A highly divergent and tiny-genomed obligate Wolbachia symbiont in an insect-parasitic nematode

Wolbachia is perhaps the most abundant host-associated microbe on the planet, infecting ~40% of insects and other terrestrial arthropod species, and ~40% of filarial nematode species. Here we report the discovery of a highly divergent strain of Wolbachia in Howardula sp., a tylenchid nematode that parasitizes woodland flies in the genus Spelobia (Diptera: Sphaeroceridae), and which represents a new lineage of hosts for Wolbachia. The symbiont is found at 100% prevalence in its nematode host. We sequenced its genome and found that it contains only 550 kilobase pairs of DNA, which is about ~30% smaller than the previous smallest known Wolbachia genome, and is a subset of all other Wolbachia genomes, with no newly acquired gene content. This work provides insights into our understanding of what are the lower limits of Wolbachia cells, as well as the role of obligate Wolbachia symbionts in the convergent evolution of diverse insect parasites.