Journey of sperms from production by males to storage by queens in Crematogaster osakensis (Hymenoptera: Formicidae)

Establishment of a rapid, cost-effective, and accurate method for assessing insect sperm viability

High-quality sperm cells are crucial to reproductive success for both males and post-mating females in animals. Sperm viability, defined as the proportion of viable sperm cells, is used as a sperm quality index and this method has provided new insights into research on reproductive strategies. Sperm viability has been assessed by fluorescent staining of sperm cells. However, current staining protocols could potentially underestimate viability due to cell damage caused by cell treatments such as high dye concentration and long time for post-mounting. In this study, we established a method that enables rapid sperm viability assessment, has low sperm cell toxicity, and provides precise results regardless of operator expertise, and cost-effective using sperm cells from an ant, Crematogaster osakensis (Hymenoptera). First, to shorten the time for observation of a sufficient number of sperm cells, the volume per field of view was increased by height elevation between the glass slide and the coverslip, thereby we increased the number of sperm cells in a field of view. Second, to reduce sperm cell toxicity, we optimized the minimum dye concentration and incubation time using acridine orange (AO) and Hoechst in addition to SYBR 14 and propidium iodide (PI), which has been used in most previous studies. We determined the optimal protocol to be 1 µg/mL AO and 150 µM PI without incubation. Besides, we automated counting sperm cells with ImageJ software and combined with manual correction for more accurate results. We employed the improved method for sperm samples from mealworm beetles (Tenebrio molitor) and silkmoths (Bombyx mori). This method, established through our study, will advance research on reproductive strategies, including sperm competition and sperm quality maintenance in females.

Long-term sperm storage in eusocial Hymenoptera

In internally fertilizing species, sperm transfer is not always immediately followed by egg fertilization, and female sperm storage (FSS) may occur. FSS is a phenomenon in which females store sperm in a specialized organ for periods lasting from a few hours to several years, depending on the species. Eusocial hymenopterans (ants, social bees, and social wasps) hold the record for FSS duration. In these species, mating takes place during a single nuptial flight that occurs early in adult life for both sexes; they never mate again. Males die quickly after copulation but survive posthumously as sperm stored in their mates' spermathecae. Reproductive females, also known as queens, have a much longer life expectancy, up to 20 years in some species. Here, we review what is currently known about the molecular adaptations underlying the remarkable FSS capacities in eusocial hymenopterans. Because sperm quality is crucial to the reproductive success of both sexes, we also discuss the mechanisms involved in sperm storage and preservation in the male seminal vesicles prior to ejaculation. Finally, we propose future research directions that should broaden our understanding of this unique biological phenomenon.

Near-anoxia induces immobilization and sustains viability of sperm stored in ant queens

After copulation, insect females store sperm in a spermatheca for some duration until fertilization. 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 nearly anoxic, which induces sperm immobilization. Furthermore, mitochondrial respiratory inhibitors suppress sperm motility, suggesting that sperm immobilization may be caused by a shortage of ATP generated from only glycolysis under near-anoxic conditions. Sperm immobilization 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 that 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 under anoxic conditions for less than 2 h. This finding indicates that factors other than anoxia are also necessary for long-term sperm preservation.