The ultrastructure and symbiotic relationships of Wolbachia of mosquitoes of the Aedes scutellaris group

Transient Introgression of Wolbachia into Aedes aegypti Populations Does Not Elicit an Antibody Response to Wolbachia Surface Protein in Community Members

Wolbachia is an endosymbiotic bacterium that can restrict the transmission of human pathogenic viruses by Aedes aegypti mosquitoes. Recent field trials have shown that dengue incidence is significantly reduced when Wolbachia is introgressed into the local Ae. aegypti population. Female Ae. aegypti are anautogenous and feed on human blood to produce viable eggs. Herein, we tested whether people who reside on Tri Nguyen Island (TNI), Vietnam developed antibodies to Wolbachia Surface Protein (WSP) following release of Wolbachia-infected Ae. aegypti, as a measure of exposure to Wolbachia. Paired blood samples were collected from 105 participants before and after mosquito releases and anti-WSP titres were measured by ELISA. We determined no change in anti-WSP titres after ~30 weeks of high levels of Wolbachia-Ae. aegypti on TNI. These data suggest that humans are not exposed to the major Wolbachia surface antigen, WSP, following introgression of Wolbachia-infected Ae. aegypti mosquitoes.

Restriction of Wolbachia Bacteria in Early Embryogenesis of Neotropical Drosophila Species via Endoplasmic Reticulum-Mediated Autophagy

Wolbachia are maternally transmitted intracellular bacteria that are not only restricted to the reproductive organs but also found in various somatic tissues of their native hosts. The abundance of the endosymbiont in the soma, usually a dead end for vertically transmitted bacteria, causes a multitude of effects on life history traits of their hosts, which are still not well understood. Thus, deciphering the host-symbiont interactions on a cellular level throughout a host's life cycle is of great importance to understand their homeostatic nature, persistence, and spreading success. Using fluorescent and transmission electron microscopy, we conducted a comprehensive analysis of Wolbachia tropism in soma and germ line of six Drosophila species at the intracellular level during host development. Our data uncovered diagnostic patterns of infections to embryonic primordial germ cells and to particular cells of the soma in three different neotropical Drosophila species that have apparently evolved independently. We further found that restricted patterns of Wolbachia tropism are determined in early embryogenesis via selective autophagy, and their spatially restricted infection patterns are preserved in adult flies. We observed tight interactions of Wolbachia with membranes of the endoplasmic reticulum, which might play a scaffolding role for autophagosome formation and subsequent elimination of the endosymbiont. Finally, by analyzing D. simulans lines transinfected with nonnative Wolbachia, we uncovered that the host genetic background regulates tissue tropism of infection. Our data demonstrate a novel and peculiar mechanism to limit and spatially restrict bacterial infection in the soma during a very early stage of host development. IMPORTANCE All organisms are living in close and intimate interactions with microbes that cause conflicts but also cooperation between both unequal genetic partners due to their different innate interests of primarily enhancing their own fitness. However, stable symbioses often result in homeostatic interaction, named mutualism, by balancing costs and benefits, where both partners profit. Mechanisms that have evolved to balance and stably maintain homeostasis in mutualistic relationships are still quite understudied; one strategy is to "domesticate" potentially beneficial symbionts by actively controlling their replication rate below a critical and, hence, costly threshold, and/or to spatially and temporally restrict their localization in the host organism, which, in the latter case, in its most extreme form, is the formation of a specialized housing organ for the microbe (bacteriome). However, questions remain: how do these mutualistic associations become established in their first place, and what are the mechanisms for symbiont control and restriction in their early stages? Here, we have uncovered an unprecedented symbiont control mechanism in neotropical Drosophila species during early embryogenesis. The fruit fly evolved selective autophagy to restrict and control the proliferation of its intracellular endosymbiont Wolbachia in a defined subset of the stem cells as soon as the host's zygotic genome is activated.

A potential third-order role of the host endoplasmic reticulum as a contact site in interkingdom microbial endosymbiosis and viral infection

The normal functioning of eukaryotic cells depends on the compartmentalization of metabolic processes within specific organelles. Interactions among organelles, such as those between the endoplasmic reticulum (ER) - considered the largest single structure in eukaryotic cells - and other organelles at membrane contact sites (MCSs) have also been suggested to trigger synergisms, including intracellular immune responses against pathogens. In addition to the ER-endogenous functions and ER-organelle MCSs, we present the perspective of a third-order role of the ER as a host contact site for endosymbiotic microbial non-pathogens and pathogens, from endosymbiont bacteria to parasitic protists and viruses. Although understudied, ER-endosymbiont interactions have been observed in a range of eukaryotic hosts, including protists, plants, algae, and metazoans. Host ER interactions with endosymbionts could be an ER function built from ancient, conserved mechanisms selected for communicating with mutualistic endosymbionts in specific life cycle stages, and they may be exploited by pathogens and parasites. The host ER-'guest' interactome and traits in endosymbiotic biology are briefly discussed. The acknowledgment and understanding of these possible mechanisms might reveal novel evolutionary perspectives, uncover the causes of unexplained cellular disorders and suggest new pharmacological targets.

