The bacterial symbiont Wolbachia induces resistance to RNA viral infections in Drosophila melanogaster

Symbiotic microbial studies in diverse populations of Aphis gossypii, existing on altered host plants in different localities during different times

Complex interactions between symbiotic bacteria and insects ultimately result in equilibrium in all aspects of life in natural insect populations. In this study, abundance of principal symbiotic bacteria was estimated using qPCR in 1553 individuals of aphids, Aphis gossypii. Aphids were sampled from primary and secondary host plants-hibiscus and cotton. Hibiscus aphids were collected from 24 different locations in April, September, and November, whereas cotton aphids were collected between 2015 and 2017 from areas with wide variations in climatic conditions. About 30%-45% aphids were recorded with the most dominant symbiont, Arsenophonus. The other symbionts were in low frequency, and about 7% of aphids were noted with Hamiltonella, Acinetobacter, and Microbacterium, and 3% of aphids were verified with Serratia and Pseudomonas. Aphids infected with Hamiltonella, Arsenophonus, and Serratia can influence Buchnera densities. Hamiltonella has positive interaction with densities of Arsenophonus and Serratia. Almost 100% coinfection of Hamiltonella and Arsenophonus was detected in Xinxiang aphids and 50% coinfection was reported in aphids from North China, while no coinfection was detected in Hainan aphids. These findings describe the prevalence pattern and richness of core community of symbiotic bacteria in naturally occurring populations of A. gossypii and provide new insights for the study of symbiotic bacteria.

Designing effective Wolbachia release programs for mosquito and arbovirus control

Mosquitoes carrying endosymbiotic bacteria called Wolbachia are being released in mosquito and arbovirus control programs around the world through two main approaches: population suppression and population replacement. Open field releases of Wolbachia-infected male mosquitoes have achieved over 95% population suppression by reducing the fertility of wild mosquito populations. The replacement of populations with Wolbachia-infected females is self-sustaining and can greatly reduce local dengue transmission by reducing the vector competence of mosquito populations. Despite many successful interventions, significant questions and challenges lie ahead. Wolbachia, viruses and their mosquito hosts can evolve, leading to uncertainty around the long-term effectiveness of a given Wolbachia strain, while few ecological impacts of Wolbachia releases have been explored. Wolbachia strains are diverse and the choice of strain to release should be made carefully, taking environmental conditions and the release objective into account. Mosquito quality control, thoughtful community awareness programs and long-term monitoring of populations are essential for all types of Wolbachia intervention. Releases of Wolbachia-infected mosquitoes show great promise, but existing control measures remain an important way to reduce the burden of mosquito-borne disease.

Wolbachia-driven selective sweep in a range expanding insect species

Background

Evolutionary processes can cause strong spatial genetic signatures, such as local loss of genetic diversity, or conflicting histories from mitochondrial versus nuclear markers. Investigating these genetic patterns is important, as they may reveal obscured processes and players. The maternally inherited bacterium Wolbachia is among the most widespread symbionts in insects. Wolbachia typically spreads within host species by conferring direct fitness benefits, and/or by manipulating its host reproduction to favour infected over uninfected females. Under sufficient selective advantage, the mitochondrial haplotype associated with the favoured maternally-inherited symbiotic strains will spread (i.e. hitchhike), resulting in low mitochondrial genetic variation across the host species range.

Method

The common bluetail damselfly (Ischnura elegans: van der Linden, 1820) has recently emerged as a model organism for genetics and genomic signatures of range expansion during climate change. Although there is accumulating data on the consequences of such expansion on the genetics of I. elegans, no study has screened for Wolbachia in the damselfly genus Ischnura. Here, we present the biogeographic variation in Wolbachia prevalence and penetrance across Europe and Japan (including samples from 17 populations), and from close relatives in the Mediterranean area (i.e. I. genei: Rambur, 1842; and I. saharensis: Aguesse, 1958).

Results

Our data reveal (a) multiple Wolbachia-strains, (b) potential transfer of the symbiont through hybridization, (c) higher infection rates at higher latitudes, and (d) reduced mitochondrial diversity in the north-west populations, indicative of hitchhiking associated with the selective sweep of the most common strain. We found low mitochondrial haplotype diversity in the Wolbachia-infected north-western European populations (Sweden, Scotland, the Netherlands, Belgium, France and Italy) of I. elegans, and, conversely, higher mitochondrial diversity in populations with low penetrance of Wolbachia (Ukraine, Greece, Montenegro and Cyprus). The timing of the selective sweep associated with infected lineages was estimated between 20,000 and 44,000 years before present, which is consistent with the end of the last glacial period about 20,000 years.

Conclusions

Our findings provide an example of how endosymbiont infections can shape spatial variation in their host evolutionary genetics during postglacial expansion. These results also challenge population genetic studies that do not consider the prevalence of symbionts in many insects, which we show can impact geographic patterns of mitochondrial genetic diversity.

Persistent Spodoptera frugiperda rhabdovirus infection in Sf9 cells is not restricted by Wolbachia wMelPop-CLA and wAlbB strains and is targeted by the RNAi machinery

The endosymbiotic bacterium Wolbachia pipientis confers RNA virus refractoriness in Drosophila and Aedes mosquitoes. Questions remain about the Wolbachia-virus restriction phenotype and how extensive this phenomenon may be within other arthropods. Here, we generated two Spodoptera frugiperda cell lines stably transinfected with two strains of Wolbachia, wAlbB and wMelPop-CLA. Despite the high density of Wolbachia in stably infected Sf9 cells, RT-PCR indicated the presence of the negative-sense RNA virus Spodoptera frugiperda rhabdovirus (SfRV) in Wolbachia-infected and uninfected cell lines. No differences in the replication of SfRV between Sf9 and Wolbachia-infected cells was found. RNA-Seq analysis of the parental Sf9 cells supported SfRV's presence in these cells with abundant 20 nt virus-derived small RNAs indicating active replication of SfRV in these cells. Overall, this study supports a growing body of evidence that Wolbachia does not restrict negative-sense RNA viruses and generates an in vitro model to examine Lepidoptera-Wolbachia virus interactions.

Frequent Drivers, Occasional Passengers: Signals of Symbiont-Driven Seasonal Adaptation and Hitchhiking in the Pea Aphid, Acyrthosiphon pisum

Insects harbor a variety of maternally inherited bacterial symbionts. As such, variation in symbiont presence/absence, in the combinations of harbored symbionts, and in the genotypes of harbored symbiont species provide heritable genetic variation of potential use in the insects' adaptive repertoires. Understanding the natural importance of symbionts is challenging but studying their dynamics over time can help to elucidate the potential for such symbiont-driven insect adaptation. Toward this end, we studied the seasonal dynamics of six maternally transferred bacterial symbiont species in the multivoltine pea aphid (Acyrthosiphon pisum). Our sampling focused on six alfalfa fields in southeastern Pennsylvania, and spanned 14 timepoints within the 2012 growing season, in addition to two overwintering periods. To test and generate hypotheses on the natural relevance of these non-essential symbionts, we examined whether symbiont dynamics correlated with any of ten measured environmental variables from the 2012 growing season, including some of known importance in the lab. We found that five symbionts changed prevalence across one or both overwintering periods, and that the same five species underwent such frequency shifts across the 2012 growing season. Intriguingly, the frequencies of these dynamic symbionts showed robust correlations with a subset of our measured environmental variables. Several of these trends supported the natural relevance of lab-discovered symbiont roles, including anti-pathogen defense. For a seventh symbiont-Hamiltonella defensa-studied previously across the same study periods, we tested whether a reported correlation between prevalence and temperature stemmed not from thermally varying host-level fitness effects, but from selection on co-infecting symbionts or on aphid-encoded alleles associated with this bacterium. In general, such "hitchhiking" effects were not evident during times with strongly correlated Hamiltonella and temperature shifts. However, we did identify at least one time period in which Hamiltonella spread was likely driven by selection on a co-infecting symbiont-Rickettsiella viridis. Recognizing the broader potential for such hitchhiking, we explored selection on co-infecting symbionts as a possible driver behind the dynamics of the remaining six species. Out of twelve examined instances of symbiont dynamics unfolding across 2-week periods or overwintering spans, we found eight in which the focal symbiont underwent parallel frequency shifts under single infection and one or more co-infection contexts. This supported the idea that phenotypic variation created by the presence/absence of individual symbionts is a direct target for selection, and that symbiont effects can be robust under co-habitation with other symbionts. Contrastingly, in two cases, we found that selection may target phenotypes emerging from symbiont co-infections, with specific species combinations driving overall trends for the focal dynamic symbionts, without correlated change under single infection. Finally, in three cases-including the one described above for Hamiltonella-our data suggested that incidental co-infection with a (dis)favored symbiont could lead to large frequency shifts for "passenger" symbionts, conferring no apparent cost or benefit. Such hitchhiking has rarely been studied in heritable symbiont systems. We propose that it is more common than appreciated, given the widespread nature of maternally inherited bacteria, and the frequency of multi-species symbiotic communities across insects.

