A Wolbachia infection from Drosophila that causes cytoplasmic incompatibility despite low prevalence and densities in males

Wolbachia infection negatively impacts Drosophila simulans heat tolerance in a strain- and trait-specific manner

The susceptibility of insects to rising temperatures has largely been measured by their ability to survive thermal extremes. However, the capacity for maternally inherited endosymbionts to influence insect heat tolerance has been overlooked. Further, while some studies have addressed the impact of heat on traits like fertility, which can decline at temperatures below lethal thermal limits, none have considered the impact of endosymbionts. Here, we assess the impact of three Wolbachia strains (wRi, wAu and wNo) on the survival and fertility of Drosophila simulans exposed to heat stress during development or as adults. The effect of Wolbachia infection on heat tolerance was generally small and trait/strain specific. Only the wNo infection significantly reduced the survival of adult males after a heat shock. When exposed to fluctuating heat stress during development, the wRi and wAu strains reduced egg-to-adult survival but only the wNo infection reduced male fertility. Wolbachia densities of all three strains decreased under developmental heat stress, but reductions occurred at temperatures above those that reduced host fertility. These findings emphasize the necessity to account for endosymbionts and their effect on both survival and fertility when investigating insect responses to heat stress.

Conserved infections and reproductive phenotypes of Wolbachia symbionts in Asian tortrix moths

Wolbachia is a ubiquitous endosymbiotic bacterium that manipulates insect reproduction. A notable feature of Wolbachia is male killing (MK), whereby sons of infected females are killed during development; however, the evolutionary processes by which Wolbachia acquired the MK ability remain unclear. The tea tortrix moth Homona magnanima (Tortricidae) harbours three non-MK Wolbachia strains (wHm-a, wHm-b and wHm-c) and an MK strain wHm-t. Although wHm-t and wHm-c are closely related, only wHm-t has an MK-associated prophage region. To understand the evolutionary processes underlying the emergence of MK wHm-t, we examined Wolbachia infections and phenotypes in 62 tortricid species collected from 39 localities across Japan, Taiwan, Vietnam and Indonesia. PCR assays detected wHm-c relatives in 51 species and triple infection of wHm-a, wHm-b and wHm-c in 31 species. Apart from Taiwanese H. magnanima, no species exhibited the MK phenotype and were positive for the wHm-t-specific prophage. While wHm-t infection was dominant in Taiwanese H. magnanima, wHm-a, wHm-b and wHm-c were dominant in Japanese H. magnanima populations. These results suggest that wHm-a, wHm-b and wHm-c strains descended from a common ancestor with repeated infection loss and that wHm-t evolved from the wHm-c acquiring MK ability in allopatric populations of H. magnanima.

A diversity of endosymbionts across Australian aphids and their persistence in aphid cultures

There is increasing interest in the use of endosymbionts in pest control, which will benefit from the identification of endosymbionts from potential donor species for transfer to pest species. Here, we screened for endosymbionts in 123 Australian aphid samples across 32 species using 16S DNA metabarcoding. We then developed a qPCR method to validate the metabarcoding data set and to monitor endosymbiont persistence in aphid cultures. Pea aphids (Acyrthosiphon pisum) were frequently coinfected with Rickettsiella and Serratia, and glasshouse potato aphids (Aulacorthum solani) were coinfected with Regiella and Spiroplasma; other secondary endosymbionts detected in samples occurred by themselves. Hamiltonella, Rickettsia and Wolbachia were restricted to a single aphid species, whereas Regiella was found in multiple species. Rickettsiella, Hamiltonella and Serratia were stably maintained in laboratory cultures, although others were lost rapidly. The overall incidence of secondary endosymbionts in Australian samples tended to be lower than recorded from aphids overseas. These results indicate that aphid endosymbionts probably exhibit different levels of infectivity and vertical transmission efficiency across hosts, which may contribute to natural infection patterns. The rapid loss of some endosymbionts in cultures raises questions about factors that maintain them under field conditions, while endosymbionts that persisted in laboratory culture provide candidates for interspecific transfers.