Dirofilaria and Wolbachia in mosquitoes (Diptera: Culicidae) in central European Russia and on the Black Sea coast

Dirofilariasis is endemic in Russia, as well as in many other European countries. The aim of this study was to assess the ability of mosquitoes to transfer Dirofilaria immitis and Dirofilaria repens in regions with temperate and subtropical climates. The possible impact of the symbiotic bacterium Wolbachia on Dirofilaria transmission was also investigated. 5333 female mosquitoes were collected at 11 points in central European Russia and on the Black Sea coast during the period 2013–2017. Out of 20 mosquito species examined, 14 were infected with D. repens and 13 with D. immitis. Both species of Dirofilaria were found in different climatic regions. The total Dirofilaria spp. estimated infection rate (EIR) in the central part of Russia varied from 3.1% to 3.7% and, in the southern region, from 1.1% to 3.0%. The highest estimated infection rate was found in Anopheles messeae, the lowest in Culex pipiens. The greatest epidemiological danger was represented by Aedes aegypti, Ae. geniculatus, An. messeae and Ae. communis. Six out of 20 mosquito species were infected with Wolbachia. Pools of Aedes albopictus, Cx. pipiens and Coquillettidia richiardii were simultaneously infected with Dirofilaria and Wolbachia. After checking mosquitoes individually, it was found that there was no development of Dirofilaria to the infective larval stage in specimens infected with Wolbachia. Twenty-two Dirofilaria-infective pools were Wolbachia-free and only two mosquito pools were Wolbachia-infected. The potential for transmission of Dirofilaria in mosquito species naturally uninfected with the symbiotic bacterium Wolbachia is higher than in species infected with the bacterium.

First report of natural Wolbachia infection in the malaria mosquito Anopheles arabiensis in Tanzania

BACKGROUND

Natural infections of the endosymbiont bacteria Wolbachia have recently been discovered in populations of the malaria mosquito Anopheles gambiae (s.l.) in Burkina Faso and Mali, West Africa. This Anopheles specific strain wAnga limits the malaria parasite Plasmodium falciparum infections in the mosquito, thus it offers novel opportunities for malaria control.

RESULTS

We investigated Wolbachia presence in Anopheles arabiensis and Anopheles funestus, which are the two main malaria vectors in the Kilombero Valley, a malaria endemic region in south-eastern Tanzania. We found 3.1% (n = 65) and 7.5% (n = 147) wAnga infection prevalence in An. arabiensis in mosquitoes collected in 2014 and 2016, respectively, while no infection was detected in An. funestus (n = 41). Phylogenetic analysis suggests that at least two distinct strains of wAnga were detected, both belonging to Wolbachia supergroup A and B.

CONCLUSIONS

To our knowledge, this is the first confirmation of natural Wolbachia in malaria vectors in Tanzania, which opens novel questions on the ecological and genetic basis of its persistence and pathogen transmission in the vector hosts. Understanding the basis of interactions between Wolbachia, Anopheles mosquitoes and malaria parasites is crucial for investigation of its potential application as a biocontrol strategy to reduce malaria transmission, and assessment of how natural wAnga infections influence pathogen transmission in different ecological settings.

Whole genome screen reveals a novel relationship between Wolbachia levels and Drosophila host translation

Wolbachia is an intracellular bacterium that infects a remarkable range of insect hosts. Insects such as mosquitos act as vectors for many devastating human viruses such as Dengue, West Nile, and Zika. Remarkably, Wolbachia infection provides insect hosts with resistance to many arboviruses thereby rendering the insects ineffective as vectors. To utilize Wolbachia effectively as a tool against vector-borne viruses a better understanding of the host-Wolbachia relationship is needed. To investigate Wolbachia-insect interactions we used the Wolbachia/Drosophila model that provides a genetically tractable system for studying host-pathogen interactions. We coupled genome-wide RNAi screening with a novel high-throughput fluorescence in situ hybridization (FISH) assay to detect changes in Wolbachia levels in a Wolbachia-infected Drosophila cell line JW18. 1117 genes altered Wolbachia levels when knocked down by RNAi of which 329 genes increased and 788 genes decreased the level of Wolbachia. Validation of hits included in depth secondary screening using in vitro RNAi, Drosophila mutants, and Wolbachia-detection by DNA qPCR. A diverse set of host gene networks was identified to regulate Wolbachia levels and unexpectedly revealed that perturbations of host translation components such as the ribosome and translation initiation factors results in increased Wolbachia levels both in vitro using RNAi and in vivo using mutants and a chemical-based translation inhibition assay. This work provides evidence for Wolbachia-host translation interaction and strengthens our general understanding of the Wolbachia-host intracellular relationship.

Proteomic analysis of a mosquito host cell response to persistent Wolbachia infection

Wolbachia pipientis, an obligate intracellular bacterium associated with arthropods and filarial worms, is a target for filarial disease treatment and provides a gene drive agent for insect vector population suppression/replacement. We compared proteomes of Aedes albopictus mosquito C/wStr1 cells persistently infected with Wolbachia strain wStr, relative to uninfected C7-10 control cells. Among approximately 2500 proteins, iTRAQ data identified 815 differentially abundant proteins. As functional classes, energy and central intermediary metabolism proteins were elevated in infected cells, while suppressed proteins with roles in host DNA replication, transcription and translation suggested that Wolbachia suppresses pathways that support host cell growth and proliferation. Vacuolar ATPase subunits were strongly elevated, consistent with high densities of Wolbachia contained individually within vacuoles. Other differential level proteins had roles in ROS neutralization, protein modification/degradation and signaling, including hypothetical proteins whose functions in Wolbachia infection can potentially be manipulated by RNAi interference or transfection. Detection of flavivirus proteins supports further analysis of poorly understood, insect-specific flaviviruses and their potential interactions with Wolbachia, particularly in mosquitoes transinfected with Wolbachia. This study provides a framework for future attempts to manipulate pathways in insect cell lines that favor production of Wolbachia for eventual genetic manipulation, transformation and transinfection of vector species.

