GEOGRAPHICAL VARIATION IN THE ACCLIMATION RESPONSES OF DROSOPHILA TO TEMPERATURE EXTREMES

Populations may adapt to climatic stresses by nonplastic or plastic changes in stress resistance. Plastic changes include a range of different acclimation mechanisms. A few previous studies with Drosophila suggest interspecific variation in plastic responses to climatic stresses, but there is not much evidence for variation within species. Tropical and temperate populations of Drosophila melanogaster and Drosophila simulans were compared for plastic responses to cold and heat stress. Tropical populations tended to be less resistant to cold stress than temperate populations. In contrast, D. melanogaster populations showed similar acclimation responses to cold stress following different acclimation treatments, which included keeping adults at low temperatures for a few hours or several days and culturing larvae at a low temperature. Populations of D. simulans also showed similar plastic responses to cold stress after adults were acclimated at a low temperature for several days. In the heat resistance experiments, there was no evidence for population differences in acclimation response in either D. melanogaster or D. simulans when adults were exposed to high and low temperatures for a few days. Genetic variation for acclimation response may therefore be mainly restricted to the interspecific level, although larger experiments are required to detect small quantitative differences between populations.