Experimental demonstration of functional divergence in mitochondrial metabolism between two finch subspecies subjected to a thermal challenge

Different environments and climates can influence physiological variation and species’ geographic ranges. It has been suggested that mitochondria may play an essential role in this adaptation. We measured mitochondrial metabolism in two geographically separate subspecies of a songbird – the Long-tailed Finch, Poephila acuticauda - before and after ten days of heat exposure at 40°C. There were significant differences between subspecies, with the Red-billed P. a. hecki having lower proton leak and energy production efficiency than the Yellow-billed P. a. acuticauda, independent of treatment. Mitochondrial metabolism increased significantly after the heat treatment in five out of six variables in both subspecies, with P. a. hecki showing steeper reaction norms in metabolic acclimation to 40°C for the electron transport system. The heat treatment did not significantly increase circulating corticosterone, and at an individual level corticosterone and mitochondrial metabolism were unrelated. Eight out of nine climatic variables differed between subspecies’ geographical ranges, but whether these or other ecological variables explain the pre- and post-heat divergent mitochondrial performance requires further research. The reduced ETS metabolic flexibility of P. a. acuticauda after heat exposure suggests that future increases in the frequency and intensity of heat waves may impose asymmetric physiological costs on the two subspecies.

Methods
The mitochondrial dataset was obtained using an O2k real-time respirometer (Oroboros Instruments). For details, see Materials and Methods.