Female accessory gland fluid promotes sperm survival in yellow dung flies

Abstract

Female and male reproductive traits co-evolve through pre- and post-copulatory sexual selection and sexual conflict. Although males typically transfer many sperm during copulation, only a small proportion reach the fertilization site because females often actively or passively reduce sperm number in their reproductive tract. Males may transfer accessory substances to protect their ejaculates against female selective processes, which benefits males but can harm females. In turn, females may use accessory gland fluids to control paternity or sperm storage. Female yellow dung flies (Scathophaga stercoraria) have paired accessory glands that produce fluids involved in fertilization and egg laying. One proposed function for these fluids is spermicide. Alternatively, female accessory gland fluid may help keep sperm alive to avoid fertilization failure or encourage sperm competition. Using yellow dung flies, we investigated the interaction of female accessory gland fluid with sperm in vitro. Significantly more sperm remained alive when exposed to accessory gland fluid compared to buffer only (63% vs. 44%). We conclude that female accessory gland fluid in yellow dung flies can help nourish rather than kill male sperm, although selective nourishment of sperm is as consistent with cryptic female choice as is selective spermicide.