"Candidatus Finniella" (Rickettsiales, Alphaproteobacteria), Novel Endosymbionts of Viridiraptorid Amoeboflagellates (Cercozoa, Rhizaria)

The Rickettsiales (Alphaproteobacteria) are obligate intracellular bacteria that colonize a wide range of eukaryotic hosts, including diverse metazoa and protists. Here, we characterize rickettsial endosymbionts discovered in the cytoplasm of the algivorous amoeboflagellates Viridiraptor invadens and Orciraptor agilis (Viridiraptoridae, Cercozoa, Rhizaria), supplying evidence of free-living, phagotrophic members of the Cercozoa serving as hosts for Rickettsiales. According to 16S rRNA gene phylogenies, the bacteria represent two closely related but distinct genotypes within a deep-branching rickettsial clade, which contains the genera "Candidatus Odyssella," "Candidatus Paracaedibacter," and "Candidatus Captivus." Using the full-cycle rRNA approach, we detected the novel bacteria in four of nine viridiraptorid strains tested. Furthermore, two specific oligonucleotide probes with a single-nucleotide-difference discriminated both bacterial genotypes by fluorescence in situ hybridization (FISH). We establish the candidate species "Candidatus Finniella inopinata" (found in Viridiraptor invadens) and "Candidatus Finniella lucida" (found in Orciraptor agilis) for the novel bacteria and propose a new, provisional family of Rickettsiales, "Candidatus Paracaedibacteraceae."

Identification and ultrastructural characterization of the Wolbachia symbiont in Litomosoides chagasfilhoi

BACKGROUND

Filarial nematodes are arthropod-transmitted parasites of vertebrates that affect more than 150 million people around the world and remain a major public health problem throughout tropical and subtropical regions. Despite the importance of these nematodes, the current treatment strategies are not efficient in eliminating the parasite. The main strategy of control is based on chemotherapy with diethylcarbamazine, albendazole and ivermectin. In the 1970s, it was found that some filarids possess endosymbiotic bacteria that are important for the development, survival and infectivity of the nematodes. These bacteria belong to the genus Wolbachia, which is a widespread and abundant intracellular symbiont in worms. Knowledge about the structure of the bacteria and their relationship with their nematode hosts may allow new perspectives for the control of filarial nematodes.

METHODS

In this study, we used transmission electron microscopy combined with three-dimensional approaches to observe the structure of the endosymbiont of the filarial nematode Litomosoides chagasfilhoi, an experimental model for the study of lymphatic filariasis. In addition, the bacterium was classified based on PCR analyses.

RESULTS

The bacterium was mainly found in the hypodermis and in the female reproductive system in close association with host cell structures, such as the nucleus and endoplasmic reticulum. Our ultrastructural data also showed that the symbiont envelope is composed of two membrane units and is enclosed in a cytoplasmic vacuole, the symbiosome. Molecular data revealed that the bacterium of L. chagasfilhoi shares 100% identity with the Wolbachia endosymbiont of Litomosoides galizai.

CONCLUSIONS

Here we described ultrastructural aspects of the relationship of the Wolbachia with the filarial nematode Litomosoides chagasfilhoi and the findings lead us to consider this relationship as a mutualistic symbiosis.

wFlu: characterization and evaluation of a native Wolbachia from the mosquito Aedes fluviatilis as a potential vector control agent

There is currently considerable interest and practical progress in using the endosymbiotic bacteria Wolbachia as a vector control agent for human vector-borne diseases. Such vector control strategies may require the introduction of multiple, different Wolbachia strains into target vector populations, necessitating the identification and characterization of appropriate endosymbiont variants. Here, we report preliminary characterization of wFlu, a native Wolbachia from the neotropical mosquito Aedes fluviatilis, and evaluate its potential as a vector control agent by confirming its ability to cause cytoplasmic incompatibility, and measuring its effect on three parameters determining host fitness (survival, fecundity and fertility), as well as vector competence (susceptibility) for pathogen infection. Using an aposymbiotic strain of Ae. fluviatilis cured of its native Wolbachia by antibiotic treatment, we show that in its natural host wFlu causes incomplete, but high levels of, unidirectional cytoplasmic incompatibility, has high rates of maternal transmission, and no detectable fitness costs, indicating a high capacity to rapidly spread through host populations. However, wFlu does not inhibit, and even enhances, oocyst infection with the avian malaria parasite Plasmodium gallinaceum. The stage- and sex-specific density of wFlu was relatively low, and with limited tissue distribution, consistent with the lack of virulence and pathogen interference/symbiont-mediated protection observed. Unexpectedly, the density of wFlu was also shown to be specifically-reduced in the ovaries after bloodfeeding Ae. fluviatilis. Overall, our observations indicate that the Wolbachia strain wFlu has the potential to be used as a vector control agent, and suggests that appreciable mutualistic coevolution has occurred between this endosymbiont and its natural host. Future work will be needed to determine whether wFlu has virulent host effects and/or exhibits pathogen interference when artificially-transfected to the novel mosquito hosts that are the vectors of human pathogens.

