Mutualistic Wolbachia infection in Aedes albopictus: accelerating cytoplasmic drive

Maternally inherited rickettsial symbionts of the genus Wolbachia occur commonly in arthropods, often behaving as reproductive parasites by manipulating host reproduction to enhance the vertical transmission of infections. One manipulation is cytoplasmic incompatibility (CI), which causes a significant reduction in brood hatch and promotes the spread of the maternally inherited Wolbachia infection into the host population (i.e., cytoplasmic drive). Here, we have examined a Wolbachia superinfection in the mosquito Aedes albopictus and found the infection to be associated with both cytoplasmic incompatibility and increased host fecundity. Relative to uninfected females, infected females live longer, produce more eggs, and have higher hatching rates in compatible crosses. A model describing Wolbachia infection dynamics predicts that increased fecundity will accelerate cytoplasmic drive rates. To test this hypothesis, we used population cages to examine the rate at which Wolbachia invades an uninfected Ae. albopictus population. The observed cytoplasmic drive rates were consistent with model predictions for a CI-inducing Wolbachia infection that increases host fecundity. We discuss the relevance of these results to both the evolution of Wolbachia symbioses and proposed applied strategies for the use of Wolbachia infections to drive desired transgenes through natural populations (i.e., population replacement strategies).     

节肢动物内共生菌Wolbachia的研究进展

沃尔巴克氏体Wolbachia为母系传播的胞内共生菌,可通过对宿主产生多种调控方式扩大其自身在宿主种群的传播。据推测,有40%~60%的节肢动物都感染有Wolbachia,并可根据不同株系间的系统发育关系将其分为多个超群。为了有助于深入研究Wolbachia对其宿主的调控方式及其调控机制及提出更为有效的害虫生物防治策略,本文综述了节肢动物内共生菌Wolbachia的研究现状。1924年Wolbachia被报道首次发现于尖音库蚊Culex pipiens的生殖组织中,1971年确认其与宿主的胞质不亲和现象有关。Wolbachia可以通过胞质不亲和、杀雄、雌性化、孤雌生殖等作用方式调控宿主的生殖。除生殖调控之外,Wolbachia对宿主的调控方式还包括调控宿主新陈代谢、抵制病原菌、影响宿主生殖力等。Wolbachia调控的胞质不亲和现象可用“修饰-营救”(modification-rescue)模型解释,且已有与Wolbachia诱导宿主胞质不亲和相关的功能基因被报道。wMel株系是首个公布全基因组序列的Wolbachia株系,随后又有数十种不同株系的Wolbachia基因组陆续被破译。wMel株系Wolbachia可起到抑制登革热病毒传播的作用;同时,Wolbachia和昆虫不育技术的结合对白纹伊蚊Aedes albopictus野外种群起到良好的控制效果。鉴于目前节肢动物内共生菌Wolbachia的研究现状,我们认为未来应开展以下研究：(1)Wolbachia基因组及生殖调控作用关键功能基因的研究;(2)Wolbachia与宿主间互作机制的研究;(3)Wolbachia在生物防治方面的应用。     

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.     

Wolbachia-induced 'hybrid breakdown' in the two-spotted spider mite Tetranychus urticae Koch

The most common post-zygotic isolation mechanism between populations of the phytophagous mite Tetranychus urticae is 'hybrid breakdown', i.e. when individuals from two different populations are crossed, F1 hybrid females are produced, but F2 recombinant male offspring suffer increased mortality. Two-spotted spider mites collected from two populations, one on rose and the other on cucumber plants, were infected with Wolbachia bacteria. These bacteria may induce cytoplasmic incompatibility in their hosts: uninfected (U) females become reproductively incompatible with infected (W) males. We report on the effect of Wolbachia infections in intra- and interstrain crosses on (i) F1 mortality and sex ratios (a test for cytoplasmic incompatibility), and (ii) the number of haploid offspring and mortality in clutches of F1 virgins (a test for hybrid breakdown). U x W crosses within the rose strain exhibited partial cvtoplasmic incompatibility. More interestingly, F2 males suffered increased mortality, a result identical to the hybrid breakdown phenomenon. The experiments were repeated using females from the cucumber strain. In interstrain U x W and U x U crosses, hybrid breakdown was much stronger in the former (80 versus 26%). This is the first report of a Wolbachia infection causing a hybrid breakdown phenotype. Our results show that Wolbhachia infections can contribute to reproductive incompatibility between populations of T. urticae.     

