Assessing population and environmental effects on thermal resistance in Drosophila melanogaster using ecologically relevant assays

To make laboratory studies of thermal resistance in ectotherms more ecologically relevant, temperature changes that reflect conditions experienced by individuals in nature should be used. Here we describe an assay that is useful for quantifying multiple measures of thermal resistance of individual adult flies. We use this approach to assess upper and lower thermal limits and functional thermal scope for Drosophila melanogaster and also show that the method can be used to (1) detect a previously described latitudinal cline for cold tolerance in D. melanogaster populations collected along the east coast of Australia, (2) demonstrate that acclimation at variable temperatures during development increases tolerance to both low and high thermal stresses and therefore increases thermal scope compared to acclimation at a constant temperature, (3) show that temperate populations adapted to variable thermal environments have wider thermal limits compared to those from the less variable tropics, at least when flies were reared under constant temperature conditions and (4) demonstrate that different measures of cold resistance are often not strongly correlated. Based on our findings, we suggest that the method could be routinely used in evaluating thermal responses potentially linked to ecological processes and evolutionary adaptation.