Wolbachia infections are distributed throughout insect somatic and germ line tissues

Transcriptomic and Metatranscriptomic Analyses Provide New Insights into the Response of the Pea Aphid Acyrthosiphon pisum (Hemiptera: Aphididae) to Acetamiprid

Acetamiprid is a broad-spectrum neonicotinoid insecticide used in agriculture to control aphids. While recent studies have documented resistance to acetamiprid in several aphid species, the underlying mechanisms are still not fully understood. In this study, we analyzed the transcriptome and metatranscriptome of a laboratory strain of the pea aphid, Acyrthosiphon pisum (Harris, 1776), with reduced susceptibility to acetamiprid after nine generations of exposure to identify candidate genes and the microbiome involved in the adaptation process. Sequencing of the transcriptome of both selected (RS) and non-selected (SS) strains allowed the identification of 14,858 genes and 4938 new transcripts. Most of the differentially expressed genes were associated with catalytic activities and metabolic pathways involving carbon and fatty acids. Specifically, alcohol-forming fatty acyl-CoA reductase (FAR) and acyl-CoA synthetase (ACSF2), both involved in the synthesis of epidermal wax layer components, were significantly upregulated in RS, suggesting that adaptation to acetamiprid involves the synthesis of a thicker protective layer. Metatranscriptomic analyses revealed subtle shifts in the microbiome of RS. These results contribute to a deeper understanding of acetamiprid adaptation by the pea aphid and provide new insights for aphid control strategies.

A multidisciplinary approach to tackling invasive species: barcoding, morphology, and metataxonomy of the leafhopper Arboridia adanae

The leafhopper genus Arboridia includes several species that feed on Vitis vinifera and cause leaf chlorosis. We report the first alien Arboridia infestation in Italy in 2021 in an Apulian vineyard. To confirm the taxonomic status of the species responsible for crop damage, and reconstruct its demographic history, we barcoded individuals from Apulia together with Arboridia spp. from Crete (Greece), A. adanae from Central Turkey and other specimens of the presumed sister species, A. dalmatina from Dalmatia (Croatia). Molecular phylogenies and barcoding gap analysis identified clades not associated with sampling locations. This result is incongruent with classical specimen assignment and is further supported by morphological analyses, which did not reveal significant differences among the populations. Therefore, we propose A. dalmatina as a junior synonym of A. adanae, which would become the only grapevine-related Arboridia species in the eastern Mediterranean. To further characterise A. adanae evolution, we performed a molecular clock analysis that suggested a radiation during the Pleistocene glaciations. Finally, to assess whether the Apulian individuals carried microorganisms of agricultural relevance, we sequenced their bacterial microbiota using 16S rRNA amplicon sequencing identifying three phytopathogens not generally associated with Arboridia activities as well as Wolbachia in one Apulian haplogroup. We discuss the agricultural implications of this infestation.

A Light in the Dark: Uncovering Wolbachia-Host Interactions Using Fluorescence Imaging

The success of microbial endosymbionts, which reside naturally within a eukaryotic "host" organism, requires effective microbial interaction with, and manipulation of, the host cells. Fluorescence microscopy has played a key role in elucidating the molecular mechanisms of endosymbiosis. For 30 years, fluorescence analyses have been a cornerstone in studies of endosymbiotic Wolbachia bacteria, focused on host colonization, maternal transmission, reproductive parasitism, horizontal gene transfer, viral suppression, and metabolic interactions in arthropods and nematodes. Fluorescence-based studies stand to continue informing Wolbachia-host interactions in increasingly detailed and innovative ways.

The wild solitary bees Andrena vaga, Anthophora plumipes, Colletes cunicularius, and Osmia cornuta microbiota are host specific and dominated by endosymbionts and environmental microorganisms

We characterized the microbial communities of the crop, midgut, hindgut, and ovaries of the wild solitary bees Andrena vaga, Anthophora plumipes, Colletes cunicularius, and Osmia cornuta through 16S rRNA gene and ITS2 amplicon sequencing and a large-scale isolation campaign. The bacterial communities of these bees were dominated by endosymbionts of the genera Wolbachia and Spiroplasma. Bacterial and yeast genera representing the remaining predominant taxa were linked to an environmental origin. While only a single sampling site was examined for Andrena vaga, Anthophora plumipes, and Colletes cunicularius, and two sampling sites for Osmia cornuta, the microbiota appeared to be host specific: bacterial, but not fungal, communities generally differed between the analyzed bee species, gut compartments and ovaries. This may suggest a selective process determined by floral and host traits. Many of the gut symbionts identified in the present study are characterized by metabolic versatility. Whether they exert similar functionalities within the bee gut and thus functional redundancy remains to be elucidated.

The cellular lives of Wolbachia

Wolbachia are successful Gram-negative bacterial endosymbionts, globally infecting a large fraction of arthropod species and filarial nematodes. Efficient vertical transmission, the capacity for horizontal transmission, manipulation of host reproduction and enhancement of host fitness can promote the spread both within and between species. Wolbachia are abundant and can occupy extraordinary diverse and evolutionary distant host species, suggesting that they have evolved to engage and manipulate highly conserved core cellular processes. Here, we review recent studies identifying Wolbachia-host interactions at the molecular and cellular levels. We explore how Wolbachia interact with a wide array of host cytoplasmic and nuclear components in order to thrive in a diversity of cell types and cellular environments. This endosymbiont has also evolved the ability to precisely target and manipulate specific phases of the host cell cycle. The remarkable diversity of cellular interactions distinguishes Wolbachia from other endosymbionts and is largely responsible for facilitating its global propagation through host populations. Finally, we describe how insights into Wolbachia-host cellular interactions have led to promising applications in controlling insect-borne and filarial nematode-based diseases.

Impact of the microbiome on mosquito-borne diseases

Mosquito-borne diseases present a significant threat to human health, with the possibility of outbreaks of new mosquito-borne diseases always looming. Unfortunately, current measures to combat these diseases such as vaccines and drugs are often either unavailable or ineffective. However, recent studies on microbiomes may reveal promising strategies to fight these diseases. In this review, we examine recent advances in our understanding of the effects of both the mosquito and vertebrate microbiomes on mosquito-borne diseases. We argue that the mosquito microbiome can have direct and indirect impacts on the transmission of these diseases, with mosquito symbiotic microorganisms, particularly Wolbachia bacteria, showing potential for controlling mosquito-borne diseases. Moreover, the skin microbiome of vertebrates plays a significant role in mosquito preferences, while the gut microbiome has an impact on the progression of mosquito-borne diseases in humans. As researchers continue to explore the role of microbiomes in mosquito-borne diseases, we highlight some promising future directions for this field. Ultimately, a better understanding of the interplay between mosquitoes, their hosts, pathogens, and the microbiomes of mosquitoes and hosts may hold the key to preventing and controlling mosquito-borne diseases.

Wolbachia Induces Structural Defects Harmful to Drosophila simulans Riverside Spermiogenesis

The relationship between cytoplasmic incompatibility and the obligate intracellular alphaproteobacteria Wolbachia has for a long time been reported. Although the molecular mechanisms responsible for this reproductive alteration are beginning to be understood, the effects of Wolbachia on germ cell structure and dynamics have not yet been fully investigated. We report here that the presence of Wolbachia in infected cysts of elongating spermatids is associated with major structural defects that become more evident in mature sperm. We find mitochondrial defects, an improper axoneme structure, reduced sperm numbers, and individualization failures. The large heterogeneous variety of the ultrastructural defects found in elongating spermatids and mature sperm provide the first cytological evidence for the reduced fertility associated with Wolbachia infection in Drosophila simulans males. The observed abnormalities could be the result of the mechanical stress induced by the high bacteria numbers during the process of spermatid elongation, rather than the result of the released factors affecting the proper morphogenesis of the germ cells. Moreover, high Wolbachia densities in male germ cells may not be appropriate for causing cytoplasmic incompatibility as the bacteria are harmful for spermatid differentiation, leading to abnormal sperm that is unlikely to be functional.

