Gammaproteobacteria as essential primary symbionts in the striped shield bug, Graphosoma Lineatum (Hemiptera: Pentatomidae)

The burrower bug Macroscytus japonensis (Hemiptera: Cydnidae) acquires obligate symbiotic bacteria from the environment

Many plant-feeding stinkbugs belonging to the infraorder Pentatomomorpha possess a specialized symbiotic organ at the posterior end of the midgut, in which mutualistic bacterial symbionts are harbored extracellularly. In species of the superfamily Pentatomoidea, these symbionts typically are verticallytransmitted from host mothers to offspring, whereas in species of the superfamilies Coreoidea and Lygaeoidea they are acquired from the environment. In the pentatomoid family Cydnidae, vertical symbiont transmission has been reported in several species. Here, we report the first case of environmental symbiont acquisition in Cydnidae, observed in the burrower bug Macroscytus japonensis. A comprehensive survey of 72 insect samples from 23 sites across the Japanese archipelago revealed that (1) symbionts exhibit remarkably high diversity, forming six distinct phylogenetic groups within the Enterobacteriaceae of the γ-Proteobacteria, (2) most symbionts are cultivable and closely related to free-living Pantoea-allied bacteria, and (3) symbiont phylogenetic groups do not reflect the host phylogeny. Microbial inspection of eggs revealed the absence of bacteria on the egg surface. These results strongly suggest that symbionts are acquired from the environment, not vertical transmission. Rearing experiments confirmed environmental symbiont acquisition. When environmental symbiont sources were experimentally withheld, nymphs became aposymbiotic and died before molting to the second instar, indicating that nymphs environmentally acquire symbionts during the first-instar stage and that symbionts are essential for nymphal growth and survival. This study highlights Cydnidae as the only pentatomoid family that includes species that environmentally acquire symbionts and those that vertically transmit symbionts, providing an ideal platform for comparative studies of the ecological and environmental factors that influence the evolution of symbiont transmission modes.

Wolbachia affect female mate preference and offspring fitness in a parasitoid wasp

BACKGROUND

Wolbachia are widespread intracellular bacteria in insects that often have high rates of spread due to their impact on insect reproduction. These bacteria may also affect the mating behavior of their host with impacts on the fitness of host progeny. In this study, we investigated the impact of Wolbachia on a preference for mating with young or old males in the parasitoid wasp Habrobracon hebetor.

RESULTS

Our results showed that uninfected females from a tetracycline-treated line preferred to mate with young males, whereas Wolbachia-infected females had no preference. Time to mating was relatively shorter in the infected lines. Regardless of Wolbachia infection status, progeny resulting from matings with young males showed higher fitness than those from crosses with old males, and infected females crossed with infected young males showed the highest performance.

CONCLUSION

These results suggest an impact of Wolbachia on female mate preference and offspring fitness although it is unclear how this phenomenon increases Wolbachia transmission of infected wasps. © 2024 Society of Chemical Industry.

Bacterial communities varied in different Coccinella transversoguttata populations located in Tibetan plateau

Coccinella transversoguttata is an important predatory beetle in Asia and America. Currently, few studies have investigated C. transversoguttata in China especially in the Tibetan plateau. In this study, full-length 16 s rRNA sequencing and qPCR experiment were performed on eight C. transversoguttata populations collected from Tibet to analyze their bacterial communities and bacteria abundance. In summary, our results revealed the microbial compositions, diversities and bacterial titers in the bacterial communities in C. transversoguttata populations in the Tibetan plateau. In future, there is a need to explore the differences in microbiota among various C. transversoguttata populations collected from different locations. These results add to our understanding of the complex bacterial communities of C. transversoguttata and their utilization as potential biocontrol factors.

Differential induction of NF-κB pathways by non-pathogenic and pathogenic bacteria in Helicoverpa armigera is critical for an efficient immune response and survival

Following pathogen infection in a host, extensive changes occur in the host's gene expression pattern to suppress infection and increase the chance of host survival. Likewise, many pathogens have evolved to evade/suppress host immunity and increase their survival within the host. In this study, we assessed the NF-κB (Imd and Toll) essential gene expression response of Helicoverpa armigera to an entomopathogenic Serratia marcescens and non-pathogenic Escherichia coli. Bacterial cells of S. marcescens or E. coli were injected into the haemocoel of fifth-instar larvae of H. armigera, whereas distilled water was injected into control insects. Our results showed that the expression levels of the Imd and Toll pathway genes (i.e., Relish, imd, spätzle and dif) and the antimicrobial peptides (i.e., gloverin, transferin, gallerimycin, and galiomicin) were differentially expressed following the bacterial injections while control larvae showed no differences. The E. coli injection induced higher and longer-lasted gene expression than the S. marcescens injected larvae, in which the gene expressions were diminished from 24 h post injection. Transcript Knockdown of Relish increased the replication rates of S. marcescens and E. coli, and lowered the infected larvae survival rates. These results showed that H. armigera NF-κB immunity pathways (particularly Imd pathway) play a vital role in immunity against bacterial infections, and S. marcescens might modulate these pathways to survive and replicate in the host.

Stepwise degradation of organic matters driven by microbial interactions in China΄s coastal wetlands: Evidence from carbon isotope analysis

The microbial "unseen majority" as drivers of carbon cycle represent a significant source of uncertain climate change. To comprehend the resilience of life forms on Earth to climate change, it is crucial to incorporate knowledge of intricate microbial interactions and their impact to carbon transformation. Combined with carbon stable isotope analysis and high-throughput sequencing technology, the underlying mechanism of microbial interactions for organic carbon degradation has been elucidated. Niche differentiation enabled archaea to coexist with bacteria mainly in a cooperative manner. Bacteria composed of specialists preferred to degrade lighter carbon, while archaea were capable of utilizing heavier carbon. Microbial resource-dependent interactions drove stepwise degradation of organic matter. Bacterial cooperation directly facilitated the degradation of algae-dominated particulate organic carbon, while competitive feeding of archaea caused by resource scarcity significantly promoted the mineralization of heavier particulate organic carbon and then the release of dissolved inorganic carbon. Meanwhile, archaea functioned as a primary decomposer and collaborated with bacteria in the gradual degradation of dissolved organic carbon. This study emphasized microbial interactions driving carbon cycle and provided new perspectives for incorporating microorganisms into carbon biogeochemical models.

