Queen honey bees combat heat stress-induced loss of stored sperm viability with ATP-independent heat shock proteins

All species need to reproduce to maintain viable populations, but heat stress kills sperm cells across the animal kingdom and rising frequencies of heatwaves are a threat to biodiversity. Honey bees (Apis mellifera) are managed micro-livestock with globally viable populations; therefore, they could serve as environmental biomonitors for heat-induced reductions in fertility. Here, we exposed queens to a range of temperatures (5 to 42 °C) and measured the impact on viability of their stored sperm. We found that exposing queens to 42 °C for 4 h reduced sperm viability by 56% and induced expression of ATP-independent heat-shock proteins (HSPs) in their spermathecae but not in their ovaries. The strict ATP-independent heat-shock response in the spermatheca points to a tightly controlled ATP economy for stored sperm. Viability of ejaculated sperm and the drones (males) themselves plummets with heat, with only 42% of drones surviving 6 h at 42 °C. Conversely, 100% of queens survived. This sex-biased heat tolerance may be a quality control mechanism to ensure that queens are inseminated only with high-quality sperm. Finally, we quantified HSP expression in queens that failed in the field and found that 45% possessed a heat-shock signature similar to the laboratory heat-shocked queens. These signatures could serve as biomarkers for heat stress to enable post-failure diagnostics for the beekeeping industry, as well as surveying the prevalence of heat-induced loss of sperm viability in diverse landscapes.