Fuels for Winter: The Role of Proline in Overwintering Bumblebee Queens (Bombus impatiens)

Abstract

The common eastern bumblebee queens (Bombus impatiens) endure cold winter months by entering a diapausal state post-fertilization. During this overwintering period, these animals use stored energy reserves while maintaining a low metabolic rate. Bumblebees are thought to use primarily lipids to fuel this critical overwintering period, despite the fact that bee mitochondria do not appear equipped to break down this metabolic fuel. For some insects, lipids stored in the fat body can be converted to the amino acid proline, and this metabolic fuel has recently been discovered to be readily oxidized by bumblebee workers. My research, therefore, investigates the role of proline during overwintering in bumblebee queens. Using cellular respirometry, I determined the metabolic capacity of the muscle cells of queens to use various fuels, and if this capacity changes throughout overwintering. Surprisingly, the tested queens showed a much lower potential to oxidize proline than workers, and their capacity did not change during a four-month overwintering period. The metabolic properties of muscle tissue were further characterized using metabolic enzymes activity profile. These results further demonstrate the low potential for proline metabolism and the limitations of bumblebee queens’ capacity to oxidize lipids. Body composition was measured to determine how the various energy stores (lipid, glycogen, protein) change during overwintering; however, no decrease in concentration was observed. Overall, this work clarifies the constraints of B. impatiens metabolism during overwintering.