Establishment of a Rapid Detection Method for Yeast-like Symbionts in Brown Planthopper Based on Droplet Digital PCR Technology

The brown planthopper Nilaparvata lugens (Stål) (BPH) is a typical monophagous sucking rice pest. Over the course of their evolution, BPH and its symbionts have established an interdependent and mutually beneficial relationship, with the symbionts being important to the growth, development, reproduction, and variation in virulence of BPH. Yeast-like symbionts (YLS), harbored in the abdomen fat body cells of BPH, are vital to the growth and reproduction of the host. In recent research, the symbionts in BPH have mainly been detected using blood cell counting, PCR, real-time quantitative PCR, and other methods. These methods are vulnerable to external interference, cumbersome, time consuming and laborious. Droplet digital PCR (ddPCR) does not need a standard curve, can achieve absolute quantification, does not rely on Cq values, and is more useful for analyzing copy number variation, gene mutations, and relative gene expression. A rapid detection method for the YLS of BPH based on ddPCR was established and optimized in this study. The results showed that the method's limits of detection for the two species of YLS (Ascomycetes symbionts and Pichia guilliermondii) were 1.3 copies/μL and 1.2 copies/μL, respectively. The coefficient of variation of the sample repetition was less than 5%; therefore, the ddPCR method established in this study had good sensitivity, specificity, and repeatability. It can be used to detect the YLS of BPH rapidly and accurately.

A male-killing Wolbachia endosymbiont is concealed by another endosymbiont and a nuclear suppressor

Bacteria that live inside the cells of insect hosts (endosymbionts) can alter the reproduction of their hosts, including the killing of male offspring (male killing, MK). MK has only been described in a few insects, but this may reflect challenges in detecting MK rather than its rarity. Here, we identify MK Wolbachia at a low frequency (around 4%) in natural populations of Drosophila pseudotakahashii. MK Wolbachia had a stable density and maternal transmission during laboratory culture, but the MK phenotype which manifested mainly at the larval stage was lost rapidly. MK Wolbachia occurred alongside a second Wolbachia strain expressing a different reproductive manipulation, cytoplasmic incompatibility (CI). A genomic analysis highlighted Wolbachia regions diverged between the 2 strains involving 17 genes, and homologs of the wmk and cif genes implicated in MK and CI were identified in the Wolbachia assembly. Doubly infected males induced CI with uninfected females but not females singly infected with CI-causing Wolbachia. A rapidly spreading dominant nuclear suppressor genetic element affecting MK was identified through backcrossing and subsequent analysis with ddRAD SNPs of the D. pseudotakahashii genome. These findings highlight the complexity of nuclear and microbial components affecting MK endosymbiont detection and dynamics in populations and the challenges of making connections between endosymbionts and the host phenotypes affected by them.

One strain may hide another: Cryptic male-killing Wolbachia

While heritable symbionts are common in insects, strains that act as male-killers are considered rare. A new study in PLOS Biology identifies a novel male-killer hidden by coinfection and host resistance, highlighting the complexity of host-microbial interactions in natural systems.

cifB-transcript levels largely explain cytoplasmic incompatibility variation across divergent Wolbachia

Divergent hosts often associate with intracellular microbes that influence their fitness. Maternally transmitted Wolbachia bacteria are the most common of these endosymbionts, due largely to cytoplasmic incompatibility (CI) that kills uninfected embryos fertilized by Wolbachia-infected males. Closely related infections in females rescue CI, providing a relative fitness advantage that drives Wolbachia to high frequencies. One prophage-associated gene (cifA) governs rescue, and two contribute to CI (cifA and cifB), but CI strength ranges from very strong to very weak for unknown reasons. Here, we investigate CI-strength variation and its mechanistic underpinnings in a phylogenetic context across 20 million years (MY) of Wolbachia evolution in Drosophila hosts diverged up to 50 MY. These Wolbachia encode diverse Cif proteins (100% to 7.4% pairwise similarity), and AlphaFold structural analyses suggest that CifB sequence similarities do not predict structural similarities. We demonstrate that cifB-transcript levels in testes explain CI strength across all but two focal systems. Despite phylogenetic discordance among cifs and the bulk of the Wolbachia genome, closely related Wolbachia tend to cause similar CI strengths and transcribe cifB at similar levels. This indicates that other non-cif regions of the Wolbachia genome modulate cif-transcript levels. CI strength also increases with the length of the host's larval life stage, presumably due to prolonged cif action. Our findings reveal that cifB-transcript levels largely explain CI strength, while highlighting other covariates. Elucidating CI's mechanism contributes to our understanding of Wolbachia spread in natural systems and to improving the efficacy of CI-based biocontrol of arboviruses and agricultural pests globally.

