Phylogeny of Wolbachia pipientis based on gltA, groEL and ftsZ gene sequences: clustering of arthropod and nematode symbionts in the F supergroup, and evidence for further diversity in the Wolbachia tree

Both asymmetric mitotic segregation and cell-to-cell invasion are required for stable germline transmission of Wolbachia in filarial nematodes

Parasitic filarial nematodes that belong to the Onchocercidae family live in mutualism with Wolbachia endosymbionts. We developed whole-mount techniques to follow the segregation patterns of Wolbachia through the somatic and germline lineages of four filarial species. These studies reveal multiple evolutionarily conserved mechanisms that are required for Wolbachia localization to the germline. During the initial embryonic divisions, Wolbachia segregate asymmetrically such that they concentrate in the posteriorly localized P₂ blastomere, a precursor to the adult germline and hypodermal lineages. Surprisingly, in the next division they are excluded from the germline precursor lineage. Rather, they preferentially segregate to the C blastomere, a source of posterior hypodermal cells. Localization to the germline is accomplished by a distinct mechanism in which Wolbachia invade first the somatic gonadal cells close to the ovarian distal tip cell, the nematode stem cell niche, from the hypodermis. This tropism is associated with a cortical F-actin disruption, suggesting an active engulfment. Significantly, germline invasion occurs only in females, explaining the lack of Wolbachia in the male germline. Once in the syncytial environment of the ovaries, Wolbachia rely on the rachis to multiply and disperse into the germ cells. The utilization of cell-to-cell invasion for germline colonization may indicate an ancestral mode of horizontal transfer that preceded the acquisition of the mutualism.

Determination of Wolbachia diversity in butterflies from Western Ghats, India, by a multigene approach

Members of the genus Wolbachia are intracellular bacteria that are widespread in arthropods and establish diverse symbiotic associations with their hosts, ranging from mutualism to parasitism. Here we present the first detailed analyses of Wolbachia in butterflies from India with screening of 56 species. Twenty-nine species (52%) representing five families were positive for Wolbachia. This is the first report of Wolbachia infection in 27 of the 29 species; the other two were reported previously. This study also provides the first evidence of infection in the family Papilionidae. A striking diversity was observed among Wolbachia strains in butterfly hosts based on five multilocus sequence typing (MLST) genes, with 15 different sequence types (STs). Thirteen STs are new to the MLST database, whereas ST41 and ST125 were reported earlier. Some of the same host species from this study carried distinctly different Wolbachia strains, whereas the same or different butterfly hosts also harbored closely related Wolbachia strains. Butterfly-associated STs in the Indian sample originated by recombination and point mutation, further supporting the role of both processes in generating Wolbachia diversity. Recombination was detected only among the STs in this study and not in those from the MLST database. Most of the strains were remarkably similar in their wsp genotype, despite divergence in MLST. Only two wsp alleles were found among 25 individuals with complete hypervariable region (HVR) peptide profiles. Although both wsp and MLST show variability, MLST gives better separation between the strains. Completely different STs were characterized for the individuals sharing the same wsp alleles.

Diversity and recombination in Wolbachia and Cardinium from Bryobia spider mites

Background

Wolbachia and Cardinium are endosymbiotic bacteria infecting many arthropods and manipulating host reproduction. Although these bacteria are maternally transmitted, incongruencies between phylogenies of host and parasite suggest an additional role for occasional horizontal transmission. Consistent with this view is the strong evidence for recombination in Wolbachia, although it is less clear to what extent recombination drives diversification within single host species and genera. Furthermore, little is known concerning the population structures of other insect endosymbionts which co-infect with Wolbachia, such as Cardinium. Here, we explore Wolbachia and Cardinium strain diversity within nine spider mite species (Tetranychidae) from 38 populations, and quantify the contribution of recombination compared to point mutation in generating Wolbachia diversity.

Results

We found a high level of genetic diversity for Wolbachia, with 36 unique strains detected (64 investigated mite individuals). Sequence data from four Wolbachia genes suggest that new alleles are 7.5 to 11 times more likely to be generated by recombination than point mutation. Consistent with previous reports on more diverse host samples, our data did not reveal evidence for co-evolution of Wolbachia with its host. Cardinium was less frequently found in the mites, but also showed a high level of diversity, with eight unique strains detected in 15 individuals on the basis of only two genes. A lack of congruence among host and Cardinium phylogenies was observed.

Conclusions

We found a high rate of recombination for Wolbachia strains obtained from host species of the spider mite family Tetranychidae, comparable to rates found for horizontally transmitted bacteria. This suggests frequent horizontal transmission of Wolbachia and/or frequent horizontal transfer of single genes. Our findings strengthens earlier reports of recombination for Wolbachia, and shows that high recombination rates are also present on strains from a restrictive host range. Cardinium was found co-infecting several spider mite species, and phylogenetic comparisons suggest also horizontal transmission of Cardinium among hosts.

Detection and characterization of Wolbachia infections in laboratory and natural populations of different species of tsetse flies (genus Glossina)

Background

Wolbachia is a genus of endosymbiotic α-Proteobacteria infecting a wide range of arthropods and filarial nematodes. Wolbachia is able to induce reproductive abnormalities such as cytoplasmic incompatibility (CI), thelytokous parthenogenesis, feminization and male killing, thus affecting biology, ecology and evolution of its hosts. The bacterial group has prompted research regarding its potential for the control of agricultural and medical disease vectors, including Glossina spp., which transmits African trypanosomes, the causative agents of sleeping sickness in humans and nagana in animals.

Results

In the present study, we employed a Wolbachia specific 16S rRNA PCR assay to investigate the presence of Wolbachia in six different laboratory stocks as well as in natural populations of nine different Glossina species originating from 10 African countries. Wolbachia was prevalent in Glossina morsitans morsitans, G. morsitans centralis and G. austeni populations. It was also detected in G. brevipalpis, and, for the first time, in G. pallidipes and G. palpalis gambiensis. On the other hand, Wolbachia was not found in G. p. palpalis, G. fuscipes fuscipes and G. tachinoides. Wolbachia infections of different laboratory and natural populations of Glossina species were characterized using 16S rRNA, the wsp (Wolbachia Surface Protein) gene and MLST (Multi Locus Sequence Typing) gene markers. This analysis led to the detection of horizontal gene transfer events, in which Wobachia genes were inserted into the tsetse flies fly nuclear genome.

