Near-anoxia induces immobilisation and sustains viability of sperm in ant queens: Novel insights into mechanisms of prolonged sperm storage

Insect females store sperm in a spermatheca after copulation for some duration to use it later for fertilisation. At the beginning of their adult lives, ant queens can preserve numerous viable sperm cells from copulation for over ten years. However, the key factors influencing long-term sperm storage have not been identified. Here we show that the spermathecal environment is near anoxic, which induces sperm immobilisation. Furthermore, mitochondrial respiratory inhibitors suppress sperm motility, suggesting that sperm immobilisation may be caused by a shortage of ATP generated from only glycolysis under near anoxic condition. Sperm immobilisation is not induced by acidification via glycolytic metabolism because the spermathecal fluid is not acidic. Finally, we show that artificial anoxic conditions rather than aerobic conditions sustain viable sperm cells. Therefore, near-anoxia is a key factor influencing long-term sperm storage in ant queens. The viability of sperm cells under artificial anoxia, however, is lower than of those dissected immediately from queens. Moreover, the immotile sperm cells under more than 4 h of anoxia do not begin swimming after aerobic exposure, unlike those that were in anoxic conditions for less than 2 h. This indicates that factors other than anoxia are also necessary for long-term sperm preservation.