Requirement of lipid II biosynthesis for cell division in cell wall-less Wolbachia, endobacteria of arthropods and filarial nematodes

Obligate Wolbachia endobacteria have a reduced genome and retained genes are hypothesized to be crucial for survival. Although intracellular bacteria do not need a stress-bearing peptidoglycan cell wall, Wolbachia encode proteins necessary to synthesize the peptidoglycan precursor lipid II. The activity of the enzymes catalyzing the last two steps of this pathway was previously shown, and Wolbachia are sensitive to inhibition of lipid II synthesis. A puzzling characteristic of Wolbachia is the lack of genes for l-amino acid racemases essential for lipid II synthesis. Transcription analysis showed the expression of a possible alternative racemase metC, and recombinant Wolbachia MetC indeed had racemase activity that may substitute for the absent l-Ala racemase. However, enzymes needed to form mature peptidoglycan are absent and the function of Wolbachia lipid II is unknown. Inhibition of lipid II biosynthesis resulted in enlargement of Wolbachia cells and redistribution of Wolbachia peptidoglycan-associated lipoprotein, demonstrating that lipid II is required for coordinated cell division and may interact with the lipoprotein. We conclude that lipid II is essential for Wolbachia cell division and that this function is potentially conserved in the Gram-negative bacteria.

Insect Sex Determination Manipulated by Their Endosymbionts: Incidences, Mechanisms and Implications

The sex-determining systems of arthropods are surprisingly diverse. Some species have male or female heterogametic sex chromosomes while other species do not have sex chromosomes. Most species are diploids but some species, including wasps, ants, thrips and mites, are haplodiploids (n in males; 2n in females). Many of the sexual aberrations, such as sexual mosaics, sex-specific lethality and conversion of sexuality, can be explained by developmental defects including double fertilization of a binucleate egg, loss of a sex chromosome or perturbation of sex-determining gene expression, which occur accidentally or are induced by certain environmental conditions. However, recent studies have revealed that such sexual aberrations can be caused by various groups of vertically-transmitted endosymbiotic microbes such as bacteria of the genera Wolbachia, Rickettsia, Arsenophonus, Spiroplasma and Cardinium, as well as microsporidian protists. In this review, we first summarize the accumulated data on endosymbiont-induced sexual aberrations, and then discuss how such endosymbionts affect the developmental system of their hosts and what kinds of ecological and evolutionary effects these endosymbionts have on their host populations.

The virulent Wolbachia strain wMelPop increases the frequency of apoptosis in the female germline cells of Drosophila melanogaster

BACKGROUND

Wolbachia are bacterial endosymbionts of many arthropod species in which they manipulate reproductive functions. The distribution of these bacteria in the Drosophila ovarian cells at different stages of oogenesis has been amply described. The pathways along which Wolbachia influences Drosophila oogenesis have been, so far, little studied. It is known that Wolbachia are abundant in the somatic stem cell niche of the Drosophila germarium. A checkpoint, where programmed cell death, or apoptosis, can occur, is located in region 2a/2b of the germarium, which comprises niche cells. Here we address the question whether or not the presence of Wolbachia in germarium cells can affect the frequency of cyst apoptosis in the checkpoint.

RESULTS

Our current fluorescent microscopic observations showed that the wMel and wMelPop strains had different effects on female germline cells of D. melanogaster. The Wolbachia strain wMel did not affect the frequency of apoptosis in cells of the germarium. The presence of the Wolbachia strain wMelPop in the D. melanogasterw1118 ovaries increased the number of germaria where cells underwent apoptosis in the checkpoint. Based on the appearance in the electron microscope, there was no difference in morphological features of apoptotic cystocytes between Wolbachia-infected and uninfected flies. Bacteria with normal ultrastructure and large numbers of degenerating bacteria were found in the dying cyst cells.

CONCLUSIONS

Our current study demonstrated that the Wolbachia strain wMelPop affects the egg chamber formation in the D. melanogaster ovaries. This led to an increase in the number of germaria containing apoptotic cells. It is suggested that Wolbachia can adversely interfere either with the cystocyte differentiation into the oocyte or with the division of somatic stem cells giving rise to follicle cells and, as a consequence, to improper ratio of germline cells to follicle cells and, ultimately, to apoptosis of cysts. There was no similar adverse effect in D. melanogaster Canton S infected with the Wolbachia strain wMel. This was taken to mean that the observed increase in frequency of apoptosis was not the general effect of Wolbachia on germline cells of D. melanogaster, it was rather induced by the virulent Wolbachia strain wMelPop.

Evolutionary ecology of vertically transmitted parasites: transovarial transmission of a microsporidian sex ratio distorter in Gammarus duebeni

