Characterization of non-cytoplasmic incompatibility inducing Wolbachia in two continental African populations of Drosophila simulans

Wolbachia is an endocellular bacterium infecting arthropods and nematodes. In arthropods, it invades host populations through various mechanisms, affecting host reproduction, the most common of which being cytoplasmic incompatibility (CI). CI is an embryonic mortality occurring when infected males mate with uninfected females or females infected by a different Wolbachia strain. This phenomenon is observed in Drosophila simulans, an intensively studied Wolbachia host, harbouring at least five distinct bacterial strains. In this study, we investigate various aspects of the Wolbachia infections occurring in two continental African populations of D. simulans: CI phenotype, phylogenetic position based on the wsp gene and associated mitochondrial haplotype. From the East African population (Tanzania), we show that (i) the siIII mitochondrial haplotype occurs in continental populations, which was unexpected based on the current views of D. simulans biogeography, (ii) the wKi strain (that rescues from CI while being unable to induce it) is very closely related to the CI-inducing strain wNo, (iii) wKi and wNo might not derive from a unique infection event, and (iv) wKi is likely to represent the same entity as the previously described wMa variant. In the West African population (Cameroon), the Wolbachia infection was found identical to the previously described wAu, which does not induce CI. This finding supports the view that wAu might be an ancient infection in D. simulans.     

Variable fitness effects of Wolbachia infection in Drosophila melanogaster

Maternally inherited Wolbachia bacteria are extremely widespread among insects and their presence is usually associated with parasitic modifications of host fitness. Wolbachia pipientis infects Drosophila melanogaster populations from all continents, but their persistence in this species occurs despite any strong parasitic effects. Here, we have investigated the symbiosis between Wolbachia and D. melanogaster and found that Wolbachia infection can have significant survival and fecundity effects. Relative to uninfected flies, infected females from three fly strains showed enhanced survival or fecundity associated with Wolbachia infection, one strain showed both and one strain responded positively to Wolbachia removal. We found no difference in egg hatch rates (cytoplasmic incompatibility) for crosses between infected males and uninfected females, although there were fecundity differences. Females from this cross consistently produced fewer eggs than infected females and these fecundity differences could promote the spread of infection just like cytoplasmic incompatibility. More surprising, we found that infected females often had the greatest fecundity when mated to uninfected males. This could also promote the spread of Wolbachia infection, though here the fitness benefits would also help to spread infection when Wolbachia are rare. We suggest that variable fitness effects, in both sexes, and which interact strongly with the genetic background of the host, could increase cytoplasmic drive rates in some genotypes and help explain the widespread persistence of Wolbachia bacteria in D. melanogaster populations. These interactions may further explain why many D. melanogaster populations are polymorphic for Wolbachia infection. We discuss our results in the context of host-symbiont co-evolution.     

Mechanisms promoting the long-term persistence of a Wolbachia infection in a laboratory-adapted population of Drosophila melanogaster

Intracellular bacteria of the genus Wolbachia are widespread endosymbionts across diverse insect taxa. Despite this prevalence, our understanding of how Wolbachia persists within populations is not well understood. Cytoplasmic incompatibility (CI) appears to be an important phenotype maintaining Wolbachia in many insects, but it is believed to be too weak to maintain Wolbachia in Drosophila melanogaster, suggesting that Wolbachia must also have other effects on this species. Here we estimate the net selective effect of Wolbachia on its host in a laboratory-adapted population of D. melanogaster, to determine the mechanisms leading to its persistence in the laboratory environment. We found i) no significant effects of Wolbachia infection on female egg-to-adult survival or adult fitness, ii) no reduced juvenile survival in males, iii) substantial levels of CI, and iv) a vertical transmission rate of Wolbachia higher than 99%. The fitness of cured females was, however, severely reduced (a decline of 37%) due to CI in offspring. Taken together these findings indicate that Wolbachia is maintained in our laboratory environment due to a combination of a nearly perfect transmission rate and substantial CI. Our results show that there would be strong selection against females losing their infection and producing progeny free from Wolbachia.     