Conclusions

Wolbachia infections were detected in both laboratory and natural populations of several different Glossina species. The characterization of these Wolbachia strains promises to lead to a deeper insight in tsetse flies-Wolbachia interactions, which is essential for the development and use of Wolbachia-based biological control methods.

Presence and distribution of the endosymbiont Wolbachia among Solenopsis spp. (Hymenoptera: Formicidae) from Brazil and its evolutionary history

Wolbachia are intracellular bacteria that commonly infect arthropods. Its prevalence among ants of the genus Solenopsis is high. In the present study, the presence and distribution of these endosymbionts was examined among populations of Solenopsis spp. from Brazil. A phylogenetic analysis based on the wsp gene was conducted to infer the evolutionary history of Wolbachia infections within the populations surveyed. A high frequency of Wolbachia bacteria was observed among the genus Solenopsis, 51% of the colonies examined were infected. Incidence was higher in populations from southern Brazil. However, little genetic variability was found among different Wolbachia strains within supergroups A and B. Our findings also suggest that horizontal transmission events can occur through the social parasite S. daguerrei.

Neorickettsial endosymbionts of the digenea: diversity, transmission and distribution

Digeneans are endoparasitic flatworms with complex life cycles and distinct life stages that parasitize different host species. Some digenean species harbour bacterial endosymbionts known as Neorickettsia (Order Rickettsiales, Family Anaplasmataceae). Neorickettsia occur in all life stages and are maintained by vertical transmission. Far from benign however, Neorickettsia may also be transmitted horizontally by digenean parasites to their vertebrate definitive hosts. Once inside, Neorickettsia can infect macrophages and other cell types. In some vertebrate species (e.g. dogs, horses and humans), neorickettsial infections cause severe disease. Taken from a mostly parasitological perspective, this article summarizes our current knowledge on the transmission ecology of neorickettsiae, both for pathogenic species and for neorickettsiae of unknown pathogenicity. In addition, we discuss the diversity, phylogeny and geographical distribution of neorickettsiae, as well as their possible evolutionary associations with various groups of digeneans. Our understanding of neorickettsiae is at an early stage and there are undoubtedly many more neorickettsial endosymbioses with digeneans waiting to be discovered. Because neorickettsiae can infect vertebrates, it is particularly important to examine digenean species that regularly infect humans. Rapid advances in molecular tools and their application towards bacterial identification bode well for our future progress in understanding the biology of Neorickettsia.

Detection and characterization of Wolbachia infections in natural populations of aphids: is the hidden diversity fully unraveled?

Aphids are a serious threat to agriculture, despite being a rather small group of insects. The about 4,000 species worldwide engage in highly interesting and complex relationships with their microbial fauna. One of the key symbionts in arthropods is Wolbachia, an α-Proteobacterium implicated in many important biological processes and believed to be a potential tool for biological control. Aphids were thought not to harbour Wolbachia; however, current data suggest that its presence in aphids has been missed, probably due to the low titre of the infection and/or to the high divergence of the Wolbachia strains of aphids. The goal of the present study is to map the Wolbachia infection status of natural aphids populations, along with the characterization of the detected Wolbachia strains. Out of 425 samples from Spain, Portugal, Greece, Israel and Iran, 37 were found to be infected. Our results, based mainly on 16S rRNA gene sequencing, indicate the presence of two new Wolbachia supergroups prevailing in aphids, along with some strains belonging either to supergroup B or to supergroup A.

Wolbachia-mediated antibacterial protection and immune gene regulation in Drosophila

The outcome of microbial infection of insects is dependent not only on interactions between the host and pathogen, but also on the interactions between microbes that co-infect the host. Recently the maternally inherited endosymbiotic bacteria Wolbachia has been shown to protect insects from a range of microbial and eukaryotic pathogens. Mosquitoes experimentally infected with Wolbachia have upregulated immune responses and are protected from a number of pathogens including viruses, bacteria, Plasmodium and filarial nematodes. It has been hypothesised that immune upregulation underpins Wolbachia-mediated protection. Drosophila is a strong model for understanding host-Wolbachia-pathogen interactions. Wolbachia-mediated antiviral protection in Drosophila has been demonstrated for a number of different Wolbachia strains. In this study we investigate whether Wolbachia-infected flies are also protected against pathogenic bacteria. Drosophila simulans lines infected with five different Wolbachia strains were challenged with the pathogenic bacteria Pseudomonas aeruginosa PA01, Serratia marcescens and Erwinia carotovora and mortality compared to paired lines without Wolbachia. No difference in mortality was observed in the flies with or without Wolbachia. Similarly no antibacterial protection was observed for D. melanogaster infected with Wolbachia. Interestingly, D. melanogaster Oregon RC flies which are naturally infected with Wolbachia showed no upregulation of the antibacterial immune genes TepIV, Defensin, Diptericin B, PGRP-SD, Cecropin A1 and Attacin D compared to paired flies without Wolbachia. Taken together these results indicate that Wolbachia-mediated antibacterial protection is not ubiquitous in insects and furthermore that the mechanisms of antibacterial and antiviral protection are independent. We suggest that the immune priming and antibacterial protection observed in Wolbachia-infected mosquitoes may be a consequence of the recent artificial introduction of the symbiont into insects that normally do not carry Wolbachia and that antibacterial protection is unlikely to be found in insects carrying long-term Wolbachia infections.

New insights into the evolution of Wolbachia infections in filarial nematodes inferred from a large range of screened species

BACKGROUND

Wolbachia are intriguing symbiotic endobacteria with a peculiar host range that includes arthropods and a single nematode family, the Onchocercidae encompassing agents of filariases. This raises the question of the origin of infection in filariae. Wolbachia infect the female germline and the hypodermis. Some evidences lead to the theory that Wolbachia act as mutualist and coevolved with filariae from one infection event: their removal sterilizes female filariae; all the specimens of a positive species are infected; Wolbachia are vertically inherited; a few species lost the symbiont. However, most data on Wolbachia and filaria relationships derive from studies on few species of Onchocercinae and Dirofilariinae, from mammals.

METHODOLOGY/PRINCIPAL FINDINGS

We investigated the Wolbachia distribution testing 35 filarial species, including 28 species and 7 genera and/or subgenera newly screened, using PCR, immunohistochemical staining, whole mount fluorescent analysis, and cocladogenesis analysis. (i) Among the newly screened Onchocercinae from mammals eight species harbour Wolbachia but for some of them, bacteria are absent in the hypodermis, or in variable density. (ii) Wolbachia are not detected in the pathological model Monanema martini and in 8, upon 9, species of Cercopithifilaria. (iii) Supergroup F Wolbachia is identified in two newly screened Mansonella species and in Cercopithifilaria japonica. (iv) Type F Wolbachia infect the intestinal cells and somatic female genital tract. (v) Among Oswaldofilariinae, Waltonellinae and Splendidofilariinae, from saurian, anuran and bird respectively, Wolbachia are not detected.