Using Wolbachia to control rice planthopper populations: progress and challenges

Wolbachia have been developed as a tool for protecting humans from mosquito populations and mosquito-borne diseases. The success of using Wolbachia relies on the facts that Wolbachia are maternally transmitted and that Wolbachia-induced cytoplasmic incompatibility provides a selective advantage to infected over uninfected females, ensuring that Wolbachia rapidly spread through the target pest population. Most transinfected Wolbachia exhibit a strong antiviral response in novel hosts, thus making it an extremely efficient technique. Although Wolbachia has only been used to control mosquitoes so far, great progress has been made in developing Wolbachia-based approaches to protect plants from rice pests and their associated diseases. Here, we synthesize the current knowledge about the important phenotypic effects of Wolbachia used to control mosquito populations and the literature on the interactions between Wolbachia and rice pest planthoppers. Our aim is to link findings from Wolbachia-mediated mosquito control programs to possible applications in planthoppers.

The Transmission Patterns of the Endosymbiont Wolbachia within the Hawaiian Drosophilidae Adaptive Radiation

The evolution of endosymbionts and their hosts can lead to highly dynamic interactions with varying fitness effects for both the endosymbiont and host species. Wolbachia, a ubiquitous endosymbiont of arthropods and nematodes, can have both beneficial and detrimental effects on host fitness. We documented the occurrence and patterns of transmission of Wolbachia within the Hawaiian Drosophilidae and examined the potential contributions of Wolbachia to the rapid diversification of their hosts. Screens for Wolbachia infections across a minimum of 140 species of Hawaiian Drosophila and Scaptomyza revealed species-level infections of 20.0%, and across all 399 samples, a general infection rate of 10.3%. Among the 44 Wolbachia strains we identified using a modified Wolbachia multi-locus strain typing scheme, 30 (68.18%) belonged to supergroup B, five (11.36%) belonged to supergroup A, and nine (20.45%) had alleles with conflicting supergroup assignments. Co-phylogenetic reconciliation analysis indicated that Wolbachia strain diversity within their endemic Hawaiian Drosophilidae hosts can be explained by vertical (e.g., co-speciation) and horizontal (e.g., host switch) modes of transmission. Results from stochastic character trait mapping suggest that horizontal transmission is associated with the preferred oviposition substrate of the host, but not the host's plant family or island of occurrence. For Hawaiian Drosophilid species of conservation concern, with 13 species listed as endangered and 1 listed as threatened, knowledge of Wolbachia strain types, infection status, and potential for superinfection could assist with conservation breeding programs designed to bolster population sizes, especially when wild populations are supplemented with laboratory-reared, translocated individuals. Future research aimed at improving the understanding of the mechanisms of Wolbachia transmission in nature, their impact on the host, and their role in host species formation may shed light on the influence of Wolbachia as an evolutionary driver, especially in Hawaiian ecosystems.

Long-term sperm storage in eusocial Hymenoptera

In internally fertilizing species, sperm transfer is not always immediately followed by egg fertilization, and female sperm storage (FSS) may occur. FSS is a phenomenon in which females store sperm in a specialized organ for periods lasting from a few hours to several years, depending on the species. Eusocial hymenopterans (ants, social bees, and social wasps) hold the record for FSS duration. In these species, mating takes place during a single nuptial flight that occurs early in adult life for both sexes; they never mate again. Males die quickly after copulation but survive posthumously as sperm stored in their mates' spermathecae. Reproductive females, also known as queens, have a much longer life expectancy, up to 20 years in some species. Here, we review what is currently known about the molecular adaptations underlying the remarkable FSS capacities in eusocial hymenopterans. Because sperm quality is crucial to the reproductive success of both sexes, we also discuss the mechanisms involved in sperm storage and preservation in the male seminal vesicles prior to ejaculation. Finally, we propose future research directions that should broaden our understanding of this unique biological phenomenon.

Detection and Assessment of Wolbachia pipientis Infection

Wolbachia pipientis is a widespread vertically transmitted intracellular bacterium naturally present in the model organism Drosophila melanogaster. As Wolbachia is present in a large number of Drosophila lines, it is critical for researchers to be able to identify which of their stocks maintain this infection to avoid any potential confounding variables. Here, we describe methods for detecting the bacterium and assessing the infection, including polymerase chain reaction (PCR) of DNA, multi-locus sequence typing (MLST) to identify strains, western blotting for protein detection, and immunohistochemistry and fluorescence in situ hybridization (FISH) of Drosophila ovaries to visually detect Wolbachia by fluorescence microscopy.

Crosstalk between the microbiota and insect postembryonic development

Insect sequential development evolves from a simple molt towards complete metamorphosis. Like any multicellular host, insects interact with a complex microbiota. In this review, factors driving the microbiota dynamics were pointed out along their development. Special focus was put on tissue renewal, shift in insect ecology, and microbial interactions. Conversely, how the microbiota modulates its host development through nutrient acquisition, hormonal control, and cellular or tissue differentiation was exemplified. Such modifications might have long-term carry-over effects on insect physiology. Finally, remarkable microbe-driven control of insect behaviors along their life cycle was highlighted. Increasing knowledge of those interactions might offer new insights on how insects respond to their environment as well as perspectives on pest- or vector-control strategies.

Variable effects of Wolbachia on alphavirus infection in Aedes aegypti

Wolbachia pipientis (=Wolbachia) has promise as a tool to suppress virus transmission by Aedes aegypti mosquitoes. However, Wolbachia can have variable effects on mosquito-borne viruses. This variation remains poorly characterized, yet the multimodal effects of Wolbachia on diverse pathogens could have important implications for public health. Here, we examine the effects of somatic infection with two strains of Wolbachia (wAlbB and wMel) on the alphaviruses Sindbis virus (SINV), O'nyong-nyong virus (ONNV), and Mayaro virus (MAYV) in Ae. aegypti. We found variable effects of Wolbachia including enhancement and suppression of viral infections, with some effects depending on Wolbachia strain. Both wAlbB- and wMel-infected mosquitoes showed enhancement of SINV infection rates one week post-infection, with wAlbB-infected mosquitoes also having higher viral titers than controls. Infection rates with ONNV were low across all treatments and no significant effects of Wolbachia were observed. The effects of Wolbachia on MAYV infections were strikingly strain-specific; wMel strongly blocked MAYV infections and suppressed viral titers, while wAlbB did not influence MAYV infection. The variable effects of Wolbachia on vector competence underscore the importance of further research into how this bacterium impacts the virome of wild mosquitoes including the emergent human pathogens they transmit.

The structure and diversity of microbial communities in Paederus fuscipes (Coleoptera: Staphylinidae): from ecological paradigm to pathobiome

BACKGROUND

Paederus fuscipes is medically the most famous rove beetle, which causes dermatitis or conjunctivitis in humans, as well as gastrointestinal toxicosis in livestock, via releasing toxic hemolymph containing pederin. Pedrin biosynthesis genes have been identified in uncultured Pseudomonas-like endosymbionts that are speculated to be acquired through a horizontal transfer. However, the composition of the P. fuscipes microbial community, especially of the gut and genital microbiome, remains unclear. This study was aimed to characterize the structure and diversity of P. fuscipes-associated bacterial communities in terms of gender, organ, and location using the Illumina HiSeq platform in the southern littorals of Caspian Sea.

