In fish, environmental pollution is one factor that induces oxidative stress, and this can disturb the natural antioxidant defense system. Oxidative stress has been well characterized in vitro, yet the in vivo effects of metal-induced oxidative stress have not been extensively studied. In two experiments we examined the impacts of copper (Cu) on gene expression, oxidative damage, and cell oxidative capacity in liver and gill of zebrafish. In the first experiment, soft water-acclimated zebrafish were exposed to 8 and 15 μg/l Cu for 48 h. This exposure resulted in significant increases in gene expression of cytochrome c oxidase subunit 17 (COX-17) and catalase, associated with both increased Cu load and protein carbonyl concentrations in the gill and liver after 48 h. In addition, we examined the potential protective effects of increased waterborne Ca²⁺(3.3 mM) and Na⁺(10 mM) on acute Cu toxicity. While both treatments were effective at reducing liver and/or gill Cu loads and attenuating oxidative damage at 48 h, 10 mM Na⁺was more protective than 3.3 mM Ca²⁺. There were variable changes in the maximal activities of COX and citrate synthase (CS), indicating possible alterations in cell oxidative capacity. Moreover, Cu affected COX-to-CS ratios in both gill and liver, suggesting that Cu alters normal mitochondrial biogenic processes, possibly though metallochaperones like COX-17. Overall, this study provides important steps in determining the transcriptional and physiological endpoints of acute Cu toxicity in a model tropical species.

In response to hypoxic stress, many animals compensate for a reduced cellular O ₂ supply by suppressing total metabolism, thereby reducing O ₂ demand. For small endotherms that are native to high-altitude environments, this is not always a viable strategy, as the capacity for sustained aerobic thermogenesis is critical for survival during periods of prolonged cold stress. For example, survivorship studies of deer mice ( Peromyscus maniculatus ) have demonstrated that thermogenic capacity is under strong directional selection at high altitude. Here, we integrate measures of whole-organism thermogenic performance with measures of metabolic enzyme activities and genomic transcriptional profiles to examine the mechanistic underpinnings of adaptive variation in this complex trait in deer mice that are native to different elevations. We demonstrate that highland deer mice have an enhanced thermogenic capacity under hypoxia compared with lowland conspecifics and a closely related lowland species, Peromyscus leucopus . Our findings suggest that the enhanced thermogenic performance of highland deer mice is largely attributable to an increased capacity to oxidize lipids as a primary metabolic fuel source. This enhanced capacity for aerobic thermogenesis is associated with elevated activities of muscle metabolic enzymes that influence flux through fatty-acid oxidation and oxidative phosphorylation pathways in high-altitude deer mice and by concomitant changes in the expression of genes in these same pathways. Contrary to predictions derived from studies of humans at high altitude, our results suggest that selection to sustain prolonged thermogenesis under hypoxia promotes a shift in metabolic fuel use in favor of lipids over carbohydrates.

The hypoxic and cold environment at high altitudes requires that small mammals sustain high rates of O₂transport for exercise and thermogenesis while facing a diminished O₂availability. We used laboratory-born and -raised deer mice ( Peromyscus maniculatus) from highland and lowland populations to determine the interactive effects of ancestry and hypoxia acclimation on exercise performance. Maximal O₂consumption (V̇o_2max) during exercise in hypoxia increased after hypoxia acclimation (equivalent to the hypoxia at ∼4,300 m elevation for 6–8 wk) and was consistently greater in highlanders than in lowlanders. V̇o_2maxduring exercise in normoxia was not affected by ancestry or acclimation. Highlanders also had consistently greater capillarity, oxidative fiber density, and maximal activities of oxidative enzymes (cytochrome c oxidase and citrate synthase) in the gastrocnemius muscle, lower lactate dehydrogenase activity in the gastrocnemius, and greater cytochrome c oxidase activity in the diaphragm. Hypoxia acclimation did not affect any of these muscle traits. The unique gastrocnemius phenotype of highlanders was associated with higher mRNA and protein abundances of peroxisome proliferator-activated receptor γ (PPARγ). Vascular endothelial growth factor (VEGFA) transcript abundance was lower in highlanders, and hypoxia acclimation reduced the expression of numerous genes that regulate angiogenesis and energy metabolism, in contrast to the observed population differences in muscle phenotype. Lowlanders exhibited greater increases in blood hemoglobin content, hematocrit, and wet lung mass (but not dry lung mass) than highlanders after hypoxia acclimation. Genotypic adaptation to high altitude, therefore, improves exercise performance in hypoxia by mechanisms that are at least partially distinct from those underlying hypoxia acclimation.

In species that are distributed across steep environmental gradients, adaptive variation in physiological performance may be attributable to transcriptional plasticity in underlying regulatory networks. Here we report the results of common‐garden experiments that were designed to elucidate the role of regulatory plasticity in evolutionary adaptation to hypoxic cold‐stress in deer mice (Peromyscus maniculatus). We integrated genomic transcriptional profiles with measures of metabolic enzyme activities and whole‐animal thermogenic performance under hypoxia in highland (4350 m) and lowland (430 m) mice from three experimental groups: (1) wild‐caught mice that were sampled at their native elevations; (2) wild‐caught/lab‐reared mice that were deacclimated to low‐elevation conditions in a common‐garden lab environment; and (3) the F₁ progeny of deacclimated mice that were maintained under the same low‐elevation common‐garden conditions. In each experimental group, highland mice exhibited greater thermogenic capacities than lowland mice, and this enhanced performance was associated with upregulation of transcriptional modules that influence several hierarchical steps in the O₂ cascade, including tissue O₂ diffusion (angiogenesis) and tissue O₂ utilization (metabolic fuel use and cellular oxidative capacity). Most of these performance‐related transcriptomic changes occurred over physiological and developmental timescales, suggesting that regulatory plasticity makes important contributions to fitness‐related physiological performance in highland deer mice.