Ant sperm storage organs do not have phenoloxidase constitutive immune activity

The prophenoloxidase system (proPO-AS) is a primordial constituent of insect innate immunity. Its broad action spectrum, rapid response time, and cytotoxic by-products induced by phenoloxidase (PO) production contribute to the effective clearing of invading pathogens. However, such immune reactions may not be optimal for insect organs that evolved to have mutualistic interactions with non-self-cells. Ant queens are long-lived, but only mate early in adult life and store the sperm in a specialized organ, the spermatheca. They never re-mate so their life-time reproductive success is ultimately sperm-limited, which maintains strong selection for high sperm viability before and after storage. The proPO-AS may therefore be inappropriate for the selective clearing of sexually transmitted infections, as it might also target sperm cells that cannot be replaced. We measured PO enzymatic activity in the sperm storage organs of three ant species before and after mating. Our data show that no PO is produced in the sperm storage organs, relative to other somatic tissues as controls, and that these negative results are not due to non-detection in small volumes as non-immune-relevant catalase activity in single spermatheca fluid samples of both virgin and mated queens was significant. The lack of PO activity in sperm storage organs across three different ant species may represent an evolutionarily conserved adaptation to life-long sperm storage by ant queens. We expect that PO activity will be similarly suppressed in queen spermathecae of other eusocial Hymenoptera (bees and wasps) and, more generally, of insect females that store sperm for long periods.