Host genotype changes bidirectional to unidirectional cytoplasmic incompatibility in Nasonia longicornis

Wolbachia are the most abundant maternally inherited endosymbionts of insects and cause various reproductive alterations in their hosts. One such manipulation is cytoplasmic incompatibility (CI), which is a sperm-egg incompatibility typically resulting in zygotic death. Nasonia longicornis (Hymenoptera: Pteromalidae) has an A supergroup and two closely related B supergroup Wolbachia infections. The B supergroup bacteria co-diverged in this host genus. Both triple (wNlonAwNlonB1wNlonB2) and double infections (wNlonAwNlonB1, wNlonAwNlonB2) have been obtained from the field. In the present study, CI was determined among the three Wolbachia types in different host genetic backgrounds. Results show that host genetic background determines whether bidirectional CI or unidirectional CI occurs between the two closely related B group Wolbachia. Results show that the wNlonB1-infected males are bidirectionally incompatible with wNlonB2 in their 'native' nuclear genetic background, whereas wNlonB1 males are compatible with wNlonB2 in two other N. longicornis genetic backgrounds, resulting in unidirectional CI. In contrast, wNlonB2-infected males are incompatible with wNlonB1 females in all three host genetic backgrounds. These changes in incompatibility are not due to the loss of the bacteria. We hypothesize that a repressor gene for sperm modification by wNlonB1 is segregating in N. longicornis populations. The relevance of these findings to the potential role of Wolbachia in host-reproductive divergence and speciation is discussed.     

Many compatible Wolbachia strains coexist within natural populations of Culex pipiens mosquito

Maternally inherited Wolbachia often manipulate the reproduction of arthropods to promote their transmission. In most species, Wolbachia exert a form of conditional sterility termed cytoplasmic incompatibility (CI), characterized by the death of embryos produced by the mating between individuals with incompatible Wolbachia infections. From a theoretical perspective, no stable coexistence of incompatible Wolbachia infections is expected within host populations and CI should induce the invasion of one strain or of a set of compatible strains. In this study, we investigated this prediction on CI dynamics in natural populations of the common house mosquito Culex pipiens. We surveyed the Wolbachia diversity and the expression of CI in breeding sites of the south of France between 1990 and 2005. We found that geographically close C. pipiens populations harbor considerable Wolbachia diversity, which is stably maintained over 15 years. We also observed a very low frequency of infertile clutches within each sampled site. Meanwhile, mating choice experiments conducted in laboratory conditions showed that assortative mating does not occur. Overall, this suggests that a large set of compatible Wolbachia strains are always locally dominant within mosquito populations thus, fitting with the theoretical expectations on CI dynamics.     

Wolbachia infections are distributed throughout insect somatic and germ line tissues

Wolbachia are intracellular microorganisms that form maternally-inherited infections within numerous arthropod species. These bacteria have drawn much attention, due in part to the reproductive alterations that they induce in their hosts including cytoplasmic incompatibility (CI), feminization and parthenogenesis. Although Wolbachia's presence within insect reproductive tissues has been well described, relatively few studies have examined the extent to which Wolbachia infects other tissues. We have examined Wolbachia tissue tropism in a number of representative insect hosts by western blot, dot blot hybridization and diagnostic PCR. Results from these studies indicate that Wolbachia are much more widely distributed in host tissues than previously appreciated. Furthermore, the distribution of Wolbachia in somatic tissues varied between different Wolbachia/host associations. Some associations showed Wolbachia disseminated throughout most tissues while others appeared to be much more restricted, being predominantly limited to the reproductive tissues. We discuss the relevance of these infection patterns to the evolution of Wolbachia/host symbioses and to potential applied uses of Wolbachia.     