Sex-specific distribution and classification of Wolbachia infections and mitochondrial DNA haplogroups in Aedes albopictus from the Indo-Pacific

The arbovirus vector Aedes albopictus (Asian tiger mosquito) is common throughout the Indo-Pacific region, where most global dengue transmission occurs. We analysed population genomic data and tested for cryptic species in 160 Ae. albopictus sampled from 16 locations across this region. We found no evidence of cryptic Ae. albopictus but found multiple intraspecific COI haplotypes partitioned into groups representing three Asian lineages: East Asia, Southeast Asia and Indonesia. Papua New Guinea (PNG), Vanuatu and Christmas Island shared recent coancestry, and Indonesia and Timor-Leste were likely invaded from East Asia. We used a machine learning trained on morphologically sexed samples to classify sexes using multiple genetic features and then characterized the wAlbA and wAlbB Wolbachia infections in 664 other samples. The wAlbA and wAlbB infections as detected by qPCR showed markedly different patterns in the sexes. For females, most populations had a very high double infection incidence, with 67% being the lowest value (from Timor-Leste). For males, the incidence of double infections ranged from 100% (PNG) to 0% (Vanuatu). Only 6 females were infected solely by the wAlbA infection, while rare uninfected mosquitoes were found in both sexes. The wAlbA and wAlbB densities varied significantly among populations. For mosquitoes from Torres Strait and Vietnam, the wAlbB density was similar in single-infected and superinfected (wAlbA and wAlbB) mosquitoes. There was a positive association between wAlbA and wAlbB infection densities in superinfected Ae. albopictus. Our findings provide no evidence of cryptic species of Ae. albopictus in the region and suggest site-specific factors influencing the incidence of Wolbachia infections and their densities. We also demonstrate the usefulness of ddRAD tag depths as sex-specific mosquito markers. The results provide baseline data for the exploitation of Wolbachia-induced cytoplasmic incompatibility (CI) in dengue control.

The mosquito Aedes albopictus transmits dengue and other arboviruses. This study investigates the genetics of these mosquitoes and their endosymbiont Wolbachia in the Indo-Pacific region, where 70% of global dengue transmission occurs. The analysis of mitochondrial DNA sequences showed no evidence of cryptic Ae. albopictus but suggested three Asian lineages: East Asia, Southeast Asia and Indonesia. Papua New Guinea, Vanuatu and Christmas Island shared recent coancestry, and Indonesia and Timor-Leste were likely invaded from East Asia. We used bioinformatics to classify sexes and then characterized the wAlbA and wAlbB Wolbachia infections via both bioinformatics and quantitative PCR. We found markedly different patterns of wAlbA and wAlbB infections in the sexes. The wAlbA and wAlbB densities varied significantly among populations, suggesting site-specific factors influencing the incidence of Wolbachia infections and their densities. We also demonstrate the usefulness of next generation sequencing data in developing molecular markers that can be repeatedly reanalysed to investigate new issues as these arise. These results provide baseline data for the exploitation of Wolbachia-induced cytoplasmic incompatibility in dengue control.

Does diet breadth affect the complexity of the phytophagous insect microbiota? The case study of Chrysomelidae

Chrysomelidae is a family of phytophagous insects with a highly variable degree of trophic specialization. The aim of this study is to test whether species feeding on different plants (generalists) harbour more complex microbiotas than those feeding on a few or a single plant species (specialists). The microbiota of representative leaf beetle species was characterized with a metabarcoding approach targeting V1–V2 and V4 regions of the bacterial 16S rRNA. Almost all the analysed species harbour at least one reproductive manipulator bacteria (e.g., Wolbachia, Rickettsia). Two putative primary symbionts, previously isolated only from a single species (Bromius obscurus), have been detected in two species of the same subfamily, suggesting a widespread symbiosis in Eumolpinae. Surprisingly, the well‐known aphid symbiont Buchnera is well represented in the microbiota of Orsodacne humeralis. Moreover, in this study, using Hill numbers to dissect the components of the microbiota diversity (abundant and rare bacteria), it has been demonstrated that generalist insect species harbour a more diversified microbiota than specialists. The higher microbiota diversity associated with a wider host‐plant spectrum could be seen as an adaptive trait, conferring new metabolic potential useful to expand the diet breath, or as a result of environmental stochastic acquisition conveyed by diet.