CONCLUSIONS/SIGNIFICANCE

The absence of Wolbachia in 63% of onchocercids, notably in the ancestral Oswaldofilariinae estimated 140 mya old, the diverse tissues or specimens distribution, and a recent lateral transfer in supergroup F Wolbachia, modify the current view on the role and evolution of the endosymbiont and their hosts. Further genomic analyses on some of the newly sampled species are welcomed to decipher the open questions.

Co-existence, phylogeny and putative role of Wolbachia and yeast-like symbiont (YLS) in Kerria lacca (Kerr)

This study reports the first detection of Wolbachia and yeast-like symbiont (YLS) harbored in Kerria lacca (Kerr), a scale insect, latter of which produces an economically important natural resin, known as lac. Wolbachia was detected using PCR amplification and sequencing of 16S rDNA; and further confirmation and phylogenetic analysis was carried out by fast evolving wsp gene. Neighbor-joining and maximum parsimonious (MP) analysis showed that this strain belongs to subgroup "ori" of Wolbachia super group B of arthropods. Wolbachia of K. lacca is hereby designated as "wKerlac" according to Wolbachia nomenclature system. Histological study revealed the presence of yeast-like endosymbiont, which was also confirmed by PCR amplification of 18S rDNA. Phylogenetic analysis revealed that YLS of K. lacca is quite distinct from YLS of aphid, planthoppers, and beetles. Putative roles of Wolbachia in lecanoid chromosome system of sex determination and in biased sex ratio of K. lacca populations; and YLS in nutritional supplementation and detoxifying substances which are deleterious to K. lacca, are hereby, suggested.

Distribution and molecular characterization of Wolbachia endosymbionts and filarial nematodes in Maryland populations of the lone star tick (Amblyomma americanum)

The lone star tick Amblyomma americanum is host to a wide diversity of endosymbiotic bacteria. We identified a novel Wolbachia symbiont infecting A. americanum. Multilocus sequence typing phylogenetically placed the endosymbiont in the increasingly diverse F supergroup. We assayed a total of 1031 ticks (119 females, 78 males and 834 nymphs in 89 pools) from 16 Maryland populations for infection. Infection frequencies in the natural populations were approximately 5% in females and <2% (minimum infection rate) in nymphs; infection was not detected in males. Infected populations were only observed in southern Maryland, suggesting the possibility that Wolbachia is currently invading Maryland A. americanum populations. Because F supergroup Wolbachia have been detected previously in filarial nematodes, tick samples were assayed for nematodes by PCR. Filarial nematodes were detected in 70% and 9% of Wolbachia-positive and Wolbachia-negative tick samples, respectively. While nematodes were more common in Wolbachia-positive tick samples, the lack of a strict infection concordance (Wolbachia-positive, nematode-negative and Wolbachia-negative, nematode-positive ticks) suggests that Wolbachia prevalence in ticks is not due to nematode infection. Supporting this hypothesis, phylogenetic analysis indicated that the nematodes were likely a novel species within the genus Acanthocheilonema, which has been previously shown to be Wolbachia-free.

Detection and phylogenetic analysis of Wolbachia in the Asiatic rice leafroller, Cnaphalocrocis medinalis, in Chinese populations

Wolbachia are a group of intracellular inherited endosymbiontic bacteria infecting a wide range of insects. In this study the infection status of Wolbachia (Rickettsiales: Rickettsiaceae) was measured in the Asiatic rice leafroller, Cnaphalocrocis medinalis (Guenée) (Lepidoptera: Pyralidae), from twenty locations in China by sequencing wsp, ftsZ and 16S rDNA genes. The results showed high infection rates of Wolbachia in C. medinalis populations. Wolbachia was detected in all geographically separate populations; the average infection rate was ~ 62.5%, and the highest rates were 90% in Wenzhou and Yangzhou populations. The Wolbachia detected in different C. medinalis populations were 100% identical to each other when wsp, ftsZ, and 16S rDNA sequences were compared, with all sequences belonging to the Wolbachia B supergroup. Based on wsp, ftsZ and 16S rDNA sequences of Wolbachia, three phylogenetic trees of similar pattern emerged. This analysis indicated the possibility of inter-species and intra-species horizontal transmission of Wolbachia in different arthropods in related geographical regions. The migration route of C. medinalis in mainland China was also discussed since large differentiation had been found between the wsp sequences of Chinese and Thai populations.

Wolbachia infection in the tribe Naupactini (Coleoptera, Curculionidae): association between thelytokous parthenogenesis and infection status

Several parthenogenetic species of broad-nosed weevils exist, some of them of economic importance because of their pest status. Screening of the maternally inherited Wolbachia bacterium in 29 weevils of the tribe Naupactini, using multilocus sequence typing allowed us to assess a significant correlation between asexuality and infection, and suggests an involvement of Wolbachia in the origin of this reproductive mode. The nine Wolbachia strains retrieved from the Naupactini belong to the B supergroup. Phylogenetic analysis of these strains, along with other 23 strains obtained from arthropods and nematodes, supports previous hypotheses that horizontal transfer of Wolbachia amongst species from unrelated taxa has been pervasive.

The native Wolbachia endosymbionts of Drosophila melanogaster and Culex quinquefasciatus increase host resistance to West Nile virus infection

BACKGROUND

The bacterial endosymbiont Wolbachia pipientis has been shown to increase host resistance to viral infection in native Drosophila hosts and in the normally Wolbachia-free heterologous host Aedes aegypti when infected by Wolbachia from Drosophila melanogaster or Aedes albopictus. Wolbachia infection has not yet been demonstrated to increase viral resistance in a native Wolbachia-mosquito host system.

