Physiological and morphological development of the spermatheca in queens of the ant Crematogaster osakensis (Hymenoptera: Formicidae: Myrmicinae)

Abstract

The spermatheca is a specialized organ for sperm storage and is essential for reproductive success, particularly in ants, where queens preserve the viability of stored sperm for over a decade. However, the process of establishing the physiological environment and morphology of the spermatheca that enable long-term sperm storage remains largely unexplored. This study demonstrated that a near-anoxic state crucial for sperm viability and an elevated pH, both key features of the spermathecal environment in mated queens of Crematogaster osakensis , were established by 8 days after eclosion, suggesting that the spermatheca becomes functionally ready for sperm storage well before mating. Subsequently, the near-anoxic condition within the spermatheca is maintained after mating. In contrast, the queen’s body was normoxic at eclosion, gradually declined to near-anoxic levels by the mating period, and subsequently recovered to normoxic levels 2 months after the nuptial flight. These findings suggest spatio-temporal regulation of oxygen levels in ant queens. Morphologically, the formation of columnar epithelial cells in the hilar region of the spermathecal reservoir, considered essential for sperm storage, was triggered by nuptial flight, unlike in other ant subfamilies, where they develop during the pupal stage. Because these cells are thought to facilitate waste removal from stored sperm, their formation soon after mating appears to occur at an appropriate time for sperm maintenance. These findings indicate that C . osakensis queens establish their sperm storage environment in two distinct phases: preparation of physiological conditions before mating, followed by the completion of morphological development after mating to support long-term sperm maintenance.