Dramatic changes in mitochondrial substrate use at critically high temperatures: a comparative study using Drosophila

Ectotherm thermal tolerance is critical to species distribution, but at present the physiological underpinnings of heat tolerance remain poorly understood. Mitochondrial function is perturbed at critically high temperatures in some ectotherms, including insects, suggesting that heat tolerance of these animals is linked to failure of oxidative phosphorylation (OXPHOS) and/or ATP production. To test this hypothesis, we measured mitochondrial oxygen consumption rate in six Drosophila species with different heat tolerance using high-resolution respirometry. Using a substrate-uncoupler-inhibitor titration protocol, we examined specific steps of the electron transport system to study how temperatures below, bracketing and above organismal heat limits affect mitochondrial function and substrate oxidation. At benign temperatures (19 and 30 degrees C), complex I-supported respiration (CI-OXPHOS) was the most significant contributor to maximal OXPHOS. At higher temperatures (34, 38, 42 and 46 degrees C), CI-OXPHOS decreased considerably, ultimately to very low levels at 42 and 46 degrees C. The enzymatic catalytic capacity of complex I was intact across all temperatures and accordingly the decreased CI-OXPHOS is unlikely to be caused directly by hyperthermic denaturation/inactivation of complex I. Despite the reduction in CI-OXPHOS, maximal OXPHOS capacity was maintained in all species, through oxidation of alternative substrates - proline, succinate and, particularly, glycerol-3-phosphate suggesting important mitochondrial flexibility at temperatures exceeding the organismal heat limit. Interestingly, this failure of CI-OXPHOS and compensatory oxidation of alternative substrates occurred at temperatures that correlated with species heat tolerance, such that heat-tolerant species could defend 'normal' mitochondrial function at higher temperatures than sensitive species. Future studies should investigate why CI-OXPHOS is perturbed and how this potentially affects ATP production rates.

Automated summary

The heat tolerance of ectotherms, such as insects, is linked to the failure of oxidative phosphorylation (OXPHOS) and/or ATP production at critically high temperatures, with complex I-supported respiration (CI-OXPHOS) decreasing significantly at higher temperatures but the enzymatic catalytic capacity of complex I remaining intact. Despite the reduction in CI-OXPHOS, all species were able to maintain maximal OXPHOS capacity through oxidation of alternative substrates, indicating important mitochondrial flexibility at temperatures exceeding organismal heat limits. Interestingly, the failure of CI-OXPHOS and compensatory oxidation of alternative substrates occurred at temperatures correlated with species heat tolerance, suggesting a potential link between mitochondrial function and thermal tolerance in ectotherms.