Discussion

Several human-infective filarial nematodes carry an intracellular alphaproteobacterial symbiont, Wolbachia. In terrestrial arthropods, Wolbachia induce a variety of reproductive manipulations that promote Wolbachia transmission. In filarial nematodes, Wolbachia do not impact host reproduction, but show features that suggest a mutualist association. However, some filarial species lack Wolbachia entirely and others have a patchy infection. While A and B supergroup Wolbachia strains only infect arthropods and C and D strains only infect filaria, supergroup F Wolbachia have been found in arthropods and filaria. Thus the history of association between Wolbachia and filarial nematodes is complex (see LeFoulon et al PeerJ. 2016;4:e1840). When one sequences a Wolbachia-infected filarial nematode genome, one gets the endiosymbiont genome "for free". Fragments of Wolbachia genomic DNA are also found in the nematodes' nuclear genomes. These horizontally-transferred fragments likely arise because Wolbachia is present in the nematode germline. We use these Wolbachia insertions to track the history of Wolbachia in filarial nematodes. We find that currently Wolbachia-free species used to carry infections, that D Wolbachia have replaced C in some species, and that Dirofilaria species carried two distinct Wolbachia (C and F). We find evidence of current dual infection by C and F Wolbachia in some but not all Dirofilaria repens. These findings suggest that the relationship between Wolbachia and filaria is not simply mutualist, and that some filarial Wolbachia may be recently-acquired manipulators.