Should Symbionts Be Nice or Selfish? Antiviral Effects of Wolbachia Are Costly but Reproductive Parasitism Is Not

Counting rare Wolbachia endosymbionts using digital droplet PCR

Wolbachia is the most widespread animal-associated intracellular microbe, living within the cells of over half of insect species. Since they can suppress pathogen replication and spread rapidly through insect populations, Wolbachia is at the vanguard of public health initiatives to control mosquito-borne diseases. Wolbachia's abilities to block pathogens and spread quickly are closely linked to their abundance in host tissues. The most common method for counting Wolbachia is quantitative polymerase chain reaction (qPCR), yet qPCR can be insufficient to count rare Wolbachia, necessitating tissue pooling and consequently compromising individual-level resolution of Wolbachia dynamics. Digital droplet PCR (ddPCR) offers superior sensitivity, enabling the detection of rare targets and eliminating the need for sample pooling. Here, we report three ddPCR assays to measure total Wolbachia abundance, Wolbachia abundance adjusted for DNA extraction efficiency, and Wolbachia density relative to host genome copies. Using Drosophila melanogaster with wMel Wolbachia as a model, we show these ddPCR assays can reliably detect as few as 7 to 12 Wolbachia gene copies in a 20 µL reaction. The designed oligos are homologous to sequences from at least 106 Wolbachia strains across supergroup A and 53 host species from the Drosophila, Scaptomyza, and Zaprionus genera, suggesting broad utility. These highly sensitive ddPCR assays are expected to significantly advance Wolbachia-host interactions research by enabling the collection of molecular data from individual insect tissues. Their ability to detect rare Wolbachia will be especially valuable in applied and natural field settings where pooling samples could obscure important variation.IMPORTANCEWolbachia bacteria live inside the cells of many animals, especially insects. In many insect species, almost every individual carries Wolbachia. How common Wolbachia becomes within a population often depends on how much of it is present in the insect's body. Therefore, accurately measuring Wolbachia levels is crucial for understanding how these bacteria interact with their hosts and spread. However, traditional molecular assays can lack the sensitivity needed for accurate, individual-level quantification of rare Wolbachia. Here, we present three highly sensitive digital droplet PCR assays for Wolbachia detection, offering superior sensitivity compared to existing methods. These assays will be useful for studies that measure Wolbachia abundance and related phenotypes in individual insects, providing enhanced resolution and improving efforts to characterize the mechanisms that govern phenotypic variation.

Wolbachia Offers Protection Against Two Common Natural Viruses of Drosophila

Wolbachia pipientis is a maternally transmitted endosymbiont infecting more than half of terrestrial arthropod species. Wolbachia can express parasitic phenotypes such as manipulation of host reproduction and mutualist phenotypes such as protection against RNA virus infections. Because Wolbachia can invade populations by reproductive manipulation and block virus infection, it is used to modify natural insect populations. However, the ecological importance of virus protection is not yet clear, especially due to scarce information on Wolbachia protection against viruses that are common in nature. We used systemic infection to investigate whether Wolbachia protects its host by suppressing the titer of DMELDAV and DMelNora virus, two viruses that commonly infect Drosophila melanogaster flies in natural populations. Antiviral protection was tested in three systems to assess the impact of Wolbachia strains across species: (1) a panel of Wolbachia strains transfected into Drosophila simulans, (2) two Wolbachia strains introgressed into the natural host D. melanogaster, and (3) two native Wolbachia strains in their natural hosts Drosophila baimaii and Drosophila tropicalis. We showed that certain Wolbachia strains provide protection against DMelNora virus and DMELDAV, and this protection is correlated with Wolbachia density, which is consistent with what has been observed in protection against other RNA viruses. Additionally, we found that Wolbachia does not protect its original host, D. melanogaster, from DMELDAV infection. While native Wolbachia can reduce DMELDAV titers in D. baimaii, this effect was not detected in D. tropicalis. Although the Wolbachia protection-induced phenotype seems to depend on the virus, the specific Wolbachia strain, and the host species, our findings suggest that antiviral protection may be one of the mutualistic effects that helps explain why Wolbachia is so widespread in arthropod populations.

Development of the Incompatible Insect Technique targeting Aedes albopictus: introgression of a wild nuclear background restores the performance of males artificially infected with Wolbachia

The bacterium Wolbachia pipientis is increasingly studied for its potential use in controlling insect vectors or pests due to its ability to induce Cytoplasmic Incompatibility (CI). CI can be exploited by establishing an opportunistic Wolbachia infection in a targeted insect species through trans-infection and then releasing the infected males into the environment as sterilizing agents. Several host life history traits (LHT) have been reported to be negatively affected by artificial Wolbachia infection. Wolbachia is often considered the causative agent of these detrimental effects, and the importance of the host's genetic origins in the outcome of trans-infection is generally overlooked. In this study, we investigated the impact of host genetic background using an Aedes albopictus line recently trans-infected with wPip from the Culex pipiens mosquito, which exhibited some fitness costs. We measured several LHTs including fecundity, egg hatch rate, and male mating competitiveness in the incompatible line after four rounds of introgression aiming at restoring genetic diversity in the nuclear genome. Our results show that introgression with a wild genetic background restored most fitness traits and conferred mating competitiveness comparable to that of wild males. Finally, we show that introgression leads to faster and stronger population suppression under laboratory conditions. Overall, our data support that the host genome plays a decisive role in determining the fitness of Wolbachia-infected incompatible males.IMPORTANCEThe bacterium Wolbachia pipientis is increasingly used to control insect vectors and pests through the Incompatible Insect Technique (IIT) inducing a form of conditional sterility when a Wolbachia-infected male mates with an uninfected or differently infected female. Wolbachia artificial trans-infection has been repeatedly reported to affect mosquitoes LHTs, which may in turn compromise the efficiency of IIT. Using a tiger mosquito (Aedes albopictus) line recently trans-infected with a Wolbachia strain from Culex pipiens and displaying reduced fitness, we show that restoring genetic diversity through introgression significantly mitigated the fitness costs associated with Wolbachia trans-infection. This was further demonstrated through experimental population suppression, showing that introgression is required to achieve mosquito population suppression under laboratory conditions. These findings are significant for the implementation of IIT programs, as an increase in female fecundity and male performance improves mass rearing productivity as well as the sterilizing capacity of released males.

Taming of the microbial beasts: Plant immunity tethers potentially pathogenic microbiota members

Plants are in intimate association with taxonomically structured microbial communities called the plant microbiota. There is growing evidence that the plant microbiota contributes to the holistic performance and general health of plants, especially under unfavorable situations. Despite the attached benefits, surprisingly, the plant microbiota in nature also includes potentially pathogenic strains, signifying that the plant hosts have tight control over these microbes. Despite the conceivable role of plant immunity in regulating its microbiota, we lack a complete understanding of its role in governing the assembly, maintenance, and function of the plant microbiota. Here, we highlight the recent progress on the mechanistic relevance of host immunity in orchestrating plant-microbiota dialogues and discuss the pluses and perils of these microbial assemblies.

