The thermotolerance effect of heat hardening (also called short-term acclimation), knockdown resistance to high temperature (KRHT) with and without heat hardening and chill-coma recovery (CCR) are important phenotypes of thermal adaptation in insects and other organisms. Drosophila melanogaster from Denmark and Australia were previously selected for low and high KRHT, respectively. These flies were crossed to construct recombinant inbred lines (RIL). KRHT was higher in heat-hardened than in nonhardened RIL. We quantify the heat-hardening effect (HHE) as the ratio in KRHT between heat-hardened and nonhardened RIL. Composite interval mapping revealed a more complex genetic architecture for KRHT without heat-hardening than for KRHT in heat-hardened insects. Five quantitative trait loci (QTL) were found for KRHT, but only two of them were significant after heat hardening. KRHT and CCR showed trade-off associations for QTL both in the middle of chromosome 2 and the right arm of chromosome 3, which should be the result of either pleiotropy or linkage. The major QTL on chromosome 2 explained 18% and 27-33% of the phenotypic variance in CCR and KRHT in nonhardened flies, respectively, but its KRHT effects decreased by heat hardening. We discuss candidate loci for each QTL. One HHE-QTL was found in the region of small heat-shock protein genes. However, HHE-QTL explained only a small fraction of the phenotypic variance. Most heat-resistance QTL did not colocalize with CCR-QTL. Large-effect QTL for CCR and KRHT without hardening (basal thermotolerance) were consistent across continents, with apparent transgressive segregation for CCR. HHE (inducible thermotolerance) was not regulated by large-effect QTL.

Chill‐coma recovery (CCR) is an important trait for thermal adaptation in insects. Multiple phenotypes could be affected by selection on CCR if the trait is genetically correlated with other adaptive traits. To test for heritable (co‐)variation in CCR, we examined direct and correlated responses to bi‐directional selection on CCR. Drosophila buzzatii Patterson & Wheeler (Diptera: Drosophilidae) was artificially selected for decreased and increased recovery time following exposure to 0 °C. After 18 selected generations, the selection response in CCR was significant but qualitatively asymmetric, with replicated lines for slow CCR showing the highest response. Knockdown resistance to high temperature was not affected by CCR selection. Starvation resistance in the adult fly showed no clear pattern of correlated responses to CCR selection. Selection on CCR had no impact on developmental time and body size. Chill‐coma recovery shows no apparent genetic trade‐offs with any of the multiple traits included in this study. These results are largely consistent with recent studies on clines in D. buzzatii, which showed that CCR is not across‐population correlated with other clinally varying traits of thermal adaptation. Cold adaptation may evolve toward increased cold resistance independent of upper thermal limits.

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