wMel Wolbachia genome remains stable after 7 years in Australian Aedes aegypti field populations

Infection of wMel Wolbachia in Aedes aegypti imparts two signature features that enable its application for biocontrol of dengue. First, the susceptibility of mosquitoes to viruses such as dengue and Zika is reduced. Second, a reproductive manipulation is caused that enables wMel introgression into wild-type mosquito populations. The long-term success of this method relies, in part, on evolution of the wMel genome not compromising the critical features that make it an attractive biocontrol tool. This study compared the wMel Wolbachia genome at the time of initial releases and 1-7 years post-release in Cairns, Australia. Our results show the wMel genome remains highly conserved up to 7 years post-release in gene sequence, content, synteny and structure. This work suggests the wMel genome is stable in its new mosquito host and, therefore, provides reassurance on the potential for wMel to deliver long-term public-health impacts.

Infections by Transovarially Transmitted DMelSV in Drosophila Have No Impact on Ovarian Transposable Element Transcripts but Increase Their Amounts in the Soma

Transposable elements (TEs) are genomic parasites, which activity is tightly controlled in germline cells. Using Sindbis virus, it was recently demonstrated that viral infections affect TE transcript amounts in somatic tissues. However, the strongest evolutionary impacts are expected in gonads, because that is where the genomes of the next generations lie. Here, we investigated this aspect using the Drosophila melanogaster Sigma virus. It is particularly relevant in the genome/TE interaction given its tropism to ovaries, which is the organ displaying the more sophisticated TE control pathways. Our results in Drosophila simulans flies allowed us to confirm the existence of a strong homeostasis of the TE transcriptome in ovaries upon infection, which, however, rely on TE-derived small RNA modulations. In addition, we performed a meta-analysis of RNA-seq data and propose that the immune pathway that is triggered upon viral infection determines the direction of TE transcript modulation in somatic tissues.

Genome analyses of four Wolbachia strains and associated mitochondria of Rhagoletis cerasi expose cumulative modularity of cytoplasmic incompatibility factors and cytoplasmic hitchhiking across host populations

Background

The endosymbiont Wolbachia can manipulate arthropod reproduction and invade host populations by inducing cytoplasmic incompatibility (CI). Some host species are coinfected with multiple Wolbachia strains which may have sequentially invaded host populations by expressing different types of modular CI factor (cif) genes. The tephritid fruit fly Rhagoletis cerasi is a model for CI and Wolbachia population dynamics. It is associated with at least four Wolbachia strains in various combinations, with demonstrated (wCer2, wCer4), predicted (wCer1) or unknown (wCer5) CI phenotypes.

Results

We sequenced and assembled the draft genomes of the Wolbachia strains wCer1, wCer4 and wCer5, and compared these with the previously sequenced genome of wCer2 which currently invades R. cerasi populations. We found complete cif gene pairs in all strains: four pairs in wCer2 (three Type I; one Type V), two pairs in wCer1 (both Type I) and wCer4 (one Type I; one Type V), and one pair in wCer5 (Type IV). Wolbachia genome variant analyses across geographically and genetically distant host populations revealed the largest diversity of single nucleotide polymorphisms (SNPs) in wCer5, followed by wCer1 and then wCer2, indicative of their different lengths of host associations. Furthermore, mitogenome analyses of the Wolbachia genome-sequenced individuals in combination with SNP data from six European countries revealed polymorphic mitogenome sites that displayed reduced diversity in individuals infected with wCer2 compared to those without.

Conclusions

Coinfections with Wolbachia are common in arthropods and affect options for Wolbachia-based management strategies of pest and vector species already infected by Wolbachia. Our analyses of Wolbachia genomes of a host naturally coinfected by several strains unravelled signatures of the evolutionary dynamics in both Wolbachia and host mitochondrial genomes as a consequence of repeated invasions. Invasion of already infected populations by new Wolbachia strains requires new sets of functionally different cif genes and thereby may select for a cumulative modularity of cif gene diversity in invading strains. Furthermore, we demonstrated at the mitogenomic scale that repeated CI-driven Wolbachia invasions of hosts result in reduced mitochondrial diversity and hitchhiking effects. Already resident Wolbachia strains may experience similar cytoplasmic hitchhiking effects caused by the invading Wolbachia strain.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-021-07906-6.

Molecular population genetics of Sex-lethal (Sxl) in the Drosophila melanogaster species group: a locus that genetically interacts with Wolbachia pipientis in Drosophila melanogaster

Sex-lethal (Sxl) is the sex determination switch in Drosophila, and also plays a critical role in germ-line stem cell daughter differentiation in Drosophila melanogaster. Three female-sterile alleles at Sxl in D. melanogaster were previously shown to genetically interact to varying degrees with the maternally inherited endosymbiont Wolbachia pipientis. Given this genetic interaction and W. pipientis’ ability to manipulate reproduction in Drosophila, we carried out a careful study of both the population genetics (within four Drosophila species) and molecular evolutionary analysis (across 20 Drosophila species) of Sxl. Consistent with earlier studies, we find that selective constraint has played a prominent role in Sxl’s molecular evolution within Drosophila, but we also observe patterns that suggest both episodic bursts of protein evolution and recent positive selection at Sxl. The episodic nature of Sxl’s protein evolution is discussed in light of its genetic interaction with W. pipientis.

Wolbachia in the spittlebug Prosapia ignipectus: Variable infection frequencies, but no apparent effect on host reproductive isolation

Animals serve as hosts for complex communities of microorganisms, including endosymbionts that live inside their cells. Wolbachia bacteria are perhaps the most common endosymbionts, manipulating host reproduction to propagate. Many Wolbachia cause cytoplasmic incompatibility (CI), which results in reduced egg hatch when uninfected females mate with infected males. Wolbachia that cause intense CI spread to high and relatively stable frequencies, while strains that cause weak or no CI tend to persist at intermediate, often variable, frequencies. Wolbachia could also contribute to host reproductive isolation (RI), although current support for such contributions is limited to a few systems. To test for Wolbachia frequency variation and effects on host RI, we sampled several local Prosapia ignipectus (Fitch) (Hemiptera: Cercopidae) spittlebug populations in the northeastern United States over two years, including closely juxtaposed Maine populations with different monomorphic color forms, "black" and "lined." We discovered a group-B Wolbachia (wPig) infecting P. ignipectus that diverged from group-A Wolbachia-like model wMel and wRi strains in Drosophila-6 to 46 MYA. Populations of the sister species Prosapia bicincta (Say) from Hawaii and Florida are uninfected, suggesting that P. ignipectus acquired wPig after their initial divergence. wPig frequencies were generally high and variable among sites and between years. While phenotyping wPig effects on host reproduction is not currently feasible, the wPig genome contains three divergent sets of CI loci, consistent with high wPig frequencies. Finally, Maine monomorphic black and monomorphic lined populations of P. ignipectus share both wPig and mtDNA haplotypes, implying no apparent effect of wPig on the maintenance of this morphological contact zone. We hypothesize P. ignipectus acquired wPig horizontally as observed for many Drosophila species, and that significant CI and variable transmission produce high but variable wPig frequencies.