Mixta mediterraneensis as a novel and abundant gut symbiont of the allergen-producing domestic mite Blomia tropicalis

Blomia tropicalis is an allergen-producing mite in the human environment in tropical regions. The microbiome of B. tropicalis was described using the barcode sequencing region of V4 16S rDNA and genome assemblage. Mixta mediterraneensis, previously isolated from human skin swabs, was identified as a B. tropicalis gut symbiont based on genome assembly. The microbiome contains two bacteria, Staphylococcus and M. mediterraneensis. The number of M. mediterraneensis 16S DNA copies was 106 per mite and 109 per feces in the rearing chamber based on qPCR quantification. The profile of this bacterium reached 50% of reads in the mite gut and feces. Genomic analyses revealed that the bacterium has several metabolic pathways that suggest metabolic cooperation with the mite host in vitamin and amino acid synthesis, nitrogen recycling, and antimicrobial defense. Lysozyme is present in the symbiotic bacterium but absent in the mite. The B. tropicalis microbiome contained Staphylococcus, which accelerates mite population growth. Mites can digest Staphylococcus by using specific enzymes with hydrolytic functions against bacterial cell walls (chitinases and cathepsin D), leading to endocytosis of bacteria and their degradation in lysosomes and phagosomes. Gene expression analysis of B. tropicalis indicated that phagocytosis was mediated by the PI3-kinase/Akt pathway interacting with the invasins produced by M. mediterraneensis. Moreover, the symbiont had metabolic pathways that allowed it to recycle the mite metabolic waste product guanine, known as a mite attractant. The mite host symbiont enhances mite aggregation in the feces, and the fecal-oral transmission route is excepted.

Microbiota of pest insect Nezara viridula mediate detoxification and plant defense repression

The Southern green shield bug, Nezara viridula, is an invasive piercing and sucking pest insect that feeds on crop plants and poses a threat to global food production. Given that insects are known to live in a close relationship with microorganisms, our study provides insights into the community composition and function of the N. viridula-associated microbiota and its effect on host-plant interactions. We discovered that N. viridula hosts both vertically and horizontally transmitted microbiota throughout different developmental stages and their salivary glands harbor a thriving microbial community that is transmitted to the plant while feeding. The N. viridula microbiota was shown to aid its host with the detoxification of a plant metabolite, namely 3-nitropropionic acid, and repression of host plant defenses. Our results demonstrate that the N. viridula-associated microbiota plays an important role in interactions between insects and plants and could therefore be considered a valuable target for the development of sustainable pest control strategies.

Episyrphus balteatus symbiont variation across developmental stages, living states, two sexes, and potential horizontal transmission from prey or environment

Introduction

Episyrphus balteatus is one representative Syrphidae insect which can provide extensive pollination and pest control services. To date, the symbiont composition and potential acquisition approaches in Syrphidae remain unclear.

Methods

Herein, we investigated microbiota dynamics across developmental stages, different living states, and two sexes in E. balteatus via full-length 16S rRNA genes sequencing, followed by an attempt to explore the possibility of symbiont transmission from prey Megoura crassicauda to the hoverfly.

Results

Overall, Proteobacteria and Firmicutes were the dominant bacteria phyla with fluctuating relative abundances across the life stage. Cosenzaea myxofaciens is dominant in adulthood, while Enterococcus silesiacus and Morganella morganii dominate in larvae and pupae of E. balteatus, respectively. Unexpectedly, Serratia symbiotica, one facultative endosymbiont commonly harbored in aphids, was one of the predominant bacteria in larvae of E. balteatus, just behind Enterococcus silesiacus. In addition, S. symbiotica was also surprisingly most dominated in M. crassicauda aphids (92.1% relative abundance), which are significantly higher than Buchnera aphidicola (4.7% relative abundance), the primary obligate symbiont of most aphid species. Approximately 25% mortality was observed among newly emerged adults, of which microbiota was also disordered, similar to normally dying individuals. Sexually biased symbionts and 41 bacteria species with pairwise co-occurrence in E. balteatus and 23 biomarker species for each group were identified eventually. Functional prediction showed symbionts of hoverflies and aphids, both mainly focusing on metabolic pathways. In brief, we comprehensively explored the microbiome in one Syrphidae hoverfly using E. balteatus reared indoors on M. morganii as the model, revealed its dominated symbiont species, identified sexually biased symbionts, and found an aphid facultative endosymbiont inhabited in the hoverfly. We also found that the dominated symbiotic bacteria in M. crassicauda are S. symbiotica other than Buchnera aphidicola.

Discussion

Taken together, this study provides new valuable resources about symbionts in hoverflies and prey aphids jointly, which will benefit further exploring the potential roles of microbiota in E. balteatus.

Complete metamorphosis and microbiota turnover in insects

The insects constitute the majority of animal diversity. Most insects are holometabolous: during complete metamorphosis their bodies are radically reorganized. This reorganization poses a significant challenge to the gut microbiota, as the gut is replaced during pupation, a process that does not occur in hemimetabolous insects. In holometabolous hosts, it offers the opportunity to decouple the gut microbiota between the larval and adult life stages resulting in high beta diversity whilst limiting alpha diversity. Here, we studied 18 different herbivorous insect species from five orders of holometabolous and three orders of hemimetabolous insects. Comparing larval and adult specimens, we find a much higher beta-diversity and hence microbiota turnover in holometabolous insects compared to hemimetabolous insects. Alpha diversity did not differ between holo- and hemimetabolous insects nor between developmental stages within these groups. Our results support the idea that pupation offers the opportunity to change the gut microbiota and hence might facilitate ecological niche shifts. This possible effect of niche shift facilitation could explain a selective advantage of the evolution of complete metamorphosis, which is a defining trait of the most speciose insect taxon, the holometabola.

Symbioses shape feeding niches and diversification across insects

For over 300 million years, insects have relied on symbiotic microbes for nutrition and defence. However, it is unclear whether specific ecological conditions have repeatedly favoured the evolution of symbioses, and how this has influenced insect diversification. Here, using data on 1,850 microbe-insect symbioses across 402 insect families, we found that symbionts have allowed insects to specialize on a range of nutrient-imbalanced diets, including phloem, blood and wood. Across diets, the only limiting nutrient consistently associated with the evolution of obligate symbiosis was B vitamins. The shift to new diets, facilitated by symbionts, had mixed consequences for insect diversification. In some cases, such as herbivory, it resulted in spectacular species proliferation. In other niches, such as strict blood feeding, diversification has been severely constrained. Symbioses therefore appear to solve widespread nutrient deficiencies for insects, but the consequences for insect diversification depend on the feeding niche that is invaded.

