Wolbachia infections in Australian ichneumonid parasitoid wasps (Hymenoptera: Ichneumonidae) : evidence for adherence to the global equilibrium hypothesis

Wolbachia are intracellular bacteria that infect a wide range of arthropod and nematode hosts, whose reproduction they manipulate to increase rates of vertical transmission. Horizontal transmission between unrelated species also occurs; pathways and mechanisms are still under debate, but parasitoid wasps have been suggested as potential vectors. Here, we perform the largest screening of Wolbachia in any parasitoid group by sampling 228 species of Ichneumonidae from Australia. The observed infection frequency of 23% is very close to the proposed global equilibrium frequency of ~20%. Rates of gain and loss are very similar in comparisons between different parasitoid lifestyles, such as ecto- vs. endoparasitoids. Genotyping Wolbachia showed identical strains in several unrelated ichneumonid species and other insect groups from around the world. A permutation test comparing Wolbachia strains in parasitoids and their host orders did not yield conclusive results, nor did a test for a latitudinal gradient. We argue that future studies should carefully consider the time scale of the processes under investigation. The speed at which Wolbachia move horizontally is probably so high that potential patterns are quickly erased and cannot be detected when sampling at the species level, nor with the slowly evolving multilocus sequence typing set of bacterial markers.