Counting rare Wolbachia endosymbionts using digital droplet PCR

Wolbachia is the most widespread animal-associated intracellular microbe, living within the cells of over half of insect species. Since they can suppress pathogen replication and spread rapidly through insect populations, Wolbachia is at the vanguard of public health initiatives to control mosquito-borne diseases. Wolbachia's abilities to block pathogens and spread quickly are closely linked to their abundance in host tissues. The most common method for counting Wolbachia is quantitative polymerase chain reaction (qPCR), yet qPCR can be insufficient to count rare Wolbachia, necessitating tissue pooling and consequently compromising individual-level resolution of Wolbachia dynamics. Digital droplet PCR (ddPCR) offers superior sensitivity, enabling the detection of rare targets and eliminating the need for sample pooling. Here, we report three ddPCR assays to measure total Wolbachia abundance, Wolbachia abundance adjusted for DNA extraction efficiency, and Wolbachia density relative to host genome copies. Using Drosophila melanogaster with wMel Wolbachia as a model, we show these ddPCR assays can reliably detect as few as 7 to 12 Wolbachia gene copies in a 20 μL reaction. The designed oligos are homologous to sequences from at least 106 Wolbachia strains across Supergroup A and 53 host species from the Drosophila, Scaptomyza, and Zaprionus genera, suggesting broad utility. These highly sensitive ddPCR assays are expected to significantly advance Wolbachia-host interactions research by enabling the collection of molecular data from individual insect tissues. Their ability to detect rare Wolbachia will be especially valuable in applied and natural field settings where pooling samples could obscure important variation.

Wolbachia in Antarctic terrestrial invertebrates: Absent or undiscovered?

Interactions between a host organism and its associated microbiota, including symbiotic bacteria, play a crucial role in host adaptation to changing environmental conditions. Antarctica provides a unique environment for the establishment and maintenance of symbiotic relationships. One of the most extensively studied symbiotic bacteria in invertebrates is Wolbachia pipientis, which is associated with a wide variety of invertebrates. Wolbachia is known for manipulating host reproduction and having obligate or facultative mutualistic relationships with various hosts. However, there is a lack of clear understanding of the prevalence of Wolbachia in terrestrial invertebrates in Antarctica. We present the outcomes of a literature search for information on the occurrence of Wolbachia in each of the major taxonomic groups of terrestrial invertebrates (Acari, Collembola, Diptera, Rotifera, Nematoda, Tardigrada). We also performed profiling of prokaryotes based on three marker genes and Kraken2 in available whole genome sequence data obtained from Antarctic invertebrate samples. We found no reports or molecular evidence of Wolbachia in these invertebrate groups in Antarctica. We discuss possible reasons underlying this apparent absence and suggest opportunities for more targeted future research to confirm bacteria's presence or absence.

Modelling suggests Wolbachia-induced cytoplasmic incompatibility in oak gall wasps with cyclical parthenogenesis

Oak gall wasps typically exhibit a life cycle with one sexual and one asexual generation each year. These wasps can carry various endosymbionts, one of which is the maternally inherited bacterium Wolbachia that can induce several reproductive manipulations on its host. Cytoplasmic incompatibility (CI) has been described as the most prominent of these manipulations. CI leads to embryonic mortality in the hosts' offspring when infected males mate with either uninfected females or with females that harbour different Wolbachia strains. It has been hypothesized that Wolbachia can induce CI in oak gall wasps. To address this hypothesis, we derived a mathematical model to investigate the spread of a bacterial infection in naive populations and to determine the plausibility of CI occurrence. To validate our model, we used published data from Wolbachia-infected Belonocnema kinseyi populations in two approaches. Our first approach uses measurements of infection frequencies and maternal transmission in the sexual generation. For the second approach, we extended the model to compare predictions to estimates of mtDNA-haplotypes, which, like Wolbachia, are maternally inherited, and can therefore be associated with the infection. Both approaches indicate that CI is present in these populations. Our model can be generalized to investigate the occurrence of CI not only for oak gall wasps but also for other species.

Rapid host switching of Wolbachia and even more rapid turnover of their phages and incompatibility-causing loci

About half of all insect species carry maternally inherited Wolbachia alphaproteobacteria, making Wolbachia the most common endosymbionts known in nature. Often Wolbachia spread to high frequencies within populations due to cytoplasmic incompatibility (CI), a Wolbachia-induced sperm modification caused by prophage-associated genes (cifs) that kill embryos without Wolbachia. Several Wolbachia variants also block viruses, including wMel from Drosophila melanogaster when transinfected into the mosquito Aedes aegypti. CI enables the establishment and stable maintenance of pathogen-blocking wMel in natural Ae. aegypti populations. These transinfections are reducing dengue disease incidence on multiple continents. While it has long been known that closely related Wolbachia occupy distantly related hosts, the timing of Wolbachia host switching and molecular evolution has not been widely quantified. We provide a new, conservative calibration for Wolbachia chronograms based on examples of co-divergence of Wolbachia and their insect hosts. Synthesizing publicly available and new genomic data, we use our calibration to demonstrate that wMel-like variants separated by only about 370,000 years have naturally colonized holometabolous dipteran and hymenopteran insects that diverged approximately 350 million years ago. Data from Wolbachia variants closely related to those currently dominant in D. melanogaster and D. simulans illustrate that cifs are rapidly acquired and lost among Wolbachia genomes, on a time scale of 104-105 years. This turnover occurs with and without the Wovirus prophages that contain them, with closely related cifs found in distantly related phages and distantly related cifs found in closely related phages. We present evidence for purifying selection on CI rescue function and on particular Cif protein domains. Our results quantify the tempo and mode of rapid host switching and horizontal gene transfer that underlie the spread and diversity of Wolbachia sampled from diverse host species. The wMel variants we highlight from hosts in different climates may offer new options for broadening Wolbachia-based biocontrol of diseases and pests.

Wolbachia endosymbionts in Drosophila regulate the resistance to Zika virus infection in a sex dependent manner

Drosophila melanogaster has been used extensively for dissecting the genetic and functional bases of host innate antiviral immunity and virus-induced pathology. Previous studies have shown that the presence of Wolbachia endosymbionts in D. melanogaster confers resistance to infection by certain viral pathogens. Zika virus is an important vector-borne pathogen that has recently expanded its range due to the wide geographical distribution of the mosquito vector. Here, we describe the effect of Wolbachia on the immune response of D. melanogaster adult flies following Zika virus infection. First, we show that the presence of Wolbachia endosymbionts promotes the longevity of uninfected D. melanogaster wild type adults and increases the survival response of flies following Zika virus injection. We find that the latter effect is more pronounced in females rather than in males. Then, we show that the presence of Wolbachia regulates Zika virus replication during Zika virus infection of female flies. In addition, we demonstrate that the antimicrobial peptide-encoding gene Drosocin and the sole Jun N-terminal kinase-specific MAPK phosphatase Puckered are upregulated in female adult flies, whereas the immune and stress response gene TotM is upregulated in male individuals. Finally, we find that the activity of RNA interference and Toll signaling remain unaffected in Zika virus-infected female and male adults containing Wolbachia compared to flies lacking the endosymbionts. Our results reveal that Wolbachia endosymbionts in D. melanogaster affect innate immune signaling activity in a sex-specific manner, which in turn influences host resistance to Zika virus infection. This information contributes to a better understanding of the complex interrelationship between insects, their endosymbiotic bacteria, and viral infection. Interpreting these processes will help us design more effective approaches for controlling insect vectors of infectious disease.