Vertically transmitted parasites are transmitted from generation to generation of hosts usually via the host's gametes. Owing to gamete size dimorphism, the major transmission route is transovarial and selection (on the parasite) favours strategies which increase the relative frequency of the transmitting (female) host sex. These strategies impose unusual selection pressures on the host, and coevolution between hosts and vertically transmitted parasites has been implicated in speciation, in the evolution of symbiosis, and in the evolution of novel systems of host reproduction and sex determination. We review the evolutionary implications of vertically transmitted parasites in arthropods before focusing on strategies of transmission of a parasitic sex ratio distorter in Gammarus duebeni. The efficiency of parasite transmission to new hosts is a key factor underlying the relationship between vertically transmitted parasites and their hosts. Vertically transmitted parasites must overcome 2 bottlenecks in order to ensure successful infection of future host generations: first, transmission from adult to gamete; and secondly, transmission to the germ-line of the infected host. We investigate these 2 processes with regard to transovarial transmission by a microsporidian parasite in Gammarus duebeni. Parasite transmission from adult to eggs is highly efficient, with 96% of eggs of infected mothers inheriting the infection, whereas transmission to germ-line within infected embryos is relatively inefficient (72%). We measure parasite distribution between cells of developing embryos, and use these distributions to infer possible mechanisms of parasite transmission to germ-line. Parasite distribution within the embryo is dependent on host cell lineage, and is not consistent with unbiased segregation between daughter cells. These results indicate that parasites segregate together at host cell division, and may reflect a strategy of differential segregation to the host germ-line. We consider alternative parasite strategies at the cell-level in terms of their evolutionary implications.

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.

Using in situ hybridization to detect endosymbiont Wolbachia in dissected tissues of mosquito host

The endosymbiont Wolbachia, extensively occurring in arthropods, usually causes reproductive distortions of the host, such as mosquitoes. In past years, detection of Wolbachia in host tissues has highly relied on transmission electron microscopy (TEM) that is tedious and usually unable to gain satisfactory results without experienced techniques and expensive instruments. Polymerase chain reaction (PCR) recently has become popular in Wolbachia identification. However, necessity of DNA extraction from host individuals or dissected tissues has limited its application in extensiveness and versatility. At present, in situ hybridization has increased its role in examination of various microbes. This report provides a technique for rapid detection and localization of Wolbachia in tissues dissected from mosquitoes and possibly other infected organisms. To detect Wolbachia and to localize them in host tissues more precisely, in situ hybridization by using digoxigenin (DIG)-labeled probes was invented and applied to Wolbachia detection in this study. The results showed that Wolbachia preferentially aggregate in ovarioles, which is consistent with previous observations by TEM. The endobacteria also were detected in salivary glands, mostly in lateral lobes. Ultrastructurally, Wolbachia has been shown to occur in the cytoplasma of salivary gland cells.

Filarial susceptibility and effects of Wolbachia in Aedes pseudoscutellaris mosquitoes

The mosquito Aedes pseudoscutellaris (Theobald), a member of the Aedes (Stegomyia) scutellaris complex (Diptera: Culicidae), is an important vector of subperiodic Wuchereria bancrofti (Cobbold) (Spirurida: Onchocercidae), causing human lymphatic filariasis, on South Pacific islands. Maternal inheritance of filarial susceptibility in the complex has previously been asserted, and larval tetracycline treatment reduced susceptibility; the maternally inherited Wolbachia in these mosquitoes were suggested to be responsible. To investigate the relationship of these two factors, we eliminated Wolbachia from a strain of Ae. pseudoscutellaris by tetracycline treatment, and tested filarial susceptibility of the adult female mosquitoes using Brugia pahangi (Edeson & Buckley). Filarial susceptibility was not significantly different in Wolbachia-free and infected lines of Ae. pseudoscutellaris, suggesting that the Wolbachia in these mosquitoes do not influence vector competence. Crosses between Wolbachia-infected males and uninfected females of Ae. pseudoscutellaris showed cytoplasmic incompatibility (CI), i.e. no eggs hatched, unaffected by larval crowding or restricted nutrient availability, whereas these factors are known to affect CI in Drosophila simulans. Reciprocal crosses between Ae. pseudoscutellaris and Ae. katherinensis Woodhill produced no progeny, even when both parents were Wolbachia-free, suggesting that nuclear factors are responsible for this interspecific sterility.

Population differentiation and Wolbachia phylogeny in mosquitoes of the Aedes scutellaris group

Mosquito species of the Aedes (Stegomyia) scutellaris (Walker) group (Diptera: Culicidae) are distributed across many islands of the South Pacific and include major regional vectors of filariasis, such as Aedes polynesiensis (Marks). Analysis of populations of Ae. polynesiensis at the extremes of its range, from Fiji and from Moorea, French Polynesia, using the rDNA ITS2 (internal transcribed spacer 2) region and six microsatellite markers showed considerable genetic differentiation between them (F(ST) = 0.298-0.357). Phylogenetic analysis of the Wolbachia endosymbionts in three members of the complex revealed that based on the wsp gene they are all very similar and belong to the Mel subgroup of the A clade, closely related to the Wolbachia strain present in the gall wasp Callyrhytis glandium (Giraud) (Hymenoptera: Cynipidae). By contrast they are only distantly related to the A-clade Wolbachia in Aedes albopictus (Skuse), a species closely allied to the Ae. scutellaris group. There was very low differentiation between the Wolbachia in the Moorea and Fiji populations of Ae. polynesiensis.