High Incidence of Related Wolbachia across Unrelated Leaf-Mining Diptera

Simple Summary

Polyphagous leaf-mining flies of the genus Liriomyza are pests that pose a serious threat to agricultural and horticultural industries. The endosymbiotic bacterium Wolbachia has been proposed as a useful biocontrol strategy for managing pests, but few studies have so far examined Wolbachia in leafminers. We find a high incidence of related Wolbachia in a survey of infections in 13 dipteran leafminer species collected from Australia and elsewhere which could potentially be useful for the incompatible insect technique (IIT) of pest suppression. We performed curing and crossing experiments on L. brassicae to demonstrate the presence of cytoplasmic incompatibility (CI) needed for IIT, providing a foundation for future transfection of CI Wolbachia from L. brassicae to other Liriomyza pests. Overall, these findings highlight a high incidence of Wolbachia in leaf-mining Diptera, potential horizontal transmission events and possible applications of Wolbachia-based biocontrol strategies for Liriomyza pests.

Abstract

The maternally inherited endosymbiont, Wolbachia pipientis, plays an important role in the ecology and evolution of many of its hosts by affecting host reproduction and fitness. Here, we investigated 13 dipteran leaf-mining species to characterize Wolbachia infections and the potential for this endosymbiont in biocontrol. Wolbachia infections were present in 12 species, including 10 species where the Wolbachia infection was at or near fixation. A comparison of Wolbachia relatedness based on the wsp/MLST gene set showed that unrelated leaf-mining species often shared similar Wolbachia, suggesting common horizontal transfer. We established a colony of Liriomyza brassicae and found adult Wolbachia density was stable; although Wolbachia density differed between the sexes, with females having a 20-fold higher density than males. Wolbachia density increased during L. brassicae development, with higher densities in pupae than larvae. We removed Wolbachia using tetracycline and performed reciprocal crosses between Wolbachia-infected and uninfected individuals. Cured females crossed with infected males failed to produce offspring, indicating that Wolbachia induced complete cytoplasmic incompatibility in L. brassicae. The results highlight the potential of Wolbachia to suppress Liriomyza pests based on approaches such as the incompatible insect technique, where infected males are released into populations lacking Wolbachia or with a different incompatible infection.

Prevalence and relationship of endosymbiotic Wolbachia in the butterfly genus Erebia

BACKGROUND

Wolbachia is an endosymbiont common to most invertebrates, which can have significant evolutionary implications for its host species by acting as a barrier to gene flow. Despite the importance of Wolbachia, still little is known about its prevalence and diversification pattern among closely related host species. Wolbachia strains may phylogenetically coevolve with their hosts, unless horizontal host-switches are particularly common. We address  these issues in the genus Erebia, one of the most diverse Palearctic butterfly genera.

RESULTS

We sequenced the Wolbachia genome from a strain infecting Erebia cassioides and showed that it belongs to the Wolbachia supergroup B, capable of infecting arthropods from different taxonomic orders. The prevalence of Wolbachia across 13 closely related Erebia host species based on extensive population-level genetic data revealed that multiple Wolbachia strains jointly infect all investigated taxa, but with varying prevalence. Finally, the phylogenetic relationships of Wolbachia strains are in some cases significantly associated to that of their hosts, especially among the most closely related Erebia species, demonstrating mixed evidence for phylogenetic coevolution.

CONCLUSIONS

Closely related host species can be infected by closely related Wolbachia strains, evidencing some phylogenetic coevolution, but the actual pattern of infection more often reflects historical or contemporary geographic proximity among host species. Multiple processes, including survival in distinct glacial refugia, recent host shifts in sympatry, and a loss of Wolbachia during postglacial range expansion seem to have jointly shaped the complex interactions between Wolbachia evolution and the diversification of its host among our studied Erebia species.