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.

Wolbachia Strain wGri From the Tea Geometrid Moth Ectropis grisescens Contributes to Its Host's Fecundity

Members of the Wolbachia genus manipulate insect-host reproduction and are the most abundant bacterial endosymbionts of insects. The tea Geometrid moth Ectropis grisescens (Warren) (Lepidoptera: Geometridae) is the most devastating insect pest of tea plants [Camellia sinensis (L.) O. Kuntze] in China. However, limited data on the diversity, typing, or phenotypes of Wolbachia in E. grisescens are available. Here, we used a culture-independent method to compare the gut bacteria of E. grisescens and other tea Geometridae moths. The results showed that the composition of core gut bacteria in larvae of the three Geometridae moth species was similar, except for the presence of Wolbachia. Moreover, Wolbachia was also present in adult female E. grisescens samples. A Wolbachia strain was isolated from E. grisescens and designated as wGri. Comparative analyses showed that this strain shared multilocus sequence types and Wolbachia surface protein hypervariable region profiles with cytoplasmic incompatibility (CI)-inducing strains in supergroup B; however, the wGri-associated phenotypes were undetermined. A reciprocal cross analysis showed that Wolbachia-uninfected females mated with infected males resulted in 100% embryo mortality (0% eggs hatched per female). Eggs produced by mating between uninfected males and infected females hatched normally. These findings indicated that wGri induces strong unidirectional CI in E. grisescens. Additionally, compared with uninfected females, Wolbachia-infected females produced approximately 30-40% more eggs. Together, these results show that this Wolbachia strain induces reproductive CI in E. grisescens and enhances the fecundity of its female host. We also demonstrated that wGri potential influences reproductive communication between E. grisescens and Ectropis obliqua through CI.

Parallel Sequencing of Wolbachia wCer2 from Donor and Novel Hosts Reveals Multiple Incompatibility Factors and Genome Stability after Host Transfers

The application of Wolbachia in insect pest and vector control requires the establishment of genotypically stable host associations. The cytoplasmic incompatibility (CI) inducing Wolbachia strain wCer2 naturally occurs in the cherry fruit fly Rhagoletis cerasi as co-infection with other strains and was transferred to other fruit fly species by embryonic microinjections. We obtained wCer2 genome data from its native and three novel hosts, Drosophila simulans, Drosophila melanogaster, and Ceratitis capitata and assessed its genome stability, characteristics, and CI factor (cif) genes. De novo assembly was successful from Wolbachia cell-enriched singly infected D. simulans embryos, with minimal host and other bacterial genome traces. The low yield of Wolbachia sequence reads from total genomic extracts of one multiply infected R. cerasi pupa and one singly infected C. capitata adult limited de novo assemblies but was sufficient for comparative analyses. Across hosts wCer2 was stable in genome synteny and content. Polymorphic nucleotide sites were found in wCer2 of each host; however, only one nucleotide was different between R. cerasi and C. capitata, and none between replicated D. simulans lines. The wCer2 genome is highly similar to wAu (D. simulans), wMel (D. melanogaster), and wRec (Drosophila recens). In contrast to wMel and wRec (each with one cif gene pair) and wAu (without any cif genes), wCer2 has three pairs of Type I cif genes, and one Type V cifB gene without a cifA complement. This may explain previously reported CI patterns of wCer2, including incomplete rescue of its own CI modification in three novel host species.

Transcriptional Response of Wolbachia to Dengue Virus Infection in Cells of the Mosquito Aedes aegypti

Aedes aegypti transmits one of the most significant mosquito-borne viruses, dengue virus (DENV). The absence of effective vaccines and clinical treatments and the emergence of insecticide resistance in A. aegypti necessitate novel vector control strategies. A new approach uses the endosymbiotic bacterium Wolbachia pipientis to reduce the spread of arboviruses. However, the Wolbachia-mediated antiviral mechanism is not well understood. To shed light on this mechanism, we investigated an unexplored aspect of Wolbachia-virus-mosquito interaction. We used RNA sequencing to examine the transcriptional response of Wolbachia to DENV infection in A. aegypti Aag2 cells transinfected with the wAlbB strain of Wolbachia. Our results suggest that genes encoding an endoribonuclease (RNase HI), a regulator of sigma 70-dependent gene transcription (6S RNA), essential cellular, transmembrane, and stress response functions and primary type I and IV secretion systems were upregulated, while a number of transport and binding proteins of Wolbachia, ribosome structure, and elongation factor-associated genes were downregulated due to DENV infection. Furthermore, bacterial retrotransposon, transposable, and phage-related elements were found among the up- and downregulated genes. We show that Wolbachia elicits a transcriptional response to virus infection and identify differentially expressed Wolbachia genes mostly at the early stages of virus infection. These findings highlight Wolbachia's ability to alter its gene expression in response to DENV infection of the host cell. IMPORTANCE Aedes aegypti is a vector of several pathogenic viruses, including dengue, Zika, chikungunya, and yellow fever viruses, which are of importance to human health. Wolbachia is an endosymbiotic bacterium currently used in transinfected mosquitoes to suppress replication and transmission of dengue viruses. However, the mechanism of Wolbachia-mediated virus inhibition is not fully understood. While several studies have shown mosquitoes' transcriptional responses to dengue virus infection, none have investigated these responses in Wolbachia, which may provide clues to the inhibition mechanism. Our results suggest changes in the expression of a number of functionally important Wolbachia genes upon dengue virus infection, including those involved in stress responses, providing insights into the endosymbiont's reaction to virus infection.

The iron transporter Transferrin 1 mediates homeostasis of the endosymbiotic relationship between Drosophila melanogaster and Spiroplasma poulsonii

Iron is involved in numerous biological processes in both prokaryotes and eukaryotes and is therefore subject to a tug-of-war between host and microbes upon pathogenic infections. In the fruit fly Drosophila melanogaster, the iron transporter Transferrin 1 (Tsf1) mediates iron relocation from the hemolymph to the fat body upon infection as part of the nutritional immune response. The sequestration of iron in the fat body renders it less available for pathogens, hence limiting their proliferation and enhancing the host ability to fight the infection. Here we investigate the interaction between host iron homeostasis and Spiroplasma poulsonii, a facultative, vertically transmitted, endosymbiont of Drosophila. This low-pathogenicity bacterium is devoid of cell wall and is able to thrive in the host hemolymph without triggering pathogen-responsive canonical immune pathways. However, hemolymph proteomics revealed an enrichment of Tsf1 in infected flies. We find that S. poulsonii induces tsf1 expression and triggers an iron sequestration response similarly to pathogenic bacteria. We next demonstrate that free iron cannot be used by Spiroplasma while Tsf1-bound iron promotes bacterial growth, underlining the adaptation of Spiroplasma to the intra-host lifestyle where iron is mostly protein-bound. Our results show that Tsf1 is used both by the fly to sequester iron and by Spiroplasma to forage host iron, making it a central protein in endosymbiotic homeostasis.

Living in the endosymbiotic world of Wolbachia: A centennial review

The most widespread intracellular bacteria in the animal kingdom are maternally inherited endosymbionts of the genus Wolbachia. Their prevalence in arthropods and nematodes worldwide and stunning arsenal of parasitic and mutualistic adaptations make these bacteria a biological archetype for basic studies of symbiosis and applied outcomes for curbing human and agricultural diseases. Here, we conduct a summative, centennial analysis of living in the Wolbachia world. We synthesize literature on Wolbachia's host range, phylogenetic diversity, genomics, cell biology, and applications to filarial, arboviral, and agricultural diseases. We also review the mobilome of Wolbachia including phage WO and its essentiality to hallmark reproductive phenotypes in arthropods. Finally, the Wolbachia system is an exemplar for discovery-based science education using biodiversity, biotechnology, and bioinformatics lessons. As we approach a century of Wolbachia research, the interdisciplinary science of this symbiosis stands as a model for consolidating and teaching the integrative rules of endosymbiotic life.