METHODOLOGY/PRINCIPAL FINDINGS

In this study, we investigated Wolbachia-induced resistance to West Nile virus (WNV; Flaviviridae) by measuring infection susceptibility in Wolbachia-infected and Wolbachia-free D. melanogaster and Culex quinquefasciatus, a natural mosquito vector of WNV. Wolbachia infection of D. melanogaster induces strong resistance to WNV infection. Wolbachia-infected flies had a 500-fold higher ID50 for WNV and produced 100,000-fold lower virus titers compared to flies lacking Wolbachia. The resistance phenotype was transmitted as a maternal, cytoplasmic factor and was fully reverted in flies cured of Wolbachia. Wolbachia infection had much less effect on the susceptibility of D. melanogaster to Chikungunya (Togaviridae) and La Crosse (Bunyaviridae) viruses. Wolbachia also induces resistance to WNV infection in Cx. quinquefasciatus. While Wolbachia had no effect on the overall rate of peroral infection by WNV, Wolbachia-infected mosquitoes produced lower virus titers and had 2 to 3-fold lower rates of virus transmission compared to mosquitoes lacking Wolbachia.

CONCLUSIONS/SIGNIFICANCE

This is the first demonstration that Wolbachia can increase resistance to arbovirus infection resulting in decreased virus transmission in a native Wolbachia-mosquito system. The results suggest that Wolbachia reduces vector competence in Cx. quinquefasciatus, and potentially in other Wolbachia-infected mosquito vectors.

The Wolbachia endosymbiont as an anti-filarial nematode target

Human disease caused by parasitic filarial nematodes is a major cause of global morbidity. The parasites are transmitted by arthropod intermediate hosts and are responsible for lymphatic filariasis (elephantiasis) or onchocerciasis (river blindness). Within these filarial parasites are intracellular alpha-proteobacteria, Wolbachia, that were first observed almost 30 years ago. The obligate endosymbiont has been recognized as a target for anti-filarial nematode chemotherapy as evidenced by the loss of worm fertility and viability upon antibiotic treatment in an extensive series of human trials. While current treatments with doxycycline and rifampicin are not practical for widespread use due to the length of required treatments and contraindications, anti-Wolbachia targeting nevertheless appears a promising alternative for filariasis control in situations where current programmatic strategies fail or are unable to be delivered and it provides a superior efficacy for individual therapy. The mechanisms that underlie the symbiotic relationship between Wolbachia and its nematode hosts remain elusive. Comparative genomics, bioinfomatic and experimental analyses have identified a number of potential interactions, which may be drug targets. One candidate is de novo heme biosynthesis, due to its absence in the genome sequence of the host nematode, Brugia malayi, but presence in Wolbachia and its potential roles in worm biology. We describe this and several additional candidate targets, as well as our approaches for understanding the nature of the host-symbiont relationship.

Variation in antiviral protection mediated by different Wolbachia strains in Drosophila simulans

Drosophila C virus (DCV) is a natural pathogen of Drosophila and a useful model for studying antiviral defences. The Drosophila host is also commonly infected with the widespread endosymbiotic bacteria Wolbachia pipientis. When DCV coinfects Wolbachia-infected D. melanogaster, virus particles accumulate more slowly and virus induced mortality is substantially delayed. Considering that Wolbachia is estimated to infect up to two-thirds of all insect species, the observed protective effects of Wolbachia may extend to a range of both beneficial and pest insects, including insects that vector important viral diseases of humans, animals and plants. Currently, Wolbachia-mediated antiviral protection has only been described from a limited number of very closely related strains that infect D. melanogaster. We used D. simulans and its naturally occurring Wolbachia infections to test the generality of the Wolbachia-mediated antiviral protection. We generated paired D. simulans lines either uninfected or infected with five different Wolbachia strains. Each paired fly line was challenged with DCV and Flock House virus. Significant antiviral protection was seen for some but not all of the Wolbachia strain-fly line combinations tested. In some cases, protection from virus-induced mortality was associated with a delay in virus accumulation, but some Wolbachia-infected flies were tolerant to high titres of DCV. The Wolbachia strains that did protect occurred at comparatively high density within the flies and were most closely related to the D. melanogaster Wolbachia strain wMel. These results indicate that Wolbachia-mediated antiviral protection is not ubiquitous, a finding that is important for understanding the distribution of Wolbachia and virus in natural insect populations.

Distribution of Wolbachia infection in Chorthippus parallelus populations within and beyond a Pyrenean hybrid zone

Two subspecies of the meadow grasshopper Chorthippus parallelus meet in a secondary hybrid zone in the Pyrenees. We have recently detected Wolbachia infection in this extensively studied species. C. p. parallelus (Cpp) and C. p. erythropus (Cpe) harbour bacteria from the B and F supergroups, but they differ noticeably in the incidence and type of infection present in their populations. We can distinguish different regional infection patterns that are associated with the distribution of pure and hybrid C. parallelus individuals. The northern pattern of low-level infection characterizes Cpp populations, whereas the southern pattern of high-level infection affects Cpe locations. These patterns converge in the hybrid zone and generate a third infection pattern featuring an extremely high degree of co-infection with the two Wolbachia types in hybrids. The distribution of Wolbachia among the studied populations encourages us to consider the possibility that this bacterium has a significant influence on the origin, maintenance and dynamics of this hybrid zone, given the reproductive alterations that are often associated with Wolbachia.

Arsenophonus, an emerging clade of intracellular symbionts with a broad host distribution

BACKGROUND

The genus Arsenophonus is a group of symbiotic, mainly insect-associated bacteria with rapidly increasing number of records. It is known from a broad spectrum of hosts and symbiotic relationships varying from parasitic son-killers to coevolving mutualists.The present study extends the currently known diversity with 34 samples retrieved mainly from hippoboscid (Diptera: Hippoboscidae) and nycteribiid (Diptera: Nycteribiidae) hosts, and investigates phylogenetic relationships within the genus.

RESULTS

The analysis of 110 Arsenophonus sequences (incl. Riesia and Phlomobacter), provides a robust monophyletic clade, characterized by unique molecular synapomorphies. On the other hand, unstable inner topology indicates that complete understanding of Arsenophonus evolution cannot be achieved with 16S rDNA. Moreover, taxonomically restricted Sampling matrices prove sensitivity of the phylogenetic signal to sampling; in some cases, Arsenophonus monophyly is disrupted by other symbiotic bacteria. Two contrasting coevolutionary patterns occur throughout the tree: parallel host-symbiont evolution and the haphazard association of the symbionts with distant hosts. A further conspicuous feature of the topology is the occurrence of monophyletic symbiont lineages associated with monophyletic groups of hosts without a co-speciation pattern. We suggest that part of this incongruence could be caused by methodological artifacts, such as intragenomic variability.