Using Baker's Yeast to Determine Functions of Novel Wolbachia (and Other Prokaryotic) Effectors

Yeasts are single-celled eukaryotic organisms classified as fungi, mostly in the phylum Ascomycota. Of about 1500 named species, Saccharomyces cerevisiae, also known as baker's yeast, domesticated by humans in the context of cooking and brewing, is a profound genetic tool for exploring functions of novel effector proteins from Wolbachia and prokaryotes in general. Wolbachia is a Gram-negative alpha-proteobacterium that infects up to ~75% of all insects as an obligate intracellular microbe (Jeyaprakash A, Hoy MA, Insect Mol Biol 9:393-405, 2000). Wolbachia's lifestyle presents unique challenges for researchers. Wolbachia cannot be axenically cultured and has never been genetically manipulated. Furthermore, many Wolbachia genes have no known function or well-annotated orthologs in other genomes. Yet given the effects of Wolbachia on host phenotypes, which have considerable practical applications for pest control, they undoubtedly involve secreted effector proteins that interact with host gene products. Studying these effectors is challenging with Wolbachia's current genetic limitations. However, some of the constraints to working with Wolbachia can be overcome by expressing candidate proteins in S. cerevisiae. This approach capitalizes on yeast's small genome (~6500 genes), typical eukaryotic cellular organization, and the sophisticated suite of genetic tools available for its manipulation in culture. Thus, yeast can serve as a powerful mock eukaryotic host background to study Wolbachia effector function. Specifically, yeast is used for recombinant protein expression, drug discovery, protein localization studies, protein interaction mapping (yeast two-hybrid system), modeling chromosomal evolution, and examining interactions between proteins responsible for complex phenotypes in less tractable prokaryotic systems. As an example, the paired genes responsible for Wolbachia-mediated cytoplasmic incompatibility (CI) encode novel proteins with limited homology to other known proteins, and no obvious function. This article details how S. cerevisiae was used as an initial staging ground to explore the molecular basis of one of Wolbachia's trademark phenotypes (CI).

Drosophila melanogaster Transcriptome Response to Different Wolbachia Strains

Wolbachia is a maternally inherited, intercellular bacterial symbiont of insects and some other invertebrates. Here, we investigated the effect of two different Wolbachia strains, differing in a large chromosomal inversion, on the differential expression of genes in D. melanogaster females. We revealed significant changes in the transcriptome of the infected flies compared to the uninfected ones, as well as in the transcriptome of flies infected with the Wolbachia strain, wMelPlus, compared to flies infected with the wMelCS112 strain. We linked differentially expressed genes (DEGs) from two pairwise comparisons, "uninfected-wMelPlus-infected" and "uninfected-wMelCS112-infected", into two gene networks, in which the following functional groups were designated: "Proteolysis", "Carbohydrate transport and metabolism", "Oxidation-reduction process", "Embryogenesis", "Transmembrane transport", "Response to stress" and "Alkaline phosphatases". Our data emphasized similarities and differences between infections by different strains under study: a wMelPlus infection results in more than double the number of upregulated DEGs and half the number of downregulated DEGs compared to a wMelCS112 infection. Thus, we demonstrated that Wolbachia made a significant contribution to differential expression of host genes and that the bacterial genotype plays a vital role in establishing the character of this contribution.

A theoretical model for host-controlled regulation of symbiont density

There is growing empirical evidence that animal hosts actively control the density of their mutualistic symbionts according to their requirements. Such active regulation can be facilitated by compartmentalization of symbionts within host tissues, which confers a high degree of control of the symbiosis to the host. Here, we build a general theoretical framework to predict the underlying ecological drivers and evolutionary consequences of host-controlled endosymbiont density regulation for a mutually obligate association between a host and a compartmentalized, vertically transmitted symbiont. Building on the assumption that the costs and benefits of hosting a symbiont population increase with symbiont density, we use state-dependent dynamic programming to determine an optimal strategy for the host, i.e., that which maximizes host fitness, when regulating the density of symbionts. Simulations of active host-controlled regulation governed by the optimal strategy predict that the density of the symbiont should converge to a constant level during host development, and following perturbation. However, a similar trend also emerges from alternative strategies of symbiont regulation. The strategy which maximizes host fitness also promotes symbiont fitness compared to alternative strategies, suggesting that active host-controlled regulation of symbiont density could be adaptive for the symbiont as well as the host. Adaptation of the framework allowed the dynamics of symbiont density to be predicted for other host-symbiont ecologies, such as for non-essential symbionts, demonstrating the versatility of this modelling approach.

Wolbachia protects Drosophila melanogaster against two naturally occurring and virulent viral pathogens

Wolbachia is a common endosymbiont that can protect insects against viral pathogens. However, whether the antiviral effects of Wolbachia have a significant effect on fitness remains unclear. We have investigated the interaction between Drosophila melanogaster, Wolbachia and two viruses that we recently isolated from wild flies, La Jolla virus (LJV; Iflaviridae) and Newfield virus (NFV; Permutotetraviridae). Flies infected with these viruses have increased mortality rates, and NFV partially sterilizes females. These effects on fitness were reduced in Wolbachia-infected flies, and this was associated with reduced viral titres. However, Wolbachia alone also reduces survival, and under our experimental conditions these costs of the symbiont can outweigh the benefits of antiviral protection. In contrast, protection against the sterilizing effect of NFV leads to a net benefit of Wolbachia infection after exposure to the virus. These results support the hypothesis that Wolbachia is an important defense against the natural pathogens of D. melanogaster. Furthermore, by reducing the cost of Wolbachia infection, the antiviral effects of Wolbachia may aid its invasion into populations and help explain why it is so common in nature.

Complex effects of environment and Wolbachia infections on the life history of Drosophila melanogaster hosts

Wolbachia bacteria are common endosymbionts of many arthropods found in gonads and various somatic tissues. They manipulate host reproduction to enhance their transmission and confer complex effects on fitness-related traits. Some of these effects can serve to increase the survival and transmission efficiency of Wolbachia in the host population. The Wolbachia-Drosophila melanogaster system represents a powerful model to study the evolutionary dynamics of host-microbe interactions and infections. Over the past decades, there has been a replacement of the ancestral wMelCS Wolbachia variant by the more recent wMel variant in worldwide D. melanogaster populations, but the reasons remain unknown. To investigate how environmental change and genetic variation of the symbiont affect host developmental and adult life-history traits, we compared effects of both Wolbachia variants and uninfected controls in wild-caught D. melanogaster strains at three developmental temperatures. While Wolbachia did not influence any developmental life-history traits, we found that both lifespan and fecundity of host females were increased without apparent fitness trade-offs. Interestingly, wMelCS-infected flies were more fecund than uninfected and wMel-infected flies. By contrast, males infected with wMel died sooner, indicating sex-specific effects of infection that are specific to the Wolbachia variant. Our study uncovered complex temperature-specific effects of Wolbachia infections, which suggests that symbiont-host interactions in nature are strongly dependent on the genotypes of both partners and the thermal environment.