Diversification of Wolbachia endosymbiont in the Culex pipiens mosquito

The α-proteobacteria Wolbachia are among the most common intracellular bacteria and have recently emerged as important drivers of arthropod biology. Wolbachia commonly act as reproductive parasites in arthropods by inducing cytoplasmic incompatibility (CI), a type of conditional sterility between hosts harboring incompatible infections. In this study, we examined the evolutionary histories of Wolbachia infections, known as wPip, in the common house mosquito Culex pipiens, which exhibits the greatest variation in CI crossing patterns observed in any insect. We first investigated a panel of 20 wPip strains for their genetic diversity through a multilocus scheme combining 13 Wolbachia genes. Because Wolbachia depend primarily on maternal transmission for spreading within arthropod populations, we also studied the variability in the coinherited Cx. pipiens mitochondria. In total, we identified 14 wPip haplotypes, which all share a monophyletic origin and clearly cluster into five distinct wPip groups. The diversity of Cx. pipiens mitochondria was extremely reduced, which is likely a consequence of cytoplasmic hitchhiking driven by a unique and recent Wolbachia invasion. Phylogenetic evidence indicates that wPip infections and mitochondrial DNA have codiverged through stable cotransmission within the cytoplasm and shows that a rapid diversification of wPip has occurred. The observed pattern demonstrates that a considerable degree of Wolbachia diversity can evolve within a single host species over short evolutionary periods. In addition, multiple signatures of recombination were found in most wPip genomic regions, leading us to conclude that the mosaic nature of wPip genomes may play a key role in their evolution.     

Wolbachia-induced unidirectional cytoplasmic incompatibility and the stability of infection polymorphism in parapatric host populations

Wolbachia are intracellular, maternally inherited bacteria that are widespread among arthropods and commonly induce a reproductive incompatibility between infected male and uninfected female hosts known as unidirectional cytoplasmic incompatibility (CI). If infected and uninfected populations occur parapatrically, CI acts as a post-zygotic isolation barrier. We investigate the stability of such infection polymorphisms in a mathematical model with two populations linked by migration. We determine critical migration rates below which infected and uninfected populations can coexist. Analytical solutions of the critical migration rate are presented for mainland-island models. These serve as lower estimations for a more general model with two-way migration. The critical migration rate is positive if either Wolbachia causes a fecundity reduction in infected female hosts or its transmission is incomplete, and is highest for intermediate levels of CI. We discuss our results with respect to local adaptations of the Wolbachia host, speciation, and pest control.     

Wolbachia distribution and cytoplasmic incompatibility based on a survey of 42 spider mite species (Acari: Tetranychidae) in Japan

Wolbachia are a group of maternally inherited bacteria that infect a wide range of arthropods. Wolbachia infections are known to result in the expression of various abnormal reproductive phenotypes, the best known being cytoplasmic incompatibility. The first systematic survey of 42 spider mite species in Japan revealed that seven species (16.7%) were infected with Wolbachia. Wolbachia in the spider mites were grouped into three subgroups in supergroup B by phylogenetic analyses of the wsp gene. Most spider mites did not show cytoplasmic incompatibility when infected males were crossed with uninfected females. However, all infected populations of Panonychus mori and Oligonychus gotohi (five and four populations, respectively) possessed modification-positive strains of Wolbachia, and the cytoplasmic incompatibility decreased egg hatchability and female ratio of the spider mites. Thus, some Wolbachia strains cause sex ratio distortion in their hosts.     

Fitness advantage and cytoplasmic incompatibility in Wolbachia single- and superinfected Aedes albopictus

Wolbachia are obligate, maternally inherited, intracellular bacteria that infect numerous insects and other invertebrates. Wolbachia infections have evolved multiple mechanisms to manipulate host reproduction and facilitate invasion of naive host populations. One such mechanism is cytoplasmic incompatibility (CI) that occurs in many insect species, including Aedes albopictus (Asian tiger mosquito). The multiple Wolbachia infections that occur naturally in A. albopictus make this mosquito a useful system in which to study CI. Here, experiments employ mosquito strains that have been introgressed to provide genetically similar strains that harbor differing Wolbachia infection types. Cytoplasmic incompatibility levels, host longevity, egg hatch rates, and fecundity are examined. Crossing results demonstrate a pattern of additive unidirectional cytoplasmic incompatibility. Furthermore, relative to uninfected females, infected females are at a reproductive advantage due to both cytoplasmic incompatibility and a fitness increase associated with Wolbachia infection. In contrast, no fitness difference was observed in comparisons of single- and superinfected females. We discuss the observed results in regard to the evolution of the Wolbachia/A. albopictus symbiosis and the observed pattern of Wolbachia infection in natural populations.     

