Lessons from the use of genetically modified Drosophila melanogaster in ecological studies: Hsf mutant lines show highly trait-specific performance in field and laboratory thermal assays

1. Laboratory studies on genetically modified strains may reveal important information on mechanisms involved in coping with thermal stress. However, to address the evolutionary significance of specific genes or physiological mechanisms, ecologically relevant field tests should also be performed. 2. We have tested the importance of inducible heat shock proteins (Hsps) under different thermal conditions using two heat shock factor (Hsf) mutant lines (either able (Hsf(+)) or unable (Hsf(0)) to mount a heat stress response) and an outbred laboratory adapted wild-type line of Drosophila melanogaster under both laboratory and field conditions. 3. In the field, there was a tendency towards better performance of Hsf(+) flies relative to Hsf(0) flies, but as compared with wild-type the performance of both mutant lines was very low. 4. In the laboratory tests, Hsf(+) flies had higher heat knock-down resistance relative to Hsf(0) flies but in other assays on heat, cold and desiccation resistance there was either no difference between the two mutant lines or the Hsf(0) line had higher performance. Also, the superiority of the wild-type flies under field conditions was trait specific. 5. The results emphasize that the ecological relevance of specific molecular mechanisms should be tested under a range of conditions both in the laboratory and in the field. Genetically modified lines cannot be assumed to represent the performance of natural populations, especially for field and/or ecologically relevant studies. 6. As evident in this study, ideal controls and adequate replication of genetically modified strains can be difficult to obtain. Thus, caution is needed when interpreting results comparing the performance of genetically modified lines with that of control lines.