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 degreesC, 25 degreesC or 28 degreesC for more than 20 years. At lower heat-shock 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 temperatures (36 and 37 degreesC), however, the 28 degreesC flies had the highest levels of HSF. These differences persisted after one generation of acclimation 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 degreesC populations. These results are similar but not identical to previously reported differences in expression of Hsp70 (the major heat-inducible stress protein in Drosophila melanogaster) among the experimental lines. We conclude that HSF activation evolves rapidly during laboratory culture at diverse temperatures and could play an important role in the evolution of the heat-shock response.