Specialized acquisition behaviors maintain reliable environmental transmission in an insect-microbial mutualism

Understanding how horizontally transmitted mutualisms are maintained is a major focus of symbiosis research.^1,2,3,4 Unlike vertical transmission, hosts that rely on horizontal transmission produce symbiont-free offspring that must find and acquire their beneficial microbes from the environment. This transmission strategy is inherently risky since hosts may not obtain the right symbiont every generation. Despite these potential costs, horizontal transmission underlies stable mutualisms involving a large diversity of both plants and animals.^5,6,7,8,9 One largely unexplored way horizontal transmission is maintained is for hosts to evolve sophisticated mechanisms to consistently find and acquire specific symbionts from the environment. Here, we examine this possibility in the squash bug Anasa tristis, an insect pest that requires bacterial symbionts in the genus Caballeronia¹⁰ for survival and development.¹¹ We conduct a series of behavioral and transmission experiments that track strain-level transmission in vivo among individuals in real-time. We demonstrate that nymphs can accurately find feces from adult bugs in both the presence and absence of those adults. Once nymphs locate the feces, they deploy feeding behavior that results in nearly perfect symbiont acquisition success. We further demonstrate that nymphs can locate and feed on isolated, cultured symbionts in the absence of feces. Finally, we show this acquisition behavior is highly host specific. Taken together, our data describe not only the evolution of a reliable horizontal transmission strategy, but also a potential mechanism that drives patterns of species-specific microbial communities among closely related, sympatric host species.

Fenoxycarb exposure affects antiviral immunity and HaNPV infection in the cotton bollworm, Helicoverpa armigera

BACKGROUND

Application of insect growth regulators (IGR) is a good option for insect pest management because of their fewer adverse effects on humans and domestic animals. These compounds are capable of interfering with normal growth and development by mimicking the actions of hormones such as juvenile hormone (JH) or ecdysone. The effect of JH and its analogs on some aspects of insect immunity has been determined, yet their possible effects on antiviral immunity response has not been investigated yet. Considering the importance of antiviral response in viral replication, in this study the effects of the JH analog (JHA), fenoxycarb on the antiviral immunity pathway core genes [i.e. micro (mi)RNA, small interfering (si)RNA and apoptosis] of Helicoverpa armigera (Hubner) larvae were investigated. The effect of fenoxycarb on the susceptibility of the larvae to H. armigera nuclear polyhedrosis virus (HaNPV) also was assessed.

RESULTS

The results showed that the transcription level of miRNA (Dicer1, Ago1), siRNA (Dicer2, Ago2) and apoptosis (Caspase1, Caspase5) core genes in H. armigera larvae were decreased significantly after 24, 48 and 96 h feeding on a diet containing lethal and sublethal doses of fenoxycarb. Moreover, the mortality rate to HaNPV in the larvae treated with fenoxycarb increased compared to the control, leading to an increased replication of HaNPV.

CONCLUSION

Together, our results suggest that the antiviral immune system could be modulated by JHA and facilitate HaNPV replication in the larvae, increasing the mortality rate of the insect larvae. Understanding the effect of JHA on antiviral immunity is an important step toward the process of exploiting JHAs and viral pathogens to control insect pests. © 2022 Society of Chemical Industry.

The endocrine disruptor, fenoxycarb modulates gut immunity and gut bacteria titer in the cotton bollworm, Helicoverpa armigera

The endocrine system modulates several physiological functions such as development and metamorphosis in insects. The normal growth and development of insects is interfered with insect growth regulators (IGRs), which act as mimics of insect hormones like juvenile hormone (JH) and ecdysone hormone. The effects of JH and its analogs on systemic immunity have been identified. However the effect of these compounds on local gut immunity is largely unknown. In this study, the effects of JH analog fenoxycarb on the local gut immunity of Helicoverpa armigera, gut bacteria population, and their role in the pathogenicity of Bacillus thuringiensis (Bt) were analyzed. The results showed that feeding fenoxycarb causes a decrease in the transcription level of IMD (Relish and PGPR-LC), ROS (DUOX and SOD) related genes and antimicrobial peptides (AMPs), followed by an overpopulation of gut bacteria. The fenoxycarb-treated larvae showed higher susceptibility to Bt compared to the control larvae. Overall, these findings collectively suggest that JH analog affects local gut immunity and gut bacteria titer.

Symbiotic Microorganisms and Their Different Association Types in Aquatic and Semiaquatic Bugs

True bugs (Hemiptera, suborder Heteroptera) constitute the largest suborder of nonholometabolous insects and occupy a wide range of habitats various from terrestrial to semiaquatic to aquatic niches. The transition and occupation of these diverse habitats impose various challenges to true bugs, including access to oxygen for the aquatic species and plant defense for the terrestrial phytophagans. Although numerous studies have demonstrated that microorganisms can provide multiple benefits to terrestrial host insects, a systematic study with comprehensive higher taxa sampling that represents aquatic and semiaquatic habitats is still lacking. To explore the role of symbiotic microorganisms in true bug adaptations, 204 samples belonging to all seven infraorders of Heteroptera were investigated, representing approximately 85% of its superfamilies and almost all known habitats. The symbiotic microbial communities of these insects were analyzed based on the full-length amplicons of the bacterial 16S rRNA gene and fungal ITS region. Bacterial communities varied among hosts inhabiting terrestrial, semiaquatic, and aquatic habitats, while fungal communities were more related to the geographical distribution of the hosts. Interestingly, co-occurrence networks showed that species inhabiting similar habitats shared symbiotic microorganism association types. Moreover, functional prediction analyses showed that the symbiotic bacterial community of aquatic species displayed richer amino acid and lipid metabolism pathways, while plant-feeding true bugs benefited more from the symbiont-provided xenobiotics biodegradation pathway. These results deepened the recognition that symbiotic microorganisms were likely to help heteropterans occupy diverse ecological habitats and provided a reference framework for further studies on how microorganisms affect host insects living in various habitats. IMPORTANCE Symbiotic bacteria and fungi generally colonize insects and provide various benefits for hosts. Although numerous studies have investigated symbionts in terrestrial plant-feeding insects, explorations of symbiotic bacterial and fungal communities in aquatic and semiaquatic insects are rare. In this study, the symbiotic microorganisms of 204 aquatic, semiaquatic, and terrestrial true bugs were explored. This comprehensive taxon sampling covers ~85% of the superfamilies of true bugs and most insect habitats. Analyses of the diversity of symbionts demonstrated that the symbiotic microbial diversities of true bugs were mainly affected by host habitats. Co-occurrence networks showed that true bugs inhabiting similar habitats shared symbiotic microbial association types. These correlations between symbionts and hosts together with the functions of bacterial communities indicated that symbiotic microbial communities may help true bugs adapt to (semi)aquatic habitats.