Mitochondrial DNA in the Drosophila Melanogaster Complex

Mitochondrial DNA in the complex Drosophila melanogaster was among the first studied in metazoans. The variability of the molecule was extensively studied using restriction enzymes, gene sequence and recently sequence of the whole coding region. Within the complex, seven major haplotypes have been described, one (me) in D. melanogaster, three in D. simulans (siI, siII, siIII), two in D. mauritiana (maI, maII), and one in D. sechellia (se). The molecular distance between the haplotypes is comprised between 1 and 5%, except for siII and maI, which are virtually identical. The nucleotide diversity within each of these haplotypes is very low, varying from 0 to 0.0005. Most of the cytoplasms are infected by the bacterium Wolbachia and different bacterial strains infect cytoplasms harboring different mtDNA types. mtDNA polymorphism is discussed in relation with Wolbachia, nuclear polymorphism and speciation events.     

Linkage disequilibria between mtDNA haplotypes and chromosomal arrangements in a natural population of Drosophila subobscura

The association between mtDNA haplotypes and chromosomal arrangements in a natural population of Drosophila subobscura from Calvia (Balearic Islands, Spain) was studied in order to search for linkage disequilibria, in an attempt to explain the populational dynamics of the mtDNA haplotypes of this species in nature. The presence of Wolbachia was not detected. Two main haplotypes (I and II) were found, as well as a series of less common ones. The Tajima D-test seemed to indicate some kind of seasonal population bottleneck. An analysis of linkage disequilibrium and factorial analysis of correspondences detected an association between haplotype I and the J(ST) inversion and haplotype II and the J(1) inversion.     

The endosymbiont Wolbachia in Eurasian populations of Drosophila melanogaster

The endosymbiotic [alpha]-proteobacteria Wolbachia is widely spread among arthropods and Filariidae nematodes. This bacterium is transmitted vertically via a transovarian route. Wolbachia is a cause of several reproductive abnormalities in the host species. We analyzed the isofemale lines created using flies collected from Drosophila melanogaster natural populations for infection with the endosymbiont Wolbachia. Wolbachia were genotyped according to five variable markers: the presence of insertion sequence IS5 in two loci, the copy number of two minisatellite repeats, and an inversion. Overall, 665 isofemale lines isolated from the populations of D. melanogaster from Ukraine, Belarus, Moldova, Caucasus, Central Asia, Ural, Udmurtia, Altai, West and East Siberia, and Far East in 1974 through 2005 were used in the work. The samples from Ukrainian, Altaian, and Middle Asian populations were largest. The infection rate of D. melanogaster populations from Middle Asia, Altaian, and Eastern Europe (Ukraine, Moldavia, and Belarus) with Wolbachia amounted to 64, 56, and 39%, respectively. The D. melanogaster population from the Caucasus displayed heterogeneity in the genotypes of this cytoplasmic infection. The Wolbachia genotype wMel, detected in all the populations studied, was the most abundant. The genotype wMelCS2 was always present in the populations from Middle Asia and Altai and was among the rare variants in the D. melanogaster populations from the Eastern Europe. Single instances of the Wolbachia genotype wMelCS occurred in a few flies from the Central Asian and Altai populations, but was not found this genotype in the other regions.     