Living in the endosymbiotic world of Wolbachia: A centennial review

The most widespread intracellular bacteria in the animal kingdom are maternally inherited endosymbionts of the genus Wolbachia. Their prevalence in arthropods and nematodes worldwide and stunning arsenal of parasitic and mutualistic adaptations make these bacteria a biological archetype for basic studies of symbiosis and applied outcomes for curbing human and agricultural diseases. Here, we conduct a summative, centennial analysis of living in the Wolbachia world. We synthesize literature on Wolbachia's host range, phylogenetic diversity, genomics, cell biology, and applications to filarial, arboviral, and agricultural diseases. We also review the mobilome of Wolbachia including phage WO and its essentiality to hallmark reproductive phenotypes in arthropods. Finally, the Wolbachia system is an exemplar for discovery-based science education using biodiversity, biotechnology, and bioinformatics lessons. As we approach a century of Wolbachia research, the interdisciplinary science of this symbiosis stands as a model for consolidating and teaching the integrative rules of endosymbiotic life.

Characterization and comparison of the bacterial microbiota of Lysiphlebia japonica parasitioid wasps and their aphid host Aphis gosypii

BACKGROUND

Endosymbiotic bacteria have been reported to mediate interactions between parasitoids and their insect hosts. How parasitic wasps influence changes in host microbial communities and the relationship between them are of great importance to the study of host-parasitoid co-evolutionary and ecological interactions. However, these interactions remain largely unreported for interactions between Aphis gossypii and Lysiphlebia japonica.

RESULTS

In this study, we characterize the bacterial microbiota of L. japonica wasps at different developmental stages and monitor changes over time in the bacterial microbiota of their parasitized and nonparasitized aphid hosts, using metagenomic analysis of 16S rDNA sequencing data. Proteobacteria, Firmicutes, and Actinobacteria were the three most abundant bacterial phyla identified in L. japonica. We found that parasitism was associated with an increased abundance of Buchnera nutritional endosymbionts, but decreased abundance of Acinetobacter, Arsenophonus, Candidatus_Hamiltonella, and Pseudomonas facultative symbionts in aphid hosts. Functional analysis of enriched pathways of parasitized aphids showed significant differences in the 'transport and metabolism of carbohydrates' and 'amino acid, lipid, and coenzyme biosynthesis' pathways. Notably, the composition of symbiotic bacteria in wasp larvae was highly similar to that of their aphid hosts, especially the high abundance of Buchnera.

CONCLUSION

The results provide a conceptual framework for L. japonica interactions with A. gossypii in which the exchange of symbiotic microbes provides a means by which microbiota can potentially serve as evolutionary drivers of complex, multilevel interactions underlying the ecology and co-evolution of these hosts and parasites. © 2021 Society of Chemical Industry.

Forward genetics in Wolbachia: Regulation of Wolbachia proliferation by the amplification and deletion of an addictive genomic island

Wolbachia is one of the most prevalent bacterial endosymbionts, infecting approximately 40% of terrestrial arthropod species. Wolbachia is often a reproductive parasite but can also provide fitness benefits to its host, as, for example, protection against viral pathogens. This protective effect is currently being applied to fight arboviruses transmission by releasing Wolbachia-transinfected mosquitoes. Titre regulation is a crucial aspect of Wolbachia biology. Higher titres can lead to stronger phenotypes and fidelity of transmission but can have a higher cost to the host. Since Wolbachia is maternally transmitted, its fitness depends on host fitness, and, therefore, its cost to the host may be under selection. Understanding how Wolbachia titres are regulated and other aspects of Wolbachia biology has been hampered by the lack of genetic tools. Here we developed a forward genetic screen to identify new Wolbachia over-proliferative mutant variants. We characterized in detail two new mutants, wMelPop2 and wMelOctoless, and show that the amplification or loss of the Octomom genomic region lead to over-proliferation. These results confirm previous data and expand on the complex role of this genomic region in the control of Wolbachia proliferation. Both new mutants shorten the host lifespan and increase antiviral protection. Moreover, we show that Wolbachia proliferation rate in Drosophila melanogaster depends on the interaction between Octomom copy number, the host developmental stage, and temperature. Our analysis also suggests that the life shortening and antiviral protection phenotypes of Wolbachia are dependent on different, but related, properties of the endosymbiont; the rate of proliferation and the titres near the time of infection, respectively. We also demonstrate the feasibility of a novel and unbiased experimental approach to study Wolbachia biology, which could be further adapted to characterize other genetically intractable bacterial endosymbionts.

Differential gene expression in Drosophila melanogaster and D. nigrosparsa infected with the same Wolbachia strain

Wolbachia are maternally inherited endosymbionts that infect nearly half of all arthropod species. Wolbachia manipulate their hosts to maximize their transmission, but they can also provide benefits such as nutrients and resistance against viruses to their hosts. The Wolbachia strain wMel was recently found to increase locomotor activities and possibly trigger cytoplasmic incompatibility in the transinfected fly Drosophila nigrosparsa. Here, we investigated, in females of both D. melanogaster and D. nigrosparsa, the gene expression between animals uninfected and infected with wMel, using RNA sequencing to see if the two Drosophila species respond to the infection in the same or different ways. A total of 2164 orthologous genes were used. The two fly species responded to the infection in different ways. Significant changes shared by the fly species belong to the expression of genes involved in processes such as oxidation-reduction process, iron-ion binding, and voltage-gated potassium-channel activity. We discuss our findings also in the light of how Wolbachia survive within both the native and the novel host.

Interactions Between Tsetse Endosymbionts and Glossina pallidipes Salivary Gland Hypertrophy Virus in Glossina Hosts

Tsetse flies are the sole cyclic vector for trypanosomosis, the causative agent for human African trypanosomosis or sleeping sickness and African animal trypanosomosis or nagana. Tsetse population control is the most efficient strategy for animal trypanosomosis control. Among all tsetse control methods, the Sterile Insect Technique (SIT) is one of the most powerful control tactics to suppress or eradicate tsetse flies. However, one of the challenges for the implementation of SIT is the mass production of target species. Tsetse flies have a highly regulated and defined microbial fauna composed of three bacterial symbionts (Wigglesworthia, Sodalis and Wolbachia) and a pathogenic Glossina pallidipes Salivary Gland Hypertrophy Virus (GpSGHV) which causes reproduction alterations such as testicular degeneration and ovarian abnormalities with reduced fertility and fecundity. Interactions between symbionts and GpSGHV might affect the performance of the insect host. In the present study, we assessed the possible impact of GpSGHV on the prevalence of tsetse endosymbionts under laboratory conditions to decipher the bidirectional interactions on six Glossina laboratory species. The results indicate that tsetse symbiont densities increased over time in tsetse colonies with no clear impact of the GpSGHV infection on symbionts density. However, a positive correlation between the GpSGHV and Sodalis density was observed in Glossina fuscipes species. In contrast, a negative correlation between the GpSGHV density and symbionts density was observed in the other taxa. It is worth noting that the lowest Wigglesworthia density was observed in G. pallidipes, the species which suffers most from GpSGHV infection. In conclusion, the interactions between GpSGHV infection and tsetse symbiont infections seems complicated and affected by the host and the infection density of the GpSGHV and tsetse symbionts.

Vector Control: Wolbachia Expands Its Protective Reach from Humans to Plants

RNA viral titers are often suppressed in insects co-infected with the bacterial endosymbiont Wolbachia. This property has been used to suppress transmission of the ragged rice stunt virus from its insect host, the brown planthopper, to the rice plant.

