An analysis of the hot mitochondria in the honeybee and wasp flight muscle

Abstract

Temperature profoundly influences life, aiding and disrupting foundational biochemical processes within thermally-sensitive energy-transforming organelles—mitochondria. Yet, some animals seemingly defy upper thermal limits through mitochondrial adaptations. Worker honeybees (Apis mellifera) can tolerate and even exploit high temperatures for several biological functions, including brood nest maintenance and protection. Some species “heat-ball” hive invaders, such as wasps, through thoracic flight muscle (FM) vibrations, warming invaders up to a putative 50°C, substantially more significant than the mitochondrial thermal range of 35-37°C for mammals and ~ 39-42°C for most birds. Therefore, several hymenopteran species are outliers in the literature regarding thermal limits. As honeybees are known to operate at high temperatures, this thesis aims to investigate the thermal upper limits (Tcrit) of honeybees relative to paper wasps (Polistes chinensis) at the whole-animal and subcellular-organelle level. This study found intact honeybees had greater respiratory control and survival to higher temperatures (49.3°C) than the wasps (47°C). Purpose- built fluorescent microscopes with heating systems investigated mitochondrial properties in permeabilised honeybee and wasp FMs. The temperature was ramped from ambient to over 55°C, then cooled. Mitochondrial membrane potential (ΔΨM), ATP synthesis and reactive oxygen species (ROS) production rates were analysed with temperature. Honeybee ΔΨM resisted disruption up to more extreme temperatures (41.6°C) than the wasp and demonstrated recovery up to 56°C. In contrast, wasps showed greater thermal sensitivity at lower temperatures (33.7°C) and poor ΔΨM recovery capacity beyond 48°C. Similarly, maximal ATP synthesis rates were identified as 10°C greater (43°C) than the wasp (33°C). The wasp also had greater ROS production. Additional probes were used to explore the mitochondrial-specific lipid cardiolipin and total FM protein thermo-stabilities, which concluded thermal tolerance up to 44.4°C in honeybees compared to 41.5°C in the wasp. Generalised protein unfolding also occurred at lower temperatures in wasps (42.9°C) compared to honeybees (46.3°C). These data indicate honeybees have remarkable thermo-tolerance and correlated mitochondrial adaptations underpinning complex life at high temperatures.