Wolbachia-mediated persistence of mtDNA from a potentially extinct species

Drosophila quinaria is polymorphic for infection with Wolbachia, a maternally transmitted endosymbiont. Wolbachia-infected individuals carry mtDNA that is only distantly related to the mtDNA of uninfected individuals, and the clade encompassing all mtDNA haplotypes within D. quinaria also includes the mtDNA of several other species of Drosophila. Nuclear gene variation reveals no difference between the Wolbachia-infected and uninfected individuals of D. quinaria, indicating that they all belong to the same interbreeding biological species. We suggest that the Wolbachia and the mtDNA with which it is associated were derived via interspecific hybridization and introgression. The sequences in the Wolbachia and the associated mtDNA are ≥6% divergent from those of any known Drosophila species. Thus, in spite of nearly complete species sampling, the sequences from which these mitochondria were derived remain unknown, raising the possibility that the donor species is extinct. The association between Wolbachia infection and mtDNA type within D. quinaria suggests that Wolbachia may be required for the continued persistence of the mtDNA from an otherwise extinct Drosophila species. We hypothesize that pathogen-protective effects conferred by Wolbachia operate in a negative frequency-dependent manner, thus bringing about a stable polymorphism for Wolbachia infection.     

Wolbachia 在花蓟马中的感染及其与寄主线粒体DNA多样性的关系

【目的】Wolbachia 是一种广泛存在于节肢动物中的胞内共生细菌,影响寄主的生物学特性。花蓟马 Frankliniella intonsa (Trybom)是重要的害虫,对农作物及园林植物造成危害。本研究旨在明确 Wolbachia 在花蓟马中的感染情况,并分析其与寄主线粒体DNA多样性的关系。【方法】采集中国境内26个花蓟马自然种群,运用多位点序列分型技术（multilocus sequence typing, MLST）对其体内 Wolbachia 感染率及株系进行分析;利用线粒体 COI 分子标记研究花蓟马的遗传分化及遗传多样性;通过比较感染和未感染 Wolbachia 个体 COI 数据,探究 Wolbachia 多样性与寄主线粒体DNA多样性之间的关系。【结果】花蓟马中 Wolbachia 的感染率为0%~60%,共检测到5种 Wolbachia 株系（wFint1,wFint2,wFint3,wFint4及wFint5）,均属于B大组且形成一个单系群。Wolbachia感染情况与这些花蓟马种群（除CC, GZ, TA和TY, N<5）的线粒体DNA多样性相关,表现为不感染 Wolbachia 的种群中线粒体DNA单倍型多样性（Hd）与核苷酸多样性（Pi）均高于感染 Wolbachia 的种群,且 Wolbachia 感染率与 Hd 呈显著负相关（ P <0.05）。AMOVA分析表明花蓟马线粒体DNA遗传分化与Wolbachia 感染情况有关。【结论】 Wolbachia 可能在侵染花蓟马种群后出现遗传分化;Wolbachia 感染与寄主线粒体DNA多样性有关。     

Can cytoplasmic incompatibility inducing Wolbachia promote the evolution of mate preferences?

The maternally inherited bacterium, Wolbachia pipientis, manipulates host reproduction by rendering uninfected females reproductively incompatible with infected males (cytoplasmic incompatibility, CI). Hosts may evolve mechanisms, such as mate preferences, to avoid fitness costs of Wolbachia infection. Despite the potential importance of mate choice for Wolbachia population dynamics, this possibility remains largely unexplored. Here we model the spread of an allele encoding female mate preference for uninfected males alongside the spread of CI inducing Wolbachia. Mate preferences can evolve but the spread of the preference allele depends on factors associated with both Wolbachia infection and the preference allele itself. Incomplete maternal transmission of Wolbachia, fitness costs and low CI, improve the spread of the preference allele and impact on the population dynamics of Wolbachia. In addition, mate preferences are found in infected individuals. These results have important consequences for the fate of Wolbachia and studies addressing mate preferences in infected populations.     

Wolbachia transfer from Drosophila melanogaster into D. simulans: Host effect and cytoplasmic incompatibility relationships.