Obligate Gut Symbiotic Association with Caballeronia in the Mulberry Seed Bug Paradieuches dissimilis (Lygaeoidea: Rhyparochromidae)

Many insects possess symbiotic bacteria in their bodies, and microbial symbionts play pivotal metabolic roles for their hosts. Members of the heteropteran superfamilies Coreoidea and Lygaeoidea stinkbugs harbor symbionts of the genus Caballeronia in their intestinal tracts. Compared with symbiotic associations in Coreoidea, those in Lygaeoidea insects are still less understood. Here, we investigated a symbiotic relationship involving the mulberry seed bug Paradieuches dissimilis (Lygaeoidea: Rhyparochromidae) using histological observations, cultivation of the symbiont, 16S rRNA gene amplicon sequencing, and infection testing of cultured symbionts. Histological observations and cultivation revealed that P. dissimilis harbors Caballeronia symbionts in the crypts of its posterior midgut. 16S rRNA gene amplicon sequencing of field-collected P. dissimilis confirmed that the genus Caballeronia is dominant in the midgut of natural populations of P. dissimilis. In addition, PCR diagnostics showed that the eggs were free of symbiotic bacteria, and hatchlings horizontally acquired the symbionts from ambient soil. Infection and rearing experiments revealed that symbiont-free aposymbiotic individuals had abnormal body color, small body size, and, strikingly, a low survival rate, wherein no individuals reached adulthood, indicating an obligate cooperative mutualism between the mulberry seed bug and Caballeronia symbionts.

Anthropogenic regulation governs nutrient cycling and biological succession in hydropower reservoirs

Hydropower plays an important role in the supply of renewable energy, but it also exerts a great influence on the river continuum. Understanding nutrient cycling and microbial community succession in hydropower reservoirs is key to weighing hydroelectric pros and cons. However, the underlying control mechanisms are still not well known, especially with respect to the impacts of hydrological conditions. Based on a comprehensive survey of hydropower reservoirs along the Wujiang River in SW China and an integration of published data, we found that reservoir physicochemical and biological stratifications and planktonic microbial community assembly were synergistically evolving, and reservoir hydraulic load (i.e., mean water depth per unit retention time) was a key factor controlling the strength of stratifications, CO₂ and N₂O fluxes, nutrient retention efficiency, and bacterioplankton diversity. Hydraulic loads are artificially designed for hydropower reservoirs, and nutrient cycling and biological succession in reservoirs are thus governed by anthropogenic regulation. This study provides a theoretical basis to mitigate the environmental impacts of hydropower dams by regulating reservoir hydraulic load.

Differences in microbiome composition and transcriptome profiles between male and female Paederus fuscipes harbouring pederin-producing bacteria

Pederin, a group of antitumor compounds, is produced by an endosymbiotic bacterium of Paederus fuscipes. Pederin content differed between male and female P. fuscipes, but the reason why these differences are maintained remains unexplored. Here, the pederin-producing bacteria (PPB) infection rate in P. fuscipes was investigated. Furthermore, we assessed the microbiota structure differences in male and female P. fuscipes harbouring PPB and sequenced the transcriptome of both sexes to shed light on genes of interest. Of the 625 analysed beetles (275 females, 350 males), 96.36% of females and 31.14% of males were positive for PPB infection. PPB accounted for 54.36%-82.70% of the bacterial population in females but showed a much lower abundance in males (0.92%-3.87%). Reproductive organs possessed the highest PPB abundance compared with other parts of females, but no such relationships existed in males. Moreover, we provide the first transcriptome analysis of male and female P. fuscipes harbouring PPB and identified 8893 differentially expressed unigenes. Our results indicated that the pederin content difference between males and females might be caused by the PPB density difference in hosts. The biosequence data would be helpful for illustrating the mechanism that regulates PPB density in P. fuscipes.

The Negative Effects of Feces-Associated Microorganisms on the Fitness of the Stored Product Mite Tyrophagus putrescentiae

Feces have been suggested as a major source of microorganisms for recolonization of the gut of stored product mites via coprophagy. The mites can host microorganisms that decrease their fitness, but their transmission is not known. To address the role of fecal microbiota on mite fitness, we performed an experimental study in which the surfaces of mite (Tyrophagus putrescentiae) eggs were sterilized. Mites eggs (15 per experimental box) were then hatched and grown on feedstock with and without feces. These experiments were conducted with four distinct T. putrescentiae populations (5L, 5K, 5N, and 5P), and mite population density after 21 day of cultivation was used to assess mite fitness and the impact of fecal microbiota on fitness. Population density was not affected by the presence of feces in two of the cultures (5L and 5K), while significant effects of feces were observed in the other cultures (5N and 5P). Mite culture microbial communities were analyzed using cultivation-independent next-generation amplicon sequencing of microbial 16S and 18S ribosomal RNA (rRNA) genes in the fitness influenced populations (5N and 5P). Several microbial taxa were associated with fecal treatments and reduced mite fitness, including Staphylococcus and Bartonella-like bacteria, and the fungal genera Yamadazyma, Candida, and Aspergillus. Although coprophagy is the transmission route mites used to obtain beneficial gut bacteria such as Bartonella-like organisms, the results of this study demonstrate that fecal-associated microorganisms can have negative effects on some populations of T. putrescentiae fitness, and this may counteract the positive effects of gut symbiont acquisition.

Environmental Temperature, but Not Male Age, Affects Wolbachia and Prophage WO Thereby Modulating Cytoplasmic Incompatibility in the Parasitoid Wasp, Habrobracon Hebetor

Wolbachia is an endosymbiotic bacterium found in many species of arthropods and manipulates its host reproduction. Cytoplasmic incompatibility (CI) is one of the most common manipulations that is induced when an uninfected female mates with a Wolbachia-infected male. The CI factors (cifA and cifB genes) are encoded by phage WO that naturally infects Wolbachia. Here, we questioned whether an environmental factor (temperature) or host factor (male age) affected the strength of the CI phenotype in the ectoparasitoid wasp, Habrobracon hebetor. We found that temperature, but not male age, results in reduced CI penetrance. Consistent with these results, we also found that the expression of the cif CI factors decreased in temperature-exposed males but was consistent across aging male wasps. Similar to studies of other insect systems, cifA showed a higher expression level than cifB, and male hosts showed increased cif expression relative to females. Our results suggest that prophage WO is present in the Wolbachia-infected wasps and expression of cif genes contributes to the induction of CI in this insect. It seems that male aging has no effect on the intensity of CI; however, temperature affects Wolbachia and prophage WO titers as well as expression levels of cif genes, which modulate the CI level.

Wolbachia promotes successful sex with siblings in the parasitoid Habrobracon hebetor

BACKGROUND

Wolbachia are intracellular α-proteobacteria that have a wide distribution among various arthropods and nematodes. They affect the host reproduction favoring their maternal transmission, which sets up a potential conflict in inbreeding situations when the host avoids sexual reproduction preventing inbreeding depression, while Wolbachia pushes it. We used the wasp Habrobracon hebetor to test the hypothesis that Wolbachia modulates inbreeding avoidance behavior and promotes sib mating.

