No abstract

Honeybee males produce ejaculates consisting of large numbers of high quality sperm. Because queens never re-mate after a single mating episode early in life, sperm are stored in a specialised organ for years but the proximate mechanisms underlying this key physiological adaptation are unknown. We quantified energy metabolism in honeybee sperm and show that the glycolytic metabolite glyceraldehyde-3-phosphate (GA3P) is a key substrate for honeybee sperm survival and energy production. This reliance on non-aerobic energy metabolism in stored sperm was further supported by our findings of very low levels of oxygen inside the spermatheca. Expression of GA3P dehydrogenase (GAPDH), the enzyme involved in catabolism of GA3P, was significantly higher in stored compared to ejaculated sperm. Therefore, long-term sperm storage seems facilitated by the maintenance of non-aerobic energy production, the need for only the ATP-producing steps of glycolysis and by avoiding sperm damage resulting from ROS production. We also confirm that honeybee sperm is capable of aerobic metabolism, which predominates in ejaculated sperm while they compete for access to the spermatheca, but is suppressed during storage. Consequently, the remarkable reproductive traits of honeybees are proximately achieved by differential usage of energy production pathways to maximise competitiveness and minimise damage of sperm.

The queens of eusocial bees, ants and wasps mate only during a very short period early in life and males therefore produce ejaculates consisting of large numbers of high quality sperm. Such extreme selection for high fecundity resulted in males investing minimally into their somatic survival, including their immune system. However, if susceptible males are unable to protect their reproductive tissue from infections, they compromise queen fitness if they transfer pathogens during mating. We used the honey bee Apis mellifera and investigated the course of infection of the sexually transmitted pathogen Nosema apis. We predicted that honey bee males are susceptible but protect their reproductive tissues from infections. We investigated the effects of N. apis infections on the midgut, the accessory glands and the accessory testes and quantified the consequences of infection on male survival and fecundity. We found that N. apis is able to infect males, and as infections progressed, it significantly impacted fertility and survival in older males. Even though we confirm males to be able to minimize N. apis infections of their reproductive tissues, the parasite is present in ejaculates of older males. Consequently N. apis evolved alternative routes to successfully infect ejaculates and get sexually transmitted.

Honey bee (Apis mellifera) males are highly susceptible to infections with the sexually transmitted fungal pathogen Nosema apis. However, they are able to suppress this parasite in the ejaculate using immune molecules in the seminal fluid. We predicted that males respond to infections by altering the seminal fluid proteome to minimize the risk to sexually transmit the parasite to the queen and her colony. We used iTRAQ isotopic labeling to compare seminal fluid proteins from infected and noninfected males and found that N. apis infections resulted in significant abundance changes in 111 of the 260 seminal fluid proteins quantitated. The largest group of proteins with significantly changed abundances consisted of 15 proteins with well-known immune-related functions, which included two significantly more abundant chitinases in the seminal fluid of infected males. Chitinases were previously hypothesized to be involved in honey bee antifungal activity against N. apis. Here we show that infection with N. apis triggers a highly specific immune response in the seminal fluid of honey bee males.