The laboratory assessments of heat and cold resistance revealed results that are qualitatively similar to those obtained on the same lines by Bubliy & Loeschcke (2005)
, despite the fact that selection had been relaxed for several generations prior to the assessments performed here. Previously, only females had been assessed (Bubliy & Loeschcke 2005
). We found that the cold and the heat resistance of both sexes had been increased by selection, and that there was a tendency towards the CS lines being more heat resistant than the C lines and the HS lines being more cold resistant than the C lines. In the literature, there is some disagreement about cross-resistance between cold and heat resistance (reviewed in Hoffmann et al. 2003
). Correlations between thermal extremes appear to depend on the selection procedures and the type of assay used to assess lines (reviewed in Hoffmann et al. 2003
A comparison between the laboratory and the field results suggests that there are three cases where differences in laboratory resistance predict field performance. Firstly, selection for increased heat resistance has enhanced the likelihood of locating resources under hot conditions in the field. Flies from the HS lines were caught in consistently higher numbers than those from the controls under hot conditions. This difference between the HS and the C lines may reflect the ability of flies to reach food (and perhaps high humidity provided by the food layer) under extreme hot conditions as well as their ability to survive these conditions. In previous releases, under hot conditions to test acclimation effects, we have shown that only flies hardened by exposure to a sub-lethal high temperature are caught in appreciable numbers (Loeschcke & Hoffmann in press
). When baits are left out, no additional flies are caught at later times when temperatures become cooler again, suggesting that flies die when ambient temperatures are higher than 38°C unless the flies locate food and high humidity conditions provided at the baits. Secondly, the relatively higher capture rates of CS flies compared with C flies under cold conditions suggest that laboratory cold resistance predicts field performance under cold conditions. Thirdly, the similar levels of laboratory cold resistance of the HS and the C lines predict their similar capture success under cold conditions.
However, there were also three differences in the field performance of the lines that were not evident from the laboratory results. Firstly, we found that at intermediate temperatures, the HS lines were less likely to be caught than the C lines, suggesting a tradeoff between high performance under extreme and benign conditions. Resistance towards stresses has traditionally been assumed to infer a fitness cost (Bergelson & Purrington 1996
; Taylor & Feyereisen 1996
; but see Coustau et al. (2000)
). For thermal stress, heat-shock proteins may be involved in observed tradeoffs. This group of proteins is known to be of adaptive importance under heat stress (Feder & Hofmann 1999
; Sørensen et al. 2003
). However, it is costly to possess extra copies of Hsp70
genes under benign conditions (Feder et al. 1996
; Roberts & Feder 2000
) and patterns of Hsp70
induction in field populations indicate that populations exposed to heat stress have low levels of Hsp70 (Sørensen et al. 2001
). Secondly, the replicate HS lines did not behave consistently in the field, unlike in the laboratory. One of the replicate lines (HS5) had a lower relative capture success compared with other HS lines under hot conditions, whereas flies from this same line are caught with one of the highest likelihoods (relative to C lines) under cold environmental conditions (). Although we only tested one C line and one of each of the selected lines in each release, C lines behaved similarly when compared directly. This indicated that the results are due to an effect of selection on the likelihood of locating resources in nature, and not due to differences between the C lines. The ‘odd behaviour’ of HS5 emphasizes the importance of biological replication in this type of studies. Thirdly, the relatively high capture success of the CS lines under hot conditions was not predicted from the laboratory results. Although the CS lines were somewhat more resistant to heat than controls in our assays and those carried out previously (Bubliy & Loeschcke 2005
), this difference was much smaller than the difference in laboratory resistance between the HS and the C lines. Yet, the capture success of the CS and the HS lines in the field relative to the controls was similar under hot conditions. Laboratory selection for cold resistance appears to be as effective as selection for heat resistance in increasing field performance under hot conditions.
In other field release experiments with similar designs to the ones used here, it has been shown that size and crowding during development (Hoffmann & Loeschcke 2006
) and hardening (Loeschcke & Hoffmann in press) affect the likelihood of locating resources in the field. The results from the study reported here show that some results from laboratory selection experiments on climatic traits can be related to field performance, emphasizing the ecological relevance of laboratory selection studies. However, our study also reveals some surprising results not predicted from the laboratory assays. The differences emphasize the importance for further development of efficient methods testing questions of physiological and evolutionary interest in the field. The approaches used and conclusion drawn from release experiments are based on the assumption that capture success at baits in a habitat that is otherwise free of natural resources reflects differential survival probabilities under different temperatures and provides a field fitness correlate in Drosophila
. However, we have not proven that is the case, and we are aware that this is just one component of fitness, although finding breeding resources may also be relevant to finding mating partners, as flies are often seen mating around resources (Partridge et al. 1987
). Fitness components such as mating success, reproduction and survival are not directly investigated in field releases. Nevertheless, the assay represents a valuable addition to the laboratory assays that are normally used by Drosophila
researchers to evaluate fitness. Another way of testing the ecological relevance of results obtained from selection experiments in the laboratory is to investigate the field distribution of alleles at candidate genes identified from selection experiments. Most knowledge on mechanisms involved in thermal resistance has so far been obtained from laboratory studies. The present study represents one attempt to move beyond the limitations of the laboratory environment.