RESULTS

The OTUs identified from P. fuscipes specimens were collapsed into 40 phyla, 112 classes, 249 orders, 365 families, 576 genera, and 106 species. The most abundant families were Pseudomonadaceae, Spiroplasmataceae, Weeksellaceae, Enterococcaceae, and Rhizobiaceae, respectively. Thirty top genera made up > 94% of the P. fuscipes microbiome, with predominating Pseudomonas, followed by the Spiroplasma, Apibacter, Enterococcus, Dysgonomonas, Sebaldella, Ruminococcus, and Wolbachia. Interesting dissimilarities were also discovered within and between the beetle microbiomes in terms of genders and organs. Analyses showed that Spiroplasma / Apibacter as well as Pseudomonas / Pseudomonas were the most abundant in the genitals / intestines of male and female beetles, respectively. Bacterial richness did not display any significant difference in the three provinces but was higher in male beetles than in females and more in the genitals than intestines.

CONCLUSIONS

The present study identified Pseudomonas-like endobacterium as a common symbiont of P. fuscipes beetles; this bacterium begins its journey from gut and genitalia of females to reach the male rove beetles. Additionally, male and female rove beetles were characterized by distinctive microbiota in different organs, likely reflecting different functions and/or adaptation processes. Evidence of the extension of P. fuscipes microbiome from the environmental paradigm to the pathobiome was also presented herein. A comprehensive survey of P. fuscipes microbiome components may eventually lead to ecological insights into the production and utilization of defensive compound of pederin and also the management of linear dermatitis with the use of available antibiotics against bacterial pathogens released by the beetles. Video Abstract.

Uncovering Active Bacterial Symbionts in Three Species of Pollen-feeding Beetles (Nitidulidae: Meligethinae)

Microbial symbionts enable many phytophagous insects to specialize on plant-based diets through a range of metabolic services. Pollen comprises one-plant tissue consumed by such herbivores. While rich in lipids and proteins, its nutrient content is often imbalanced and difficult-to-access due to a digestibly recalcitrant cell wall. Pollen quality can be further degraded by harmful allelochemicals. To identify microbes that may aid in palynivory, we performed cDNA-based 16S rRNA metabarcoding on three related pollen beetles (Nitidulidae: Meligethinae) exhibiting different dietary breadths: Brassicogethes aeneus, B. matronalis, and Meligethes atratus. Nine bacterial symbionts (i.e., 97% OTUs) exhibited high metabolic activity during active feeding. Subsequent PCR surveys revealed varying prevalence of those from three Rickettsialles genera-Lariskella, Rickettsia, and Wolbachia-within beetle populations. Our findings lay the groundwork for future studies on the influence of phylogeny and diet on palynivorous insect microbiomes, and roles of symbionts in the use of challenging diets.

Habitat and Host Species Drive the Structure of Bacterial Communities of Two Neotropical Trap-Jaw Odontomachus Ants : Habitat and Host Species Drive the Structure of Bacterial Communities of Two Neotropical Trap-Jaw Odontomachus Ants

Ants have long been known for their associations with other taxa, including macroscopic fungi and symbiotic bacteria. Recently, many ant species have had the composition and function of their bacterial communities investigated. Due to its behavioral and ecological diversity, the subfamily Ponerinae deserves more attention regarding its associated microbiota. Here, we used the V4 region of the 16S rRNA gene to characterize the bacterial communities of Odontomachus chelifer (ground-nesting) and Odontomachus hastatus (arboreal), two ponerine trap-jaw species commonly found in the Brazilian savanna ("Cerrado") and Atlantic rainforest. We investigated habitat effects (O. chelifer in the Cerrado and the Atlantic rainforest) and species-specific effects (both species in the Atlantic rainforest) on the bacterial communities' structure (composition and abundance) in two different body parts: cuticle and gaster. Bacterial communities differed in all populations studied. Cuticular communities were more diverse, while gaster communities presented variants common to other ants, including Wolbachia and Candidatus Tokpelaia hoelldoblerii. Odontomachus chelifer populations presented different communities in both body parts, highlighting the influence of habitat type. In the Atlantic rainforest, the outcome depended on the body part targeted. Cuticular communities were similar between species, reinforcing the habitat effect on bacterial communities, which are mainly composed of environmentally acquired taxa. Gaster communities, however, differed between the two Odontomachus species, suggesting species-specific effects and selective filters. Unclassified Firmicutes and uncultured Rhizobiales variants are the main components accounting for the observed differences. Our study indicates that both host species and habitat act synergistically, but to different degrees, to shape the bacterial communities in these Odontomachus species.

Developmental Integration of Endosymbionts in Insects

In endosymbiosis, two independently existing entities are inextricably intertwined such that they behave as a single unit. For multicellular hosts, the endosymbiont must be integrated within the host developmental genetic network to maintain the relationship. Developmental integration requires innovations in cell type, gene function, gene regulation, and metabolism. These innovations are contingent upon the existing ecological interactions and may evolve mutual interdependence. Recent studies have taken significant steps toward characterizing the proximate mechanisms underlying interdependence. However, the study of developmental integration is only in its early stages of investigation. Here, we review the literature on mutualistic endosymbiosis to explore how unicellular endosymbionts developmentally integrate into their multicellular hosts with emphasis on insects as a model. Exploration of this process will help gain a more complete understanding of endosymbiosis. This will pave the way for a better understanding of the endosymbiotic theory of evolution in the future.

Increased biting rate and decreased Wolbachia density in irradiated Aedes mosquitoes

Background

Releasing considerable numbers of radiation-sterilized males is a promising strategy to suppress mosquito vectors. However, releases may also include small percentages of biting females, which translate to non-negligible numbers when releases are large. Currently, the effects of irradiation on host-seeking and host-biting behaviors have not been exhaustively investigated. Information is also lacking regarding the effects of sterilizing treatment on the endosymbiotic bacterium Wolbachia, which is known to affect the vector competence of infected mosquitos.

Methods

To ascertain the effects of irradiation on females, the pupae of two Aedes albopictus strains, differing in their natural or artificial Wolbachia infection type, and Aedes aegypti—which is not infected by Wolbachia—were treated with various doses of X-rays and monitored for key fitness parameters and biting behavior over a period of 2 weeks. The effect of radiation on Wolbachia was investigated by quantitative polymerase chain reaction (qPCR) and fluorescence in situ hybridization (FISH) analysis.

Results

Partial Aedes albopictus female sterility was achieved at 28 Gy, but the number of weekly bites more than doubled compared to that of the controls. Radiation doses of 35 and 45 Gy completely inhibited progeny production but did not significantly affect the survival or flight ability of Ae. albopictus females and caused a tripling of the number of bites per female per week (compared to untreated controls). These results were also confirmed in Ae. aegypti after treatment at 50 Gy. Wolbachia density decreased significantly in 45-Gy-irradiated females, with the greatest decreases in the early irradiation group (26 ± 2-h-old pupae). Wolbachia density also decreased as adults aged. This trend was confirmed in ovaries but not in extra-ovarian tissues. FISH analysis showed a strongly reduced Wolbachia-specific fluorescence in the ovaries of 13 ± 1-day-old females.

Conclusions

These results suggest that, under sterile insect technique (SIT) programs, the vector capacity of a target population could increase with the frequency of the irradiated females co-released with the sterile males due to an increased biting rate. In the context of successful suppression, the related safety issues are expected to be generally negligible, but they should be conservatively evaluated when large-scale programs relying on imperfect sexing and high overflooding release ratios are run for long periods in areas endemic for arboviral diseases. Also, the effects of irradiation on the vector competence deserve further investigation.

Graphical Abstract

Supplementary Information

The online version contains supplementary material available at 10.1186/s13071-022-05188-9.