Coupled population dynamics of endosymbionts within and between hosts

Many insect species are infected with maternally transmitted endosymbionts, the most widely documented being Wolbachia . The rate of spread and equilibrium of prevalence of these infections depend on two parameters – maternal transmission fidelity and relative fitness of infected cytoplasmic lineages. Both transmission fidelity and the phenotypic effect of endosymbionts often increase with endosymbiont density within hosts. Thus, the dynamics of infection prevalence in host populations depends on processes affecting within-host density of endosymbionts. In theory, the equilibrium prevalence of infection by male-killing endosymbionts is much more sensitive to changes in transmission fidelity and relative fitness than is that of endosymbionts that cause cytoplasmic incompatibility. In natural populations, male-killers exhibit much greater temporal and spatial variation in the prevalence of infection than do endosymbionts that cause cytoplasmic incompatibility. Thus, the population dynamics of endosymbiont infections, especially those that cause male-killing, is likely to be governed by environmental and genetic variables that affect within-host density of these infections.     

Strain-specific regulation of intracellular Wolbachia density in multiply infected insects

Vertically transmitted symbionts suffer a severe reduction in numbers when they pass through host generations, resulting in genetic homogeneity or even clonality of their populations. Wolbachia endosymbionts that induce cytoplasmic incompatibility in their hosts depart from this rule, because cytoplasmic incompatibility actively maintains multiple infection within hosts. Hosts and symbionts are thus probably under peculiar selective pressures that must shape the way intracellular bacterial populations are regulated. We studied the density and location of Wolbachia within adult Leptopilina heterotoma, a haplodiploid wasp that is parasitic on Drosophila and that is naturally infected with three Wolbachia strains, but for which we also obtained one simply infected and two doubly infected lines. Comparison of these four lines by quantitative polymerase chain reaction using a real-time detection system showed that total Wolbachia density varies according to the infection status of individuals, while the specific density of each Wolbachia strain remains constant regardless of the presence of other strains. This suggests that Wolbachia strains do not compete with one another within the same host individual, and that a strain-specific regulatory mechanism is operating. We discuss the regulatory mechanisms that are involved, and how this process might have evolved as a response to selective pressures acting on both partners.     

Generation of a novel Wolbachia infection in Aedes albopictus (Asian tiger mosquito) via embryonic microinjection

Genetic strategies that reduce or block pathogen transmission by mosquitoes are being investigated as a means to augment current control measures. Strategies of vector suppression and replacement are based upon intracellular Wolbachia bacteria, which occur naturally in many insect populations. Maternally inherited Wolbachia have evolved diverse mechanisms to manipulate host insect reproduction and promote infection invasion. One mechanism is cytoplasmic incompatibility (CI) through which Wolbachia promotes infection spread by effectively sterilizing uninfected females. In a prior field test, releases of Wolbachia-infected males were used to suppress a field population of Culex pipiens. An additional strategy would employ Wolbachia as a vehicle to drive desired transgenes into vector populations (population replacement). Wolbachia-based population suppression and population replacement strategies require an ability to generate artificial Wolbachia associations in mosquitoes. Here, we demonstrate a technique for transferring Wolbachia (transfection) in a medically important mosquito species: Aedes albopictus (Asian tiger mosquito). Microinjection was used to transfer embryo cytoplasm from a double-infected Ae. albopictus line into an aposymbiotic line. The resulting mosquito line is single-infected with the wAlbB Wolbachia type. The artificially generated infection type is not known to occur naturally and displays a new CI crossing type and the first known example of bidirectional CI in Aedes mosquitoes. We discuss the results in relation to applied mosquito control strategies and the evolution of Wolbachia infections in Ae. albopictus.     

Wolbachia affects oviposition and mating behaviour of its spider mite host

Wolbachia bacteria are transmitted from mother to offspring via the cytoplasm of the egg. When mated to males infected with Wolbachia bacteria, uninfected females produce unviable offspring, a phenomenon called cytoplasmic incompatibility (CI). Current theory predicts that ‘sterilization’ of uninfected females by infected males confers a fitness advantage to Wolbachia in infected females. When the infection is above a threshold frequency in a panmictic population, CI reduces the fitness of uninfected females below that of infected females and, consequently, the proportion of infected hosts increases. CI is a mechanism that benefits the bacteria but, apparently, not the host. The host could benefit from avoiding incompatible mates. Parasite load and disease resistance are known to be involved in mate choice. Can Wolbachia also be implicated in reproductive behaviour? We used the two‐spotted spider mite – Wolbachia symbiosis to address this question. Our results suggest that uninfected females preferably mate to uninfected males while infected females aggregate their offspring, thereby promoting sib mating. Our data agrees with other results that hosts of Wolbachia do not necessarily behave as innocent bystanders – host mechanisms that avoid CI can evolve.     