Characterization of Wolbachia infections and interspecific crosses of Aedes (Stegomyia) polynesiensis and Ae. (Stegomyia) riversi (Diptera: Culicidae)

Prior studies have identified a complicated pattern of interspecific hybridization between members of the Aedes (Stegomyia) scutellaris (Walker) mosquito group, which includes medically important vectors of bancroftian filariasis and dengue. Here, we report that two members of the group, Aedes polynesiensis Marks and Aedes riversi Bohart & Ingram, are both infected with intracellular Wolbachia bacteria. Sequencing of the Wolbachia wsp gene demonstrates that the infections differ from each other and from Wolbachia infections previously reported in mosquitoes. Aedes polynesiensis is the first mosquito identified with a wMel Wolbachia type. Intraspecific crosses of infected and aposymbiotic lines generated via antibiotic treatment show that the Wolbachia infections in both species cause high levels of cytoplasmic incompatibility. Interspecific crosses show that the two species are reproductively isolated. However, repeating the interspecific crosses with aposymbiotic mosquito strains demonstrates that the Wolbachia infections play a role in preventing hybrid offspring. We discuss Wolbachia infections in relation to better defining the evolutionary relationships and causes of speciation within the group, understanding the basis for the observed east-to-west gradient in filarial refractoriness, and developing novel genetic control measures.

Strain-specific quantification of Wolbachia density in Aedes albopictus and effects of larval rearing conditions

The density of the endosymbiont Wolbachia can influence the expression of the crossing sterilities known as cytoplasmic incompatibility (CI), and also its rate of maternal transmission. Aedes albopictus mosquitoes contain a superinfection with the Wolbachia strains wAlbA and wAlbB. A strain-specific real-time quantitative PCR assay was developed and used to quantify relative Wolbachia strain densities within individual mosquitoes. The wAlbB strain was consistently found to be at higher density than wAlbA, which can explain a slightly lower rate of maternal transmission reported for wAlbA. The effects of larval crowding and nutritional stress were also examined. Larval crowding always reduced adult size, but reduced the density of Wolbachia strains relative to uncrowded conditions only if crowding was accompanied by restricted nutrient availability. Crowded rearing conditions never resulted in strain segregation or in a reduction in the penetrance of CI, however. The rate of maternal transmission and the penetrance of CI are the two most important variables that determine relative Wolbachia population invasion dynamics, and both are considerably higher here than have been reported in the Drosophila simulans model system.

Identification of Wolbachia--host interacting factors through cytological analysis

Manipulation of host reproduction and efficient maternal transmission have facilitated the global spread of Wolbachia through millions of insect species. Cytological studies of the most common Wolbachia-induced phenotype, cytoplasmic incompatibility (CI), demonstrate that Wolbachia induce CI by altering host cell cycle timing. Cytological analyses also suggest that microtubules and motor proteins may play a role in the maternal and somatic transmission of Wolbachia.

Host age effect and expression of cytoplasmic incompatibility in field populations of Wolbachia-superinfected Aedes albopictus

The Asian tiger mosquito, Aedes albopictus (Skuse), is a known vector of dengue in South America and Southeast Asia. It is naturally superinfected with two strains of Wolbachia endosymbiont that are able to induce cytoplasmic incompatibility (CI). In this paper, we report the strength of CI expression in crosses involving field-caught males. CI expression was found to be very strong in all crosses between field males and laboratory-reared uninfected or wAlbA infected young females. In addition, crossing experiments with laboratory colonies showed that aged super-infected males could express strong CI when mated with young uninfected or wAlbA infected females. These results provide additional evidence that the CI properties of Wolbachia infecting Aedes albopictus are well suited for applied strategies that seek to utilise Wolbachia for host population modification.

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).

Wolbachia infection and cytoplasmic incompatibility in the cricket Teleogryllus taiwanemma

Wolbachia are cytoplasmically inherited bacteria found in many arthropods. They induce various reproductive alterations in their hosts, including cytoplasmic incompatibility, thelytokous parthenogenesis, feminization and male-killing. In this study, we examined Wolbachia infection and its effects on the host cricket Teleogryllus taiwanemma. In a phylogenetic study based on the wsp gene coding for a Wolbachia surface protein, the Wolbachia strain harboured by T. taiwanemma was clustered together with those harboured by Laodelphax striatellus, Tribolium confusum, Acraea encedon, Trichogramma deion and Adalia bipunctata. Crossing experiments using the Wolbachia-infected and uninfected strains of cricket showed that the infection is associated with the expression of unidirectional cytoplasmic incompatibility: the egg hatch rate in the incompatible cross between the infected males and uninfected females was 20.3 %. We also examined the distribution of Wolbachia within the host using polymerase chain reaction assays; they were detected in the antennae, heads, forewings, hindwings, testes, ovaries, Malpighian tubules, foot muscles and fat bodies. Quantitative polymerase chain reaction assays showed that the bacterial density was highest in the fat bodies, followed by the ovaries and testes. Wolbachia were not detected in the haemolymph or in mature spermatozoa. The spermatozoa of the infected male may be modified by the presence of Wolbachia during its development. To examine this possibility, we compared the profiles of sperm proteins between the infected and uninfected males using two-dimensional gel electrophoresis. However, no differences in the protein profiles were observed.