RESULTS

Our results showed no obvious pre-copulatory inbreeding avoidance in this wasp. However, H. hebetor showed a strong post-copulatory inbreeding avoidance behavior that resulted in a low fertilization rate of uninfected siblings and therefore high rate of production of male progeny was obtained. We observed higher rates of fertilization success in the Wolbachia-infected lines that resulted in significantly higher female progeny production compared to the uninfected sib mates. Since diploid females are the result of successful fertilization due to haplodiploidy sex determination system in this insect, our results indicate that Wolbachia promoted fertile sib mating in H. hebetor. Interestingly, the rate of adult emergence in the progeny of Wolbachia-infected sib mates were almost similar to the non-sib mate crosses and significantly more than those observed in the uninfected sib mate crosses.

CONCLUSION

Our results support the idea that Wolbachia modulates inbreeding avoidance and promotes sib mating and also mitigates inbreeding depression. By promoting successful sex with siblings and increasing the probability of female progeny, Wolbachia enhances its transmission to the next generation. This is an undescribed effect of Wolbachia on the host reproduction. © 2021 Society of Chemical Industry.

Co-occurrence of planktonic bacteria and archaea affects their biogeographic patterns in China's coastal wetlands

Planktonic bacteria and archaea play a key role in maintaining ecological functions in aquatic ecosystems; however, their biogeographic patterns and underlying mechanisms have not been well known in coastal wetlands including multiple types and at a large space scale. Therefore, planktonic bacteria and archaea and related environmental factors were investigated in twenty-one wetlands along China's coast to understand the above concerns. The results indicated that planktonic bacteria had different biogeographic pattern from planktonic archaea, and both patterns were not dependent on the wetland's types. Deterministic selection shapes the former's community structure, whereas stochastic processes regulate the latter's, being consistent with the fact that planktonic archaea have a larger niche breadth than planktonic bacteria. Planktonic bacteria and archaea co-occur, and their co-occurrence rather than salinity is more important in shaping their community structure although salinity is found to be a main environmental deterministic factor in the coastal wetland waters. This study highlights the role of planktonic bacteria-archaea co-occurrence on their biogeographic patterns, and thus provides a new insight into studying underlying mechanisms of microbial biogeography in coastal wetlands.

Plant-bacteria associations are phylogenetically structured in the phyllosphere

Determining whether and how global change will lead to novel interactions between hosts and microbes is an important issue in ecology and evolution. Understanding the contribution of host and microbial ecologies and evolutionary histories in driving their contemporary associations is an important step towards addressing this challenge and predicting the fitness consequences of novel associations. Using shotgun metagenomic and amplicon sequencing of bacterial communities from the leaf surfaces (phyllosphere) of trees, we investigated how phylogenetic relatedness among hosts and among their associated bacteria influences the distribution of bacteria among hosts. We also evaluated whether the functional traits of trees and bacteria explained these associations across multiple host species. We show that phylogenetically similar hosts tended to associate with the same bacteria and that phylogenetically similar bacteria tended to associate with the same host species. Phylogenetic interactions between tree and bacterial taxa also explained variation in their associations. The effect of host and symbiont evolutionary histories on bacterial distribution across hosts were observed across phylogenetic scales, but prominently explained variation among higher taxonomic categories of hosts and symbionts. These results suggest that ecological variation arising early in the plant and bacterial phylogenies have been particularly important for driving their contemporary associations. Variation in bacterial functional genes associated with the biosynthesis of aromatic amino acids and compounds and with cell motility were notably important in explaining bacterial community turnover among gymnosperm and angiosperm hosts. Overall, our results suggest an influence of host and bacterial traits and evolutionary histories in driving their contemporary associations.

Environmental Acquisition of Gut Symbiotic Bacteria in the Saw-Toothed Stinkbug, Megymenum gracilicorne (Hemiptera: Pentatomoidea: Dinidoridae)

Many plant-sucking stinkbugs possess a specialized symbiotic organ with numerous crypts in a posterior region of the midgut. In stinkbugs of the superfamily Pentatomoidea, specific γ-proteobacteria are hosted in the crypt cavities, which are vertically transmitted through host generations and essential for normal growth and survival of the host insects. Here we report the discovery of an exceptional gut symbiotic association in the saw-toothed stinkbug, Megymenum gracilicorne (Hemiptera: Pentatomoidea: Dinidoridae), in which specific γ-proteobacterial symbionts are not transmitted vertically but acquired environmentally. Histological inspection identified a very thin and long midgut symbiotic organ with two rows of tiny crypts whose cavities harbor rod-shaped bacterial cells. Molecular phylogenetic analyses of bacterial 16S rRNA gene sequences from the symbiotic organs of field-collected insects revealed that (i) M. gracilicorne is stably associated with Pantoea-allied γ-proteobacteria within the midgut crypts, (ii) the symbiotic bacteria exhibit a considerable level of diversity across host individuals and populations, (iii) the major symbiotic bacteria represent an environmental bacterial lineage that was reported to be capable of symbiosis with the stinkbug Plautia stali, and (iv) the minor symbiotic bacteria also represent several bacterial lineages that were reported as cultivable symbionts of P. stali and other stinkbugs. The symbiotic bacteria were shown to be generally cultivable. Microbial inspection of ovipositing adult females and their eggs and nymphs uncovered the absence of stable vertical transmission of the symbiotic bacteria. Rearing experiments showed that symbiont-supplemented newborn nymphs exhibit improved survival, suggesting the beneficial nature of the symbiotic association.

Midgut bacterial diversity of a leaf-mining beetle, Dactylispa xanthospila (Gestro) (Coleoptera: Chrysomelidae: Cassidinae)

Microorganisms play an essential role in the growth and development of numerous insect species. In this study, the total DNA from the midgut of adults of Dactylispaxanthospila were isolated and bacterial 16S rRNA sequenced using the high-throughput Illumina MiSeq platform. Then, the composition and diversity of the midgut bacterial community were analysed with QIIME2. The results showed the midgut bacteria of D.xanthospila belong to 30 phyla, 64 classes, 135 orders, 207 families and 369 genera. At the phylum level, Proteobacteria, Bacteroidetes and Firmicutes were the dominant bacteria, accounting for 91.95%, 3.44% and 2.53%, respectively. The top five families are Enterobacteriaceae (69.51%), Caulobacteraceae (5.24%), Rhizobiaceae (4.61%), Sphingomonadaceae (4.23%) and Comamonadaceae (2.67%). The bacterial community's primary functions are carbohydrate metabolism, amino acid metabolism and cofactor and vitamin metabolism, which are important for the nutritional requirements of plant-feeding insects.

