Transition in the production of diploid-female to haploid-male eggs in bumblebee colonies: sperm quality or depletion?

Bumblebees establish annual colonies that start with the emergence of workers in spring and end with the production of sexuals, the majority of which are males, in late summer. To date, the causes responsible for the transition in the production of diploid-female offspring to haploid-male offspring during the decline phase of colonies remain elusive. Using flow cytometry, we tested whether such a caste shift is correlated with a decline in sperm number and quality (i.e., sperm viability and sperm DNA fragmentation) in the queen spermatheca over time, from mating to the emergence of the first males. We found that sperm number and viability significantly decreased, while sperm DNA fragmentation increased in the spermatheca over time. These results suggest that the shift towards male production during the decline phase of a bumblebee colony stems at least partly from a combination of a drop in sperm count and sperm quality in queens’ spermatheca. In social Hymenoptera, sex determination is based on the haplodiploidy system in which males develop from unfertilized eggs and are haploid, and females develop from fertilized eggs and are diploid. So far, the proximal mechanisms responsible for the social transition from diploid-female egg production to haploid-male egg production remain unknown. We show that the shift towards male production during the decline phase of bumblebee colonies is associated with a reduction in the quantity and quality of the sperm stored in the queen spermatheca over time. More generally, it suggests that sperm conservation is limited and likely adapted to the relatively short lifespan of bumblebee queens, which produce small and annual colonies.