GEOGRAPHICAL VARIATION IN THE ACCLIMATION RESPONSES OF DROSOPHILA TO TEMPERATURE EXTREMES

Populations may adapt to climatic stresses by nonplastic or plastic ch anges in stress resistance. Plastic changes include a range of differe nt acclimation mechanisms. A few previous studies with Drosophila sugg est 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 s imulans were compared for plastic responses to cold and heat stress. T ropical populations tended to be less resistant to cold stress than te mperate populations. In contrast, D. melanogaster populations showed s imilar acclimation responses to cold stress following different acclim ation treatments, which included keeping adults at low temperatures fo r a few hours or several days and culturing larvae at a low temperatur e. Populations of D. simulans also showed similar plastic responses to cold stress after adults were acclimated at a low temperature for sev eral days. In the heat resistance experiments, there was no evidence f or population differences in acclimation response in either D. melanog aster or D. simulans when adults were exposed to high and low temperat ures for a few days. Genetic variation for acclimation response may th erefore be mainly restricted to the interspecific level, although larg er experiments are required to detect small quantitative differences b etween populations.