Costs and benefits of cold acclimation in field released Drosophila – Associating laboratory and field results.

Physiological and evolutionary responses to thermal variation are often investigated under controlled laboratory conditions. However, this approach may fail to account for the complexity of natural environments. Here we investigated the costs and benefits of developmental or adult cold acclimation using the ability of field released Drosophila melanogaster to find a resource as a proxy of fitness. Measurements were carried out on two continents across a range of temperatures. Cold acclimation improved the flies' ability to find resources at low temperatures. However, this came at a cost at higher temperatures where cold acclimated flies were up to 36 times less likely to find a resource under warm conditions. These costs were not detected in standard laboratory tests but indicate that physiological acclimation may improve fitness only over a narrow set of thermal conditions while it may have the opposite effect once conditions extend outside this range. In a second study we released 10,000 flies from a single population under cold field conditions. Flies caught at either the release or the resource station were subsequently compared with respect to cold performance. This study showed that the ability to locate a field resource has a genetic basis with a high heritability since only round of selection on parental flies (F0) revealed clear differences in the ability of offspring (F1 and F2) to locate field resources at cold temperatures. Again we found a poor association between field and laboratory performance emphasising the importance of testing thermal resistance under relevant/natural conditions.