Antagonistic interaction between male-killing and cytoplasmic incompatibility induced by Cardinium and Wolbachia in the whitefly, Bemisia tabaci

Cardinium and Wolbachia are maternally inherited bacterial symbionts of arthropods that can manipulate host reproduction by increasing the fitness of infected females. Here, we report that Cardinium and Wolbachia coinfection induced male-killing and cytoplasmic incompatibility (CI) when they coexisted in a cryptic species of whitefly, Bemisia tabaci Asia II7. Cardinium and Wolbachia symbionts were either singly or simultaneously localized in the bacteriocytes placed in the abdomen of B. tabaci nymphs and adults. Cardinium-Wolbachia coinfection induced male-killing and resulted in a higher female sex ratio in the intraspecific amphigenetic progeny of Asia II7 I_CWH and I_CWL lines; interestingly, male-killing induction was enhanced with increased Cardinium titer. Moreover, single infection of Wolbachia induced partial CI in the Asia II7 I_W line and resulted in reduced fecundity, higher embryonic mortality, and lower female sex ratio. The uninfected Asia II7 I_U line had significantly higher fecundity, lower embryonic and nymphal mortalities, and a lower level of CI than both the Wolbachia-infected Asia II7 I_W line and the Cardinium-Wolbachia-coinfected Asia II7 I_CWH line. Our findings indicate that Cardinium-Wolbachia coinfection induced male-killing, which may have had antagonistic effects on Wolbachia-induced CI in the Asia II7 whiteflies. For the first time, our study revealed that B. tabaci Asia II7 reproduction is co-manipulated by Cardinium and Wolbachia endosymbionts.

Wolbachia Endosymbionts of Fleas Occur in All Females but Rarely in Males and Do Not Show Evidence of Obligatory Relationships, Fitness Effects, or Sex-Distorting Manipulations

The widespread temporal and spatial persistence of endosymbionts in arthropod host populations, despite potential conflicts with their hosts and fluctuating environmental conditions, is puzzling. Here, we disentangled three main mechanisms that are commonly proposed to explain such persistence, namely, obligatory relationships, in which the host is fully dependent on its endosymbiont, fitness advantages conferred by the endosymbiont, and reproductive manipulations imposed by the endosymbiont. Our model system reflects an extreme case, in which the Wolbachia endosymbiont persists in all female flea hosts but rarely in male ones. We cured fleas of both sexes of Wolbachia but found no indications for either lower reproduction, offspring survival, or a change in the offspring sex ratio, compared to Wolbacia-infected fleas. These results do not support any of the suggested mechanisms. We highlight future directions to advance our understanding of endosymbiont persistence in fleas, as well as in other model systems, with extreme sex-differences in endosymbiont persistence. Insights from such studies are predicted to shed light on the evolution and ecology of arthropod-endosymbiont interactions in nature.

Environmental and Genetic Contributions to Imperfect wMel-Like Wolbachia Transmission and Frequency Variation

Maternally transmitted Wolbachia bacteria infect about half of all insect species. They usually show imperfect maternal transmission and often produce cytoplasmic incompatibility (CI). Irrespective of CI, Wolbachia frequencies tend to increase when rare only if they benefit host fitness. Several Wolbachia, including wMel that infects Drosophila melanogaster, cause weak or no CI and persist at intermediate frequencies. On the island of São Tomé off West Africa, the frequencies of wMel-like Wolbachia infecting Drosophila yakuba (wYak) and Drosophila santomea (wSan) fluctuate, and the contributions of imperfect maternal transmission, fitness effects, and CI to these fluctuations are unknown. We demonstrate spatial variation in wYak frequency and transmission on São Tomé. Concurrent field estimates of imperfect maternal transmission do not predict spatial variation in wYak frequencies, which are highest at high altitudes where maternal transmission is the most imperfect. Genomic and genetic analyses provide little support for D. yakuba effects on wYak transmission. Instead, rearing at cool temperatures reduces wYak titer and increases imperfect transmission to levels observed on São Tomé. Using mathematical models of Wolbachia frequency dynamics and equilibria, we infer that temporally variable imperfect transmission or spatially variable effects on host fitness and reproduction are required to explain wYak frequencies. In contrast, spatially stable wSan frequencies are plausibly explained by imperfect transmission, modest fitness effects, and weak CI. Our results provide insight into causes of wMel-like frequency variation in divergent hosts. Understanding this variation is crucial to explain Wolbachia spread and to improve wMel biocontrol of human disease in transinfected mosquito systems.