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.

Reduced competence to arboviruses following the sustainable invasion of Wolbachia into native Aedes aegypti from Southeastern Brazil

Field release of Wolbachia-infected Aedes aegypti has emerged as a promising solution to manage the transmission of dengue, Zika and chikungunya in endemic areas across the globe. Through an efficient self-dispersing mechanism, and the ability to induce virus-blocking properties, Wolbachia offers an unmatched potential to gradually modify wild Ae. aegypti populations turning them unsuitable disease vectors. Here we describe a proof-of-concept field trial carried out in a small community of Niterói, greater Rio de Janeiro, Brazil. Following the release of Wolbachia-infected eggs, we report here a successful invasion and long-term establishment of the bacterium across the territory, as denoted by stable high-infection indexes (> 80%). We have also demonstrated that refractoriness to dengue and Zika viruses, either thorough oral-feeding or intra-thoracic saliva challenging assays, was maintained over the adaptation to the natural environment of Southeastern Brazil. These findings further support Wolbachia's ability to invade local Ae. aegypti populations and impair disease transmission, and will pave the way for future epidemiological and economic impact assessments.

Wolbachia: endosymbiont of onchocercid nematodes and their vectors

Background

Wolbachia is an obligate intracellular maternally transmitted, gram-negative bacterium which forms a spectrum of endosymbiotic relationships from parasitism to obligatory mutualism in a wide range of arthropods and onchocercid nematodes, respectively. In arthropods Wolbachia produces reproductive manipulations such as male killing, feminization, parthenogenesis and cytoplasmic incompatibility for its propagation and provides an additional fitness benefit for the host to protect against pathogens, whilst in onchocercid nematodes, apart from the mutual metabolic dependence, this bacterium is involved in moulting, embryogenesis, growth and survival of the host.

Methods

This review details the molecular data of Wolbachia and its effect on host biology, immunity, ecology and evolution, reproduction, endosymbiont-based treatment and control strategies exploited for filariasis. Relevant peer-reviewed scientic papers available in various authenticated scientific data bases were considered while writing the review.

Conclusions

The information presented provides an overview on Wolbachia biology and its use in the control and/or treatment of vectors, onchocercid nematodes and viral diseases of medical and veterinary importance. This offers the development of new approaches for the control of a variety of vector-borne diseases.

Graphic Abstract

Diversification, selective sweep, and body size in the invasive Palearctic alfalfa weevil infected with Wolbachia

The alfalfa weevil Hypera postica, native to the Western Palearctic, is an invasive legume pest with two divergent mitochondrial clades in its invading regions, the Western clade and the Eastern/Egyptian clade. However, knowledge regarding the native populations is limited. The Western clade is infected with the endosymbiotic bacteria Wolbachia that cause cytoplasmic incompatibility in host weevils. Our aim was to elucidate the spatial genetic structure of this insect and the effect of Wolbachia on its population diversity. We analyzed two mitochondrial and two nuclear genes of the weevil from its native ranges. The Western clade was distributed in western/central Europe, whereas the Eastern/Egyptian clade was distributed from the Mediterranean basin to central Asia. Intermediate mitotypes were found from the Balkans to central Asia. Most Western clade individuals in western Europe were infected with an identical Wolbachia strain. Mitochondrial genetic diversity of the infected individuals was minimal. The infected clades demonstrated a higher nonsynonymous/synonymous substitution rate ratio than the uninfected clades, suggesting a higher fixation of nonsynonymous mutations due to a selective sweep by Wolbachia. Trans-Mediterranean and within-European dispersal routes were supported. We suggest that the ancestral populations diversified by geographic isolation due to glaciations and that the diversity was reduced in the west by a recent Wolbachia-driven sweep(s). The intermediate clade exhibited a body size and host plant that differed from the other clades. Pros and cons of the possible use of infected-clade males to control uninfected populations are discussed.

Pervasive effects of Wolbachia on host activity

Heritable symbionts have diverse effects on the physiology, reproduction and fitness of their hosts. Maternally transmitted Wolbachia are one of the most common endosymbionts in nature, infecting about half of all insect species. We test the hypothesis that Wolbachia alter host behaviour by assessing the effects of 14 different Wolbachia strains on the locomotor activity of nine Drosophila host species. We find that Wolbachia alter the activity of six different host genotypes, including all hosts in our assay infected with wRi-like Wolbachia strains (wRi, wSuz and wAur), which have rapidly spread among Drosophila species in about the last 14 000 years. While Wolbachia effects on host activity were common, the direction of these effects varied unpredictably and sometimes depended on host sex. We hypothesize that the prominent effects of wRi-like Wolbachia may be explained by patterns of Wolbachia titre and localization within host somatic tissues, particularly in the central nervous system. Our findings support the view that Wolbachia have wide-ranging effects on host behaviour. The fitness consequences of these behavioural modifications are important for understanding the evolution of host-symbiont interactions, including how Wolbachia spread within host populations.

Dual proteomics of Drosophila melanogaster hemolymph infected with the heritable endosymbiont Spiroplasma poulsonii

Insects are frequently infected with heritable bacterial endosymbionts. Endosymbionts have a dramatic impact on their host physiology and evolution. Their tissue distribution is variable with some species being housed intracellularly, some extracellularly and some having a mixed lifestyle. The impact of extracellular endosymbionts on the biofluids they colonize (e.g. insect hemolymph) is however difficult to appreciate because biofluid composition can depend on the contribution of numerous tissues. Here we investigate Drosophila hemolymph proteome changes in response to the infection with the endosymbiont Spiroplasma poulsonii. S. poulsonii inhabits the fly hemolymph and gets vertically transmitted over generations by hijacking the oogenesis in females. Using dual proteomics on infected hemolymph, we uncovered a weak, chronic activation of the Toll immune pathway by S. poulsonii that was previously undetected by transcriptomics-based approaches. Using Drosophila genetics, we also identified candidate proteins putatively involved in controlling S. poulsonii growth. Last, we also provide a deep proteome of S. poulsonii, which, in combination with previously published transcriptomics data, improves our understanding of the post-transcriptional regulations operating in this bacterium.

Comparative genome sequencing reveals insights into the dynamics of Wolbachia in native and invasive cherry fruit flies

Wolbachia is a maternally inherited obligate endosymbiont that can induce a wide spectrum of effects in its host, ranging from mutualism to reproductive parasitism. At the genomic level, recombination within and between strains, transposable elements, and horizontal transfer of strains between host species make Wolbachia an evolutionarily dynamic bacterial system. The invasive cherry fruit fly Rhagoletis cingulata arrived in Europe from North America ~40 years ago, where it now co‐occurs with the native cherry pest R. cerasi. This shared distribution has been proposed to have led to the horizontal transfer of different Wolbachia strains between the two species. To better understand transmission dynamics, we performed a comparative genome study of the strain wCin2 in its native United States and invasive European populations of R. cingulata with wCer2 in European R. cerasi. Previous multilocus sequence genotyping (MLST) of six genes implied that the source of wCer2 in R. cerasi was wCin2 from R. cingulata. However, we report genomic evidence discounting the recent horizontal transfer hypothesis for the origin of wCer2. Despite near identical sequences for the MLST markers, substantial sequence differences for other loci were found between wCer2 and wCin2, as well as structural rearrangements, and differences in prophage, repetitive element, gene content, and cytoplasmic incompatibility inducing genes. Our study highlights the need for whole‐genome sequencing rather than relying on MLST markers for resolving Wolbachia strains and assessing their evolutionary dynamics.

