Insights into female sperm storage from the spermathecal fluid proteome of the honeybee Apis mellifera

Background: Female animals are often able to store sperm inside their body-in some species even for several decades. The molecular basis of how females keep non-own cells alive is largely unknown, but since sperm cells are reported to be transcriptionally silenced and therefore limited in their ability to maintain their own function, it is likely that females actively participate in sperm maintenance. Because female contributions are likely to be of central importance for sperm survival, molecular insights into the process offer opportunities to observe mechanisms through which females manipulate sperm. Results: We used the honeybee, Apis mellifera, in which queens are highly polyandrous and able to maintain sperm viable for several years. We identified over a hundred proteins representing the major constituents of the spermathecal fluid, which females contribute to sperm in storage. We found that the gel profile of proteins from spermathecal fluid is very similar to the secretions of the spermathecal gland and concluded that the spermathecal glands are the main contributors to the spermathecal fluid proteome. A detailed analysis of the spermathecal fluid proteins indicate that they fall into a range of different functional groups, most notably enzymes of energy metabolism and antioxidant defense. A metabolic network analysis comparing the proteins detected in seminal fluid and spermathecal fluid showed a more integrated network is present in the spermathecal fluid that could facilitate long-term storage of sperm. Conclusions: We present a large-scale identification of proteins in the spermathecal fluid of honeybee queens and provide insights into the molecular regulation of female sperm storage.