Deleterious Wolbachiain the ant Formica truncorum

Wolbachiais a maternally inherited bacterium that may manipulate the reproduction of its arthropod hosts. In insects, it is known to lead to inviable matings, cause asexual reproduction or kill male offspring, all to its own benefit, but to the detriment of its host. In social Hymenoptera, Wolbachiaoccurs widely, but little is known about its fitness effects. We report on a Wolbachiainfection in the wood ant Formica truncorum, and evaluate whether it influences reproductive patterns. All 33 colonies of the study population were infected, suggesting that Wolbachiainfection is at, or close to, fixation. Interestingly, in colonies with fewer infected workers, significantly more sexuals are produced, indicating that Wolbachiahas deleterious effects in this species. In addition, adult workers are shown to have significantly lower infection rates (45) than worker pupae (87) or virgin queens (94), suggesting that workers lose their infection over life. Clearance of Wolbachiainfection has, to our knowledge, never been shown in any other natural system, but we argue that it may, in this case, represent an adaptive strategy to reduce colony load. The cause of fixation requires further study, but our data strongly suggest that Wolbachiahas no influence on the sex ratio in this species.