Abstract:
Bumblebees (Bombus terrestris) are an enigmatic species, once considered too large to fly, they now fuel scientific discussion with their thermogenic capacity. The mechanism of this is not known, however two hypotheses have been put forward. The first, futile cycling of glycolysis intermediates, is incomplete as it simply does not produce enough heat. The second, shivering, while potent, is not effective below 15°C. Given that bumblebees fly below this temperature, a pre-shivering thermogenic mechanism must exist. Mitochondrial function analyses uncovered differences between the two major respiratory pathways in bumblebee mitochondria; Complex I (CI) and mitochondrial glycerophosphate dehydrogenase (mGPDH). LEAK respiration was 5-fold higher in mGPDH relative to CI. Furthermore, the respiratory control ratio (RCR) of CI was 9.95, much higher than the RCR of mGPDH, 1.68. This suggested inefficiency of mGPDH in converting ingested fuels into ATP. ATP flux further showed that at 25°C and 35°C, CI produced 7.5 and 4.1-fold more ATP, respectively, than mGPDH. To assess whether this ineffiency resulted in significant heat output, calorimetric analysis was conducted using a purpose-built calorimeter. In the LEAK state, mGPDH respiration released 5-times more heat than CI, enough to warm an insect thorax 10°C in 15 mins. This is pertinent, given that the absence of ADP in LEAK respiration mimics the low [ADP] during pre-flight. To assess whether mGPDH has a thermogenic function, respiratory analyses were conducted at a range of temperatures, and the Q10 values were determined. In the LEAK state between 15°C and 25°C, the Q10 of CI was 3.1, while the Q10 of mGPDH was 1.8. This reveals reduced temperature sensitivity in mGPDH relative to CI, which is key for a thermogenic function. Furthermore, differences in binding affinities between CI and mGPDH suggested that an accumulation of substrate may activate the mGPDH pathway in response to the cold. This in turn activates the thermogenic function of the pathway, and allows the bee to warm enough for shivering to begin.