Cryptic Wolbachia (Rickettsiales: Rickettsiaceae) Detection and Prevalence in Culicoides (Diptera: Ceratopogonidae) Midge Populations in the United States

Culicoides midges vector numerous veterinary and human pathogens. Many of these diseases lack effective therapeutic treatments or vaccines to limit transmission. The only effective approach to limit disease transmission is vector control. However, current vector control for Culicoides midges is complicated by the biology of many Culicoides species and is not always effective at reducing midge populations and impacting disease transmission. The endosymbiont Wolbachia pipientis Hertig may offer an alternative control approach to limit disease transmission and affect Culicoides populations. Here the detection of Wolbachia infections in nine species of Culicoides midges is reported. Infections were detected at low densities using qPCR. Wolbachia infections were confirmed with the sequencing of a partial region of the 16S gene. Fluorescence in situ hybridization of Culicoides sonorensis Wirth and Jones adults and dissected ovaries confirm the presence of Wolbachia infections in an important vector of Bluetongue and Epizootic hemorrhagic disease viruses. The presence of Wolbachia in Culicoides populations in the United States suggests the need for further investigation of Wolbachia as a strategy to limit transmission of diseases vectored by Culicoides midges.

Titer regulation in arthropod-Wolbachia symbioses

Symbiosis between intracellular bacteria (endosymbionts) and animals are widespread. The alphaproteobacterium Wolbachia pipientis is known to maintain a variety of symbiotic associations, ranging from mutualism to parasitism, with a wide range of invertebrates. Wolbachia infection might deeply affect host fitness (e.g. reproductive manipulation and antiviral protection), which is thought to explain its high prevalence in nature. Bacterial loads significantly influence both the infection dynamics and the extent of bacteria-induced host phenotypes. Hence, fine regulation of bacterial titers is considered as a milestone in host-endosymbiont interplay. Here, we review both environmental and biological factors modulating Wolbachia titers in arthropods.

Heatwaves cause fluctuations in wMel Wolbachia densities and frequencies in Aedes aegypti

Aedes aegypti mosquitoes infected with the wMel strain of Wolbachia are being released into natural mosquito populations in the tropics as a way of reducing dengue transmission. High temperatures adversely affect wMel, reducing Wolbachia density and cytoplasmic incompatibility in some larval habitats that experience large temperature fluctuations. We monitored the impact of a 43.6°C heatwave on the wMel infection in a natural population in Cairns, Australia, where wMel was first released in 2011 and has persisted at a high frequency. Wolbachia infection frequencies in the month following the heatwave were reduced to 83% in larvae sampled directly from field habitats and 88% in eggs collected from ovitraps, but recovered to be near 100% four months later. Effects of the heatwave on wMel appeared to be stage-specific and delayed, with reduced frequencies and densities in field-collected larvae and adults reared from ovitraps but higher frequencies in field-collected adults. Laboratory experiments showed that the effects of heatwaves on cytoplasmic incompatibility and density are life stage-specific, with first instar larvae being the most vulnerable to temperature effects. Our results indicate that heatwaves in wMel-infected populations will have only temporary effects on Wolbachia frequencies and density once the infection has established in the population. Our results are relevant to ongoing releases of wMel-infected Ae. aegypti in several tropical countries.

Mosquitoes infected with Wolbachia bacteria are being released in the tropics to replace natural mosquito populations and suppress dengue transmission. Aedes aegypti mosquitoes with the wMel strain of Wolbachia were first released in Cairns, Australia in 2011 and releases were then expanded to the entire city and surrounding suburbs. Today, wMel is at a high frequency within the Ae. aegypti population and local dengue transmission in Cairns has declined to nearly zero. Wolbachia infections are vulnerable to high temperatures and the ability of wMel to persist in populations and block dengue may be constrained by climate. Cairns experienced a record heatwave of 43.6°C in November 2018 and we wanted to see whether this affected the wMel-infected Ae. aegypti population. Our results show that the frequency and density of wMel declined after the heatwave, with effects depending on the mosquito life stage tested. When we monitored the population again in April 2019, wMel had returned to a high frequency. We suggest that heatwaves of the magnitude experienced in Cairns will not have long-term impacts on the wMel infection but may affect invasion during releases or interfere with dengue blockage. Heatwaves may affect interventions with wMel-infected Ae. aegypti that are being deployed in several countries. Effects may depend on the proportion of larval habitats that are protected from extreme temperature fluctuations.

Transfection of Culicoides sonorensis biting midge cell lines with Wolbachia pipientis

Background

Biting midges of the genus Culicoides vector multiple veterinary pathogens and are difficult to control. Endosymbionts particularly Wolbachia pipientis may offer an alternative to control populations of Culicoides and/or impact disease transmission in the form of population suppression or replacement strategies.