Evidence for female mortality in Wolbachia-mediated cytoplasmic incompatibility in haplodiploid insects: epidemiologic and evolutionary consequences

Until now, only two Wolbachia-mediated cytoplasmic incompatibility (CI) types have been described in haplodiploid species, the first in Nasonia (Insect) and the second in Tetranychus (Acari). They both induce a male-biased sex ratio in the incompatible cross. In Nasonia, CI does not reduce fertility since incompatible eggs develop as haploid males, whereas in Tetranychus CI leads to a partial mortality of incompatible eggs, thus reducing the fertility of females. Here, we study Wolbachia infection in a Drosophila parasitoid, Leptopilina heterotoma (Hymenoptera: Figitidae). A survey of Wolbachia infection shows that all natural populations tested are totally infected. Crosses between infected males and cured females show complete incompatibility: almost no females are produced. Moreover, incompatible eggs die early during their development, unlike Nasonia. This early death allows the parasitized Drosophila larva to achieve its development and to emerge. Thus, uninfected females crossed with infected males have reduced offspring production consisting only of males. Evidence of this CI type in insects demonstrates that the difference in CI types of Nasonia and Tetranychus is not due to specific factors of insects or acari. Using theoretical models, we compare the invasion processes of different strategies of Wolbachia: CI in diploid species, the two CI types in haplodiploid species, and parthenogenesis (the classical effect in haplodiploid species). Models show that CI in haplodiploid species is less efficient than in diploid ones. However, the Leptopilina type is advantageous compared to the Nasonia type. Parthenogenesis may be more or less advantageous, depending on the infection cost and on the proportion of fertilized eggs. Finally, we can propose different processes of Wolbachia strategy evolution in haplodiploid species from Nasonia CI type to Leptopilina CI type or parthenogenesis.

Phylogenetic analysis of the 16S rDNA of the cytoplasmic bacterium Wolbachia from the novel host Folsomia candida (Hexapoda, Collembola) and its implications for wolbachial taxonomy

Wolbachia pipientis are intracellular, transovarially inherited alpha-Proteobacteria in invertebrates. Four major Wolbachia groups exist: A, B (contained in divergent arthropods), C and D (harbored by Nematoda). By means of transmission electron microscopy, we observed Wolbachia-like bacteria in a primitive insect, Folsomia candida (Hexapoda, Collembola, Isotomidae). 16S rDNA analysis proved them to constitute a novel lineage, henceforth named group E, in the wolbachial phylogenetic tree. It shares 97.8% 16S rDNA homology with its nearest neighbors, groups A and B, which diverged from it more recently. We propose (i) a new taxon E for the Wolbachia strain in F. candida, (ii) that the single-described Wolbachia pipientis fall apart into at least three species: C, D and the large E-A-B complex. F. candida's group E Wolbachia rekindle the question about invasive capacities of free-living ancestral wolbachiae and horizontal transfer.

Wolbachia pipientis: microbial manipulator of arthropod reproduction

The alpha-proteobacterium Wolbachia pipientis is a very common cytoplasmic symbiont of insects, crustaceans, mites, and filarial nematodes. To enhance its transmission, W. pipientis has evolved a large scale of host manipulations: parthenogenesis induction, feminization, and male killing. W. pipientis's most common effect is a crossing incompatibility between infected males and uninfected females. Little is known about the genetics and biochemistry of these symbionts because of their fastidious requirements. The affinity of W. pipientis for the microtubules associated with the early divisions in eggs may explain some of their effects. Such inherited microorganisms are thought to have been major factors in the evolution of sex determination, eusociality, and speciation. W. pipientis isolates are also of interest as vectors for the modification of wild insect populations, in the improvement of parasitoid wasps in biological pest control, and as a new method for interfering with diseases caused by filarial nematodes.

Wolbachia, normally a symbiont of Drosophila, can be virulent, causing degeneration and early death

Wolbachia, a maternally transmitted microorganism of the Rickettsial family, is known to cause cytoplasmic incompatibility, parthenogenesis, or feminization in various insect species. The bacterium-host relationship is usually symbiotic: incompatibility between infected males and uninfected females can enhance reproductive isolation and evolution, whereas the other mechanisms enhance progeny production. We have discovered a variant Wolbachia carried by Drosophila melanogaster in which this cozy relationship is abrogated. Although quiescent during the fly's development, it begins massive proliferation in the adult, causing widespread degeneration of tissues, including brain, retina, and muscle, culminating in early death. Tetracycline treatment of carrier flies eliminates both the bacteria and the degeneration, restoring normal life-span. The 16s rDNA sequence is over 98% identical to Wolbachia known from other insects. Examination of laboratory strains of D. melanogaster commonly used in genetic experiments reveals that a large proportion actually carry Wolbachia in a nonvirulent form, which might affect their longevity and behavior.

Organization of Wolbachia pipientis in the Drosophila fertilized egg and embryo revealed by an anti-Wolbachia monoclonal antibody

Cytoplasmic incompatibility (CI) in Drosophila is related to the presence of Wolbachia, an intracellular microorganism found in many species of insects. In order to study the intracellular localization of Wolbachia in eggs and embryos, we have purified the bacteria from fly embryos and subsequently generated a monoclonal antibody (Mab Wol-1) specific for Wolbachia. Indirect immunofluorescence staining using Wol-1 reveals that during mitosis, Wolbachia are localized near spindle poles and centrosomes. Double label immunofluorescence experiments using anti-tubulin and anti-Wolbachia antibodies show that Wolbachia co-localize with centrosomal microtubules throughout the cell cycle. Direct interactions between the bacteria and centrosome-organized microtubules are implied from seven observations: (1) throughout the mitotic cycle, the position and movement of Wolbachia precisely mimic the behavior of the centrosome and apparently associated with centrosome-organized microtubules; (2) Wolbachia segregate equally to each spindle pole during mitosis; (3) Wolbachia do not associate with spindle microtubules during mitosis; (4) Wolbachia located in the egg cortex localize to the domains of cytoplasm organized by microtubules during blastoderm formation; (5) polar body nuclei that lack centrosomes but contain associated microtubules do not contain Wolbachia; (6) Wolbachia no longer associated with yolk nuclei, following differentiation and loss of centrosomes; (7) during pole cell formation, Wolbachia co-localize with the centrosome on the apical side of the nucleus as pole cells form. Quantitative data indicates that no Wolbachia growth occurs during the preblastoderm period even though rapid nuclear, and subsequent cellular, proliferation takes place during this same period. This indicates that Wolbachia are under strict growth regulation by the host suggesting that host factors play a role in regulating growth of Wolbachia in the egg. Further cellular and molecular studies of the extensive, global interactions between host and symbiont observed in this egg should provide important new insights into the evolution of host/symbiosis and the cell biology of cytoplasmic incompatibility.

