Matching mitochondrial respiration to changing environments through acclimatization and the evolution of fixed genetic differences

Aerobic respiration in mitochondria is the source for most of the energy that powers complex animals, and maintaining energy flow from mitochondria near the optimum needed for life processes presents challenges for complex animals. Environments of most animals change rapidly. Moreover, individuals pass through developmental stages with different energy demands, and they shift life-history states that require modified production of adenosine triphosphate (ATP). To adjust to changing conditions, all complex animals display some capacity for acclimatization through phenotypic flexibility, whereby key aspects of mitochondrial respiration are reversibly altered. Phenotypic flexibility is a universal feature of the energy-production mechanisms of animals, but all animals face limitations in the range of environments and circumstances to which they can acclimatize. We discuss multiple examples of such phenotypic flexibility in animals, focusing on the different mechanisms employed that acclimatize mitochondrial respiration to exogenous and endogenous challenges. Genotype sets the range of phenotypes related to mitochondrial respiration that is available to an animal. Numerous studies document adaptive evolution of both mitochondrial and nuclear genes that directly affect the range of environments that will support oxidative phosphorylation. Phenotypic flexibility can obscure evolutionary changes in response to changing energy demands, and understanding the interplay of capacity for acclimatization and adaptive evolution of mitochondrial systems presents major challenges for physiological and evolutionary biologists.