Transmission-Blocking Strategies Against Malaria Parasites During Their Mosquito Stages

Malaria is still the most widespread parasitic disease and causes the most infections globally. Owing to improvements in sanitary conditions and various intervention measures, including the use of antimalarial drugs, the malaria epidemic in many regions of the world has improved significantly in the past 10 years. However, people living in certain underdeveloped areas are still under threat. Even in some well-controlled areas, the decline in malaria infection rates has stagnated or the rates have rebounded because of the emergence and spread of drug-resistant malaria parasites. Thus, new malaria control methods must be developed. As the spread of the Plasmodium parasite is dependent on the part of its life cycle that occurs in mosquitoes, to eliminate the possibility of malaria infections, transmission-blocking strategies against the mosquito stage should be the first choice. In fact, after the gametocyte enters the mosquito body, it undergoes a series of transformation processes over a short period, thus providing numerous potential blocking targets. Many research groups have carried out studies based on targeting the blocking of transmission during the mosquito phase and have achieved excellent results. Meanwhile, the direct killing of mosquitoes could also significantly reduce the probability of malaria infections. Microorganisms that display complex interactions with Plasmodium, such as Wolbachia and gut flora, have shown observable transmission-blocking potential. These could be used as a biological control strategy and play an important part in blocking the transmission of malaria.

Diet-induced changes in titer support a discrete response of Wolbachia-associated plastic recombination in Drosophila melanogaster

Plastic recombination in Drosophila melanogaster has been associated with a variety of extrinsic and intrinsic factors such as temperature, starvation, and parasite infection. The bacterial endosymbiont Wolbachia pipientis has also been associated with plastic recombination in D. melanogaster. Wolbachia infection is pervasive in arthropods and this infection induces a variety of phenotypes in its hosts, the strength of which can depend on bacterial titer. Here, we test the hypothesis that the magnitude of Wolbachia-associated plastic recombination in D. melanogaster depends on titer. To manipulate titer, we raised Wolbachia-infected and uninfected flies on diets that have previously been shown to increase or decrease Wolbachia titer relative to controls. We measured recombination in treated and control individuals using a standard backcrossing scheme with two X-linked visible markers. Our results recapitulate previous findings that Wolbachia infection is associated with increased recombination rate across the yellow-vermillion interval of the X chromosome. Our data show no significant effect of diet or diet by Wolbachia interactions on recombination, suggesting that diet-induced changes in Wolbachia titer have no effect on the magnitude of plastic recombination. These findings represent one of the first steps toward investigating Wolbachia-associated plastic recombination and demonstrate that the phenotype is a discrete response rather than a continuous one.

Molecular phylogeny of heritable symbionts and microbiota diversity analysis in phlebotominae sand flies and Culex nigripalpus from Colombia

Background

Secondary symbionts of insects include a range of bacteria and fungi that perform various functional roles on their hosts, such as fitness, tolerance to heat stress, susceptibility to insecticides and effects on reproduction. These endosymbionts could have the potential to shape microbial communites and high potential to develop strategies for mosquito-borne disease control.

Methodology/Principal findings

The relative frequency and molecular phylogeny of Wolbachia, Microsporidia and Cardinium were determined of phlebotomine sand flies and mosquitoes in two regions from Colombia. Illumina Miseq using the 16S rRNA gene as a biomarker was conducted to examine the microbiota. Different percentages of natural infection by Wolbachia, Cardinium, and Microsporidia in phlebotomines and mosquitoes were detected. Phylogenetic analysis of Wolbachia shows putative new strains of Lutzomyia gomezi (wLgom), Brumptomyia hamata (wBrham), and a putative new group associated with Culex nigripalpus (Cnig) from the Andean region, located in Supergroup A and Supergroup B, respectively. The sequences of Microsporidia were obtained of Pi. pia and Cx. nigripalpus, which are located on phylogeny in the IV clade (terrestrial origin). The Cardinium of Tr. triramula and Ps. shannoni were located in group C next to Culicoides sequences while Cardinium of Mi. cayennensis formed two putative new subgroups of Cardinium in group A. In total were obtained 550 bacterial amplicon sequence variants (ASVs) and 189 taxa to the genus level. The microbiota profiles of Sand flies and mosquitoes showed mainly at the phylum level to Proteobacteria (67.6%), Firmicutes (17.9%) and Actinobacteria (7.4%). High percentages of relative abundance for Wolbachia (30%-83%) in Lu. gomezi, Ev. dubitans, Mi. micropyga, Br. hamata, and Cx. nigripalpus were found. ASVs assigned as Microsporidia were found in greater abundance in Pi. pia (23%) and Cx. nigripalpus (11%). An important finding is the detection of Rickettsia in Pi. pia (58,8%) and Bartonella sp. in Cx. nigripalpus.

Conclusions/Significance

We found that Wolbachia infection significantly decreased the alpha diversity and negatively impacts the number of taxa on sand flies and Culex nigripalpus. The Principal Coordinate Analysis (PCoA) is consistent, which showed statistically significant differences (PERMANOVA, F = 2.4744; R2 = 0.18363; p-value = 0.007) between the microbiota of sand flies and mosquitoes depending on its origin, host and possibly for the abundance of some endosymbionts (Wolbachia, Rickettsia).

The secondary endosymbionts can positively influence the metabolism of many compounds essential for the survival of the insect vectors, provide resistance to pathogens and impact susceptibility to insecticides, as also the tolerance to heat stress. We provide information from new records of natural infection of secondary endosymbionts, such as Wolbachia, Cardinium, Microsporidia, Flavobacterium, and Rickettsia in phlebotomine sand flies and mosquitoes from Colombia. An important finding is the detection of Bartonella sp. in Cx. nigripalpus. Clear differences were found in the composition and diversity of microbiota at the intra-specific and interspecific levels in sand flies and Cx. nigripalpus, which may depend in the of the load of natural infection of endosymbionts (as Wolbachia), the geographical distribution and host.

RNA virome diversity and Wolbachia infection in individual Drosophila simulans flies

The endosymbiont bacteria of the genus Wolbachia are associated with multiple mutualistic effects on insect biology, including nutritional and antiviral properties. Members of the genus Wolbachia naturally occur in fly species of the genus Drosophila, providing an operational model host for studying how virome composition may be affected by its presence. Drosophila simulans populations can carry a variety of strains of members of the genus Wolbachia, with the wAu strain associated with strong antiviral protection under experimental conditions. We used D. simulans sampled from the Perth Hills, Western Australia, to investigate the potential virus protective effect of the wAu strain of Wolbachia on individual wild-caught flies. Our data revealed no appreciable variation in virus composition and abundance between individuals infected or uninfected with Wolbachia associated with the presence or absence of wAu. However, it remains unclear whether wAu might affect viral infection and host survival by increasing tolerance rather than inducing complete resistance. These data also provide new insights into the natural virome diversity of D. simulans. Despite the small number of individuals sampled, we identified a repertoire of RNA viruses, including nora virus, galbut virus, thika virus and La Jolla virus, that have been identified in other species of the genus Drosophila. Chaq virus-like sequences associated with galbut virus were also detected. In addition, we identified five novel viruses from the families Reoviridae, Tombusviridae, Mitoviridae and Bunyaviridae. Overall, this study highlights the complex interaction between Wolbachia and RNA virus infections and provides a baseline description of the natural virome of D. simulans.

