Impact of immune activation on stored sperm viability in ant queens

Abstract

Ant queens mate on a single occasion early in life and store millions of sperm cells in their spermatheca. By carefully using stored sperm to fertilize eggs, they can produce large colonies of thousands of individuals. Queens can live for decades and their lifetime reproductive success is dependent on their ability to keep stored sperm alive. Maintaining high sperm viability requires metabolic energy which could trade-off with other costly processes such as immunity. We tested the impact of immune activation on the survival of stored sperm by prompting Lasius niger ant queens to mount a melanization response and subsequently measuring sperm viability in their spermatheca. Since queens face different challenges that influence energy allocation depending on the life stage of their colony, we measured sperm viability after immune activation in both newly mated queens (incipient) and in queens 1 year after mating (established). We found that immune activation reduced sperm viability in established queens but not in incipient queens, showing that the cost of immunity on sperm preservation depends on the life stage. Unexpectedly, established queens had significantly higher sperm viability in their spermatheca compared to incipient queens suggesting that ant queens are able to remove dead sperm from their spermatheca.

Keywords: ant queen; immunity; sperm storage; trade-off.

Figure 1.

Melanized scarring caused by the immune challenge. (a) Control queens did not mount a melanization reaction. (b) The needle prick through the intersegmental membrane of the 2nd and 3rd tergite of the abdomen (immune challenge) caused queens to mount a melanization reaction, indicated by the white arrow. (Online version in colour.)

Figure 2.

Sperm viability of control and immune-challenged incipient and established queens. The percentage of live sperm cells (box and whiskers plot) in Lasius niger ant queen spermatheca is represented for age, incipient and established, and treatment, control and immune challenge, groups. The midline of the box and whiskers plot is the median of each data group; the lower and upper edges of the boxes are the first and third quartiles, respectively. As there are no outliers, the whiskers extend to the lowest and highest data points in each group. Statistically significant differences between control and immune-challenged queens are shown with an asterisk. (Online version in colour.)