Cross-tissue and generation predictability of relative Wolbachia densities in the mosquito Aedes aegypti

BACKGROUND

The insect endosymbiotic bacterium Wolbachia is being deployed in field populations of the mosquito Aedes aegypti for biological control. This microbe prevents the replication of human disease-causing viruses inside the vector, including dengue, Zika and chikungunya. Relative Wolbachia densities may in part predict the strength of this 'viral blocking' effect. Additionally, Wolbachia densities may affect the strength of the reproductive manipulations it induces, including cytoplasmic incompatibility (CI), maternal inheritance rates or induced fitness effects in the insect host. High rates of CI and maternal inheritance and low rates of fitness effects are also key to the successful spreading of Wolbachia through vector populations and its successful use in biocontrol. The factors that control Wolbachia densities are not completely understood.

METHODS

We used quantitative PCR-based methods to estimate relative density of the Wolbachia wAlbB strain in both the somatic and reproductive tissues of adult male and female mosquitoes, as well as in eggs. Using correlation analyses, we assessed whether densities in one tissue predict those in others within the same individual, but also across generations.

RESULTS

We found little relationship among the relative Wolbachia densities of different tissues in the same host. The results also show that there was very little relationship between Wolbachia densities in parents and those in offspring, both in the same and different tissues. The one exception was with ovary-egg relationships, where there was a strong positive association. Relative Wolbachia densities in reproductive tissues were always greater than those in the somatic tissues. Additionally, the densities were consistent in females over their lifetime regardless of tissue, whereas they were generally higher and more variable in males, particularly in the testes.

CONCLUSIONS

Our results indicate that either stochastic processes or local tissue-based physiologies are more likely factors dictating Wolbachia densities in Ae. aegypti individuals, rather than shared embryonic environments or heritable genetic effects of the mosquito genome. These findings have implications for understanding how relative Wolbachia densities may evolve and/or be maintained over the long term in Ae. aegypti.

Why and how do protective symbionts impact immune priming with pathogens in invertebrates?

Growing evidence demonstrates that invertebrates display adaptive-like immune abilities, commonly known as "immune priming". Immune priming is a process by which a host improves its immune defences following an initial pathogenic exposure, leading to better protection after a subsequent infection with the same - or different - pathogens. Nevertheless, beneficial symbionts can enhance similar immune priming processes in hosts, such as when they face repeated infections with pathogens. This "symbiotic immune priming" protects the host against pathogenic viruses, bacteria, fungi, or eukaryotic parasites. In this review, we explore the extent to which protective symbionts interfere and impact immune priming against pathogens from both a mechanical (proximal) and an evolutionary (ultimate) point of view. We highlight that the immune priming of invertebrates is the cornerstone of the tripartite interaction of hosts/symbionts/pathogens. The main shared mechanism of immune priming (induced by symbionts or pathogens) is the sustained immune response at the beginning of host-microbial interactions. However, the evolutionary outcome of immune priming leads to a specific discrimination, which provides enhanced tolerance or resistance depending on the type of microbe. Based on several studies testing immune priming against pathogens in the presence or absence of protective symbionts, we observed that both types of immune priming could overlap and affect each other inside the same hosts. As protective symbionts could be an evolutionary force that influences immune priming, they may help us to better understand the heterogeneity of pathogenic immune priming across invertebrate populations and species.

A single mutation weakens symbiont-induced reproductive manipulation through reductions in deubiquitylation efficiency

Animals interact with microbes that affect their performance and fitness, including endosymbionts that reside inside their cells. Maternally transmitted Wolbachia bacteria are the most common known endosymbionts, in large part because of their manipulation of host reproduction. For example, many Wolbachia cause cytoplasmic incompatibility (CI) that reduces host embryonic viability when Wolbachia-modified sperm fertilize uninfected eggs. Operons termed cifs control CI, and a single factor (cifA) rescues it, providing Wolbachia-infected females a fitness advantage. Despite CI's prevalence in nature, theory indicates that natural selection does not act to maintain CI, which varies widely in strength. Here, we investigate the genetic and functional basis of CI-strength variation observed among sister Wolbachia that infect Drosophila melanogaster subgroup hosts. We cloned, Sanger sequenced, and expressed cif repertoires from weak CI-causing wYak in Drosophila yakuba, revealing mutations suspected to weaken CI relative to model wMel in D. melanogaster A single valine-to-leucine mutation within the deubiquitylating (DUB) domain of the wYak cifB homolog (cidB) ablates a CI-like phenotype in yeast. The same mutation reduces both DUB efficiency in vitro and transgenic CI strength in the fly, each by about twofold. Our results map hypomorphic transgenic CI to reduced DUB activity and indicate that deubiquitylation is central to CI induction in cid systems. We also characterize effects of other genetic variation distinguishing wMel-like cifs Importantly, CI strength determines Wolbachia prevalence in natural systems and directly influences the efficacy of Wolbachia biocontrol strategies in transinfected mosquito systems. These approaches rely on strong CI to reduce human disease.

The impact of artificial selection for Wolbachia-mediated dengue virus blocking on phage WO

Wolbachia is currently at the forefront of global efforts to control arbovirus transmission from the vector Aedes aegypti. The use of Wolbachia relies on two phenotypes—cytoplasmic incompatibility (CI), conferred by cifA and cifB genes in prophage WO, and Wolbachia-mediated pathogen blocking (WMPB). These traits allow for local, self-sustaining reductions in transmission of dengue (DENV) following release of Wolbachia-infected A. aegypti. Here, aided by previous artificial selection experiment that generated Low and High pathogen blocking lines, we examined the potential link between WMPB and phage WO. We found no evidence that Wolbachia or phage WO relative densities predict DENV blocking strength across selected lines. However, selection resulted in reduced phage WO relative density for the Low WMPB line. The Low blocking line was previously shown to have reduced fitness as a result of selection. Through subsequent genomic analyses, we demonstrate that SNP variation underpinning selection for low blocking led to elevated frequency of potential deleterious SNPs on chromosome 1. The key region on chromosome 1 contains genes relating to cell cycle regulation, oxidative stress, transcriptional pausing, among others, that may have cascading effects on Wolbachia intracellular environment. We hypothesize that reduction in phage WO may be driven by changes in the loci directly under selection for blocking, or by the accumulation of predicted deleterious alleles in linkage disequilibrium with blocking loci resulting from hitchhiking. For the Low line with fewer phage WO, we also detected reduced expression of cifA and cifB CI genes, with patterns of expression varying between somatic and reproductive tissues. In conclusion, we propose that artificial selection for WMPB trait had corresponding impacts on phage WO densities, and also the transcription of CI-causing genes. Future studies may include a more detailed analysis of the regions the A. aegypti chromosome 1’s ability to affect WMPB and other Wolbachia-associated intrinsic factors such as phage WO.

Wolbachia are widespread endosymbiotic bacteria of insects that cause Wolbachia-mediated pathogen blocking (WMPB) and cytoplasmic incompatibility (CI). The latter mediated by cif genes localized in the prophage WO region. Because of that, Wolbachia-infected mosquitoes are currently being used in field to fight the transmission of vector-borne viruses such as Dengue (DENV) to human populations. Aided by a previous artificial selection experiment that generated lines with variable (High and Low) DENV blocking strength, we tested for a potential link between WMPB and phage WO. There was no evidence that Wolbachia nor phage WO densities predict DENV blocking strength. However, we found that the Low blocking line had reduced phage WO density, and lower expression of the cif genes in a tissue-specific manner. We demonstrate that in addition to previous report of reduced fitness, the Low blocking line also exhibited increased frequency of potential deleterious SNPs on chromosome 1. Our hypotheses are that reduction in phage WO may have resulted from changes in the loci directly under selection for blocking, or by linkage disequilibrium events linked to the accumulation of mosquito predicted deleterious alleles. Our results highlight the importance of chromosome 1 for WMPB and its potential impact for other Wolbachia-associated factors like phage WO.

