Functional Divergence of Mitochondria and Coevolution of Genomes: Cool Mitochondria in Hot Lizards

Mitochondria play a key role in the ecology and evolution of species through their influence on aerobic metabolism. Mitochondrial DNA (mtDNA) and nuclear genomes must interact for optimal functioning of oxidative phosphorylation to produce ATP, and breakdown of coadaptation components from each may have important evolutionary consequences for hybridization. Introgression of mitochondria in natural populations through hybridization with unidirectional backcrossing allows the testing of coadaptation of mitochondria to different nuclear backgrounds. We compared the function of mitochondria isolated from two species of Urosaurus lizards and hybrid populations. Due to past introgression, hybrids contain the nuclear genome of the "hot-adapted" species (U. graciosus) but the mtDNA of the less heat-tolerant species (U. ornatus). It was found that the function of the parental forms of mitochondria had significantly diverged with the hot-adapted species. There was significant genotype × genotype × environment interactions for mitochondrial membrane potential and genotype × genotype interactions for ATP production. Membrane potential decreased less at a higher temperature, while ATP production was higher at both temperatures in introgressed mitochondria. Oxygen consumption was lower in U. graciosus than in U. ornatus parental-type mitochondria, indicating a likely response to living in hotter environments. Respiratory control ratio values, which provide an indication of the functional quality of isolated mitochondria, were lower in introgressed mitochondria than in parental U. ornatus types, indicating a negative impact on biological function in introgressed mitochondria.