Impact of immune activation on stored sperm viability in ant queens
Sarah Chérasse, Sarah Chérasse , Serge Aron
Evolutionary Biology and Ecology, Université Libre de Bruxelles, Belgium; Evolutionary Biology and Ecology, Université Libre de Bruxelles, Belgium ; Evolutionary Biology and Ecology, Université Libre de Bruxelles, Belgium
Ant queens mate on a single occasion early in life and store millions of sperm cells in their spermatheca. Queens can live for decades and by carefully using stored sperm to fertilize their eggs, they can mother large colonies. Fertilization requires live spermatozoa meaning queens will have to maintain high sperm viability in their spermatheca. Such a process is likely to require metabolic energy and could thus risk being traded-off against other costly physiological requirements such as immune mechanisms that are essential for survival. We tested the impact of immune system activation on the maintenance of stored sperm by artificially inducing Lasius niger queens to mount a melanization response and subsequently measuring sperm viability in their spermatheca. Since queens face different challenges that influence energy allocation (e.g. intensity of pathogen pressure) depending on the life stage of their colony, we measured sperm viability after immune activation in both newly mated queens (incipient) and in queens one year after mating (established). In both incipient and established queens, we found that melanization activation had no effect on the percentage of viable sperm stored in the spermatheca, showing that queens continue to preserve irreplaceable sperm even when faced with a costly immune challenge. Interestingly, queen life stage did however influence sperm viability as established queens had a significantly higher percentage of viable sperm cells in their spermatheca compared to incipient queens. This suggests that ant queens discard dead sperm from their spermatheca probably to provide optimal conditions for the live sperm that remains in storage.