In Vivo Microbial Coevolution Favors Host Protection and Plastic Downregulation of Immunity

Microbiota can protect their hosts from infection. The short timescales in which microbes can evolve presents the possibility that “protective microbes” can take-over from the immune system of longer-lived hosts in the coevolutionary race against pathogens. Here, we found that coevolution between a protective bacterium (Enterococcus faecalis) and a virulent pathogen (Staphylococcus aureus) within an animal population (Caenorhabditis elegans) resulted in more disease suppression than when the protective bacterium adapted to uninfected hosts. At the same time, more protective E. faecalis populations became costlier to harbor and altered the expression of 134 host genes. Many of these genes appear to be related to the mechanism of protection, reactive oxygen species production. Crucially, more protective E. faecalis populations downregulated a key immune gene, , known to be effective against S. aureus infection. These results suggest that a microbial line of defense is favored by microbial coevolution and may cause hosts to plastically divest of their own immunity.

Comparative Genomics Reveals Factors Associated with Phenotypic Expression of Wolbachia

Wolbachia is a widespread, vertically transmitted bacterial endosymbiont known for manipulating arthropod reproduction. Its most common form of reproductive manipulation is cytoplasmic incompatibility (CI), observed when a modification in the male sperm leads to embryonic lethality unless a compatible rescue factor is present in the female egg. CI attracts scientific attention due to its implications for host speciation and in the use of Wolbachia for controlling vector-borne diseases. However, our understanding of CI is complicated by the complexity of the phenotype, whose expression depends on both symbiont and host factors. In the present study, we perform a comparative analysis of nine complete Wolbachia genomes with known CI properties in the same genetic host background, Drosophila simulans STC. We describe genetic differences between closely related strains and uncover evidence that phages and other mobile elements contribute to the rapid evolution of both genomes and phenotypes of Wolbachia. Additionally, we identify both known and novel genes associated with the modification and rescue functions of CI. We combine our observations with published phenotypic information and discuss how variability in cif genes, novel CI-associated genes, and Wolbachia titer might contribute to poorly understood aspects of CI such as strength and bidirectional incompatibility. We speculate that high titer CI strains could be better at invading new hosts already infected with a CI Wolbachia, due to a higher rescue potential, and suggest that titer might thus be a relevant parameter to consider for future strategies using CI Wolbachia in biological control.

Host microbiota can facilitate pathogen infection

Animals live in symbiosis with numerous microbe species. While some can protect hosts from infection and benefit host health, components of the microbiota or changes to the microbial landscape have the potential to facilitate infections and worsen disease severity. Pathogens and pathobionts can exploit microbiota metabolites, or can take advantage of a depletion in host defences and changing conditions within a host, to cause opportunistic infection. The microbiota might also favour a more virulent evolutionary trajectory for invading pathogens. In this review, we consider the ways in which a host microbiota contributes to infectious disease throughout the host's life and potentially across evolutionary time. We further discuss the implications of these negative outcomes for microbiota manipulation and engineering in disease management.

Intraspecific variation in immune gene expression and heritable symbiont density

Host genetic variation plays an important role in the structure and function of heritable microbial communities. Recent studies have shown that insects use immune mechanisms to regulate heritable symbionts. Here we test the hypothesis that variation in symbiont density among hosts is linked to intraspecific differences in the immune response to harboring symbionts. We show that pea aphids (Acyrthosiphon pisum) harboring the bacterial endosymbiont Regiella insecticola (but not all other species of symbionts) downregulate expression of key immune genes. We then functionally link immune expression with symbiont density using RNAi. The pea aphid species complex is comprised of multiple reproductively-isolated host plant-adapted populations. These ‘biotypes’ have distinct patterns of symbiont infections: for example, aphids from the Trifolium biotype are strongly associated with Regiella. Using RNAseq, we compare patterns of gene expression in response to Regiella in aphid genotypes from multiple biotypes, and we show that Trifolium aphids experience no downregulation of immune gene expression while hosting Regiella and harbor symbionts at lower densities. Using F1 hybrids between two biotypes, we find that symbiont density and immune gene expression are both intermediate in hybrids. We propose that in this system, Regiella symbionts are suppressing aphid immune mechanisms to increase their density, but that some hosts have adapted to prevent immune suppression in order to control symbiont numbers. This work therefore suggests that antagonistic coevolution can play a role in host-microbe interactions even when symbionts are transmitted vertically and provide a clear benefit to their hosts. The specific immune mechanisms that we find are downregulated in the presence of Regiella have been previously shown to combat pathogens in aphids, and thus this work also highlights the immune system’s complex dual role in interacting with both beneficial and harmful microbes.

Insects frequently form beneficial partnerships with heritable microbes that are passed from mothers to offspring. Natural populations exhibit a great deal of variation in the frequency of heritable microbes and in the within-host density of these infections. Uncovering the mechanisms underlying variation in host-microbe interactions is key to understanding how they evolve. We study a model host-microbe interaction: the pea aphid and a heritable bacterium that makes aphids resistant to fungal pathogens. We show that aphids harboring bacteria show sharply reduced expression of innate immune system genes, and that this leads to increased densities of symbionts. We further show that populations of aphids that live on different species of plants vary in differential immune gene expression and in the density of their symbiont infections. This study contributes to our mechanistic understanding of an important model of host-microbe symbiosis and suggests that hosts and heritable microbes are evolving antagonistically. This work also sheds light on how invertebrate immune systems evolve to manage the complex task of combatting harmful pathogens while accommodating potentially beneficial microbes.

Meta-analysis suggests the microbiome responds to Evolve and Resequence experiments in Drosophila melanogaster

BACKGROUND

Experimental evolution has a long history of uncovering fundamental insights into evolutionary processes, but has largely neglected one underappreciated component--the microbiome. As eukaryotic hosts evolve, the microbiome may also respond to selection. However, the microbial contribution to host evolution remains poorly understood. Here, we re-analyzed genomic data to characterize the metagenomes from ten Evolve and Resequence (E&R) experiments in Drosophila melanogaster to determine how the microbiome changed in response to host selection.

RESULTS

Bacterial diversity was significantly different in 5/10 studies, primarily in traits associated with metabolism or immunity. Duration of selection did not significantly influence bacterial diversity, highlighting the importance of associations with specific host traits.

CONCLUSIONS

Our genomic re-analysis suggests the microbiome often responds to host selection; thus, the microbiome may contribute to the response of Drosophila in E&R experiments. We outline important considerations for incorporating the microbiome into E&R experiments. The E&R approach may provide critical insights into host-microbiome interactions and fundamental insight into the genomic basis of adaptation.