Wolbachia are maternally transmitted endocellular bacteria causing a reproductive incompatibility called cytoplasmic incompatibility (CI) in several arthropod species, including Drosophila. CI results in embryonic mortality in incompatible crosses. The only bacterial strain known to infect Drosophila melanogaster (wDm) was transferred from a D. melanogaster isofemale line into uninfected D. simulans isofemale lines by embryo microinjections. Males from the resulting transinfected lines induce >98% embryonic mortality when crossed with uninfected D. simulans females. In contrast, males from the donor D. melanogaster line induce only 18-32% CI on average when crossed with uninfected D. melanogaster females. Transinfected D. simulans lines do not differ from the D. melanogaster donor line in the Wolbachia load found in the embryo or in the total bacterial load of young males. However, >80% of cysts are infected by Wolbachia in the testes of young transinfected males, whereas only 8% of cysts are infected in young males from the D. melanogaster donor isofemale line. This difference might be caused by physiological differences between hosts, but it might also involve tissue-specific control of Wolbachia density by D. melanogaster. The wDm-transinfected D. simulans lines are unidirectionally incompatible with strains infected by the non-CI expressor Wolbachia strains wKi, wMau, or wAu, and they are bidirectionally incompatible with strains infected by the CI-expressor Wolbachia strains wHa or wNo. However, wDm-infected males do not induce CI toward females infected by the CI-expressor strain wRi, which is found in D. simulans continental populations, while wRi-infected males induce partial CI toward wDm-infected females. This peculiar asymmetrical pattern could reflect an ongoing divergence between the CI mechanisms of wRi and wDm. It would also confirm other results indicating that the factor responsible for CI induction in males is distinct from the factor responsible for CI rescue in females.     

Evolutionary history of a mosquito endosymbiont revealed through mitochondrial hitchhiking

Due to cytoplasmic inheritance, spread of maternally inherited Wolbachia symbionts can result in reduction of mitochondrial variation in populations. We examined sequence diversity of the mitochondrial NADH dehydrogenase subunit 4 (ND4) gene in Wolbachia-infected (South Africa (SA), California and Thailand) and uninfected (SA) Culex pipiens complex populations. In total, we identified 12 haplotypes (A-L). In infected populations, 99% of individuals had haplotype K. In the uninfected SA population, 11 haplotypes were present, including K. Nuclear allozyme diversity was similar between infected and uninfected SA populations. Analysis of nuclear DNA sequences suggested that haplotype K presence in uninfected SA Cx. pipiens was probably due to a shared ancestral polymorphism rather than hybrid introgression. These data indicate that Wolbachia spread has resulted in drastic reduction of mitochondrial variability in widely separated Cx. pipiens complex populations. In contrast, the uninfected SA population is probably a cryptic species where Wolbachia introgression has been prevented by reproductive isolation, maintaining ancestral levels of mitochondrial diversity. Molecular clock analyses suggest that the Wolbachia sweep occurred within the last 47000 years. The effect of Wolbachia on mitochondrial dynamics can provide insight on the potential for Wolbachia to spread transgenes into mosquito populations to control vector-borne diseases.     

Effects of Wolbachia on mtDNA variation in two fire ant species

Wolbachia are endosymbiotic bacteria that infect arthropods. As they are maternally transmitted, the spread of Wolbachia variants within host populations may affect host mtDNA evolution. We sequenced a portion of the mitochondrial cytochrome oxidase I gene from numerous individuals of two Wolbachia-infected fire ant species, Solenopsis invicta and S. richteri, to determine how these bacteria influence patterns of mtDNA variation. As predicted, there was a strong association between Wolbachia strain and host mtDNA lineage within and between these fire ant species. However, there was no consistent association between the presence of Wolbachia and a reduction in mtDNA diversity. Moreover, patterns of mtDNA variation within Wolbachia-infected populations did not differ consistently from neutral expectations, despite our prediction that strong positive selection acting on Wolbachia influences the evolutionary dynamics of other cytoplasmic genomes. Specifically, while values of Tajima's D consistently were less than zero for all six samples of fire ants harbouring Wolbachia, MacDonald-Kreitman tests suggested that the patterns of variation were different from those expected under neutrality in only two of the samples. We conclude that these neutrality tests do not unambiguously reveal a clear effect of Wolbachia infection on patterns of mtDNA variation and substitution in fire ants. Finally, consistent with an earlier study, our data revealed the presence of two divergent mtDNA haplotype lineages and Wolbachia strains within S. invicta. Recognition of these two lineages has important consequences for interpreting patterns of mtDNA evolution and genetic differentiation between conspecific social forms of this species.     