Benefits and costs of hosting facultative symbionts in plant-sucking insects: A meta-analysis

Many animals have evolved associations with symbiotic microbes that benefit the host through increased growth, lifespan, and survival. Some interactions are obligate (essential for survival) while others are facultative (usually beneficial but not essential). Not all individuals host all facultative symbionts in a population, and thus there is probably a trade-off between the cost of hosting these symbionts and the benefits they confer to the host. Plant-sucking insects have been one of the most important models to test these costs and benefits experimentally. This research is now moving beyond the description of symbiont effects towards understanding the mechanisms of action, and their role in the wider ecological community. We present a quantitative and systematic analysis of the published evidence exploring this question. We found that whitefly and true bugs experience benefits through increased growth and fecundity, whereas aphids experience costs to their fecundity but benefits through increased resistance to natural enemies. We also report the lack of data in some plant-sucking groups, and explore variation in effect strengths and directions across aphid host, symbiont and plant species thus highlighting the importance of considering the context dependency of these interactions.

Multiple concurrent and convergent stages of genome reduction in bacterial symbionts across a stink bug family

Nutritional symbioses between bacteria and insects are prevalent and diverse, allowing insects to expand their feeding strategies and niches. A common consequence of long-term associations is a considerable reduction in symbiont genome size likely influenced by the radical shift in selective pressures as a result of the less variable environment within the host. While several of these cases can be found across distinct insect species, most examples provide a limited view of a single or few stages of the process of genome reduction. Stink bugs (Pentatomidae) contain inherited gamma-proteobacterial symbionts in a modified organ in their midgut and are an example of a long-term nutritional symbiosis, but multiple cases of new symbiont acquisition throughout the history of the family have been described. We sequenced the genomes of 11 symbionts of stink bugs with sizes that ranged from equal to those of their free-living relatives to less than 20%. Comparative genomics of these and previously sequenced symbionts revealed initial stages of genome reduction including an initial pseudogenization before genome reduction, followed by multiple stages of progressive degeneration of existing metabolic pathways likely to impact host interactions such as cell wall component biosynthesis. Amino acid biosynthesis pathways were retained in a similar manner as in other nutritional symbionts. Stink bug symbionts display convergent genome reduction events showing progressive changes from a free-living bacterium to a host-dependent symbiont. This system can therefore be used to study convergent genome evolution of symbiosis at a scale not previously available.

Removal of gut symbiotic bacteria negatively affects life history traits of the shield bug, Graphosoma lineatum

The shield bug, Graphosoma lineatum (Heteroptera, Pentatomidae), harbors extracellular Pantoea-like symbiont in the enclosed crypts of the midgut. The symbiotic bacteria are essential for normal longevity and fecundity of this insect. In this study, life table analysis was used to assess the biological importance of the gut symbiont in G. lineatum. Considering vertical transmission of the bacterial symbiont through the egg surface contamination, we used surface sterilization of the eggs to remove the symbiont. The symbiont population was decreased in the newborn nymphs hatched from the surface-sterilized eggs (the aposymbiotic insects), and this reduction imposed strongly negative effects on the insect host. We found significant differences in most life table parameters between the symbiotic insects and the aposymbiotics. The intrinsic rate of increase in the control insects (0.080 ± 0.003 day-1) was higher than the aposymbiotic insects (0.045 ± 0.007 day-1). Also, the net reproductive and gross reproductive rates were decreased in the aposymbiotic insects (i.e., 20.770 ± 8.992 and 65.649 ± 27.654 offspring/individual, respectively), compared with the symbiotic insects (i.e., 115.878 ± 21.624 and 165.692 ± 29.058 offspring/individual, respectively). These results clearly show biological importance of the symbiont in G. lineatum.

Wolbachia and Spiroplasma could influence bacterial communities of the spider mite Tetranychus truncatus

The structures of arthropod bacterial communities are complex. These microbiotas usually provide many beneficial services to their hosts, whereas occasionally they may be parasitical. To date, little is known about the bacterial communities of Tetranychus truncatus and the factors contributing to the structure of its bacterial communities are unexplored yet. Here, we used four symbiont-infected T. truncatus strains-including one Wolbachia and Spiroplasma co-infected strain, two symbiont singly-infected strains and one symbiont uninfected strain-to investigate the influence of endosymbionts on the structure of the host mites' microbiota. Based on 16S rRNA genes sequencing analysis, we found Wolbachia and Spiroplasma were the two most abundant bacteria in T. truncatus and the presence of both symbionts could not change the diversity of bacterial communities (based on alpha-diversity indexes such as ACE, Chao1, Shannon and Simpson diversity index). Symbiont infection did alter the abundance of many other bacterial genera, such as Megamonas and Bacteroides. The structures of bacterial communities differed significantly among symbiont-infected strains. These results suggested a prominent effect of Wolbachia and Spiroplasma on bacterial communities of the host T. truncatus. These findings advance our understanding of T. truncatus microbiota and will be helpful for further study on bacterial communities of spider mites.

Optimization of probiotic therapeutics using machine learning in an artificial human gastrointestinal tract

The gut microbiota's metabolome is composed of bioactive metabolites that confer disease resilience. Probiotics' therapeutic potential hinges on their metabolome altering ability; however, characterizing probiotics' metabolic activity remains a formidable task. In order to solve this problem, an artificial model of the human gastrointestinal tract is introduced coined the ABIOME (A Bioreactor Imitation of the Microbiota Environment) and used to predict probiotic formulations' metabolic activity and hence therapeutic potential with machine learning tools. The ABIOME is a modular yet dynamic system with real-time monitoring of gastrointestinal conditions that support complex cultures representative of the human microbiota and its metabolome. The fecal-inoculated ABIOME was supplemented with a polyphenol-rich prebiotic and combinations of novel probiotics that altered the output of bioactive metabolites previously shown to invoke anti-inflammatory effects. To dissect the synergistic interactions between exogenous probiotics and the autochthonous microbiota a multivariate adaptive regression splines (MARS) model was implemented towards the development of optimized probiotic combinations with therapeutic benefits. Using this algorithm, several probiotic combinations were identified that stimulated synergistic production of bioavailable metabolites, each with a different therapeutic capacity. Based on these results, the ABIOME in combination with the MARS algorithm could be used to create probiotic formulations with specific therapeutic applications based on their signature metabolic activity.