Recent advances in understanding tick and rickettsiae interactions

Ticks are haematophagous arthropods with unique molecular mechanisms for digesting host blood meal while acting as vectors for various pathogens of public health significance. The tick's pharmacologically active saliva plays a fundamental role in modulating the host's immune system for several days to weeks, depending on the tick species. The vector tick has also developed sophisticated molecular mechanisms to serve as a competent vector for pathogens, including the spotted fever group (SFG) rickettsiae. Evidence is still inadequate concerning tick-rickettsiae-host interactions and saliva-assisted transmission of the pathogen to the mammalian host. Rickettsia parkeri, of the SFG rickettsia, can cause a milder version of Rocky Mountain spotted fever known as American Boutonneuse fever. The Gulf Coast tick (Amblyomma maculatum) often transmits this pathogenic rickettsia in the USA. This review discusses the knowledge gap concerning tick-rickettsiae-host interactions by highlighting the SFG rickettsia and the Am maculatum model system. Filling this knowledge gap will provide a better understanding of the tick-rickettsiae-host interactions in disease causation, which will be crucial for developing effective methods for preventing tick-borne diseases.

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.

Gut Bacterial and Fungal Communities of the Wild and Laboratory-Reared Thitarodes Larvae, Host of the Chinese Medicinal Fungus Ophiocordyceps sinensis on Tibetan Plateau

Simple Summary

The ghost moth, Thitarodes sp., is an obligate host of the most precious fungus Ophiocordyceps sinensis on Tibetan plateau. Artificial rearing of the ghost moth at low-altitude laboratory by mimicking the environment of the wild habitat for the cultivation of the Chinese cordyceps has been realized. However, the high mortality of ghost moth larvae by pathogens, low and slow infection, and mummification rate by O. sinensis still constrain the efficient cultivation of the Chinese cordyceps. Both larval gut microbiota and their exploitation in the Thitarodes artificial rearing system have attracted a renewed interest. In the present study, the gut bacterial and fungal communities of the wild and laboratory-reared populations were characterized using both culture-dependent and -independent approaches. The discovery of apparent microbial community shifts between the wild and laboratory-reared ghost moth larvae, many opportunistic pathogenic bacteria and fungi in the gut of the laboratory-reared ghost moth larvae, and the dominant bacteria enriched in the wild ghost moth provide interesting cues for selecting beneficial probiotic bacteria to improve the effectiveness of Thitarodes rearing system and the cultivation of the Chinese cordyceps.

Abstract

By employing a culture-dependent and -independent 16S rRNA and ITS gene high-throughput sequencing analyses, comprehensive information was obtained on the gut bacterial and fungal communities in the ghost moth larvae of three different geographic locations from high-altitude on Tibet plateau and from low-altitude laboratory. Twenty-six culturable bacterial species belonging to 21 genera and 14 fungal species belonging to 12 genera were identified from six populations by culture-dependent method. Carnobacterium maltaromaticum was the most abundant bacterial species from both the wild and laboratory-reared larvae. The most abundant OTUs in the wild ghost moth populations were Carnobacteriaceae, Enterobacteriaceae for bacteria, and Ascomycota and Basidiomycota for fungi. Larval microbial communities of the wild ghost moth from different geographic locations were not significantly different from each other but significant difference in larval microbial community was detected between the wild and laboratory-reared ghost moth. The larval gut of the wild ghost moth was dominated by the culturable Carnobacterium. However, that of the laboratory-reared ghost moth exhibited significantly abundant Wolbachia, Rhizobium, Serratia, Pseudomonas, and Flavobacterium. Furthermore, the larval gut of the wild ghost moth had a significantly higher abundance of Ophiocordyceps but lower abundance of Candida and Aspergillus than that of the laboratory-reared ghost moth.

Sensing and signalling viral infection in drosophila

The fruitfly Drosophila melanogaster is a valuable model to unravel mechanisms of innate immunity, in particular in the context of viral infections. RNA interference, and more specifically the small interfering RNA pathway, is a major component of antiviral immunity in drosophila. In addition, the contribution of inducible transcriptional responses to the control of viruses in drosophila and other invertebrates is increasingly recognized. In particular, the recent discovery of a STING-IKKβ-Relish signalling cassette in drosophila has confirmed that NF-κB transcription factors play an important role in the control of viral infections, in addition to bacterial and fungal infections. Here, we review recent developments in the field, which begin to shed light on the mechanisms involved in sensing of viral infections and in signalling leading to production of antiviral effectors.

Torix Rickettsia are widespread in arthropods and reflect a neglected symbiosis

BACKGROUND

Rickettsia are intracellular bacteria best known as the causative agents of human and animal diseases. Although these medically important Rickettsia are often transmitted via haematophagous arthropods, other Rickettsia, such as those in the Torix group, appear to reside exclusively in invertebrates and protists with no secondary vertebrate host. Importantly, little is known about the diversity or host range of Torix group Rickettsia.

RESULTS

This study describes the serendipitous discovery of Rickettsia amplicons in the Barcode of Life Data System (BOLD), a sequence database specifically designed for the curation of mitochondrial DNA barcodes. Of 184,585 barcode sequences analysed, Rickettsia is observed in ∼0.41% of barcode submissions and is more likely to be found than Wolbachia (0.17%). The Torix group of Rickettsia are shown to account for 95% of all unintended amplifications from the genus. A further targeted PCR screen of 1,612 individuals from 169 terrestrial and aquatic invertebrate species identified mostly Torix strains and supports the "aquatic hot spot" hypothesis for Torix infection. Furthermore, the analysis of 1,341 SRA deposits indicates that Torix infections represent a significant proportion of all Rickettsia symbioses found in arthropod genome projects.

CONCLUSIONS

This study supports a previous hypothesis that suggests that Torix Rickettsia are overrepresented in aquatic insects. In addition, multiple methods reveal further putative hot spots of Torix Rickettsia infection, including in phloem-feeding bugs, parasitoid wasps, spiders, and vectors of disease. The unknown host effects and transmission strategies of these endosymbionts make these newly discovered associations important to inform future directions of investigation involving the understudied Torix Rickettsia.

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.

Wolbachia and Virus Alter the Host Transcriptome at the Interface of Nucleotide Metabolism Pathways

Wolbachia is a maternally transmitted bacterium that manipulates arthropod and nematode biology in myriad ways. The Wolbachia strain colonizing Drosophila melanogaster creates sperm-egg incompatibilities and protects its host against RNA viruses, making it a promising tool for vector control. Despite successful trials using Wolbachia-transfected mosquitoes for dengue control, knowledge of how Wolbachia and viruses jointly affect insect biology remains limited. Using the Drosophila melanogaster model, transcriptomics and gene expression network analyses revealed pathways with altered expression and splicing due to Wolbachia colonization and virus infection. Included are metabolic pathways previously unknown to be important for Wolbachia-host interactions. Additionally, Wolbachia-colonized flies exhibit a dampened transcriptomic response to virus infection, consistent with early blocking of virus replication. Finally, using Drosophila genetics, we show that Wolbachia and expression of nucleotide metabolism genes have interactive effects on virus replication. Understanding the mechanisms of pathogen blocking will contribute to the effective development of Wolbachia-mediated vector control programs.IMPORTANCE Recently developed arbovirus control strategies leverage the symbiotic bacterium Wolbachia, which spreads in insect populations and blocks viruses from replicating. While this strategy has been successful, details of how this "pathogen blocking" works are limited. Here, we use a combination of virus infections, fly genetics, and transcriptomics to show that Wolbachia and virus interact at host nucleotide metabolism pathways.

