Proteomic analysis of spermathecal fluid reveals factors related to long-term sperm storage in ant queens

Ant queens can maintain a large number of sperm cells for over a decade after mating at the beginning of their adult lives until they die. This ability is prominent because sperm cells cannot maintain their fertilization ability long after ejaculation in animals; however, the cellular mechanisms remain unclear. Sperm cells are maintained in the female sperm storage organ, the spermatheca, which supplies a suitable environment for sperm cells. To reveal the molecular basis of the long-term sperm storage mechanisms in ant queens, protein profiles enriched in the spermathecal fluid relative to the hemolymph were identified in Lasius japonicus using mass spectrometry-based proteomics. Proteins related to the extracellular matrix, antioxidant, metabolic pathways, proteases, and with uncharacterized functions were enriched in the spermathecal fluid relative to the hemolymph. These enriched proteins were shared with highly expressed genes previously detected by transcriptome analyses of the spermatheca in queens of Crematogaster osakensis belonging to a different subfamily than L. japonicus . It is considered that the ability for long-term sperm storage has evolved in the early ant lineage; therefore, the common proteins identified in the two ant species are crucial for this ability.