Host-endoparasitoid-endosymbiont relationships: concealed Strepsiptera provide new twist to Wolbachia in Australian tephritid fruit flies

Wolbachia are widespread endosymbionts that affect arthropod reproduction and fitness. Mostly maternally inherited, Wolbachia are occasionally transferred horizontally. Previously, two Wolbachia strains were reported at low prevalence and titres across seven Australian tephritid species, possibly indicative of frequent horizontal transfer. Here, we performed whole-genome sequencing of field-caught Wolbachia-positive flies. Unexpectedly, we found complete mitogenomes of an endoparasitic strepsipteran, Dipterophagus daci, suggesting that Wolbachia in the flies are linked to concealed parasitization. We performed the first genetic characterization of D. daci and detected D. daci in Wolbachia-positive flies not visibly parasitized, and most but not all Wolbachia-negative flies were D. daci-negative, presumably reflecting polymorphism for the Wolbachia infections in D. daci. We dissected D. daci from stylopized flies and confirmed that Wolbachia infects D. daci, but also found Wolbachia in stylopized fly tissues, likely somatic, horizontally transferred, non-heritable infections. Furthermore, no Wolbachia cif and wmk genes were detected and very low mitogenomic variation in D. daci across its distribution. Therefore, Wolbachia may influence host fitness without reproductive manipulation. Our study of 13 tephritid species highlights that concealed early stages of strepsipteran parasitization led to the previous incorrect assignment of Wolbachia co-infections to tephritid species, obscuring ecological studies of this common endosymbiont and its horizontal transmission by parasitoids.

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

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.

Does getting defensive get you anywhere?-Seasonal balancing selection, temperature, and parasitoids shape real-world, protective endosymbiont dynamics in the pea aphid

Facultative, heritable endosymbionts are found at intermediate prevalence within most insect species, playing frequent roles in their hosts' defence against environmental pressures. Focusing on Hamiltonella defensa, a common bacterial endosymbiont of aphids, we tested the hypothesis that such pressures impose seasonal balancing selection, shaping a widespread infection polymorphism. In our studied pea aphid (Acyrthosiphon pisum) population, Hamiltonella frequencies ranged from 23.2% to 68.1% across a six-month longitudinal survey. Rapid spikes and declines were often consistent across fields, and we estimated that selection coefficients for Hamiltonella-infected aphids changed sign within this field season. Prior laboratory research suggested antiparasitoid defence as the major Hamiltonella benefit, and costs under parasitoid absence. While a prior field study suggested these forces can sometimes act as counter-weights in a regime of seasonal balancing selection, our present survey showed no significant relationship between parasitoid wasps and Hamiltonella prevalence. Field cage experiments provided some explanation: parasitoids drove modest ~10% boosts to Hamiltonella frequencies that would be hard to detect under less controlled conditions. They also showed that Hamiltonella was not always costly under parasitoid exclusion, contradicting another prediction. Instead, our longitudinal survey - and two overwintering studies - showed temperature to be the strongest predictor of Hamiltonella prevalence. Matching some prior lab discoveries, this suggested that thermally sensitive costs and benefits, unrelated to parasitism, can shape Hamiltonella dynamics. These results add to a growing body of evidence for rapid, seasonal adaptation in multivoltine organisms, suggesting that such adaptation can be mediated through the diverse impacts of heritable bacterial endosymbionts.

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.

Wolbachia infection in wild mosquitoes (Diptera: Culicidae): implications for transmission modes and host-endosymbiont associations in Singapore

Background

Wolbachia are intracellular bacterial endosymbionts found in most insect lineages. In mosquitoes, the influence of these endosymbionts on host reproduction and arboviral transmission has spurred numerous studies aimed at using Wolbachia infection as a vector control technique. However, there are several knowledge gaps in the literature and little is known about natural Wolbachia infection across species, their transmission modes, or associations between various Wolbachia lineages and their hosts. This study aims to address these gaps by exploring mosquito-Wolbachia associations and their evolutionary implications.

Methods

We conducted tissue-specific polymerase chain reaction screening for Wolbachia infection in the leg, gut and reproductive tissues of wild mosquitoes from Singapore using the Wolbachia surface protein gene (wsp) molecular marker. Mosquito-Wolbachia associations were explored using three methods—tanglegram, distance-based, and event-based methods—and by inferred instances of vertical transmission and host shifts.

Results

Adult mosquitoes (271 specimens) representing 14 genera and 40 species were screened for Wolbachia. Overall, 21 species (51.2%) were found positive for Wolbachia, including five in the genus Aedes and five in the genus Culex. To our knowledge, Wolbachia infections have not been previously reported in seven of these 21 species: Aedes nr. fumidus, Aedes annandalei, Uranotaenia obscura, Uranotaenia trilineata, Verrallina butleri, Verrallina sp. and Zeugnomyia gracilis. Wolbachia were predominantly detected in the reproductive tissues, which is an indication of vertical transmission. However, Wolbachia infection rates varied widely within a mosquito host species. There was no clear signal of cophylogeny between the mosquito hosts and the 12 putative Wolbachia strains observed in this study. Host shift events were also observed.

Conclusions

Our results suggest that the mosquito-Wolbachia relationship is complex and that combinations of transmission modes and multiple evolutionary events likely explain the observed distribution of Wolbachia diversity across mosquito hosts. These findings have implications for a better understanding of the diversity and ecology of Wolbachia and for their utility as biocontrol agents.

Pervasive Effects of Wolbachia on Host Temperature Preference

Heritable symbionts can modify a range of ecologically important host traits, including behavior. About half of all insect species are infected with maternally transmitted Wolbachia, a bacterial endosymbiont known to alter host reproduction, nutrient acquisition, and virus susceptibility. Here, we broadly test the hypothesis that Wolbachia modifies host behavior by assessing the effects of eight different Wolbachia strains on the temperature preference of six Drosophila melanogaster subgroup species. Four of the seven host genotypes infected with A-group Wolbachia strains (wRi in Drosophila simulans, wHa in D. simulans, wSh in Drosophila sechellia, and wTei in Drosophila teissieri) prefer significantly cooler temperatures relative to uninfected genotypes. Contrastingly, when infected with divergent B-group wMau, Drosophila mauritiana prefers a warmer temperature. For most strains, changes to host temperature preference do not alter Wolbachia titer. However, males infected with wSh and wTei tend to experience an increase in titer when shifted to a cooler temperature for 24 h, suggesting that Wolbachia-induced changes to host behavior may promote bacterial replication. Our results indicate that Wolbachia modifications to host temperature preference are likely widespread, which has important implications for insect thermoregulation and physiology. Understanding the fitness consequences of these Wolbachia effects is crucial for predicting evolutionary outcomes of host-symbiont interactions, including how Wolbachia spreads to become common.IMPORTANCE Microbes infect a diversity of species, influencing the performance and fitness of their hosts. Maternally transmitted Wolbachia bacteria infect most insects and other arthropods, making these bacteria some of the most common endosymbionts in nature. Despite their global prevalence, it remains mostly unknown how Wolbachia influence host physiology and behavior to proliferate. We demonstrate pervasive effects of Wolbachia on Drosophila temperature preference. Most hosts infected with A-group Wolbachia prefer cooler temperatures, whereas the one host species infected with divergent B-group Wolbachia prefers warmer temperatures, relative to uninfected genotypes. Changes to host temperature preference generally do not alter Wolbachia abundance in host tissues, but for some A-group strains, adult males have increased Wolbachia titer when shifted to a cooler temperature. This suggests that Wolbachia-induced changes to host behavior may promote bacterial replication. Our results help elucidate the impact of endosymbionts on their hosts amid the global Wolbachia pandemic.

The effect of the endosymbiont Wolbachia on the behavior of insect hosts

As one of the most successful intracellular symbiotic bacteria, Wolbachia can infect many arthropods and nematodes. Wolbachia infection usually affects the reproduction of their hosts to promote their own proliferation and transmission. Currently, most of the studies focus on the mechanisms of Wolbachia interactions with host reproduction. However, in addition to distribution in the reproductive tissues, Wolbachia also infect various somatic tissues of their hosts, including the brain. This raises the potential that Wolbachia may influence some somatic processes, such as behaviors in their hosts. So far, information about the effects of Wolbachia infection on host behavior is still very limited. The present review presents the current literature on different aspects of the influence of Wolbachia on various behaviors, including sleep, learning and memory, mating, feeding and aggression in their insect hosts. We then highlight ongoing scientific efforts in the field that need addressing to advance this field, which can have significant implications for further developing Wolbachia as environmentally friendly biocontrol agents to control insect-borne diseases and agricultural pests.