Rapid molecular evolution of Spiroplasma symbionts of Drosophila

Spiroplasma is a genus of Mollicutes whose members include plant pathogens, insect pathogens and endosymbionts of animals. Spiroplasma phenotypes have been repeatedly observed to be spontaneously lost in Drosophila cultures, and several studies have documented a high genomic turnover in Spiroplasma symbionts and plant pathogens. These observations suggest that Spiroplasma evolves quickly in comparison to other insect symbionts. Here, we systematically assess evolutionary rates and patterns of Spiroplasma poulsonii, a natural symbiont of Drosophila. We analysed genomic evolution of sHy within flies, and sMel within in vitro culture over several years. We observed that S. poulsonii substitution rates are among the highest reported for any bacteria, and around two orders of magnitude higher compared with other inherited arthropod endosymbionts. The absence of mismatch repair loci mutS and mutL is conserved across Spiroplasma, and likely contributes to elevated substitution rates. Further, the closely related strains sMel and sHy (>99.5 % sequence identity in shared loci) show extensive structural genomic differences, which potentially indicates a higher degree of host adaptation in sHy, a protective symbiont of Drosophila hydei. Finally, comparison across diverse Spiroplasma lineages confirms previous reports of dynamic evolution of toxins, and identifies loci similar to the male-killing toxin Spaid in several Spiroplasma lineages and other endosymbionts. Overall, our results highlight the peculiar nature of Spiroplasma genome evolution, which may explain unusual features of its evolutionary ecology.

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.

Bacterial Symbionts of Tsetse Flies: Relationships and Functional Interactions Between Tsetse Flies and Their Symbionts

Tsetse flies (Glossina spp.) act as the sole vectors of the African trypanosome species that cause Human African Trypanosomiasis (HAT or African Sleeping Sickness) and Nagana in animals. These flies have undergone a variety of specializations during their evolution including an exclusive diet consisting solely of vertebrate blood for both sexes as well as an obligate viviparous reproductive biology. Alongside these adaptations, Glossina species have developed intricate relationships with specific microbes ranging from mutualistic to parasitic. These relationships provide fundamental support required to sustain the specializations associated with tsetse's biology. This chapter provides an overview on the knowledge to date regarding the biology behind these relationships and focuses primarily on four bacterial species that are consistently associated with Glossina species. Here their interactions with the host are reviewed at the morphological, biochemical and genetic levels. This includes: the obligate symbiont Wigglesworthia, which is found in all tsetse species and is essential for nutritional supplementation to the blood-specific diet, immune system maturation and facilitation of viviparous reproduction; the commensal symbiont Sodalis, which is a frequently associated symbiont optimized for survival within the fly via nutritional adaptation, vertical transmission through mating and may alter vectorial capacity of Glossina for trypanosomes; the parasitic symbiont Wolbachia, which can manipulate Glossina via cytoplasmic incompatibility and shows unique interactions at the genetic level via horizontal transmission of its genetic material into the genome in two Glossina species; finally, knowledge on recently observed relations between Spiroplasma and Glossina is explored and potential interactions are discussed based on knowledge of interactions between this bacterial Genera and other insect species. These flies have a simple microbiome relative to that of other insects. However, these relationships are deep, well-studied and provide a window into the complexity and function of host/symbiont interactions in an important disease vector.

Thermal sensitivity of the Spiroplasma-Drosophila hydei protective symbiosis: The best of climes, the worst of climes

The outcome of natural enemy attack in insects is commonly influenced by the presence of protective symbionts in the host. The degree to which protection functions in natural populations, however, will depend on the robustness of the phenotype and symbiosis to variation in the abiotic environment. We studied the impact of a key environmental parameter-temperature-on the efficacy of the protective effect of the symbiont Spiroplasma on its host Drosophila hydei, against attack by the parasitoid wasp Leptopilina heterotoma. In addition, we investigated the thermal sensitivity of the symbiont's vertical transmission, which may be a key determinant of the ability of the symbiont to persist. We found that vertical transmission was more robust than previously considered, with Spiroplasma being maintained at 25°C, at 18°C and with 18/15°C diurnal cycles, with rates of segregational loss only increasing at 15°C. Protection against wasp attack was ablated before symbiont transmission was lost, with the symbiont failing to rescue the fly host at 18°C. We conclude that the presence of a protective symbiosis in natural populations cannot be simply inferred from the presence of a symbiont whose protective capacity has been tested under narrow controlled conditions. More broadly, we argue that the thermal environment is likely to represent an important determinant of the evolutionary ecology of defensive symbioses in natural environments, potentially driving seasonal, latitudinal and altitudinal variation in symbiont frequency.

A Review: Aedes-Borne Arboviral Infections, Controls and Wolbachia-Based Strategies

Arthropod-borne viruses (Arboviruses) continue to generate significant health and economic burdens for people living in endemic regions. Of these viruses, some of the most important (e.g., dengue, Zika, chikungunya, and yellow fever virus), are transmitted mainly by Aedes mosquitoes. Over the years, viral infection control has targeted vector population reduction and inhibition of arboviral replication and transmission. This control includes the vector control methods which are classified into chemical, environmental, and biological methods. Some of these control methods may be largely experimental (both field and laboratory investigations) or widely practised. Perceptively, one of the biological methods of vector control, in particular, Wolbachia-based control, shows a promising control strategy for eradicating Aedes-borne arboviruses. This can either be through the artificial introduction of Wolbachia, a naturally present bacterium that impedes viral growth in mosquitoes into heterologous Aedes aegypti mosquito vectors (vectors that are not natural hosts of Wolbachia) thereby limiting arboviral transmission or via Aedes albopictus mosquitoes, which naturally harbour Wolbachia infection. These strategies are potentially undermined by the tendency of mosquitoes to lose Wolbachia infection in unfavourable weather conditions (e.g., high temperature) and the inhibitory competitive dynamics among co-circulating Wolbachia strains. The main objective of this review was to critically appraise published articles on vector control strategies and specifically highlight the use of Wolbachia-based control to suppress vector population growth or disrupt viral transmission. We retrieved studies on the control strategies for arboviral transmissions via arthropod vectors and discussed the use of Wolbachia control strategies for eradicating arboviral diseases to identify literature gaps that will be instrumental in developing models to estimate the impact of these control strategies and, in essence, the use of different Wolbachia strains and features.

Coexistence of Two Male-Killers and Their Impact on the Development of Oriental Tea Tortrix Homona magnanima

Male-killing, the death of male offspring induced by maternally transmitted microbes, is classified as early, or late, male-killing. The primary advantage afforded by early male-killing, which typically occurs during embryogenesis, is the reallocation of resources to females, that would have otherwise been consumed by males. Meanwhile, the key advantage of late male-killing, which typically occurs during late larval development, is the maximized potential for horizontal transmission. To date, no studies have reported on the associated developmental and physiological effects of host coinfection with early and late male-killers, which may have a significant impact on the population dynamics of the male-killers. Here we used a lepidopteran tea pest Homona magnanima as a model, which is a unique system wherein an early male-killer (a Spiroplasma bacterium) and a late male-killer (an RNA virus) can coexist in nature. An artificially established matriline, coinfected with both Spiroplasma and RNA virus, exhibited embryonic death (early male-killing) as seen in the host line singly infected with Spiroplasma. Moreover, the coinfected line also exhibited developmental retardation and low pupal weight similar to the host line singly infected with the RNA virus. A series of field surveys revealed that Spiroplasma-RNA virus coinfection occurs in nature at a low frequency. Hence, although the two male-killers are capable of coexisting within the H. magnanima population independently, high associated fitness cost appears to limit the prevalence of male-killer coinfection in the field host population.

