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Aerobic metabolism and temperature : intraspecific variation and thermal acclimation in Atlantic killifish Healy, Timothy Mark


Environmental temperatures impact the performance of ectothermic organisms, such that changes in performance with temperature often influence range limits. Aerobic scope (the difference between maximum and standard aerobic metabolic rates) has been proposed as a physiological mechanism that may underlie the effects of temperature on performance. However, the effects of phenotypic plasticity and genetic divergence on aerobic scope curves, and the physiological mechanisms that constrain changes in the shapes of these curves remain poorly understood. Here, I assess the responses of aerobic scope to temperature in two subspecies of the eurythermal Atlantic killifish (Fundulus heteroclitus) through measurements of routine and maximum oxygen consumption. I demonstrate that killifish maintain aerobic scope over wide ranges of temperatures even during acute thermal exposures (5-33°C), but that thermal acclimation increases aerobic scope within optimal temperature ranges and at extreme temperatures (Chapter 2). Differences in aerobic scope as a result of thermal acclimation and intraspecific divergence in killifish are primarily associated with differences in routine oxygen consumption. Northern killifish have higher routine oxygen consumption than southern killifish, whereas cold-acclimated killifish have lower routine oxygen consumption than acutely cold-exposed warm-acclimated killifish. I also demonstrate that intraspecific variation in routine oxygen consumption is not associated with differences in acute thermal tolerance and hypoxia tolerance in admixed killifish (Chapter 3), and that decreases in routine oxygen consumption as a result of cold acclimation are paralleled by lower expression levels of genes involved in oxidative phosphorylation in muscle tissue (Chapter 4). Interestingly, despite these changes in oxidative phosphorylation gene expression, positive regulators of mitochondrial biogenesis are induced in cold-acclimated killifish (Chapters 4 & 5), suggesting that changes in mitochondrial volume density may improve oxygen delivery to mitochondria rather than compensating the effects of low temperature on cellular respiration. Taken together, my data indicate that cold acclimation may result in inverse compensation of metabolism in killifish, whereas intraspecific divergence results in countergradient variation in metabolism (i.e., thermal compensation). These opposite patterns may reflect latitudinal differences in selection associated with overwinter survival, and may contribute to the differences between the killifish subspecies that maintain intraspecific range boundaries.

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