Phylogeny and Density Dynamics of Wolbachia Infection of the Health Pest Paederus fuscipes Curtis (Coleoptera: Staphylinidae)

The maternally inherited obligate intracellular bacteria Wolbachia infects the reproductive tissues of a wide range of arthropods and affects host reproduction. Wolbachia is a credible biocontrol agent for reducing the impact of diseases associated with arthropod vectors. Paederus fuscipes is a small staphylinid beetle that causes dermatitis linearis and conjunctivitis in humans when they come into contact with skin. Wolbachia occur in this beetle, but their relatedness to other Wolbachia, their infection dynamics, and their potential host effects remain unknown. In this study, we report the phylogenetic position and density dynamics of Wolbachia in P. fuscipes. The phylogeny of Wolbachia based on an analysis of MLST genotyping showed that the bacteria from P. fuscipes belong to supergroup B. Quantitative PCR indicated that the infection density in adults was higher than in any other life stage (egg, larva or pupa), and that reproductive tissue in adults had the highest infection densities, with similar densities in the sexes. These findings provide a starting point for understanding the Wolbachia infection dynamics in P. fuscipes, and interactions with other components of the microbiota.

Phenotypic Response of Wolbachia pipientis in a Cell-Free Medium

Wolbachia, an obligate intracellular bacterium estimated to infect millions of arthropod species worldwide, is currently being utilized in novel control strategies to limit the transmission of Dengue and Zika viruses. A limitation for Wolbachia-based control approaches is the difficulty of transferring Wolbachia to novel hosts and the lack of tools for the genetic transformation of Wolbachia due to the inability to culture Wolbachia outside the insect host cell in an axenic media. Here, we applied extracellular Wolbachia to phenotypic microarrays to measure the metabolic response of Wolbachia in media formulations with different pH levels and supplementation with Casamino acids. Results suggested a pH of 6.5–6.8 and showed that the supplementation of 1 mg/mL casamino acids increased the survival and longevity of Wolbachia in an axenic medium. In addition, phenotypic microarrays are a useful tool to measure the phenotypic response of Wolbachia under different media conditions, as well as determine specific components that may be required for an axenic medium. This study is an initial step toward the development of a potential Wolbachia axenic culture system.

Wolbachia pipientis Associated With Tephritid Fruit Fly Pests: From Basic Research to Applications

Members of the true fruit flies (family Tephritidae) are among the most serious agricultural pests worldwide, whose control and management demands large and costly international efforts. The need for cost-effective and environmentally friendly integrated pest management (IPM) has led to the development and implementation of autocidal control strategies. These approaches include the widely used sterile insect technique and the incompatible insect technique (IIT). IIT relies on maternally transmitted bacteria (namely Wolbachia) to cause a conditional sterility in crosses between released mass-reared Wolbachia-infected males and wild females, which are either uninfected or infected with a different Wolbachia strain (i.e., cytoplasmic incompatibility; CI). Herein, we review the current state of knowledge on Wolbachia-tephritid interactions including infection prevalence in wild populations, phenotypic consequences, and their impact on life history traits. Numerous pest tephritid species are reported to harbor Wolbachia infections, with a subset exhibiting high prevalence. The phenotypic effects of Wolbachia have been assessed in very few tephritid species, due in part to the difficulty of manipulating Wolbachia infection (removal or transinfection). Based on recent methodological advances (high-throughput DNA sequencing) and breakthroughs concerning the mechanistic basis of CI, we suggest research avenues that could accelerate generation of necessary knowledge for the potential use of Wolbachia-based IIT in area-wide integrated pest management (AW-IPM) strategies for the population control of tephritid pests.

Quenching autofluorescence in the alimentary canal tissues of Bactericera cockerelli (Hemiptera: Triozidae) for immunofluorescence labeling

Immunofluorescence has been widely used to localize microbes or specific molecules in insect tissues or cells. However, significant autofluorescence is frequently observed in tissues which can interfere with the fluorescent identification of target antigens, leading to inaccurate or even false positive fluorescent labeling. The alimentary canal of the potato psyllid, Bactericera cockerelli Šulc, exhibits intense autofluorescence, hindering the application of immunolocalization for the detection and localization of the economically important pathogen transmitted by this insect, "Candidatus Liberibacter solanacearum" (Lso). In the present study, we tested the use of irradiation, hydrogen peroxide (H₂ O₂ ) and Sudan black B (SBB) treatments to reduce the autofluorescence in the B. cockerelli alimentary canal tissues. Furthermore, we assessed the compatibility of the above-mentioned treatments with Lso immunolocalization and actin staining using phalloidin. Our results showed that the autofluorescence in the alimentary canal was reduced by irradiation, H₂ O₂ , or SBB treatments. The compatibility assays indicated that irradiation and H₂ O₂ treatment both greatly reduced the fluorescent signal associated with Lso and actin. However, the SBB incubation preserved those target signals, while efficiently eliminating autofluorescence in the psyllid alimentary canal. Therefore, herein we propose a robust method for reducing the autofluorescence in the B. cockerelli alimentary canal with SBB treatment, which may improve the use of immunofluorescence labeling in this organism. This method may also have a wide range of uses by reducing the autofluorescence in other arthropod species.

Age, tissue, genotype and virus infection regulate Wolbachia levels in Drosophila

The bacterial symbiont Wolbachia can protect insects against viral pathogens, and the varying levels of antiviral protection are correlated with the endosymbiont load within the insects. To understand why Wolbachia strains differ in their antiviral effects, we investigated the factors controlling Wolbachia density in five closely related strains in their natural Drosophila hosts. We found that Wolbachia density varied greatly across different tissues and between flies of different ages, and these effects depended on the host-symbiont association. Some endosymbionts maintained largely stable densities as flies aged while others increased, and these effects in turn depended on the tissue being examined. Measuring Wolbachia rRNA levels in response to viral infection, we found that viral infection itself also altered Wolbachia levels, with Flock House virus causing substantial reductions in symbiont loads late in the infection. This effect, however, was virus-specific as Drosophila C virus had little impact on Wolbachia in all of the five host systems. Because viruses have strong tissue tropisms and antiviral protection is thought to be cell-autonomous, these effects are likely to affect the virus-blocking phenomenon. However, we were unable to find any evidence of a correlation between Wolbachia and viral titres within the same tissues. We conclude that Wolbachia levels within flies are regulated in a complex host-symbiont-virus-dependent manner and this trinity is likely to influence the antiviral effects of Wolbachia.

The environment and species affect gut bacteria composition in laboratory co-cultured Anopheles gambiae and Aedes albopictus mosquitoes

The midgut microbiota of disease vectors plays a critical role in the successful transmission of human pathogens. The environment influences the microbiota composition; however, the relative mosquito-species contribution has not been rigorously disentangled from the environmental contribution to the microbiota structure. Also, the extent to which the microbiota of the adult sugar food source and larval water can predict that of the adult midgut and vice versa is not fully understood. To address these relationships, larvae and adults of Anopheles gambiae and Aedes albopictus were either reared separately or in a co-rearing system, whereby aquatic and adult stages of both species shared the larval water and sugar food source, respectively. Despite being reared under identical conditions, clear intra- and interspecies differences in midgut microbiota-composition were observed across seven cohorts, collected at different time points over a period of eight months. Fitting a linear model separately for each OTU in the mosquito midgut showed that two OTUs significantly differed between the midguts of the two mosquito species. We also show an effect for the sugar food source and larval water on the adult midgut microbiota. Our findings suggest that the mosquito midgut microbiota is highly dynamic and controlled by multiple factors.