Intraspecific variation in symbiont density in an insect-microbe symbiosis

Many insects host vertically transmitted microbes, which can confer benefits to their hosts but are costly to maintain and regulate. A key feature of these symbioses is variation: for example, symbiont density can vary among host and symbiont genotypes. However, the evolutionary forces maintaining this variation remain unclear. We studied variation in symbiont density using the pea aphid (Acyrthosiphon pisum) and the bacterium Regiella insecticola, a symbiont that can protect its host against fungal pathogens. We found that relative symbiont density varies both between two Regiella phylogenetic clades and among aphid "biotypes." Higher density symbiont infections are correlated with stronger survival costs, but variation in density has little effect on the protection Regiella provides against fungi. Instead, we found that in some aphid genotypes, a dramatic decline in symbiont density precedes the loss of a symbiont infection. Together, our data suggest that the optimal density of a symbiont infection is likely different from the perspective of aphid and microbial fitness. Regiella might prevent loss by maintaining high within-host densities, but hosts do not appear to benefit from higher symbiont numbers and may be advantaged by losing costly symbionts in certain environments. The standing variation in symbiont density observed in natural populations could therefore be maintained by antagonistic coevolutionary interactions between hosts and their symbiotic microbes.

The Antiviral Effects of the Symbiont Bacteria Wolbachia in Insects

Wolbachia is a maternally transmitted bacterium that lives inside arthropod cells. Historically, it was viewed primarily as a parasite that manipulates host reproduction, but more recently it was discovered that Wolbachia can also protect Drosophila species against infection by RNA viruses. Combined with Wolbachia's ability to invade insect populations due to reproductive manipulations, this provides a way to modify mosquito populations to prevent them transmitting viruses like dengue. In this review, we discuss the main advances in the field since Wolbachia's antiviral effect was discovered 12 years ago, identifying current research gaps and potential future developments. We discuss that the antiviral effect works against a broad range of RNA viruses and depends on the Wolbachia lineage. We describe what is known about the mechanisms behind viral protection, and that recent studies suggest two possible mechanisms: activation of host immunity or competition with virus for cellular resources. We also discuss how association with Wolbachia may influence the evolution of virus defense on the insect host genome. Finally, we investigate whether the antiviral effect occurs in wild insect populations and its ecological relevance as a major antiviral component in insects.

The Role of Bacterial Symbionts in Triatomines: An Evolutionary Perspective

Insects have established mutualistic symbiotic interactions with microorganisms that are beneficial to both host and symbiont. Many insects have exploited these symbioses to diversify and expand their ecological ranges. In the Hemiptera (i.e., aphids, cicadas, and true bugs), symbioses have established and evolved with obligatory essential microorganisms (primary symbionts) and with facultative beneficial symbionts (secondary symbionts). Primary symbionts are usually intracellular microorganisms found in insects with specialized diets such as obligate hematophagy or phytophagy. Most Heteroptera (true bugs), however, have gastrointestinal (GI) tract extracellular symbionts with functions analogous to primary endosymbionts. The triatomines, are vectors of the human parasite, Trypanosoma cruzi. A description of their small GI tract microbiota richness was based on a few culturable microorganisms first described almost a century ago. A growing literature describes more complex interactions between triatomines and bacteria with properties characteristic of both primary and secondary symbionts. In this review, we provide an evolutionary perspective of beneficial symbioses in the Hemiptera, illustrating the context that may drive the evolution of symbioses in triatomines. We highlight the diversity of the triatomine microbiota, bacterial taxa with potential to be beneficial symbionts, the unique characteristics of triatomine-bacteria symbioses, and the interactions among trypanosomes, microbiota, and triatomines.

Multiple phenotypes conferred by a single insect symbiont are independent

Many microbial symbionts have multiple phenotypic consequences for their animal hosts. However, the ways in which different symbiont-mediated phenotypes combine to affect fitness are not well understood. We investigated whether there are correlations between different symbiont-mediated phenotypes. We used the symbiont Spiroplasma, a striking example of a bacterial symbiont conferring diverse phenotypes on insect hosts. We took 11 strains of Spiroplasma infecting pea aphids (Acyrthosiphon pisum) and assessed their ability to provide protection against the fungal pathogen Pandora neoaphidis and the parasitoids Aphidius ervi and Praon volucre. We also assessed effects on male offspring production for five of the Spiroplasma strains. All but one of the Spiroplasma strains provided very strong protection against the parasitoid P. volucre. As previously reported, variable protection against P. neoaphidis and A. ervi was also present; male-killing was likewise a variable phenotype. We find no evidence of any correlation, positive or negative, between the different phenotypes, nor was there any evidence of an effect of symbiont phylogeny on protective phenotype. We conclude that multiple symbiont-mediated phenotypes can evolve independently from one another without trade-offs between them.

Wolbachia megadiversity: 99% of these microorganismic manipulators unknown

Wolbachia (Alphaproteobacteria) are the most widespread endosymbionts of arthropods, manipulating their hosts by various means to maximize the number of host individuals infected. Based on quantitative analyzes of the published literature from Web of Science® and of DNA sequences of arthropod-hosted Wolbachia from GenBank, we made plausible that less than 1% of the expected 100 000 strains of Wolbachia in arthropods is known. Our findings suggest that more and globally better coordinated efforts in screening arthropods are needed to explore the true Wolbachia diversity and to help us understand the ecology and evolution of these host-endosymbiont interactions.

A Review: Wolbachia-Based Population Replacement for Mosquito Control Shares Common Points with Genetically Modified Control Approaches

The growing expansion of mosquito vectors has made mosquito-borne arboviral diseases a global threat to public health, and the lack of licensed vaccines and treatments highlight the urgent need for efficient mosquito vector control. Compared to genetically modified control strategies, the intracellular bacterium Wolbachia, endowing a pathogen-blocking phenotype, is considered an environmentally friendly strategy to replace the target population for controlling arboviral diseases. However, the incomplete knowledge regarding the pathogen-blocking mechanism weakens the reliability of a Wolbachia-based population replacement strategy. Wolbachia infections are also vulnerable to environmental factors, temperature, and host diet, affecting their densities in mosquitoes and thus the virus-blocking phenotype. Here, we review the properties of the Wolbachia strategy as an approach to control mosquito populations in comparison with genetically modified control methods. Both strategies tend to limit arbovirus infections but increase the risk of selecting arbovirus escape mutants, rendering these strategies less reliable.

Long-Term Dynamics Among Wolbachia Strains During Thermal Adaptation of Their Drosophila melanogaster Hosts

Climate change is a major evolutionary force triggering thermal adaptation in a broad range of species. While the consequences of global warming are being studied for an increasing number of species, limited attention has been given to the evolutionary dynamics of endosymbionts in response to climate change. Here, we address this question by studying the dynamics of Wolbachia, a well-studied endosymbiont of Drosophila melanogaster. D. melanogaster populations infected with 13 different Wolbachia strains were exposed to novel hot and cold laboratory environments for up to 180 generations. The short-term dynamics suggested a temperature-related fitness difference resulting in the increase of clade V strains in the cold environment only. Our long-term analysis now uncovers that clade V dominates in all replicates after generation 60 irrespective of temperature treatment. We propose that adaptation of the Drosophila host to either temperature or Drosophila C virus (DCV) infection are the cause of the replicated, temporally non-concordant Wolbachia dynamics. Our study provides an interesting case demonstrating that even simple, well-controlled experiments can result in complex, but repeatable evolutionary dynamics, thus providing a cautionary note on too simple interpretations on the impact of climate change.