CONCLUSION

The sample of currently available molecular data presents the genus Arsenophonus as one of the richest and most widespread clusters of insect symbiotic bacteria. The analysis of its phylogenetic lineages indicates a complex evolution and apparent ecological versatility with switches between entirely different life styles. Due to these properties, the genus should play an important role in the studies of evolutionary trends in insect intracellular symbionts. However, under the current practice, relying exclusively on 16S rRNA sequences, the phylogenetic analyses are sensitive to various methodological artifacts that may even lead to description of new Arsenophonus lineages as independent genera (e.g. Riesia and Phlomobacter). The resolution of the evolutionary questions encountered within the Arsenophonus clade will thus require identification of new molecular markers suitable for the low-level phylogenetics.

Extensive genomic diversity of closely related Wolbachia strains

Using microarray-based comparative genome hybridization (mCGH), the genomic content of Wolbachia pipientis wMel from Drosophila melanogaster was compared to the closely related Wolbachia from D. innubila (wInn), D. santomea (wSan), and three strains from D. simulans (wAu, wRi, wSim). A large number of auxiliary genes are identified in these five strains, with most absent/divergent genes being unique to a given strain. Each strain caused an average of approximately 60 genes to be removed from the core genome. As such, these organisms do not appear to have the streamlined genomes expected of obligate intracellular bacteria. Prophage, hypothetical and ankyrin repeat genes are over-represented in the absent/divergent genes, with 21-87% of absent/divergent genes coming from prophage regions. The only wMel region absent/divergent in all five query strains is that containing WD_0509 to WD_0511, including a DNA mismatch repair protein MutL-2, a degenerate RNase, and a conserved hypothetical protein. A region flanked by the two portions of the WO-B prophage in wMel is found in four of the five Wolbachia strains as well as on a plasmid of a rickettsial endosymbiont of Ixodes scapularis, suggesting lateral gene transfer between these two obligate intracellular species. Overall, these insect-associated Wolbachia have highly mosaic genomes, with lateral gene transfer playing an important role in their diversity and evolution.

The genetics and cell biology of Wolbachia-host interactions

Wolbachia are gram-negative bacteria that are widespread in nature, carried by the majority of insect species as well as some mites, crustaceans, and filarial nematodes. Wolbachia can range from parasitic to symbiotic, depending upon the interaction with the host species. The success of Wolbachia is attributed to efficient maternal transmission and manipulations of host reproduction that favor infected females, such as sperm-egg cytoplasmic incompatibility (CI). Much remains unknown about the mechanistic basis for Wolbachia-host interactions. Here we summarize the current understanding of Wolbachia interaction with insect hosts, with a focus on Drosophila. The areas of discussion include Wolbachia transmission in oogenesis, Wolbachia distribution in spermatogenesis, induction and rescue of the CI phenotype, Wolbachia genomics, and Wolbachia-membrane interactions.

How diverse is the genus Wolbachia? Multiple-gene sequencing reveals a putatively new Wolbachia supergroup recovered from spider mites (Acari: Tetranychidae)

At least 20% of all arthropods and some nematode species are infected with intracellular bacteria of the genus Wolbachia. This highly diverse genus has been subdivided into eight "supergroups" (A to H) on the basis of nucleotide sequence data. Here, we report the discovery of a new Wolbachia supergroup recovered from the spider mite species Bryobia species V (Acari: Tetranychidae), based on the sequences of three protein-coding genes (ftsZ, gltA, and groEL) and the 16S rRNA gene. Other tetranychid mites possess supergroup B Wolbachia strains. The discovery of another Wolbachia supergroup expands the known diversity of Wolbachia and emphasizes the high variability of the genus. Our data also clarify the existing supergroup structure and highlight the use of multiple gene sequences for robust phylogenetic analysis. In addition to previous reports of recombination between the arthropod-infecting supergroups A and B, we provide evidence for recombination between the nematode-infecting supergroups C and D. Robust delineation of supergroups is essential for understanding the origin and spread of this common reproductive parasite and for unraveling mechanisms of host adaptation and manipulation across a wide range of hosts.

Infection by Wolbachia: from passengers to residents

Wolbachia are endosymbiotic alpha-proteobacteria harboured by terrestrial arthropods and filarial nematodes, where they are maternally transmitted through egg cytoplasm. According to the host group, Wolbachia have developed two contrasting symbiotic strategies. In arthropods, symbiosis is secondary (i.e. facultative), and Wolbachia insure their transmission as reproduction parasites. However, despite of the efficiency of the manipulation mechanisms used, Wolbachia are limited to the state of passenger because some factors can prevent the association between Wolbachia and their hosts to become permanent. On the contrary, symbiosis is primary (i.e. obligatory) in filarial nematodes where Wolbachia insure their transmission via a mutualistic relationship, leading them to become permanent residents of their hosts. However, a few examples show that in arthropods too some Wolbachia have started to present the first stages of a mutualistic behaviour, or are even truly indispensable to their host. Whatever its strategy, Wolbachia infection is a spectacular evolutionary success, this symbiotic bacterium representing one of the most important biomass of its kind.

Identification and characterization of Wolbachia in Solenopsis saevissima fire ants (Hymenoptera: Formicidae) in southeastern Brazil

The genus Solenopsis appears to have evolved and radiated very rapidly in South America and then spread throughout the rest of the continent. As part of the expansion process, distribution patterns and different degrees of geographic isolation among populations of S. saevissima can be observed. We have investigated the presence of Wolbachia in 52 colonies and 1623 individuals in southeastern Brazil. Detection of Wolbachia infection was based on amplification of the 16S rRNA and wsp genes by polymerase chain reactions. Wolbachia was found in only one of the four locations investigated and it was observed that the populations were polymorphic for infection. The infection level observed increased during the period of screening. In particular, double infection (16SWA and B) increased from 44% in 2005 to 90% in 2006. The A-group of Wolbachia from the wsp sequences was determined by sequencing. However, two variant wsp sequences were detected in Wolbachia present in these populations. The alignment of our sequences with those deposited in GenBank indicated significant differences in relation to homologous sequences. Phylogenetic relationships were inferred using parsimony, and confidence intervals were estimated by bootstrapping. Then the divergence of the Wolbachia of S. saevissima in the populations studied with other variants allowed us to verify that wSS1 and dwSS2 formed a distinct clade within the A-group (>75%). These results can be useful in studies on the dynamics of ant populations.