An elusive endosymbiont: Does Wolbachia occur naturally in Aedes aegypti?

Wolbachia are maternally inherited endosymbiotic bacteria found within many insect species. Aedes mosquitoes experimentally infected with Wolbachia are being released into the field for Aedes-borne disease control. These Wolbachia infections induce cytoplasmic incompatibility which is used to suppress populations through incompatible matings or replace populations through the reproductive advantage provided by this mechanism. However, the presence of naturally occurring Wolbachia in target populations could interfere with both population replacement and suppression programs depending on the compatibility patterns between strains. Aedes aegypti were thought to not harbor Wolbachia naturally but several recent studies have detected Wolbachia in natural populations of this mosquito. We therefore review the evidence for natural Wolbachia infections in A. aegypti to date and discuss limitations of these studies. We draw on research from other mosquito species to outline the potential implications of natural Wolbachia infections in A. aegypti for disease control. To validate previous reports, we obtained a laboratory population of A. aegypti from New Mexico, USA, that harbors a natural Wolbachia infection, and we conducted field surveys in Kuala Lumpur, Malaysia, where a natural Wolbachia infection has also been reported. However, we were unable to detect Wolbachia in both the laboratory and field populations. Because the presence of naturally occurring Wolbachia in A. aegypti could have profound implications for Wolbachia-based disease control programs, it is important to continue to accurately assess the Wolbachia status of target Aedes populations.

Wolbachia infection in Argentinean populations of Anastrepha fraterculus sp1: preliminary evidence of sex ratio distortion by one of two strains

Background

Wolbachia, one of the most abundant taxa of intracellular Alphaproteobacteria, is widespread among arthropods and filarial nematodes. The presence of these maternally inherited bacteria is associated with modifications of host fitness, including a variety of reproductive abnormalities, such as cytoplasmic incompatibility, thelytokous parthenogenesis, host feminization and male-killing. Wolbachia has attracted much interest for its role in biological, ecological and evolutionary processes as well as for its potential use in novel and environmentally-friendly strategies for the control of insect pests and disease vectors including a major agricultural pest, the South American fruit fly, Anastrepha fraterculus Wiedemann (Diptera: Tephritidae).

Results

We used wsp, 16S rRNA and a multilocus sequence typing (MLST) scheme including gatB, coxA, hcpA, fbpA, and ftsZ genes to detect and characterize the Wolbachia infection in laboratory strains and wild populations of A. fraterculus from Argentina. Wolbachia was found in all A. fraterculus individuals studied. Nucleotide sequences analysis of wsp gene allowed the identification of two Wolbachia nucleotide variants (named wAfraCast1_A and wAfraCast2_A). After the analysis of 76 individuals, a high prevalence of the wAfraCast2_A variant was found both, in laboratory (82%) and wild populations (95%). MLST analysis identified both Wolbachia genetic variants as sequence type 13. Phylogenetic analysis of concatenated MLST datasets clustered wAfraCast1/2_A in the supergroup A. Paired-crossing experiments among single infected laboratory strains showed a phenotype specifically associated to wAfraCast1_A that includes slight detrimental effects on larval survival, a female-biased sex ratio; suggesting the induction of male-killing phenomena, and a decreased proportion of females producing descendants that appears attributable to the lack of sperm in their spermathecae.

Conclusions

We detected and characterized at the molecular level two wsp gene sequence variants of Wolbachia both in laboratory and wild populations of A. fraterculus sp.1 from Argentina. Crossing experiments on singly-infected A. fraterculus strains showed evidence of a male killing-like mechanism potentially associated to the wAfraCast1_A - A. fraterculus interactions. Further mating experiments including antibiotic treatments and the analysis of early and late immature stages of descendants will contribute to our understanding of the phenotypes elicited by the Wolbachia variant wAfraCast1_A in A. fraterculus sp.1.

Tissue- and Population-Level Microbiome Analysis of the Wasp Spider Argiope bruennichi Identified a Novel Dominant Bacterial Symbiont

Many ecological and evolutionary processes in animals depend upon microbial symbioses. In spiders, the role of the microbiome in these processes remains mostly unknown. We compared the microbiome between populations, individuals, and tissue types of a range-expanding spider, using 16S rRNA gene sequencing. Our study is one of the first to go beyond targeting known endosymbionts in spiders and characterizes the total microbiome across different body compartments (leg, prosoma, hemolymph, book lungs, ovaries, silk glands, midgut, and fecal pellets). Overall, the microbiome differed significantly between populations and individuals, but not between tissue types. The microbiome of the wasp spider Argiope bruennichi features a novel dominant bacterial symbiont, which is abundant in every tissue type in spiders from geographically distinct populations and that is also present in offspring. The novel symbiont is affiliated with the Tenericutes, but has low sequence identity (<85%) to all previously named taxa, suggesting that the novel symbiont represents a new bacterial clade. Its presence in offspring implies that it is vertically transmitted. Our results shed light on the processes that shape microbiome differentiation in this species and raise several questions about the implications of the novel dominant bacterial symbiont on the biology and evolution of its host.

Links between metamorphosis and symbiosis in holometabolous insects

Many animals depend on microbial symbionts to provide nutrition, defence or other services. Holometabolous insects, as well as other animals that undergo metamorphosis, face unique constraints on symbiont maintenance. Microbes present in larvae encounter a radical transformation of their habitat and may also need to withstand chemical and immunological challenges. Metamorphosis also provides an opportunity, in that symbiotic associations can be decoupled over development. For example, some holometabolous insects maintain the same symbiont as larvae and adults, but house it in different tissues; in other species, larvae and adults may harbour entirely different types or numbers of microbes, in accordance with shifts in host diet or habitat. Such flexibility may provide an advantage over hemimetabolous insects, in which selection on adult-stage microbial associations may be constrained by its negative effects on immature stages, and vice versa. Additionally, metamorphosis itself can be directly influenced by symbionts. Across disparate insect taxa, microbes protect hosts from pathogen infection, supply nutrients essential for rebuilding the adult body and provide cues regulating pupation. However, microbial associations remain completely unstudied for many families and even orders of Holometabola, and future research will undoubtedly reveal more links between metamorphosis and microbiota, two widespread features of animal life. This article is part of the theme issue 'The evolution of complete metamorphosis'.

Assessment of the role of Wolbachia in mtDNA paraphyly and the evolution of unisexuality in Calligrapha (Coleoptera: Chrysomelidae)

Calligrapha is a New World leaf beetle genus that includes several unisexual species in northeastern North America. Each unisexual species had an independent hybrid origin involving different combinations of bisexual species. However, surprisingly, they all cluster in a single mtDNA clade and with some individuals of their parental species, which are in turn deeply polyphyletic for mtDNA. This pattern is suggestive of a selective sweep which, together with mtDNA taxonomic incongruence and occurrence of unisexuality in Calligrapha, led to hypothesize that Wolbachia might be responsible. I tested this hypothesis studying the correlation between diversity of Wolbachia and well-established mtDNA lineages in >500 specimens of two bisexual species of Calligrapha and their derived unisexual species. Wolbachia appears highly prevalent (83.4%), and fifteen new supergroup-A strains of the bacteria are characterized, belonging to three main classes: wCallA, occupying the whole species ranges, and wCallB and wCallC, narrowly parapatric, infecting beetles with highly divergent mtDNAs where they coexist. Most beetles (71.6%) carried double infections of wCallA with another sequence class. Bayesian inference of ancestral character states and association tests between bacterial diversity and the mtDNA genealogy show that each mtDNA lineage of Calligrapha has specific types of infection. Moreover, shifts can be explained by horizontal or vertical transfer from local populations to an expanding lineage and cytoplasmic incompatibility between wCallB and wCallC types, suggesting that the symbionts hitchhike with the host and are not responsible for selective mtDNA sweeps. Lack of evidence for sweeps and the fact that individuals in the unisexual clade are uninfected or infected by the widespread wCallA type indicate that Wolbachia does not induce unisexuality in Calligrapha, although they may manipulate host reproduction through cytoplasmic incompatibility.