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.     

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 and genetic variability in the birdnest blowfly Protocalliphora sialia

Wolbachia are widespread cytoplasmically inherited bacteria that induce various reproductive alterations in host arthropods, including cytoplasmic incompatibility (CI), an incompatibility between sperm and egg that typically results in embryonic death. CI has been invoked as a possible mechanism for reproductive isolation and speciation in arthropods, by restricting gene flow and promoting maintenance (and evolution) of genetic divergence between populations. Here we investigate patterns of Wolbachia infection and nuclear and mitochondrial differentiation in geographical populations of the birdnest blowfly Protocalliphora sialia. Blowflies in western North America are infected with two A-group Wolbachia, with some individuals singly and others doubly infected. Individuals in eastern North America mostly show single infections with a B-group Wolbachia. Populations in the Midwest are polymorphic for infections and show A- or B-group infection. There is a low level of mitochondrial divergence and perfect concordance of mitochondrial haplotype with infection type, suggesting that two Wolbachia-associated selective sweeps of the mitochondrion have occurred in this species. Amplified fragment length polymorphism analysis of nuclear genetic variation shows genetic differentiation between the eastern-Midwestern and western populations. Both Midwestern and eastern flies infected with A-Wolbachia show eastern nuclear genetic profiles. Current results therefore suggest that Wolbachia has not acted as a major barrier to gene flow between western and eastern-Midwestern populations, although some genetic differentiation between A-Wolbachia infected and B-Wolbachia infected individuals in eastern-Midwestern populations cannot be ruled out.     

Wolbachia symbiont infections induce strong cytoplasmic incompatibility in the tsetse fly Glossina morsitans

Tsetse flies are vectors of the protozoan parasite African trypanosomes, which cause sleeping sickness disease in humans and nagana in livestock. Although there are no effective vaccines and efficacious drugs against this parasite, vector reduction methods have been successful in curbing the disease, especially for nagana. Potential vector control methods that do not involve use of chemicals is a genetic modification approach where flies engineered to be parasite resistant are allowed to replace their susceptible natural counterparts, and Sterile Insect technique (SIT) where males sterilized by chemical means are released to suppress female fecundity. The success of genetic modification approaches requires identification of strong drive systems to spread the desirable traits and the efficacy of SIT can be enhanced by identification of natural mating incompatibility. One such drive mechanism results from the cytoplasmic incompatibility (CI) phenomenon induced by the symbiont Wolbachia. CI can also be used to induce natural mating incompatibility between release males and natural populations. Although Wolbachia infections have been reported in tsetse, it has been a challenge to understand their functional biology as attempts to cure tsetse of Wolbachia infections by antibiotic treatment damages the obligate mutualistic symbiont (Wigglesworthia), without which the flies are sterile. Here, we developed aposymbiotic (symbiont-free) and fertile tsetse lines by dietary provisioning of tetracycline supplemented blood meals with yeast extract, which rescues Wigglesworthia-induced sterility. Our results reveal that Wolbachia infections confer strong CI during embryogenesis in Wolbachia-free (Gmm(Apo)) females when mated with Wolbachia-infected (Gmm(Wt)) males. These results are the first demonstration of the biological significance of Wolbachia infections in tsetse. Furthermore, when incorporated into a mathematical model, our results confirm that Wolbachia can be used successfully as a gene driver. This lays the foundation for new disease control methods including a population replacement approach with parasite resistant flies. Alternatively, the availability of males that are reproductively incompatible with natural populations can enhance the efficacy of the ongoing sterile insect technique (SIT) applications by eliminating the need for chemical irradiation.     

沃尔巴克氏体Wolbachia对宿主的生殖调控作用及其研究进展

沃尔巴克氏体Wolbachia是广泛分布于节肢动物体内的共生微生物,可通过宿主卵的细胞质传递给子代。Wolbachia通过多种方式调控其宿主的生殖活动,包括细胞质不亲和、孤雌生殖、雌性化、杀雄性和增强雄性或雌性的生殖力。通过这些调控作用促进其在宿主种群内的广泛传播。文章简要综述Wolbachia对宿主的生殖调控作用、水平传播、Wolbachia基因和应用方面的研究。