Parasitism and mutualism in Wolbachia: what the phylogenomic trees can and cannot say

Ecological and evolutionary theories predict that parasitism and mutualism are not fixed endpoints of the symbiotic spectrum. Rather, parasitism and mutualism may be host or environment dependent, induced by the same genetic machinery, and shifted due to selection. These models presume the existence of genetic or environmental variation that can spur incipient changes in symbiotic lifestyle. However, for obligate intracellular bacteria whose genomes are highly reduced, studies specify that discrete symbiotic associations can be evolutionarily stable for hundreds of millions of years. Wolbachia is an inherited obligate, intracellular infection of invertebrates containing taxa that act broadly as both parasites in arthropods and mutualists in certain roundworms. Here, we analyze the ancestry of mutualism and parasitism in Wolbachia and the evolutionary trajectory of this variation in symbiotic lifestyle with a comprehensive, phylogenomic analysis. Contrary to previous claims, we show unequivocally that the transition in lifestyle cannot be reconstructed with current methods due to long-branch attraction (LBA) artifacts of the distant Anaplasma and Ehrlichia outgroups. Despite the use of 1) site-heterogenous phylogenomic methods that can overcome systematic error, 2) a taxonomically rich set of taxa, and 3) statistical assessments of the genes, tree topologies, and models of evolution, we conclude that the LBA artifact is serious enough to afflict past and recent claims including the root lies in the middle of the Wolbachia mutualists and parasites. We show that different inference methods yield different results and high bootstrap support did not equal phylogenetic accuracy. Recombination was rare among this taxonomically diverse data set, indicating that elevated levels of recombination in Wolbachia are restricted to specific coinfecting groups. In conclusion, we attribute the inability to root the tree to rate heterogeneity between the ingroup and outgroup. Site-heterogenous models of evolution did improve the placement of aberrant taxa in the ingroup phylogeny. Finally, in the unrooted topology, the distribution of parasitism and mutualism across the tree suggests that at least two interphylum transfers shaped the origins of nematode mutualism and arthropod parasitism. We suggest that the ancestry of mutualism and parasitism is not resolvable without more suitable outgroups or complete genome sequences from all Wolbachia supergroups.

Absence of Wolbachia endobacteria in the non-filariid nematodes Angiostrongylus cantonensis and A. costaricensis

The majority of filarial nematodes harbour Wolbachia endobacteria, including the major pathogenic species in humans, Onchocerca volvulus, Brugia malayi and Wuchereria bancrofti. These obligate endosymbionts have never been demonstrated unequivocally in any non-filariid nematode. However, a recent report described the detection by PCR of Wolbachia in the metastrongylid nematode, Angiostrongylus cantonensis (rat lungworm), a leading cause of eosinophilic meningitis in humans. To address the intriguing possibility of Wolbachia infection in nematode species distinct from the Family Onchocercidae, we used both PCR and immunohistochemistry to screen samples of A. cantonensis and A. costaricensis for the presence of this endosymbiont. We were unable to detect Wolbachia in either species using these methodologies. In addition, bioinformatic and phylogenetic analyses of the Wolbachia gene sequences reported previously from A. cantonensis indicate that they most likely result from contamination with DNA from arthropods and filarial nematodes. This study demonstrates the need for caution in relying solely on PCR for identification of new endosymbiont strains from invertebrate DNA samples.

Wolbachia-based technologies for insect pest population control

Wolbachia are a group of obligatory intracellular and maternally inherited bacteria found in many arthropod species, including insects, mites, spiders, springtails, crustaceans, as well as in certain nematodes. Several PCR-based surveys suggest that over 20% of the arthropod species may be Wolbachia-infected, rendering this bacterium the most ubiquitous intracellular symbiont yet described. Wolbachia have recently attracted attention for their potential as novel and environmentally friendly bio-control agents. Wolbachia are able to invade and maintain themselves in the arthropod species through manipulation of the host's reproduction. Several strategies can be distinguished, one of which is cytoplasmic incompatibility (CI). Wolbachia-induced cytoplasmic incompatibility can be used beneficially in the following ways: (a) as a tool for insect pest population control in a way analogous to the "Sterile Insect technique" (SIT) and (b) as a drive system to spread desirable genotypes in field arthropod populations. In addition, virulent Wolbachia strains offer the potential to control vector species by modifying their population age structure. In the present chapter, I summarize the recent developments in Wolbachia research with an emphasis on the applied biology of Wolbachia and conclude with the challenges that Wolbachia researchers will face if they want to use and/or introduce Wolbachia into pest and vector species ofeconomic, environmental and public health relevance and, through Wolbachia-based technologies, to suppress or modify natural populations.

16S rRNA-, GroEL- and MucZ-based assessment of the taxonomic position of 'Rickettsiella melolonthae' and its implications for the organization of the genus Rickettsiella

'Rickettsiella melolonthae' is an intracellularly multiplying bacterial pathogen of European cockchafers, Melolontha melolontha (Linnaeus, 1758) and Melolontha hippocastani (Fabricius, 1801) (Coleoptera: Scarabaeidae). We report the first determination of nucleotide sequences from this organism, i.e. the 16S rRNA encoding rrs gene, the chaperonin encoding groEL gene and the mucZ gene encoding the orthologue of a capsule synthesis-inducing factor of Coxiella burnetii. Within the genus Rickettsiella, the pathotype 'Rickettsiella melolonthae' is currently classified as a synonym of the nomenclatural type species Rickettsiella popilliae. Previous sequencing of a 16S rRNA gene from a different species, Rickettsiella grylli, has motivated the transfer of the entire genus from the alphaproteobacterial order Rickettsiales to the gammaproteobacterial order Legionellales, family Coxiellaceae. We investigated the validity of this taxonomic reorganization beyond the species Rickettsiella grylli by reconstructing the organismal phylogeny from comparisons of 16S rRNA gene and GroEL and MucZ protein sequences from a selected set of alpha- and gammaproteobacteria as well as bacterial pathogens from the order Chlamydiales. Our analysis strongly supported the transfer of the genus Rickettsiella to the order Legionellales, but not its classification in one of the recognized families present in this order. Furthermore, our results substantiated inconsistencies in the internal organization of the genus. In particular, the currently accepted delineation of Rickettsiella species and the claimed synonymy of 'Rickettsiella melolonthae' with Rickettsiella popilliae are not simultaneously consistent with our findings.