Methods

Culicoides sonorensis cell lines were transfected with a Wolbachia infection using a modified shell vial technique. Infections were confirmed using PCR and cell localization using fluorescent in situ hybridization (FISH). The stability of Wolbachia infections and density was determined by qPCR. qPCR was also used to examine immune genes in the IMD, Toll and JACK/STAT pathways to determine if Wolbachia were associated with an immune response in infected cells.

Results

Here we have transfected two Culicoides sonorensis cell lines (W3 and W8) with a Wolbachia infection (walbB) from donor Aedes albopictus Aa23 cells. PCR and FISH showed the presence of Wolbachia infections in both C. sonorensis cell lines. Infection densities were higher in the W8 cell lines when compared to W3. In stably infected cells, genes in the immune Toll, IMD and JAK/STAT pathways were upregulated, along with Attacin and an Attacin-like anti-microbial peptides.

Conclusions

The successful introduction of Wolbachia infections in C. sonorensis cell lines and the upregulation of immune genes, suggest the utility of using Wolbachia for a population replacement and/or population suppression approach to limit the transmission of C. sonorensis vectored diseases. Results support the further investigation of Wolbachia induced pathogen inhibitory effects in Wolbachia-infected C. sonorensis cell lines and the introduction of Wolbachia into C. sonorensis adults via embryonic microinjection to examine for reproductive phenotypes and host fitness effects of a novel Wolbachia infection.

Evolutionary Ecology of Wolbachia Releases for Disease Control

Wolbachia is an endosymbiotic Alphaproteobacteria that can suppress insect-borne diseases through decreasing host virus transmission (population replacement) or through decreasing host population density (population suppression). We contrast natural Wolbachia infections in insect populations with Wolbachia transinfections in mosquitoes to gain insights into factors potentially affecting the long-term success of Wolbachia releases. Natural Wolbachia infections can spread rapidly, whereas the slow spread of transinfections is governed by deleterious effects on host fitness and demographic factors. Cytoplasmic incompatibility (CI) generated by Wolbachia is central to both population replacement and suppression programs, but CI in nature can be variable and evolve, as can Wolbachia fitness effects and virus blocking. Wolbachia spread is also influenced by environmental factors that decrease Wolbachia titer and reduce maternal Wolbachia transmission frequency. More information is needed on the interactions between Wolbachia and host nuclear/mitochondrial genomes, the interaction between invasion success and local ecological factors, and the long-term stability of Wolbachia-mediated virus blocking.

Quantitative methods for assessing local and bodywide contributions to Wolbachia titer in maternal germline cells of Drosophila

BACKGROUND

Little is known about how bacterial endosymbionts colonize host tissues. Because many insect endosymbionts are maternally transmitted, egg colonization is critical for endosymbiont success. Wolbachia bacteria, carried by approximately half of all insect species, provide an excellent model for characterizing endosymbiont infection dynamics. To date, technical limitations have precluded stepwise analysis of germline colonization by Wolbachia. It is not clear to what extent titer-altering effects are primarily mediated by growth rates of Wolbachia within cell lineages or migration of Wolbachia between cells.

RESULTS

The objective of this work is to inform mechanisms of germline colonization through use of optimized methodology. The approaches are framed in terms of nutritional impacts on Wolbachia. Yeast-rich diets in particular have been shown to suppress Wolbachia titer in the Drosophila melanogaster germline. To determine the extent of Wolbachia sensitivity to diet, we optimized 3-dimensional, multi-stage quantification of Wolbachia titer in maternal germline cells. Technical and statistical validation confirmed the identity of Wolbachia in vivo, the reproducibility of Wolbachia quantification and the statistical power to detect these effects. The data from adult feeding experiments demonstrated that germline Wolbachia titer is distinctly sensitive to yeast-rich host diets in late oogenesis. To investigate the physiological basis for these nutritional impacts, we optimized methodology for absolute Wolbachia quantification by real-time qPCR. We found that yeast-rich diets exerted no significant effect on bodywide Wolbachia titer, although ovarian titers were significantly reduced. This suggests that host diets affects Wolbachia distribution between the soma and late stage germline cells. Notably, relative qPCR methods distorted apparent wsp abundance, due to altered host DNA copy number in yeast-rich conditions. This highlights the importance of absolute quantification data for testing mechanistic hypotheses.

