Laboratory selection at different temperatures modifies heat-shock transcription factor (HSF) activation in Drosophila melanogaster

The magnitude and time course of activation of the heat-shock transcription factor (HSF) differ among Drosophila melanogaster lines evolving at 18 deg reesC, 25 degreesC or 28 degreesC for more than 20 years. At lower heat-sho ck temperatures (27-35 degreesC), flies from the 18 degreesC population had higher levels of activated HSF (as detected by an electrophoretic mobility shift assay) than those reared at 25 degreesC and 28 degreesC. At higher t emperatures (36 and 37 degreesC), however, the 28 degreesC flies had the hi ghest levels of HSF. These differences persisted after one generation of ac climation at 25 degreesC, suggesting that phenotypic plasticity was limited . In addition, larvae from the 28 degreesC lines activated HSF less rapidly after a 35 degreesC heat shock than those from the 18 degreesC and 25 degr eesC populations. These results are similar but not identical to previously reported differences in expression of Hsp70 (the major heat-inducible stre ss protein in Drosophila melanogaster) among the experimental lines. We con clude that HSF activation evolves rapidly during laboratory culture at dive rse temperatures and could play an important role in the evolution of the h eat-shock response.