Fruit flies usually like it warm, but you can have too much of a good thing and the temperature of the insect's surroundings can have a profound effect on the diminutive creatures’ pace of life. Fortunately, heat is less of an issue for some flies that can withstand temperatures in excess of 40°C, but it wasn't clear which aspect of the insect's physiology allows some fruit fly species to thrive in the Mojave Desert, while others struggle to survive above 35°C. Knowing that high temperatures can disrupt the mini cellular powerhouses within the cell that produce ATP to fuel life – mitochondria – Lisa Jørgensen and Johannes Overgaard from Aarhus University, Denmark, teamed up with Florence Hunter-Manseau and Nicolas Pichaud from Université de Moncton, Canada, to find out how well the mitochondria of six species of fruit flies, ranging from sensitive Drosophila immigrans to robust Drosophila mojavensis, cope as the mercury rises.
Working with her colleagues at home in Denmark, and in Canada as part of a JEB Travelling Fellowship, Jørgensen measured the oxygen consumption of the insects’ mitochondria at temperatures ranging from a comfortable 19°C up to a lethal 46°C. Then, she used a series of drugs and compounds to examine different stages in the ATP production cascade. In addition, she measured how much oxygen the mitochondria consumed – ‘an indication of how fast the process is running’, says Jørgensen – to find out which events in the chain might fail at high temperatures and whether other stages might compensate to keep the mitochondria functioning.
Intriguingly, the insects’ mitochondria turned out to be more robust than the insects themselves; ‘All species were able to maintain high oxygen consumption rates at temperatures above their organismal heat limit’, says Jørgensen, although she adds that other researchers have also made this observation. The team then wanted to know more about the mechanisms that allow mitochondria to continue consuming oxygen at apparently fatally high temperatures. Measuring the oxygen consumption of the mitochondria, they realised that the cellular structures were switching from using two main fuels – pyruvate and malate – at cooler temperatures to consuming proline, succinate and glycerol-3-phosphate at the higher temperatures. ‘These three alternative substrates, which were not used a lot at benign temperatures, now replaced the role of others at high temperatures to an extent where the mitochondria use the same amount of oxygen’, says Jørgensen.
In short, other stages in the cascade are able to step in to plug the gap when some ATP synthesis pathways begin to fail as temperatures climb. However, the team points out that the tiny powerhouses may not be able to produce the same amount of ATP using these alternative mechanisms, despite maintaining their oxygen consumption. They also suspect that these mitochondria may be generating high levels of toxins – as a by-product of oxygen consumption – which could prove fatal. Even though adaptable mitochondria are still capable of consuming oxygen, high temperatures are still fatal for the insects, and the team is keen to find out why and how some components of the ATP synthesis cascade are able to compensate when others fail.