Comparing phenotypic effects and molecular correlates of developmental, gradual and rapid cold acclimation responses in Drosophila melanogaster

1. To cope with stressful environmental temperatures, organisms can enhance thermotolerance when exposed to sub-lethal temperatures before thermal stress, a phenomenon referred to as thermal acclimation. Acclimation includes different forms (developmental, gradual or rapid) that vary in ecological importance depending on patterns of diurnal and seasonal thermal variation. 2. Here, we complete a comprehensive assessment of how the different forms of acclimation based on simulated field temperatures affect cold tolerance in Drosophila melanogaster under different levels of cold stress (-4.5 degrees C/2 h and 0 degrees C/10 h). 3. We predict that (i) combinations of acclimation treatments may be particularly beneficial and (ii) benefits of different acclimation types may differ for acute vs. chronic cold stress. We also investigate whether distinct forms of acclimation promote differential molecular responses to stress. 4. Acclimation treatments had very large effects on cold tolerance and resulted in phenotypes ranging from sensitive to tolerant individuals within the specific cold stress applied (-4.5 degrees C/2 h and 0 degrees C/10 h). Acclimation also influenced expression of several genes (Hsp23, Hsp70, Hsp40, Hsp68, Starvin and Frost) during recovery from cold stress but effects depended on the nature of the acclimation treatment. 5. Cumulative effects occurred between different forms of acclimation, and these as well as the different molecular responses point to different underlying mechanisms. 6. These results highlight that combined acclimation treatments may strongly impact field stress resistance.