Comparative Analysis of Bacterial Communities in Lutzomyia ayacuchensis Populations with Different Vector Competence to Leishmania Parasites in Ecuador and Peru

Differences in the gut microbial content of Lutzomyia (Lu.) ayacuchensis, a primary vector of Andean-type cutaneous leishmaniasis in Ecuador and Peru, may influence the susceptibility of these sand flies to infection by Leishmania. As a first step toward addressing this hypothesis, a comparative analysis of bacterial and fungal compositions from Lu. ayacuchensis populations with differential susceptibilities to Leishmania was performed. Bacterial 16S rRNA gene amplification and Illumina MiSeq sequencing approaches were used to characterize the bacterial composition in wild-caught populations from the Andean areas of Ecuador and southern Peru at which the sand fly species transmit Leishmania (Leishmania) mexicana and Leishmania (Viannia) peruviana, respectively, and a population from the northern Peruvian Andes at which the transmission of Leishmania by Lu. ayacuchensis has not been reported. In the present study, 59 genera were identified, 21 of which were widely identified and comprised more than 95% of all bacteria. Of the 21 dominant bacterial genera identified in the sand flies collected, 10 genera had never been detected in field sand flies. The Ecuador and southern Peru populations each comprised individuals of particular genera, while overlap was clearly observed between microbes isolated from different sites, such as the number of soil organisms. Similarly, Corynebacterium and Micrococcus were slightly more dominant bacterial genera in the southern Peru population, while Ochrobactrum was the most frequently isolated from other populations. On the other hand, fungi were only found in the southern Peru population and dominated by the Papiliotrema genus. These results suggest that variation in the insect gut microbiota may be elucidated by the ecological diversity of sand flies in Peru and Ecuador, which may influence susceptibility to Leishmania infection. The present study provides key insights for understanding the role of the microbiota during the course of L. (L.) mexicana and L. (V.) peruviana infections in this important vector.

A review of organic waste enrichment for inducing palatability of black soldier fly larvae: Wastes to valuable resources

The increase of annual organic wastes generated worldwide has become a major problem for many countries since the mismanagement could bring about negative effects on the environment besides, being costly for an innocuous disposal. Recently, insect larvae have been investigated to valorize organic wastes. This entomoremediation approach is rising from the ability of the insect larvae to convert organic wastes into its biomass via assimilation process as catapulted by the natural demand to complete its lifecycle. Among the insect species, black soldier fly or Hermetia illucens is widely researched since the larvae can grow in various environments while being saprophagous in nature. Even though black soldier fly larvae (BSFL) can ingest various decay materials, some organic wastes such as sewage sludge or lignocellulosic wastes such as waste coconut endosperm are destitute of decent nutrients that could retard the BSFL growth. Hence, blending with nutrient-rich low-cost substrates such as palm kernel expeller, soybean curd residue, etc. is employed to fortify the nutritional contents of larval feeding substrates prior to administering to the BSFL. Alternatively, microbial fermentation can be adopted to breakdown the lignocellulosic wastes, exuding essential nutrients for growing BSFL. Upon reaching maturity, the BSFL can be harvested to serve as the protein and lipid feedstock. The larval protein can be made into insect meal for farmed animals, whilst the lipid source could be extracted and transesterified into larval biodiesel to cushion the global energy demands. Henceforth, this review presents the influence of various organic wastes introduced to feed BSFL, targeting to reduce wastes and producing biochemicals from mature larvae through entomoremediation. Modification of recalcitrant organic wastes via fermentation processes is also unveiled to ameliorate the BSFL growth. Lastly, the sustainable applications of harvested BSFL biomass are as well covered together with the immediate shortcomings that entail further researches.

The effects of taxonomy, diet, and ecology on the microbiota of riverine macroinvertebrates

Freshwater macroinvertebrates play key ecological roles in riverine food webs, such as the transfer of nutrients to consumers and decomposition of organic matter. Although local habitat quality drives macroinvertebrate diversity and abundance, little is known about their microbiota. In most animals, the microbiota provides benefits, such as increasing the rate at which nutrients are metabolized, facilitating immune system development, and defending against pathogenic attack. Our objectives were to identify the bacteria within aquatic invertebrates and determine whether their composition varied with taxonomy, habitat, diet, and time of sample collection. In 2016 and 2017, we collected 264 aquatic invertebrates from the mainstem Saint John (Wolastoq) River in New Brunswick, Canada, representing 15 orders. We then amplified the V3-V4 hypervariable region of the 16S rRNA gene within each individual, which revealed nearly 20,000 bacterial operational taxonomic units (OTUs). The microbiota across all aquatic invertebrates were dominated by Proteobacteria (69.25% of the total sequence reads), but they differed significantly in beta diversity, both among host invertebrate taxa (genus-, family-, and order-levels) and temporally. In contrast to previous work, we observed no microbiota differences among functional feeding groups or traditional feeding habits, and neither water velocity nor microhabitat type structured microbiota variability. Our findings suggest that host invertebrate taxonomy was the most important factor in modulating the composition of the microbiota, likely through a combination of vertical and horizontal bacterial transmission, and evolutionary processes. This is one of the most comprehensive studies of freshwater invertebrate microbiota to date, and it underscores the need for future studies of invertebrate microbiota evolution and linkages to environmental bacteria and physico-chemical conditions.

Host-shift as the cause of emerging infectious diseases: Experimental approaches using Drosophila-virus interactions

Host shifts, when a cross-species transmission of a pathogen can lead to successful infections, are the main cause of emerging infectious diseases, such as COVID-19. A complex challenge faced by the scientific community is to address the factors that determine whether the cross-species transmissions will result in spillover or sustained onwards infections. Here we review recent literature and present a perspective on current approaches we are using to understand the mechanisms underlying host shifts. We highlight the usefulness of the interactions between Drosophila species and viruses as an ideal study model. Additionally, we discuss how cross-infection experiments - when pathogens from a natural reservoir are intentionally injected in novel host species- can test the effect cross-species transmissions may have on the fitness of virus and host, and how the host phylogeny may influence this response. We also discuss experiments evaluating how cooccurrence with other viruses or the presence of the endosymbiont bacteria Wolbachia may affect the performance of new viruses in a novel host. Finally, we discuss the need of surveys of virus diversity in natural populations using next-generation sequencing technologies. In the long term, these approaches can contribute to a better understanding of the basic biology of host shifts.

Larval density affects phenotype and surrounding bacterial community without altering gut microbiota in Drosophila melanogaster

Larval crowding represents a complex stressful situation arising from inter-individual competition for time- and space-limited resources. The foraging of a large number of individuals may alter the chemical and bacterial composition of food and in turn affect individual's traits. Here we used Drosophila melanogaster to explore these assumptions. First, we used a wide larval density gradient to investigate the impact of crowding on phenotypical traits. We confirmed that high densities increased development time and pupation height, and decreased viability and body mass. Next, we measured concentrations of common metabolic wastes (ammonia, uric acid) and characterized bacterial communities, both in food and in larvae, for three contrasting larval densities (low, medium and high). Ammonia concentration increased in food from medium and high larval densities, but remained low in larvae regardless of the larval density. Uric acid did not accumulate in food but was detected in larvae. Surprisingly, bacterial composition remained stable in guts of larvae whatever their rearing density, although it drastically changed in the food. Overall, these results indicate that crowding deeply affects individuals, and also their abiotic and biotic surroundings. Environmental bacterial communities likely adapt to altered nutritional situations resulting from crowding, putatively acting as scavengers of larval metabolic wastes.

Growing Ungrowable Bacteria: Overview and Perspectives on Insect Symbiont Culturability

Insects are often involved in endosymbiosis, that is, the housing of symbiotic microbes within their tissues or within their cells. Endosymbionts are a major driving force in insects' evolution, because they dramatically affect their host physiology and allow them to adapt to new niches, for example, by complementing their diet or by protecting them against pathogens. Endosymbiotic bacteria are, however, fastidious and therefore difficult to manipulate outside of their hosts, especially intracellular species. The coevolution between hosts and endosymbionts leads to alterations in the genomes of endosymbionts, limiting their ability to cope with changing environments. Consequently, few insect endosymbionts are culturable in vitro and genetically tractable, making functional genetics studies impracticable on most endosymbiotic bacteria. However, recently, major progress has been made in manipulating several intracellular endosymbiont species in vitro, leading to astonishing discoveries on their physiology and the way they interact with their host. This review establishes a comprehensive picture of the in vitro tractability of insect endosymbiotic bacteria and addresses the reason why most species are not culturable. By compiling and discussing the latest developments in the design of custom media and genetic manipulation protocols, it aims at providing new leads to expand the range of tractable endosymbionts and foster genetic research on these models.