Ants show a unique reproductive system among insects. Males finish sperm production, and their testes degenerate at a young stage. After copulation, spermatozoa are transferred into the queens, who store the received sperm cells throughout their long lifespan without additional mating. In the present study, we investigated the reproductive biology of Crematogaster osakensis from male sexual maturation to sperm transfer, and sperm storage in queens. The sperm production was completed by eclosion and all produced spermatozoa had migrated to the seminal vesicle and the testes shrank until 10 days after eclosion. Sperm were not connected with bundles in mature males. The sperm cells were immobilized in the seminal vesicle. The sperm cells with the spermatophore were ejaculated into the bursa copulatrix and remained immotile during transfer from the bursa copulatrix to the spermathecal reservoir via the spermathecal duct. These findings provide significant insights into the importance of sperm immobilization, which prevents sperm damage and/or production of reactive oxygen species rather than swimming faster competed with rival male spermatozoa to reach into the sperm storage site, even in the polyandrous species, C. osakensis. Immobilization was also observed in the spermathecal reservoir 5 years after mating. This observation suggests that sperm immobilization is one of the important factors for successful long-term sperm storage and maintaining low levels of sperm metabolism.
Keywords: Ants; Male sexual maturation; Sperm immobilization; Sperm storage; Sperm transfer; Spermatogenesis.
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