Variations in the relative abundance of Wolbachia in the gut of Nasutitermes arborum across life stages and castes

There are multiple forms of interactions between termites and bacteria. In addition to their gut microbiota, which has been intensively studied, termites host intracellular symbionts such as Wolbachia. These distinct symbioses have been so far approached independently and mostly in adult termites. We addressed the dynamics of Wolbachia and the microbiota of the eggs and gut for various life stages and castes of the wood-feeding termite, Nasutitermes arborum, using deep-sequencing of the 16S rRNA gene. Wolbachia was dominant in eggs as expected. Unexpectedly, it persisted in the gut of nearly all stages and castes, indicating a wide somatic distribution in termites. Wolbachia-related sequences clustered into few operational taxonomic units, but these were within the same genotype, acquired maternally. Wolbachia was largely dominant in DNA extracts from the guts of larvae and pre-soldiers (59.1%-99.1% of reads) where gut-resident lineages were less represented and less diverse. The reverse was true for the adult castes. This is the first study reporting the age-dependency of the relative abundance of Wolbachia in the termite gut and its negative correlation with the diversity of the microbiota. The possible mechanisms underlying this negative interaction are discussed.

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

Background

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

Results

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

Conclusions

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

Electronic supplementary material

The online version of this article (10.1186/s12866-019-1579-3) contains supplementary material, which is available to authorized users.

16S rRNA Gene Sequencing Reveals a Shift in the Microbiota of Diaphorina citri During the Psyllid Life Cycle

The Asian citrus psyllid (Diaphorina citri) is a major pest of citrus trees as it transmits Candidatus Liberibacter asiaticus (CLas). The composition of a host’s microbiota can affect the evolution and ecological distribution of the host. This study monitored the compositional shifts in the citrus psyllid microbiota through all the life stages (egg, nymph 1–5 stages, and adult) by next-generation sequencing (NGS) and quantitative real-time PCR. There were clear differences in both α- and β-diversity of microbiota through the psyllid life stages. Microbiota diversity was markedly higher in the nymph 2–5 stages than in the adult, egg, and nymph 1 stages. Proteobacteria were dominant in all the life stages of D. citri, representing >97.5% of the total bacterial community, and Candidatus Profftella armature was the dominant genus in all the life stages. Data from the qPCR analysis showed an exponential increase in the populations of three D. citri endosymbionts: Candidatus Profftella armature, Candidatus Carsonella ruddii, and Wolbachia. The gut bacterium Pantoea was present in all the life stages, but it was markedly higher in the nymph 2–5 stages. The microbiota composition substantially differed among the egg–nymph 1, nymphs 2–5, and adult stages. Therefore, we successfully characterized the microbiota dynamics and thus identified a microbiota shift during the life cycle of D. citri by 16S rRNA gene sequencing and quantitative PCR. Moreover, 16S rRNA gene sequencing suggested that D. citri acquired the ability to bear CLas in the nymph 1 stage. This study enhances our understanding of microbial establishment in the developing D. citri and provides a reference resource for the identification of potential biocontrol approaches against this pest.

Co-occurrence of pederin-producing and Wolbachia endobacteria in Paederus fuscipes Curtis, 1840 (Coleoptera: Staphilinidae) and its evolutionary consequences

The dual occurrence of Pseudomonas-like and Wolbachia endobacteria has not been investigated in the Pederus beetles yet. We investigated pederin-producing bacteria (PPB) infection in Paederus fuscipes specimens from the southern margins of the Caspian Sea by designed genus-specific (OprF) and species-specific (16S rRNA) primers. Wolbachia infection was studied through a nested-PCR assay of Wolbachia surface protein (wsp) gene. Of the 125 analyzed beetles, 42 females (82.35%) and 15 males (20.27%) were positive to PPB infection; this is the first study reporting male P. fuscipes infection to PPB. Wolbachia infection was found in 45 female (88.23%) and 50 male (67.57%) analyzed beetles. Surprisingly, a number of 36 females (70.59%) and 13 males (17.57%) were found to be infected with both PPB and Wolbachia endosymbionts. In general, population infection rates to PPB and Wolbachia were determined to be 45.6% and 76%, respectively. The infection rates of female beetles to PPB and PPB-Wolbachia were significantly higher than males. In Paederus species, only female beetles shelter PPB and the discovery of this bacterium in adult males may reflect their cannibalistic behavior on the contaminated stages. Phylogenetic analysis showed that the sequences of OprF gene were unique among Pseudomonas spp.; however, sequences of 16S rRNA gene were related to the PPB of Pederus species. The co-occurrence and random distribution of these endobacteria may imply putative tripartite interactions among PPB, Wolbachia, and Paederus. In order to elucidate these possible tripartite interactions, further studies are required even at gender level.

Wolbachia localization during Laodelphax striatellus embryogenesis

Wolbachia are intracellular bacteria carried by thousands of arthropod species. The success of Wolbachia is due to efficient vertical transmission by the host maternal germline. Wolbachia's behavior during host oogenesis is well characterized, although their behavior during embryogenesis is unclear. Vertical transmission of Wolbachia wStri in the small brown planthopper, Laodelphax striatellus is extraordinarily efficient. To understand why, we investigated its localization and dynamics in L. striatellus embryos. Microscopic observations indicated that the Wolbachia were mainly localized at the anterior region of the embryo during early embryogenesis. The distribution of Wolbachia within the anterior region was established during oogenesis, and according to a phylogenetic analysis, may be due to intrinsic factors in Wolbachia. We observed that wStri migrated to the posterior part cells during late embryogenesis, in the region where gonads were formed. An expression profile of Wolbachia-infected host embryonic development genes revealed Ddx1 mRNAs, which is required for host viability and in the germ line, accumulated in the posterior region of 3-day-old embryos, while other development genes mRNAs were significantly more abundant in the posterior region of 6-day-old embryos. These genes thus appear to be associated with the localization of Wolbachia wStri in the anterior region, although their functions remain unclear. These results can explain Wolbachia wStri high prevalence in L. striatellus.

The hemolymph microbiome of insects

Hemolymph has long been recognized as a key mediator of nutritional and immunological homeostasis in insects, with the tacit understanding that hemolymph is a hostile environment for microorganisms, and microbiologically sterile in healthy insects. Recent research is overturning the conventional wisdom, and there is now overwhelming evidence that various non-pathogenic microorganisms can stably or transiently inhabit hemolymph in a diversity of insects. Most is known about Spiroplasma, especially in Drosophila species, and secondary symbionts of the Enterobacteriaceae, notably Hamiltonella defensa, in aphids. These bacteria require many nutrients, representing a likely drain on host nutritional resources, and they persist in the hemolymph by a combination of evasion and tolerance of insect immune effectors. These traits can be costly to the insect host. For some hemolymph microorganisms, these costs are balanced by other traits beneficial to the insect, notably protection against natural enemies mediated by specific toxins or competition for key nutrients. Three key priorities for future research are: to investigate the prevalence and taxonomic diversity of hemolymph microorganisms in insects; to establish the role of host nutritional and immune factors as determinants of the abundance and proliferation rates of hemolymph microorganisms; and to integrate the developing understanding of these microorganisms and their impacts (both costs and benefits) on insect nutrition and immune function into the wider study of insect physiology.