Long-term sperm storage mechanisms in ant queens

Abstract

Females of social Hymenoptera mate only at the beginning of their adult lives and produce offspring until their death without additional mating. In most ant species, queens live for over a decade, indicating that ant queens can store large numbers of spermatozoa throughout their long lives. Because morphology of the spermatheca (sperm storage organ) in ant queens is very unique among social Hymenoptera, this organ should be important for long-term sperm storage mechanisms in ants. Sperm cells are immotile in the spermatheca of queens at 5 years after mating. This may be effective to maintain a low metabolic activity of spermatozoa that prevents cellular damage and inhibits reactive oxygen species production. The immotile spermatozoa begin to swim when they are exposed to PBS buffer. This indicates that spermatozoa do not lack flagellum functions and continue to survive even after storage for 5 years. Sperm morphology of ants is similar to that of other hymenopteran species, however differences of the cellular characterization is still unknown (e.g. tolerance to oxidative stress) . Highly expressed genes in the ant spermatheca relative to those in body samples have been identified. The genes identified include those encoding antioxidant enzymes, chaperones, and energy metabolism enzymes as well as novel genes that have no similar sequences in the public databases. In future study, it should be necessary to identify the genes responsible for the sperm longevity in ant queens and shed light on molecular and cellular mechanisms of the long-term sperm storage.