An effect of 16S rRNA intercistronic variability on coevolutionary analysis in symbiotic bacteria: molecular phylogeny of Arsenophonus triatominarum

The genes of ribosomal RNA are the most popular and frequently used markers for bacterial phylogeny and reconstruction of insect-symbiont coevolution. In primary symbionts, such as Buchnera and Wigglesworthia, genome economization leads to the establishment of a single copy of these sequences. In phylogenetic studies, they provide sufficient information and yield phylogenetic trees congruent with host evolution. In contrast, other symbiotic lineages (e.g., the genus Arsenophonus) carry a higher number of rRNA copies in their genomes, which may have serious consequences for phylogenetic inference. In this study, we show that in Arsenophonus triatominarum the degree of heterogeneity can affect reconstruction of phylogenetic relationships and mask possible coevolution between the symbiont and its host. Phylogenetic arrangement of individual rRNA copies was used, together with a calculation of their divergence time, to demonstrate that the incongruent 16S rDNA trees and low nucleotide diversity in the secondary symbiont could be reconciled with the coevolutionary scenario.

Molecular identification of Wolbachia from the filarial nematode Mansonella perstans

Wolbachiae are bacterial endosymbionts of insects and many filarial nematodes whose products trigger inflammation in filarial infections. The dependence of the parasites on their endosymbionts has also led to the use of antibiotics directed against the Wolbachiae, therapy that has been demonstrated to have a profound salutary effect on filarial infections. The identification of Wolbachiae in Mansonella species has been conclusively shown for Mansonella ozzardi (Mo), but not for Mansonella perstans (Mp). Using primers known to amplify the 16S ribosomal DNA of other filarial Wolbachiae, an identical 1393bp band was found in all samples tested. Sequence analysis of these samples demonstrated a single consensus sequence for Mp Wolbachia 16S rDNA that was most similar to Wolbachia sequences from other filarial nematodes. When aligned with the only other Mansonella Wolbachia sequence (Mo) there were only 8 nucleotide differences in the 1369bp overlapping sequence. Phylogenetic dendrograms, examining the relationship of the Mp Wolbachia to other Wolbachia 16S rDNA, showed that the Wolbachia tracked almost identically to the 5S rRNA of their parasite host. Wolbachia surface protein (WSP) was also demonstrated in protein extracted from Mp-containing whole blood. In advance of a treatment trial of Mp, a method for the quantitation of Mp Wolbachia was developed and used to demonstrate not only a relationship between microfilarial numbers and Wolbachia copy numbers, but also to demonstrate the effect of antibiotic on ridding Mp of Wolbachia.

Brugia malayi: whole genome amplification for genomic characterization of filarial parasites

Genetic characterization of field isolates and clinical specimens of filarial nematodes is often limited by a shortage of DNA; therefore, we evaluated a multiple displacement amplification (MDA) based whole genome amplification method. The quality of amplified DNA was examined by conventional PCR, real-time PCR, and DNA hybridization. MDA of 5.0 ng of adult Brugia malayi DNA and one-fifteenth of the DNA isolated from a single microfilaria resulted in 6.3 and 4.2 microg of amplified DNA, respectively. Amplified DNA was equivalent to native genomic DNA for hybridization to B. malayi BAC library clones or to an oligonucleotide microarray with approximately 18,000 filarial DNA sequences. MDA is useful for whole genome amplification of filarial DNA from very small amounts of starting material. This technology will permit detailed studies of genetic diversity that were not previously feasible.

Diversity of Wolbachia isolated from the Cubitermes sp. affinis subarquatus complex of species (Termitidae), revealed by multigene phylogenies

Wolbachia are endosymbiotic bacteria that may alter the reproductive mechanisms of arthropod hosts. Eusocial termites provide considerable scope for Wolbachia studies owing to their ancient origin, their great diversity and their considerable ecological, biological and behavioral plasticity. This article describes the phylogenetic distribution of Wolbachia infecting termites of the Cubitermes genus, which are particularly abundant soil-feeders in equatorial Africa. Fourteen colonies of the Cubitermes sp. affinis subarquatus complex of species were screened using five bacterial genes (wsp, ftsZ, coxA, fbpA and 16S rRNA genes) and a striking diversity of Wolbachia strains was identified within these closely related species. In the host complex, three Wolbachia variants were found that were not in the super groups usually reported for termites (F and H), each infecting one or two Cubitermes species.

Wolbachia Are Present in Southern African Scorpions and Cluster with Supergroup F

The presence and distribution of the intracellular bacteria Wolbachia in the arthropod subphylum Chelicerata (including class Arachnida) has not been extensively explored. Here we report the discovery of Wolbachia in scorpions. Five strains found in host species of the genus Opistophthalmus (Southern African burrowing scorpions) have been characterized by Multilocus Sequence Typing and by Wolbachia Surface Protein. Phylogenetic analyses indicate clustering in the supergroup F and a high genetic relatedness among all scorpion strains as a result of a potential transmission within the host genus. The F-group is an uncommon lineage compared to the A and B supergroups, although it is present in a broad range of hosts (including insects, filarial nematodes, and now arachnids) and across a large geographical area (e.g., North America, Africa, Europe, and Australia). It also shows no evidence of recombination and has a significantly higher genetic diversity than supergroup A and B. Overall, this pattern suggests an older radiation of F-strains with respect to A and B-strains, followed by limited horizontal transmission across host genera and reduced genetic flux among strains. A more extensive sampling of supergroup F-strains is required to confirm this scenario.

Absence of Wolbachia in Nonfilariid Worms Parasitizing Arthropods

Wolbachia are strictly intracellular maternally inherited alpha-proteobacteria, largely widespread among arthropods and filariids (i.e., filarial nematodes). Wolbachia capacities to infect new host species have been greatly evidenced and the transfer of Wolbachia between arthropods and filariids has probably occurred more than once. Interestingly, among nematode species, Wolbachia infection was found in filariids but not in closely related lineages. Their occurrence in filariids has been supposed a consequence of the parasitic lifestyle of worms within Wolbachia-infected arthropods, implying that nonfilariid worms parasitizing arthropods are also likely to be infected by some Wolbachia acquired from their hosts. To further investigate this hypothesis, we have examined seven species of nonfilariid worms of Nematoda and Nematomorpha phyla, all interacting intimately with arthropods. Wolbachia infection in nonfilariid parasitic worms was never detected by polymerase chain reaction assays of the 16S rDNA and wsp genes. By contrast, some arthropod hosts are well infected by Wolbachia of the B supergroup. Then the intimate contact with infected arthropods is not a sufficient condition to explain the Wolbachia occurrence in filariids and could underline a physiological singularity or a particular evolutionary event to acquire and maintain Wolbachia infection.