Molecular variation at the In(2L)t proximal breakpoint site in natural populations of Drosophila melanogaster and D. simulans

A previous study of nucleotide polymorphism in a Costa Rican population of Drosophila melanogaster found evidence for a nonneutral deficiency in the number of haplotypes near the proximal breakpoint of In(2L)t, a common inversion polymorphism in this species. Another striking feature of the data was a window of unusually high nucleotide diversity spanning the breakpoint site. To distinguish between selective and neutral demographic explanations for the observed patterns in the data, we sample alleles from three additional populations of D. melanogaster and one population of D. simulans. We find that the strength of associations among sites found at the breakpoint varies between populations of D. melanogaster. In D. simulans, analysis of the homologous region reveals unusually elevated levels of nucleotide polymorphism spanning the breakpoint site. As with American populations of D. melanogaster, our D. simulans sample shows a marked reduction in the number of haplotypes but not in nucleotide diversity. Haplotype tests reveal a significant deficiency in the number of haplotypes relative to the neutral expectation in the D. simulans sample and some populations of D. melanogaster. At the breakpoint site, the level of divergence between haplotype classes is comparable to interspecific divergence. The observation of interspecific polymorphisms that differentiate major haplotype classes in both species suggests that haplotype classes at this locus are considerably old. When considered in the context of other studies on patterns of variation within and between populations of D. melanogaster and D. simulans, our data appear more consistent with the operation of selection than with simple demographic explanations.     

我国三地区黑腹果蝇中Wolbachia的系统发育关系及其对宿主生殖的影响

【目的】Wolbachia 是广泛存在于节肢动物和丝状线虫体内的一类共生菌, 能够以多种方式对宿主产生影响。精卵细胞质不亲和（CI）是其引起的最普遍的表型, 即感染Wolbachia的雄性宿主与未感染或感染不同品系的雌性宿主交配后, 不能产生后代或后代极少, 而感染同品系Wolbachia的雌雄宿主交配后则能正常产生后代。我们前期研究发现, 湖北武汉、 云南六库和天津3个地区黑腹果蝇Drosophila melanogaster被Wolbachia感染。本研究旨在明确这3个地区黑腹果蝇中Wolbachia的系统发育关系及其对宿主生殖的影响。【方法】利用Clustal X软件对Wolbachia的wsp基因序列进行比对, 利用MEGA软件构建系统发育树。采用多位点序列分型（MLST）的方法对Wolbachia进行分型。通过区内交配和区之间杂交的方式研究不同地区黑腹果蝇体内Wolbachia 的关系及其对果蝇生殖的影响。【结果】湖北武汉、 云南六库和天津3个地区黑腹果蝇中感染的Wolbachia都是属于A大组的Mel亚群。这3个地区果蝇感染的Wolbachia的序列类型（ST）不同, Wolbachia之间存在一定的差异。湖北武汉和天津果蝇中的Wolbachia能引起强烈的CI表型, 而云南六库果蝇中的Wolbachia引起的CI强度相对较弱。武汉果蝇中Wolbachia不能完全挽救天津果蝇中Wolbachia引起的CI表型, 而天津果蝇中Wolbachia也不能完全挽救武汉果蝇中Wolbachia引起的CI表型。【结论】武汉和天津地区黑腹果蝇中的Wolbachia可能距离较远。Wolbachia的长期共生可能对黑腹果蝇的进化产生了一定的影响, 湖北武汉与云南六库的黑腹果蝇中感染的Wolbachia属于不同的序列类型, 这2个地区的黑腹果蝇已发生了一定的分歧, 产生了一定的生殖隔离。     