Wolbachia affect behavior and possibly reproductive compatibility but not thermoresistance, fecundity, and morphology in a novel transinfected host, Drosophila nigrosparsa

Wolbachia, intracellular endosymbionts, are estimated to infect about half of all arthropod species. These bacteria manipulate their hosts in various ways for their maximum benefits. The rising global temperature may accelerate species migration, and thus, horizontal transfer of Wolbachia may occur across species previously not in contact. We transinfected and then cured the alpine fly Drosophila nigrosparsa with Wolbachia strain wMel to study its effects on this species. We found low Wolbachia titer, possibly cytoplasmic incompatibility, and an increase in locomotion of both infected larvae and adults compared with cured ones. However, no change in fecundity, no impact on heat and cold tolerance, and no change in wing morphology were observed. Although Wolbachia increased locomotor activities in this species, we conclude that D. nigrosparsa may not benefit from the infection. Still, D. nigrosparsa can serve as a host for Wolbachia because vertical transmission is possible but may not be as high as in the native host of wMel, Drosophila melanogaster.

Virus evolution in Wolbachia-infected Drosophila

Wolbachia, a common vertically transmitted symbiont, can protect insects against viral infection and prevent mosquitoes from transmitting viral pathogens. For this reason, Wolbachia-infected mosquitoes are being released to prevent the transmission of dengue and other arboviruses. An important question for the long-term success of these programmes is whether viruses can evolve to escape the antiviral effects of Wolbachia. We have found that Wolbachia altered the outcome of competition between strains of the DCV virus in Drosophila. However, Wolbachia still effectively blocked the virus genotypes that were favoured in the presence of the symbiont. We conclude that Wolbachia did cause an evolutionary response in viruses, but this has little or no impact on the effectiveness of virus blocking.

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.

When the exception becomes the rule: An integrative approach to symbiosis

Symbiosis, mainly due to the advances in -omics technology and to the microbiome revolution, is being increasingly acknowledged as fundamental to explain any aspect of life existence. Previously considered an exception, a peculiar characteristic of few systems like lichens, corals and mycorrhizas, symbiosis is nowadays recognized as the rule, with the microbiome being part of all living entities and systems. However, our knowledge of the ecological meaning and functioning of many symbiotic systems is still limited. Here, we discuss a new, integrative approach based on current findings that looks at commonalities among symbiotic systems to produce theoretical models and conceptual knowledge that would allow a more efficient exploitation of symbiosis-based biotechnologies. The microbiome recruitment and assemblage processes are indicated as one of the potential targets where a holistic approach could bring advantages. Finally, we reflect on the potential socio-economic and environmental consequences of a symbiotic view of the world, where co-dependence is the matrix of life.

Contrasting Patterns of Virus Protection and Functional Incompatibility Genes in Two Conspecific Wolbachia Strains from Drosophila pandora

Wolbachia infections can present different phenotypes in hosts, including different forms of reproductive manipulation and antiviral protection, which may influence infection dynamics within host populations. In populations of Drosophila pandora two distinct Wolbachia strains coexist, each manipulating host reproduction: strain wPanCI causes cytoplasmic incompatibility (CI), whereas strain wPanMK causes male killing (MK). CI occurs when a Wolbachia-infected male mates with a female not infected with a compatible type of Wolbachia, leading to nonviable offspring. wPanMK can rescue wPanCI-induced CI but is unable to induce CI. The antiviral protection phenotypes provided by the wPanCI and wPanMK infections were characterized; the strains showed differential protection phenotypes, whereby cricket paralysis virus (CrPV)-induced mortality was delayed in flies infected with wPanMK but enhanced in flies infected with wPanCI compared to their respective Wolbachia-cured counterparts. Homologs of the cifA and cifB genes involved in CI identified in wPanMK and wPanCI showed a high degree of conservation; however, the CifB protein in wPanMK is truncated and is likely nonfunctional. The presence of a likely functional CifA in wPanMK and wPanMK's ability to rescue wPanCI-induced CI are consistent with the recent confirmation of CifA's involvement in CI rescue, and the absence of a functional CifB protein further supports its involvement as a CI modification factor. Taken together, these findings indicate that wPanCI and wPanMK have different relationships with their hosts in terms of their protective and CI phenotypes. It is therefore likely that different factors influence the prevalence and dynamics of these coinfections in natural Drosophila pandora hosts.IMPORTANCEWolbachia strains are common endosymbionts in insects, with multiple strains often coexisting in the same species. The coexistence of multiple strains is poorly understood but may rely on Wolbachia organisms having diverse phenotypic effects on their hosts. As Wolbachia is increasingly being developed as a tool to control disease transmission and suppress pest populations, it is important to understand the ways in which multiple Wolbachia strains persist in natural populations and how these might then be manipulated. We have therefore investigated viral protection and the molecular basis of cytoplasmic incompatibility in two coexisting Wolbachia strains with contrasting effects on host reproduction.

Symbiont-Driven Male Mating Success in the Neotropical Drosophila paulistorum Superspecies

Microbial symbionts are ubiquitous associates of living organisms but their role in mediating reproductive isolation (RI) remains controversial. We addressed this knowledge gap by employing the Drosophila paulistorum-Wolbachia model system. Semispecies in the D. paulistorum species complex exhibit strong RI between each other and knockdown of obligate mutualistic Wolbachia bacteria in female D. paulistorum flies triggers loss of assortative mating behavior against males carrying incompatible Wolbachia strains. Here we set out to determine whether de novo RI can be introduced by Wolbachia-knockdown in D. paulistorum males. We show that Wolbachia-knockdown D. paulistorum males (i) are rejected as mates by wild type females, (ii) express altered sexual pheromone profiles, and (iii) are devoid of the endosymbiont in pheromone producing cells. Our findings suggest that changes in Wolbachia titer and tissue tropism can induce de novo premating isolation by directly or indirectly modulating sexual behavior of their native D. paulistorum hosts.

Combining Wolbachia-induced sterility and virus protection to fight Aedes albopictus-borne viruses

Among the strategies targeting vector control, the exploitation of the endosymbiont Wolbachia to produce sterile males and/or invasive females with reduced vector competence seems to be promising. A new Aedes albopictus transinfection (ARwP-M) was generated by introducing wMel Wolbachia in the ARwP line which had been established previously by replacing wAlbA and wAlbB Wolbachia with the wPip strain. Various infection and fitness parameters were studied by comparing ARwP-M, ARwP and wild-type (SANG population) Ae. albopictus sharing the same genetic background. Moreover, the vector competence of ARwP-M related to chikungunya, dengue and zika viruses was evaluated in comparison with ARwP. ARwP-M showed a 100% rate of maternal inheritance of wMel and wPip Wolbachia. Survival, female fecundity and egg fertility did not show to differ between the three Ae. albopictus lines. Crosses between ARwP-M males and SANG females were fully unfertile regardless of male age while egg hatch in reverse crosses increased from 0 to about 17% with SANG males aging from 3 to 17 days. When competing with SANG males for SANG females, ARwP-M males induced a level of sterility significantly higher than that expected for an equal mating competitiveness (mean Fried index of 1.71 instead of 1). The overall Wolbachia density in ARwP-M females was about 15 fold higher than in ARwP, mostly due to the wMel infection. This feature corresponded to a strongly reduced vector competence for chikungunya and dengue viruses (in both cases, 5 and 0% rates of transmission at 14 and 21 days post infection) with respect to ARwP females. Results regarding Zika virus did not highlight significant differences between ARwP-M and ARwP. However, none of the tested ARwP-M females was capable at transmitting ZIKV. These findings are expected to promote the exploitation of Wolbachia to suppress the wild-type Ae. albopictus populations.

