Population specific differential thermal stress response in Drosophila melanogaster in a lab versus field setting

Background/Question/Methods

Temperature is an important physical factor that affects the life-history strategies and  range distributions of ectothermic insects . Photoperiodic cues also play an important role in enabling insects to predict long-term changes in temperature and adjust their development, physiology and behavior. Studies that focus on modulating temperature and other environmental parameters aim to interpret the results in the context of the environmental conditions that the organism is expected to meet in the field. However, the physiological and ecological responses usually investigated under controlled laboratory conditions may not capture the true complexity of the natural environment. In order to resolve this discrepancy, we compared laboratory and field results of temperature manipulation on Drosophila melanogaster.

For the purpose of our study we chose three natural populations of D. melanogaster collected from Maine, Pennsylvania and Florida and set up population cages using one hundred isofemale lines from each location. Virgin females were collected and placed  in cages for a five day outdoor treatment in a common garden experiment.  Flies were also placed in incubators in the laboratory  under fixed temperature and photoperiod regimes. After the initial field or lab treatment flies were assayed for heat shock tolerance, chill coma resistance and starvation tolerance.

Results/Conclusions

Phenotypic stress response results from the lab show that northern and southern populations of flies respond differentially to environmental cues. The data demonstrates that southern populations are generally more starvation tolerant, whereas northern populations are more resistant to heat shock. In the outdoor experiments we see similar patterns of differential response, with the Northern populations performing better in the chill coma and heat shock assay in certain months of the year.  The most striking result is that the variation in response correlates with the high variation in the temperature cycle and not the absolute hot or cold treatment as indicated by the lab results.

Our results suggest that it is important to consider the variation in temperature when testing for thermal tolerance in flies. The response that we get under laboratory conditions at fixed temperatures is highly variable when compared to the field experiments. The Northern populations of flies which have been exposed to a certain temperature regime perform better i.e. have higher survivorship, because of the underlying genetic mechanism to cope with extreme environmental conditions. Thus, we show that experiments done in the field can detect subtle differences in thermal performance not evident in simple laboratory tests.