in control imaginal discs, embryos and adult ovaries of the positive lines was also assayed. The selected insertions included: a small class expressed only in discs undergoing regeneration and apparently not at any other stage, possibly representing genes active exclusively in regeneration; a larger class expressed in the embryo or during oogenesis, but not normally in imaginal discs, as expected for functions recruited from earlier stages of the developmental program; and finally a class with spatially patterned expression in normal discs. This class included several insertions with expres- sion associated with compartment boundaries, including one at the decapentaplegic (dpp) , and one at the crumbs (crb) locus, a growth factor homologue, and an EGF- repeat gene respectively. Some of the expression pat- terns observed in cultured disc fragments provide evi- dence for cell communication in the regeneration blastema. Key words: imaginal discs; regeneration; enhancer detection, Drosophila SUMMARY Gene expression during imaginal disc regeneration detected using enhancer-sensitive P-elements William J. Brook, Lisa M. Ostafichuk, Jana Piorecky, Mark D. Wilkinson, Daralyn J. Hodgetts and Michael A. Russell Department of Genetics, University of Alberta, Edmonton, Alberta, CANADA T6G 2E9