CONCLUSIONS

We demonstrate that absolute quantification of Wolbachia, using well-controlled cytological and qPCR-based methods, creates new opportunities to determine how bacterial abundance within the germline relates to bacterial distribution within the body. This methodology can be applied to further test germline infection dynamics in response to chemical treatments, genetic conditions, new host/endosymbiont combinations, or potentially adapted to analyze other cell and tissue types.

Exposure to opposing temperature extremes causes comparable effects on Cardinium density but contrasting effects on Cardinium-induced cytoplasmic incompatibility

Terrestrial arthropods, including insects, commonly harbor maternally inherited intracellular symbionts that confer benefits to the host or manipulate host reproduction to favor infected female progeny. These symbionts may be especially vulnerable to thermal stress, potentially leading to destabilization of the symbiosis and imposing costs to the host. For example, increased temperatures can reduce the density of a common reproductive manipulator, Wolbachia, and the strength of its crossing incompatibility (cytoplasmic incompatibility, or CI) phenotype. Another manipulative symbiont, Cardinium hertigii, infects ~ 6–10% of Arthropods, and also can induce CI, but there is little homology between the molecular mechanisms of CI induced by Cardinium and Wolbachia. Here we investigated whether temperature disrupts the CI phenotype of Cardinium in a parasitic wasp host, Encarsia suzannae. We examined the effects of both warm (32°C day/ 29°C night) and cool (20°C day/ 17°C night) temperatures on Cardinium CI and found that both types of temperature stress modified aspects of this symbiosis. Warm temperatures reduced symbiont density, pupal developmental time, vertical transmission rate, and the strength of both CI modification and rescue. Cool temperatures also reduced symbiont density, however this resulted in stronger CI, likely due to cool temperatures prolonging the host pupal stage. The opposing effects of cool and warm-mediated reductions in symbiont density on the resulting CI phenotype indicates that CI strength may be independent of density in this system. Temperature stress also modified the CI phenotype only if it occurred during the pupal stage, highlighting the likely importance of this stage for CI induction in this symbiosis.

Insects often harbor heritable symbiotic bacteria that infect their cells and/or bodily fluids. These heritable bacteria are passed from mother to offspring and can have substantial effects on host insect biology, and include bacteria like Cardinium that cause mating incompatibilities between symbiont-infected and uninfected individuals. Often, the extent of these symbiont-conferred modifications correlates with the bacterial density in the host. The appearance of these phenotypes is also affected by temperature stress, which often reduces bacterial density. However, here we find that temperature-altered strength of Cardinium-induced mating incompatibility in a whitefly parasitoid wasp can be independent of Cardinium density. While heat treatment reduced the symbiont density and the phenotype, as expected, cold treatment also reduced symbiont density but increased the degree of mating incompatibility. Here, the prolonged duration of the host pupal development in the cold treatments appeared to be more important for phenotype strength. These results suggest that the connection between bacterial density and phenotype strength may not be as general as previously thought. Furthermore, the modification of this manipulative phenotype has implications for the effectiveness of the host, Encarsia suzannae, as a biological control agent.

Effects of Alternative Blood Sources on Wolbachia Infected Aedes aegypti Females within and across Generations

Wolbachia bacteria have been identified as a tool for reducing the transmission of arboviruses transmitted by Aedes aegypti. Research groups around the world are now mass rearing Wolbachia-infected Ae. aegypti for deliberate release. We investigated the fitness impact of a crucial element of mass rearing: the blood meal required by female Ae. aegypti to lay eggs. Although Ae. aegypti almost exclusively feed on human blood, it is often difficult to use human blood in disease-endemic settings. When females were fed on sheep or pig blood rather than human blood, egg hatch rates decreased in all three lines tested (uninfected, or infected by wMel, or wAlbB Wolbachia). This finding was particularly pronounced when fed on sheep blood, although fecundity was not affected. Some of these effects persisted after an additional generation on human blood. Attempts to keep populations on sheep and pig blood sources only partly succeeded, suggesting that strong adaptation is required to develop a stably infected line on an alternative blood source. There was a decrease in Wolbachia density when Ae. aegypti were fed on non-human blood sources. Density increased in lines kept for multiple generations on the alternate sources but was still reduced relative to lines kept on human blood. These findings suggest that sheep and pig blood will entail a cost when used for maintaining Wolbachia-infected Ae. aegypti. These costs should be taken into account when planning mass release programs.