Hosts do not simply outsource pathogen resistance to protective symbionts

Microbial symbionts commonly protect their hosts from natural enemies, but it is unclear how protective symbionts influence the evolution of host immunity to pathogens. One possibility is that 'extrinsic' protection provided by symbionts allows hosts to reduce investment in 'intrinsic' immunological resistance mechanisms. We tested this idea using pea aphids (Acyrthosiphon pisum) and their facultative bacterial symbionts that increase host resistance to the fungal pathogen Pandora neoaphidis. The pea aphid taxon is composed of multiple host plant associated populations called biotypes, which harbor characteristic communities of symbionts. We found that biotypes that more frequently carry protective symbionts have higher, rather than lower, levels of intrinsic resistance. Within a biotype there was no difference in intrinsic resistance between clones that did and did not carry a protective symbiont. The host plant on which an aphid feeds did not strongly influence intrinsic resistance. We describe a simple conceptual model of the interaction between intrinsic and extrinsic resistance and suggest that our results may be explained by selection favoring both the acquisition of protective symbionts and enhanced intrinsic resistance in habitats with high pathogen pressure. Such combined protection is potentially more robust than intrinsic resistance alone.

Mitochondria and Wolbachia titers are positively correlated during maternal transmission

Mothers provide their offspring with symbionts. Maternally transmitted, intracellular symbionts must disperse from mother to offspring with other cytoplasmic elements, like mitochondria. Here, we investigated how the intracellular symbiont Wolbachia interacts with mitochondria during maternal transmission. Mitochondria and Wolbachia may interact antagonistically and compete as each population tries to ensure its own evolutionary success. Alternatively, mitochondria and Wolbachia may cooperate as both benefit from ensuring the fitness of the mother. We characterized the relationship between mitochondria and Wolbachia titers in ovaries of Drosophila melanogaster. We found that mitochondria and Wolbachia titers are positively correlated in common laboratory genotypes of D. melanogaster. We attempted to perturb this covariation through the introduction of Wolbachia variants that colonize at different titers. We also attempted to perturb the covariation through manipulating the female reproductive tract to disrupt maternal transmission. Finally, we also attempted to disrupt the covariation by knocking down gene expression for two loci involved in mitochondrial metabolism: NADH dehydrogenase and a mitochondrial transporter. Overall, we find that mitochondria and Wolbachia titers are commonly positively correlated, but this positive covariation is disrupted at high titers of Wolbachia. Our results suggest that mitochondria and Wolbachia have likely evolved mechanisms to stably coexist, but the competitive dynamics change at high Wolbachia titers. We provide future directions to better understand how their interaction influences the maintenance of the symbiosis.

The role of defensive symbionts in host-parasite coevolution

Understanding the coevolution of hosts and parasites is a long-standing goal of evolutionary biology. There is a well-developed theoretical framework to describe the evolution of host-parasite interactions under the assumption of direct, two-species interactions, which can result in arms race dynamics or sustained genotype fluctuations driven by negative frequency dependence (Red Queen dynamics). However, many hosts rely on symbionts for defence against parasites. Whilst the ubiquity of defensive symbionts and their potential importance for disease control are increasingly recognized, there is still a gap in our understanding of how symbionts mediate or possibly take part in host-parasite coevolution. Herein we address this question by synthesizing information already available from theoretical and empirical studies. First, we briefly introduce current hypotheses on how defensive mutualisms evolved from more parasitic relationships and highlight exciting new experimental evidence showing that this can occur very rapidly. We go on to show that defensive symbionts influence virtually all important determinants of coevolutionary dynamics, namely the variation in host resistance available to selection by parasites, the specificity of host resistance, and the trade-off structure between host resistance and other components of fitness. In light of these findings, we turn to the limited theory and experiments available for such three-species interactions to assess the role of defensive symbionts in host-parasite coevolution. Specifically, we discuss under which conditions the defensive symbiont may take over from the host the reciprocal adaptation with parasites and undergo its own selection dynamics, thereby altering or relaxing selection on the hosts' own immune defences. Finally, we address potential effects of defensive symbionts on the evolution of parasite virulence. This is an important problem for which there is no single, clear-cut prediction. The selection on parasite virulence resulting from the presence of defensive symbionts in their hosts will depend on the underlying mechanism of defence. We identify the evolutionary predictions for different functional categories of symbiont-conferred resistance and we evaluate the empirical literature for supporting evidence. We end this review with outstanding questions and promising avenues for future research to improve our understanding of symbiont-mediated coevolution between hosts and parasites.

Natural Variation in Resistance to Virus Infection in Dipteran Insects

The power and ease of Drosophila genetics and the medical relevance of mosquito-transmitted viruses have made dipterans important model organisms in antiviral immunology. Studies of virus-host interactions at the molecular and population levels have illuminated determinants of resistance to virus infection. Here, we review the sources and nature of variation in antiviral immunity and virus susceptibility in model dipteran insects, specifically the fruit fly Drosophila melanogaster and vector mosquitoes of the genera Aedes and Culex. We first discuss antiviral immune mechanisms and describe the virus-specificity of these responses. In the following sections, we review genetic and microbiota-dependent variation in antiviral immunity. In the final sections, we explore less well-studied sources of variation, including abiotic factors, sexual dimorphism, infection history, and endogenous viral elements. We borrow from work on other pathogen types and non-dipteran species when it parallels or complements studies in dipterans. Understanding natural variation in virus-host interactions may lead to the identification of novel restriction factors and immune mechanisms and shed light on the molecular determinants of vector competence.

Evolution of reproductive parasites with direct fitness benefits

Maternally inherited symbionts such as Wolbachia have long been seen mainly as reproductive parasites, with deleterious effects on host fitness. It is becoming clear, however, that, frequently, these symbionts also have beneficial effects on host fitness, either along with reproductive parasitism or not. Using the examples of cytoplasmic incompatibility (CI) and male-killing (MK), we here analyze the effect of direct fitness benefits on the evolution of reproductive parasites. By means of a simple theoretical framework, we synthesize and extend earlier modeling approaches for CI and MK, which usually ignore fitness benefits. Moreover, our framework is not restricted to a particular mechanism underlying the fitness benefit (e.g., protection against pathogens). We derive invasion conditions and equilibrium frequencies for the different infection scenarios. Our results demonstrate the importance of a symbiont's "effective fecundity" (i.e., the product of the relative fecundity of an infected female and her transmission efficiency) for a symbiont's invasion success. In particular, we adopt the concept of effective fecundity to scenarios where CI and MK co-occur in one host population. We confirm that direct fitness benefits substantially facilitate the invasion and spread of infections (for example, by lowering or removing the invasion threshold) or even make invasion possible in the first place (for example, if reproductive parasitism is weak or absent). Finally, we discuss the role of direct fitness benefits in long-term evolutionary dynamics of reproductive phenotypes and highlight their potential to resolve genetic conflicts between maternally inherited symbionts and their hosts.