A prokaryotic dnaA sequence in Drosophila melanogaster: Wolbachia infection and cytoplasmic incompatibility among laboratory strains

Using oligonucleotide primers derived from the aligned polypeptide sequences of several prokaryotic dnaA genes, we amplified from Drosophila melanogaster DNA a 557 bp fragment containing a single open reading frame. The predicted peptide sequence shows a significant similarity to previously characterized protein sequences that are encoded by the dnaA genes of several prokaryotes. The dnaA sequences are also detectable by PCR in DNA from Drosophila simulans and Nasonia vitripennis flies which are infected by a symbiotic bacterium assigned to the type species Wolbachia pipientis. A tetracycline treatment that eradicates bacterial parasites from insects, abolishes the dnaA sequences from Drosophila and Nasonia DNA. In addition, dnaA-positive Drosophila melanogaster contain numerous rod-shaped bacteria in embryos, which are abolished in subsequent generations after treatment with tetracycline. Combined with phylogenetic analysis of DnaA and 16S rRNA sequences, these results show that the dnaA cognate comes from Wolbachia. A survey of Drosophila stocks using PCR amplification of dnaA and 16S rRNA sequences showed that Wolbachia is widely spread among D. melanogaster laboratory strains but absent from several established strains of the Mediterranean fruit fly Ceratitis capitata. Evidence is also presented that presence of the bacterium can cause partial cytoplasmic incompatibility between infected and non-infected D. melanogaster strains.

The distribution of cytoplasmic bacteria in the early Drosophila embryo is mediated by astral microtubules

Maternally inherited cytoplasmic bacteria have occasionally been observed in embryos and adults of different strains of several Drosophila species. While there is a considerable body of data on the relationship between bacteria and embryo viability, little is known about the behavior of these bacteria during the early development of Drosophila. In eggs laid by infected Drosophila melanogaster females we showed that cytoplasmic bacteria were initially concentrated in a thin cortical layer and scattered in the yolk region. During the following syncytial blastoderm mitoses the bacteria mainly accumulated towards the poles of the mitotic spindles, suggesting that astral microtubules play a role in localizing bacteria. This is supported by the observation that treatment of the infected embryos with the microtubule-disrupting drug colchicine led to the partial dissociation of the bacteria from the spindle poles, whereas cytochalasin treatment left almost all the bacterial clusters intact. Moreover, bacteria were not found near the polar bodies and yolk nuclei, which were without astral microtubules. In mitosis-defective embryos, with centrosomes dissociated from the nuclei, the bacteria were concentrated in association with the isolated astral microtubules, and in cold-treated embryos, in which microtubules regrew from isolated centrosomes after recovering, the bacteria clustered around the newly formed asters. These observations, also supported by electron microscope analysis, indicate a close relationship between cytoplasmic bacteria and astral microtubules, and suggest that the latter were able to build discrete cytoplasmic domains ensuring the proper distribution of cytoplasmic components during the blastoderm mitoses, despite the lack of cell membranes.

Phylogeny of cytoplasmic incompatibility micro-organisms in the parasitoid wasp genus Nasonia (Hymenoptera: Pteromalidae) based on 16S ribosomal DNA sequences

Cytoplasmic incompatibility results in embryo mortality in diploids, or all male offspring in haplodiploids, when individuals carrying different cytoplasmic factors are crossed. Cytoplasmic factors have been identified as intracellular micro-organisms. Microbe-induced cytoplasmic incompatibility is found in many insect taxa and may play a role in reproductive isolation between populations. Such micro-organisms cause bidirectional incompatibility between species of the parasitoid wasp genus Nasonia. The phylogenetic relationship of cytoplasmic incompatibility microorganisms (CIM) of different Nasonia species was analysed using their 16S ribosomal DNA (rDNA) sequence. Two 16S rDNA operons were detected in the CIM of each Nasonia species. Sequence analysis indicates that the Nasonia CIM are closely related and belong to the alpha group of the Proteobacteria.

Occurrence of rickettsia-like symbionts among species of the Aedes scutellaris group (Diptera:Culicidae)

Rickettsia-like organisms were seen by electron microscopy in the oocytes and nurse cells of Aedes hebrideus and A. scutellaris scutellaris and in the oocytes of A. pseudoscutellaris, a Samoan stock of A. polynesiensis and in three Fijian stocks of A. polynesiensis. None were found in the ovaries of A. alcasidi, A. s. katherinensis, A. cooki nor in a second stock of Samoan A. polynesiensis. It is suggested that the distribution of rickettsia-like symbionts among species of the A. scutellaris group conflicts in some cases with the hypothesis that these organisms are responsible for cytoplasmic incompatibility in the group.