Wolbachia Infections in Drosophila Melanogaster and D. Simulans: Polymorphism and Levels of Cytoplasmic Incompatibility

Wolbachia are endosymbiotic bacteria, widespread in terrestrial Arthropods. They are mainly transmitted vertically, from mothers to offspring and induce various alterations of their hosts' sexuality and reproduction, the most commonly reported phenomenon being Cytoplasmic Incompatibility (CI), observed in Drosophila melanogaster and D. simulans. Basically, CI results in a more or less intense embryonic mortality, occurring in crosses between males infected by Wolbachia and uninfected females. In D. simulans, Wolbachia and CI were observed in 1986. Since then, this host species has become a model system for investigating the polymorphism of Wolbachia infections and CI. In this review we describe the different Wolbachia infections currently known to occur in D. melanogaster and D. simulans. The two species are highly contrasting with regard to symbiotic diversity: while five Wolbachia variants have been described in D. simulans natural populations, D. melanogaster seems to harbor one Wolbachia variant only. Another marked difference between these two Drosophila species is their permissiveness with regard to CI, which seems to be fully expressed in D. simulans but partially or totally repressed in D. melanogaster, demonstrating the involvement of host factors in the control of CI levels. The potential of the two host species regarding the understanding of CI and its evolution is also discussed.     

Diversity in and evidence for selection on the mitochondrial genome of Phytophthora infestans

Two extant nomenclature systems were reconciled to relate six mitochondrial DNA (mtDNA) haplotypes of Phytophthora infestans, the oomycete pathogen causing late blight disease on potato and tomato. Carter's haplotypes I-a and I-b were included in Goodwin's haplotype A, while Carter's haplotypes II-a and II-b were included in Goodwin's haplotype B. In addition, haplotypes E and F were included in Carter's haplotype I-b. The mutational differences separating the various haplotypes were determined, and we propose that either haplotype I-b(A) or haplotype I-a(A) is the putative ancestral mtDNA of P. infestans, because either can center all the other haplotypes in a logical stepwise network of mutational changes. The occurrence of the six haplotypes in 548 isolates worldwide was determined. Haplotypes I-a and II-a were associated with diverse genotypes worldwide. As previously suggested, haplotype I-b was found only in the US-1 clonal lineage and its variants (n = 99 isolates from 16 countries on 5 continents), and haplotype II-b was limited to the US-6 clonal lineage and its derivatives (n = 36). In a confirmation of a previous suggestion, the randomly mating population in the Toluca Valley of central Mexico (n = 78) was monomorphic for mtDNA haplotype I-a(A). We hypothesize that selection there may be driving the dominance of that single mtDNA haplotype.     

Evidence for a global Wolbachia replacement in Drosophila melanogaster

Wolbachia are maternally inherited intracellular alpha-Proteobacteria found in numerous arthropod and filarial nematode species. They influence the biology of their hosts in many ways. In some cases, they act as obligate mutualists and are required for the normal development and reproduction of the host. They are best known, however, for the various reproductive parasitism traits that they can generate in infected hosts. These include cytoplasmic incompatibility (CI) between individuals of different infection status, the parthenogenetic production of females, the selective killing of male embryos, and the feminization of genetic males. Wolbachia infections of Drosophila melanogaster are extremely common in both wild populations and long-term laboratory stocks. Utilizing the newly completed genome sequence of Wolbachia pipientis wMel, we have identified a number of polymorphic markers that can be used to discriminate among five different Wolbachia variants within what was previously thought to be the single clonal infection of D. melanogaster. Analysis of long-term lab stocks together with wild-caught flies indicates that one of these variants has replaced the others globally within the last century. This is the first report of a global replacement of a Wolbachia strain in an insect host species. The sweep is at odds with current theory that cannot explain how Wolbachia can invade this host species given the observed cytoplasmic incompatibility characteristics of Wolbachia infections in D. melanogaster in the field.     

