Filarial nematodes are arthropod borne parasitic worms that infect hundreds of millions of people throughout the tropics and sub-tropics and are responsible for a great deal of morbidity in humans and domestic animals. Many filarial pathogens, such as the agents of lymphatic filariasis and river blindness, require a bacterial endosymbiont, Wolbachia pipientis, to carry out their life cycle [1–4]. In these species, depletion of the endosymbiont causes defects in growth, molting and fertility, leading to the death of the worm [5–7]. Other filarial species, some of which are very closely related to Wolbachia-dependent sister taxa, are naturally Wolbachia-free [1, 2, 8–10]. Thus far, there are no discernable patterns in Wolbachia distribution (e.g., based on host species, vector species, tissue tropism, geographic distribution, etc.), and the reasons for this disparity are poorly understood. Presumably, some genetic function(s) must be missing or reduced in Wolbachia-dependent worms in comparison to their Wolbachia-free counterparts, forcing them to rely on the bacteria as an alternative source of vital gene products. The processes underpinning Wolbachia-dependence are of biological and medical interest, as the Wolbachia products required by the dependent worm may represent useful targets for novel anti-filarial chemotherapies.
Wolbachia endobacteria and the eukaryotic mitochondria share many common features, including the intracellular lifestyle, obligatory mutualism, reduced genome size, vertical transmission, etc. These shared features, as well as their shared ancestry in the order Rickettsiales [11–13], lead us to hypothesize that Wolbachia may contribute to energy metabolism in the filarial host. Previous studies have shown that antibiotic-mediated Wolbachia depletion leads to upregulation in genes related to energy metabolism, including mitochondrially encoded subunits of the respiratory chain . This impact on host mitochondrial gene expression, and presumably energy production, suggests that Wolbachia may serve as an alternative energy source or mitochondrial “supplement,” necessitating increased activity when the endosymbiont is removed. If so, differences in mitochondrial function may account for discrepancies in Wolbachia status in the filarial lineage.
The mitochondrial genome (mtDNA) is particularly sensitive to evolutionary pressure exerted by the Wolbachia infection. Vertically-transmitted Wolbachia are able to expand rapidly through insect populations due to the mechanisms of reproductive parasitism . Wolbachia and mitochondria are co-transmitted. Thus, the mtDNA(s) of the first infected individual(s) presumably expand concurrently with the Wolbachia infection. Such Wolbachia-mitochondria “sweeps,” characterized by unusually low degrees of variation in the mtDNA of infected populations, have been noted in many insect species [16–20]. A similar lack of mtDNA diversity is seen in populations of Dirofilaria immitis (canine heartworm) in comparison to Wolbachia-free, non-filarial nematodes . A Wolbachia-induced genetic bottleneck may have led to the fixation of different mtDNA types among infected filarial species as compared to uninfected species.
The mtDNA sequences of 4 species of filarial nematodes, Onchocerca volvulus, D. immitis, Brugia malayi, and Setaria digitata, have been published. This report details the sequencing and analysis of the mtDNA sequences of 5 more species: Acanthocheilonema viteae
Chandlerella quiscali, Loa loa, Onchocerca flexuosa and Wuchereria bancrofti. Studies of the distribution of Wolbachia within filarial nematodes have shown that the infection is prevalent among 2 of the 8 filarial subfamiles, the Onchocercinae and the Dirofilariinae [1, 2]. Agreement between the phylogenies of Wolbachia and their filarial hosts suggests that Wolbachia entered the filarial lineage prior to the diversification of these 2 subfamilies [1, 26]. 7 of the 9 species included in this study are members of the the Onchocercinae and Dirofilariinae. Four of these, B. malayi
O. volvulus and W. bancrofti, are Wolbachia-dependent [1, 3, 4, 27]. The other 3, A. viteae
L. loa and O. flexuosa, are Wolbachia-free [1, 8, 10, 28], presumably due to secondary loss of the endosymbiont [1, 29]. Conversely, C. quiscali and S. digitata are Wolbachia-free and belong to subfamilies (Splendidofilariinae and Setariinae, respectively) that have not been shown to contain Wolbachia-infected species, suggesting that these subfamilies split from the lineage prior to the introduction of Wolbachia endobacteria [2, 9].
In light of the presumed impact of Wolbachia on the host mitochondria, we hypothesized that the mtDNAs of Wolbachia-dependent filaria may differ in gene content, arrangement or sequence as compared to those found in Wolbachia-free species whose ancestor(s) may not have undergone a Wolbachia-induced genetic bottleneck or evolved in the presence of an endobacterial partner capable of affecting host energy metabolism. The purpose of the reported study was to compare mtDNA from Wolbachia-dependent and independent filarial species in search of sequence level differences indicative of altered mitochondrial function. Our analyses revealed no differences that could be attributed to Wolbachia status. Future studies will be required to discover subtler affects of Wolbachia on the sequence or function of filarial nematode mitochondria.