Disruption of Host-Symbiont Associations for the Symbiotic Control and Management of Pentatomid Agricultural Pests-A Review

The family Pentatomidae (Hemiptera: Heteroptera) includes several invasive stink bug species capable to attack a large number of wild and cultivated plants, causing several damages to different crops. Pentatomids rely on obligate symbiotic associations with bacteria of the family Enterobacteriaceae, mainly of the genus Pantoea. A distinctive trait of these associations is the transmission route: during oviposition, females smear egg masses with symbiont-containing secretions, which are ingested by newly hatched nymphs, allowing the symbiont to pass through their digestive tract and establish in the crypts of the posterior midgut. Preventing newborns from orally acquiring symbionts seriously affects their fitness and survival. This symbiont inheritance process can be manipulated to develop innovative pest control measures by sterilization of egg masses prior to nymph hatching. This review summarizes the recent knowledge advances concerning the gut primary symbionts of pentatomids, with a specific focus on the most troubling pest species for agriculture. Current understanding of host colonization dynamics in pentatomids is presented, as well as the phenotypic effects determined in different insect species by the alteration of vertical transmission. Details on the current knowledge on the whole bacterial communities accompanying primary symbionts are analyzed. The recent research exploiting the perturbation of symbiont acquisition by pentatomid nymphs is discussed, by considering published work on laboratory and field trials with several active substances. These translational strategies are presently regarded as promising for limiting the populations of many important pentatomid pests in a sustainable way.

The synergy effect of arbuscular mycorrhizal fungi symbiosis and exogenous calcium on bacterial community composition and growth performance of peanut (Arachis hypogaea L.) in saline alkali soil

Peanut (Arachis hypogaea. L) is an important oil seed crop. Both arbuscular mycorrhizal fungi (AMF) symbiosis and calcium (Ca²⁺) application can ameliorate the impact of saline soil on peanut production, and the rhizosphere bacterial communities are also closely correlated with peanut salt tolerance; however, whether AMF and Ca²⁺ can withstand high-salinity through or partially through modulating rhizosphere bacterial communities is unclear. Here, we used the rhizosphere bacterial DNA from saline alkali soil treated with AMF and Ca²⁺ alone or together to perform high-throughput sequencing of 16S rRNA genes. Taxonomic analysis revealed that AMF and Ca²⁺ treatment increased the abundance of Proteobacteria and Firmicutes at the phylum level. The nitrogen-fixing bacterium Sphingomonas was the dominant genus in these soils at the genus level, and the soil invertase and urease activities were also increased after AMF and Ca²⁺ treatment, implying that AMF and Ca²⁺ effectively improved the living environment of plants under salt stress. Moreover, AMF combined with Ca²⁺ was better than AMF or Ca²⁺ alone at altering the bacterial structure and improving peanut growth in saline alkali soil. Together, AMF and Ca²⁺ applications are conducive to peanut salt adaption by regulating the bacterial community in saline alkali soil.

Control of Hydraulic Load on Bacterioplankton Diversity in Cascade Hydropower Reservoirs, Southwest China

Hydroelectric reservoirs are highly regulated ecosystems, where the understanding on bacterioplankton has been very limited so far. In view of significant changes in river hydrological conditions by dam construction, hydraulic load (i.e., the ratio of mean water depth to water retention time) was assumed to control bacterioplankton diversity in cascading hydropower reservoirs. To evaluate this hypothesis, we investigated bacterioplankton composition and diversity using high-throughput sequencing and related environmental variables in eleven reservoirs on the Wujiang River, Southwest China. Our results showed a decrease of bacterioplankton diversity index with an increase of reservoir hydraulic load. This is because hydraulic load governs dissolved oxygen variation in the water column, which is a key factor shaping bacterioplankton composition in these hydroelectric reservoirs. In contrast, bacterioplankton abundance was mainly affected by nutrient-related environmental factors. Therefore, from a hydrological perspective, hydraulic load is a decisive factor for the bacterioplankton diversity in the hydroelectric reservoirs. This study can improve the understanding of reservoir bacterial ecology, and the empirical relationship between hydraulic load and bacterioplankton diversity index will help to quantitatively evaluate ecological effects of river damming.

The innate immune gene Relish and Caudal jointly contribute to the gut immune homeostasis by regulating antimicrobial peptides in Galleria mellonella

Gut microbiota modulates various physiologic processes in insects, such as nutrition, metabolic homeostasis, and pathogen exclusion. Maintaining a normal microbiome is an essential element of the gut homeostasis, requiring an extensive network of regulatory immune responses. The molecular mechanisms driving these various effects and the events leading to the establishment of a normal microbiota in insects are still largely unknown. In this study, the NF-kB (IMD and Toll) signaling pathways in the gut of Galleria mellonella and their roles in the regulation of its gut microbes were assessed. For this, the transcript levels of the IMD pathway (Imd and Relish) and the Toll pathway (Spätzle and Dif/Dorsal) genes were analyzed and the results showed that all the genes were expressed in the gut of G. mellonella. Silencing of Relish resulted in reduced expression levels of the IMD pathway genes and antimicrobial peptides (AMPs) followed by overpopulation of gut bacteria. Antibiotics-treated larvae showed lower expression levels of the IMD and Toll pathway genes followed by lower AMPs expression levels. The expression level of caudal decreased in the antibiotics-treated larvae compared with the controls. Together, these data suggest that the IMD and Toll pathways are active in the gut of G. mellonella. The IMD pathway gene, relish functions in the regulation of gut microbes in this insect model.

Characterization of a novel Pantoea symbiont allows inference of a pattern of convergent genome reduction in bacteria associated with Pentatomidae

Phytophagous stink bugs typically harbor nutritional symbiotic bacteria in their midgut, to integrate their unbalanced diet. In the Pentatomidae, most symbionts are affiliated to the genus Pantoea, and are polyphyletic. This suggests a scenario of an ancestral establishment of symbiosis, followed by multiple symbiont replacement events by akin environmental bacteria in different host lineages. In this study, a novel Pantoeaspecies ('CandidatusPantoea persica') was characterized from the gut of the pentatomid Acrosternum arabicum, and shown to be highly abundant in a specific portion of the gut and necessary for the host development. The genome of the symbiont (2.9 Mb), while presenting putative host-supportive metabolic pathways, including those for amino acids and vitamin synthesis, showed a high level of pseudogenization, indicating ongoing genome reduction. Comparative analyses with other free-living and symbiotic Pantoea highlighted a convergent pattern of genome reduction in symbionts of pentatomids, putatively following the typical phases modelized in obligate nutritional symbionts of insects. Additionally, this system has distinctive traits, as hosts are closely related, and symbionts originated multiple independent times from closely related free-living bacteria, displaying convergent and independent conspicuous genome reduction. Due to such peculiarities, this may become an ideal model to study genome evolutionary processes in insect symbionts.