High levels of multiple Wolbachia infection and recombination in the ant Formica exsecta

Wolbachia bacteria are intracellular symbionts of many arthropod species. Their spread through host populations is promoted by drastic alterations imposed on their hosts' reproductive physiology. In the present study, we analyzed the association between Wolbachia strains and host mitochondrial haplotypes in a Swiss population of the ant Formica exsecta. In this species, female dispersal is extremely limited and the mitochondrial haplotypes are strongly differentiated between and within subpopulations. Our study revealed exceptionally high levels of multiple infection, with all ants harboring four or five distinct Wolbachia strains. Four of these strains were present in all ants analyzed. A fifth strain was associated with only three of the five mitochondrial haplotypes. An analysis of the Wolbachia gene wsp further revealed an unexpected high rate of recombination, with three of the five Wolbachia strains appearing to have arisen by homologous recombination.     

Wolbachia-mediated sperm modification is dependent on the host genotype in Drosophila.

Estimates of Wolbachia density in the eggs, testes and whole flies of drosophilid hosts have been unable to predict the lack of cytoplasmic incompatibility (CI) expression in so-called mod(-) variants. Consequently, the working hypothesis has been that CI expression, although related to Wolbachia density, is also governed by unknown factors that are influenced by both host and bacterial genomes. Here, we compare the behaviour of the mod(-) over-replicating Wolbachia popcorn strain in its native Drosophila melanogaster host to the same strain transinfected into a novel host, namely Drosophila simulans. We report that (i) the popcorn strain is a close relative of other D. melanogaster infections, (ii) the mod(-) status of popcorn in D. melanogaster appears to result from its inability to colonize sperm bundles, (iii) popcorn is present in the bundles in D. simulans and induces strong CI expression, which demonstrates that the bacterial strain does not lack the genetic machinery for inducing CI and that there is host-species-specific control over Wolbachia tissue tropism, and (iv) infection of sperm bundles by the mod(-) D. simulans wCof strain indicates that there are several independent routes by which a strain can be a CI non-expressor.     

Within- and between-population variation for Wolbachia-induced reproductive incompatibility in a haplodiploid mite

Wolbachia pipientis is a bacterium that induces cytoplasmic incompatibility (CI), the phenomenon in which infected males are reproductively incompatible with uninfected females. CI spreads in a population of hosts because it reduces the fitness of uninfected females relative to infected females. CI encompasses two steps: modification (mod) of sperm of infected males and rescuing (resc) of these chromosomes by Wolbachia in the egg. Infections associated with CI have mod+ resa+ phenotypes. However, mod- resc+ phenotypes also exist; these do not result in CI. Assuming mod/resc phenotypes are properties of the symbiont, theory predicts that mod- resc+ infections can only spread in a host population where a mod+ resc+ infection already occurs. A mod- resc+ infection spreads if the cost it imposes on the infected females is lower than the cost inflicted by the resident (mod+ resc+) infection. Furthermore, introduction of a mod- Wolbachia eventually drives infection to extinction. The uninfected population that results can be recolonized by a CI-causing Wolbachia. Here, we investigated whether variability for induction of CI was present in two Tetranychus urticae populations. In one population all isofemale lines tested were mod-. In the other, mod+ resc+ and mod- resc+ isofemale lines coexisted. We found no evidence for a cost difference to females expressing either type (mod-/-). Infections in the two populations could not be distinguished based on sequences of two Wolbachia genes. We consider the possibility that mod- is a host effect through a population dynamics model. A mod- host allele leads to infection extinction in the absence of fecundity differences. Furthermore, the uninfected population that results is immune to reestablishment of the (same) CI-causing Wolbachia.