Spatial and temporal pattern of hsp26 expression during normal development.

The tissue-specific patterns of developmental expression of hsp26-lacZ fusion genes inserted into Drosophila melanogaster by germline transformation were analyzed in several transformant lines utilizing a histochemical assay for beta-galactosidase activity on whole animals. We compared this pattern to the tissue-specific distribution of endogenous hsp26 RNA determined using hybridization of probes to RNA in situ in tissue sections. Both assays reveal that hsp26 is expressed in numerous tissues during development including spermatocytes, nurse cells, epithelium, imaginal discs, proventriculus and neurocytes. The ease and resolution of the whole-animal beta-galactosidase assay makes it particularly attractive for the elucidation of sequences involved in such complex regulation. The original hsp26-lacZ fusion gene contained 2 kb of sequence upstream of the transcription start. A construct containing only 278 bp upstream was still expressed in spermatocytes but no longer in nurse cells. In a few instances, the fusion genes were expressed in tissues for which there was no evidence for expression of the endogenous hsp26 gene. These novel patterns appear to be a result of chromosomal position since they were observed in only one or a subset of transformant lines containing identical inserts.