Onchocerca flexuosa WGS sequencing
Endosymbiont DNA in endobacteria-free filarial nematodes indicates past symbiosis and ancient horizontal genetic transfer
Infecting up to 66% of insect species, including species from every order, as well as spiders, mites, crustaceans and filarial nematodes, Wolbachia are among the most abundant symbiotic microbes on earth. These vertically transmitted endobacteria infect the host germline, facilitating horizontal DNA transfer. Many filarial nematode species, including several pathogens of medical and veterinary importance, depend on Wolbachia for proper development and survival. The mechanisms behind this interdependence are not understood. A few filarial species are naturally endosymbiont-free. In this study, we used 454 pyrosequencing to survey the genomes of two distantly related Wolbachia-free filarial species, Acanthocheilonema viteae and Onchocerca flexuosa. This screen identified 49 Wolbachia-like DNA sequences in A. viteae and 114 in O. flexuosa. qRT-PCR reactions detected expression of 30 Wolbachia-like sequences in A. viteae and 56 in O. flexuosa. Approximately half of these appear to be transcribed from pseudogenes. In situ hybridization showed that two of these pseudo-mRNAs were specifically expressed in developing embryos and testes of both species. Our results strongly suggest that the last common ancestor of extant filarial nematodes was infected with Wolbachia and that this former endosymbiont contributed to their genome evolution. Horizontally transferred Wolbachia sequences may someday explain the ability of some filarial species to live and reproduce without the endosymbiont while other species cannot.