Heat exposure improves acute copper tolerance in the intertidal copepod Tigriopus californicus.

Cross-protection, the phenomenon of exposure to one stressor conferring temporarily increased resistance to a second stressor, is a key mechanism that may help organisms cope with multiple stressor challenges. Here, we tested for cross-protection between acute copper and heat stresses in the intertidal copepod Tigriopus californicus, which exhibits local adaptation to the Northern Hemisphere's latitudinal thermal gradient. Curiously, copper tolerance in this species also increases at lower latitudes, although there is little evidence for a latitudinal gradient of copper pollution. Because both heat and copper induce oxidative stress, we hypothesized that the co-occurring tolerance patterns are driven by a shared molecular response mechanism that could also generate cross-protection during sequential stressor exposure. To test this hypothesis, we measured the acute tolerances of seven allopatric T. californicus populations after isolated and sequential stress exposures and analyzed the transcriptomic responses of the most geographically distant populations to both stressors. We observed unidirectional cross-protection between copper and heat, wherein acute copper tolerance increased when copepods were exposed to heat prior to toxicity testing, but not when stressor order was reversed. Gene expression during exposure to both stressors was highly population-specific, but there were commonalities in how the populations modulated the oxidative defense system, catabolic reactions, and other processes in response to each stressor that could contribute to the observed cross-protection. Together, these findings highlight how shared stress-response pathways can shape tolerance to sequential environmental challenges, with implications for predicting organismal resilience in a changing climate.