Sperm can't take the heat: Short-term temperature exposures compromise fertility of male bumble bees (Bombus impatiens)

Authors: CAMPION, CLAIRE - University Of Wyoming; Rajamohan, Arun; DILLON, MICHAEL - University Of Wyoming

Submitted to: Journal of Insect Physiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/7/2023
Publication Date: 2/9/2023
Citation: Campion, C., Rajamohan, A., Dillon, M. 2023. Sperm can't take the heat: Short-term temperature exposures compromise fertility of male bumble bees (Bombus impatiens). Journal of Insect Physiology. 146(2023). Article 104491. https://doi.org/10.1016/j.jinsphys.2023.104491.
DOI: https://doi.org/10.1016/j.jinsphys.2023.104491

Interpretive Summary: Bumble bees are one of the most important native pollinators of wild as well as crop plants. These bees prefer to remain in a small area where they have their preferred temperature range as well as landscape that provides them with food and nesting spots. However, with changing climatic conditions, the bees are now exposed frequently to extreme high as well as low temperatures. Bees, just as other insects, have thermal limits for optimal activity and if those limits are exceeded then their physical activities will stall for a certain period of time before the exposure period becomes lethal. If the exposure temperature is cold, then the bees go into chill coma and if exposed to heat, they will be heat shocked. Such non-lethal temperatures are considered as sublethal temperatures. The effect of exposure to such sublethal temperatures even when the bees are still active were studied in the male bumble bees to understand how the sperm cells that are stored in the bees are affected. High temperatures of 45°C (115°F) for just 85 minutes affected the bee sperm cells both when the cells were within the bee as well as when removed from the bee and tested. This suggests that high temperature exposure of male bumble bees could affect their fertility and could potentially affect local populations of bumble bees.

Technical Abstract: Bumble bee (genus Bombus) populations are increasingly under threat from habitat fragmentation, pesticides, pathogens, and climate change. Climate change is likely a prime driver of bumble bee declines but the mechanisms by which changing climates alter local abundance, leading to shifts in geographic range are unclear. Heat tolerance is quite high in bumble bees (CTmax ~ 48-55°C), making it unlikely for them to experience temperatures in these range, even with climate warming. However, the thermal tolerance of whole organisms often exceeds that of the developing and mature gametes in their reproductive system. Many insects can be sterilized by exposure to temperatures well below their upper thermal tolerance. Male bumble bees are independent from the colony and may encounter more frequent temperature extremes, but whether these exposures compromise the spermatozoa contained in the reproductive system is still unclear. Using commercially-reared Bombus impatiens colonies, males were reared in the lab and spermatozoa were exposed (in vivo and isolated in vitro) to sublethal temperatures near lower and upper thermal tolerance (CTmin and CTmax, respectively). Heat exposure (45°C for up to 85 minutes) reduced spermatozoa viability both for whole males (in vivo; control= 79.5%, heat exposed= 58%, heat stupor= 57.7%) and isolated seminal vesicles (in vitro; control = 85.5%, heat exposed= 62.9%). Whole males exposed to 4°C for 85 min (in vivo; control= 79.2%, cold= 72.4%), isolated seminal vesicles exposed to 4°C for 85 min (in vitro; control = 85.5%, cold= 85.1%), and whole males exposed to for 4°C for 48 hours (in vivo; control= 88.7%, cold= 84.3%) had no significant difference in spermatozoa viability. After less than 85 min at 45°C, males had significantly reduced spermatozoa viability, suggesting that short-term heat waves below CTmax could strongly reduce the fertility of male bumble bees with potential population-level impacts.