Costs and benefits of endosymbiont infection in a doubly-infected parasitoid

Theory suggests that maternally-inherited endosymbionts can promote their spread and persistence in host populations by enhancing the production of daughters by infected hosts, either by improving overall host fitness, or through reproductive manipulation to favor production of daughters. In the doubly-infected parasitoid Encarsia inaron, we have found that Wolbachia manipulates host reproduction, but Cardinium does not. We therefore investigated the fitness costs and/or benefits of infection by each bacterium in this parasitoid, as a potential explanation for persistence of Cardinium in the population. We differentially cured E. inaron of Wolbachia, Cardinium, or both, introgressed the infected lines with the fully cured line to create a similar genetic backround, and evaluated several fitness parameters. We found that wasps infected with Cardinium (either alone or in combination with Wolbachia) had lower initial eggloads than wasps without Cardinium, but that wasps bearing Cardinium alone lived longer than the other lines, and produced more male offspring over their lifetime than wasps in the other treatments. Female production did not differ among treatments, however, so Cardinium persistence in the population cannot be explained by this apparent fitness benefit. We suggest that Cardinium infection of E. inaron may be an example of the theoretically hypothesized “hitchhiking” symbiont that is maintained in the population via co-infection with Wolbachia.