Revisiting Wolbachia Supergroup Typing Based on WSP: Spurious Lineages and Discordance with MLST

The obligate intracellular bacteria Wolbachia are taxonomically subdivided into eight supergroups (named A-H). Supergroup typing of strains has been mostly based on phylogenetic inference of the Wolbachia surface protein (wsp), a gene that recently has been shown to experience high rates of recombination. This brings into question its suitability not only for microtaxonomy, but also for supergroup classification of the genus. A Multilocus Sequence Typing (MLST) scheme for Wolbachia has recently been developed that types strains based on five conserved genes, thus providing a rigorous supergroup annotation of strains. Here we report striking discrepancies in supergroup designation between MLST and wsp inferences, and propose a revision of current methods for Wolbachia supergroup typing. Transfer of whole wsp gene sequences between supergroups A and B has occurred. Furthermore, as a result of intragenic recombination, wsp phylogeny creates spurious basal lineages that are not supported by MLST. For example, the proposed supergroup G, based upon wsp alone, likely represents only a wsp recombinant clade. Removal of supergroup G is advised until and unless the existence of this lineage is substantiated by other sequence information (e.g., MLST). We recommend a full characterization MLST for a correct strain typing, while, based on the current data set, use of a single MLST gene can be effective for supergroup designation of A and B strains. Finally, we note that the sharing of wsp sequences between A and B strains indicates a strong genetic cohesiveness of Wolbachia strains, supporting designation of these bacteria within the same species, W. pipientis.

Bidirectional incompatibility among divergent Wolbachia and incompatibility level differences among closely related Wolbachia in Nasonia

Most insect groups harbor obligate bacterial symbionts from the alpha-proteobacterial genus Wolbachia. These bacteria alter insect reproduction in ways that enhance their cytoplasmic transmission. One of the most common alterations is cytoplasmic incompatibility (CI) - a post-fertilization modification of the paternal genome that renders embryos inviable or unable to complete diploid development in crosses between infected males and uninfected females or infected females harboring a different strain. The parasitic wasp species complex Nasonia (N. vitripennis, N. longicornis and N. giraulti) harbor at least six different Wolbachia that cause CI. Each species have double infections with a representative from both the A and B Wolbachia subgroups. CI relationships of the A and B Wolbachia of N. longicornis with those of N. giraulti and N. vitripennis are investigated here. We demonstrate that all pairwise crosses between the divergent A strains are bidirectionally incompatible. We were unable to characterize incompatibility between the B Wolbachia, but we establish that the B strain of N. longicornis induces no or very weak CI in comparison to the closely related B strain in N. giraulti that expresses complete CI. Taken together with previous studies, we show that independent acquisition of divergent A Wolbachia has resulted in three mutually incompatible strains, whereas codivergence of B Wolbachia in N. longicornis and N. giraulti is associated with differences in CI level. Understanding the diversity and evolution of new incompatibility strains will contribute to a fuller understanding of Wolbachia invasion dynamics and Wolbachia-assisted speciation in certain groups of insects.

Taxonomic status of the intracellular bacterium Wolbachia pipientis

Wolbachia pipientis is a maternally inherited, intracellular bacterium found in more than 20 % of all insects, as well as numerous other arthropods and filarial nematodes. It has been the subject of a growing number of studies in recent decades, because of the remarkable effects it has on its arthropod hosts, its potential as a tool for biological control of arthropods of agricultural and medical importance and its use as a target for treatment of filariasis. W. pipientis was originally discovered in cells of the mosquito Culex pipiens and is the only formally described member of the genus. Molecular sequence-based studies have revealed a number of phylogenetically diverse strains of W. pipientis. Owing to uncertainty about whether W. pipientis comprises more than one species, researchers in the field now commonly refer to W. pipientis simply as Wolbachia. In this note, we briefly review higher-level phylogenetic and recombination studies of W. pipientis and propose that all the intracellular symbionts known to cluster closely with the type strain of W. pipientis, including those in the currently recognized supergroups (A–H), are officially given this name.

Phylogenetic relationships of the Wolbachia of nematodes and arthropods

Wolbachia are well known as bacterial symbionts of arthropods, where they are reproductive parasites, but have also been described from nematode hosts, where the symbiotic interaction has features of mutualism. The majority of arthropod Wolbachia belong to clades A and B, while nematode Wolbachia mostly belong to clades C and D, but these relationships have been based on analysis of a small number of genes. To investigate the evolution and relationships of Wolbachia symbionts we have sequenced over 70 kb of the genome of wOvo, a Wolbachia from the human-parasitic nematode Onchocerca volvulus, and compared the genes identified to orthologues in other sequenced Wolbachia genomes. In comparisons of conserved local synteny, we find that wBm, from the nematode Brugia malayi, and wMel, from Drosophila melanogaster, are more similar to each other than either is to wOvo. Phylogenetic analysis of the protein-coding and ribosomal RNA genes on the sequenced fragments supports reciprocal monophyly of nematode and arthropod Wolbachia. The nematode Wolbachia did not arise from within the A clade of arthropod Wolbachia, and the root of the Wolbachia clade lies between the nematode and arthropod symbionts. Using the wOvo sequence, we identified a lateral transfer event whereby segments of the Wolbachia genome were inserted into the Onchocerca nuclear genome. This event predated the separation of the human parasite O. volvulus from its cattle-parasitic sister species, O. ochengi. The long association between filarial nematodes and Wolbachia symbionts may permit more frequent genetic exchange between their genomes.

Filarial nematode worms cause hundreds of millions of cases of disease in humans worldwide. As part of efforts to identify new drug targets in these parasites, the Filarial Genome Project rediscovered that these worms carry within them a symbiotic bacterium, which may be a novel target. Fenn et al. investigated the relationships of these bacteria, from the genus Wolbachia, to those previously identified in arthropods using a new dataset of genome sequence data from the human parasite Onchocerca volvulus. O. volvulus causes river blindness in West Africa. The authors found that the Wolbachia strains found in nematodes are more closely related to each other than they are to the Wolbachia in insects, suggesting that the nematodes and their bacterial partners have been coevolving for some considerable evolutionary time and may indeed be good targets. In addition, the authors identified a fragment of Wolbachia DNA that was inserted in the genome of its nematode host and has subsequently degenerated. The insertion occurred before O. volvulus diverged from another nematode species, O. ochengi, found in cattle.