Male-killing Wolbachia do not protect Drosophila bifasciata against viral infection

Background: Insect symbionts employ multiple strategies to enhance their spread through populations, and some play a dual role as both a mutualist and a reproductive manipulator. It has recently been found that this is the case for some strains of Wolbachia, which both cause cytoplasmic incompatibility and protect their hosts against viruses. Here, we carry out the first test as to whether a male-killing strain of Wolbachia also provides a direct benefit to its host by providing antiviral protection to its host Drosophila bifasciata. We infected flies with two positive sense RNA viruses known to replicate in a range of Drosophila species (Drosophila C virus and Flock House virus) and measure the rate of death in Wolbachia positive and negative host lines with the same genetic background. Results: Both viruses caused considerable mortality to D. bifasciata flies, with Drosophila C virus killing 43% more flies than the uninfected controls and Flock House virus killing 78% more flies than the uninfected controls. However, viral induced mortality was unaffected by the presence of Wolbachia. Conclusion: In the first male-killing Wolbachia strain tested for antiviral effects, we found no evidence that it conferred protection against two RNA viruses. We show that although antiviral resistance is widespread across the Wolbachia phylogeny, the trait seems to have been lost or gained along some lineages. We discuss the potential mechanisms of this, and can seemingly discount protection against these viruses as a reason why this symbiont has spread through Drosophila populations.