Compartmentalization drives the evolution of symbiotic cooperation

Across the tree of life, hosts have evolved mechanisms to control and mediate interactions with symbiotic partners. We suggest that the evolution of physical structures that allow hosts to spatially separate symbionts, termed compartmentalization, is a common mechanism used by hosts. Such compartmentalization allows hosts to: (i) isolate symbionts and control their reproduction; (ii) reward cooperative symbionts and punish or stop interactions with non-cooperative symbionts; and (iii) reduce direct conflict among different symbionts strains in a single host. Compartmentalization has allowed hosts to increase the benefits that they obtain from symbiotic partners across a diversity of interactions, including legumes and rhizobia, plants and fungi, squid and Vibrio, insects and nutrient provisioning bacteria, plants and insects, and the human microbiome. In cases where compartmentalization has not evolved, we ask why not. We argue that when partners interact in a competitive hierarchy, or when hosts engage in partnerships which are less costly, compartmentalization is less likely to evolve. We conclude that compartmentalization is key to understanding the evolution of symbiotic cooperation. This article is part of the theme issue 'The role of the microbiome in host evolution'.

Morphology and composition of the midgut bacterial community of Scaptocoris castanea Perty, 1830 (Hemiptera: Cydnidae)

The burrower bug Scaptocoris castanea is an important soybean and pasture pest in Brazil, with an underground habit feeding directly on the sap of the roots. Underground habit hinders control and knowledge of the biology and physiology of this pest. This study describes the anatomy, histology, ultrastructure and symbionts of the midgut of S. castanea. The midgut of S. castanea is anatomically divided into five regions (ventricles). Ventricles 1-3 are similar between males and females, with cells specialized in digestion and absorption of nutrients, water transport and homeostasis. Ventricle 4 has squamous epithelium forming crypts and harboring bacteria in the lumen. Ventricle 5 of males is small with cells containing apical microvilli and broad basal folds with many openings for hemocoel, while in females, this region of the midgut is well developed and colonized by intracellular bacteria, characterizing bacteriocytes. The main bacteria are Gammaproteobacteria. The results show sexual dimorphism in ventricle 5 of the midgut of S. castanea, with formation of bacteriocytes in the females, while the other regions are involved in digestive processes in both sexes.

Habitat visualization, acquisition features and necessity of the gammaproteobacterial symbiont of pistachio stink Bug, Acrosternum heegeri (Hem.: Pentatomidae)

Plant-sucking stinkbugs are especially associated with mutualistic gut bacterial symbionts. Here, we explored the symbiotic relationship of a pistachio stinkbug, Acrosternum heegeri Fieber by histological, fluorescence in situ hybridization (FISH), real-time PCR and molecular phylogenetic techniques. Furthermore, the effects of the symbiont on the resting/wandering behaviors of the newborn nymphs, pre-adult survival rates, and stage compositions were investigated. Transmission electron microscopy and real-time PCR analyses showed that a rod-shaped gammaproteobacterium was persistently located within the posterior midgut crypts. Molecular phylogenetic and FISH techniques strongly suggested that this symbiont should be placed in the genus Pantoea of the Enterobacteriales. Scanning electron microscopy confirmed the presence of the bacterial cells on the egg surface which the surface sterilization of the eggs resulted in the successful removal of the symbiont from the eggs. Symbiotic and aposymbiotic A. heegeri showed no significant differences in the wandering behaviors of the first nymphal stages, while the symbiont-free insects suffered retarded growth and lower survivability. Together, the results highlight the habitat and acquisition features of Pantoea symbiont and its contribution in A. heegeri biology that might help us for better pest management in the future.

Coinfection of the secondary symbionts, Hamiltonella defensa and Arsenophonus sp. contribute to the performance of the major aphid pest, Aphis gossypii (Hemiptera: Aphididae)

Bacterial endosymbionts play important roles in ecological traits of aphids. In this study, we characterize the bacterial endosymbionts of A. gossypii collected in Karaj, Iran and their role in the performance of the aphid. Our results indicated that beside Buchnera aphidicola, A. gossypii, also harbors both Hamiltonella defensa and Arsenophonus sp. Quantitative PCR (qPCR) results revealed that the populations of the endosymbionts increased throughout nymphal development up to adult emergence; thereafter, populations of Buchnera and Arsenophonus were diminished while the density of H. defensa constantly increased. Buchnera reduction caused prolonged development and no progeny production. Furthermore, secondary symbiont reduction led to reduction of the total life span and intrinsic rate of natural increase as well as appearance of the deformed dead offspring in comparison with the control insects. Reduction of the secondary symbionts did not affect parasitism rate of the aphid by the parasitic wasp Aphidius matricariae. Together these findings showed that H. defensa and Arsenophonus contributed to the fitness of A. gossypii by enhancing its performance, but not through parasitoid resistance.

Bacterial communities in digestive and excretory organs of cicadas

Bacteriocyte-associated symbionts are essential for the health of many sap-sucking insects, such as cicadas, leafhoppers and treehoppers, etc., but little is known about the bacterial community in the gut and other related organs in these insects. We characterized the bacterial communities in the salivary glands, alimentary canal and the Malpighian tubules of two populations of the cicada Subpsaltria yangi occurring in different habitats and feeding on different hosts. A high degree of similarity of core microbiota was revealed between the two populations, both with the top three bacteria belonging to Meiothermus, Candidatus Sulcia and Halomonas. The bacterial communities in various organs clustered moderately by populations possibly reflect adaptive changes in the microbiota of related S. yangi populations, which provide a better understanding of the speciation and adaptive mechanism of this species to different diets and habitats. When compared with two phylogenetically distant cicada species, Hyalessa maculaticollis and Meimuna mongolica, the core microbiota in S. yangi was significantly different to that of these species. In addition, our results confirm that Ca. Sulcia distributes in the digestive and excretory organs besides the bacteriomes and gonads, which provide potential important information onto the trophic functions of this obligate endosymbiont to the host insects.

Induction of DNA methyltransferase genes in Helicoverpa armigera following injection of pathogenic bacteria modulates expression of antimicrobial peptides and affects bacterial proliferation

Following pathogen attack in a host, widespread changes are induced in the host's gene expression, in particular those involved in the immune system, growth and survival. Epigenetic mechanisms have been suggested to be involved in the regulation of these changes through a number of mechanisms. DNA methylation is one of the important epigenetic processes that is carried out by DNA (cytosine-5) methyltransferase (DNMT) and alters expression of target genes. Here, we identified two putative sequences of DNMT (i.e. DNMT1 and DNMT2) from the transcriptome dataset of Helicoverpa armigera that showed high similarity to the homologous sequences in Bombyx mori. Domain architectures of DNMT1 and DNMT2 exhibit the unique pattern of DNMTs that highlights conserved function of these genes in different insects. To see if these genes play any role in bacterial infection, we challenged the fifth instar larvae of H. armigera by injecting Bacillus thuringiensis and Serratia marcescens cells into the hemolymph. Transcript levels of the DNMTs were analyzed by RT-qPCR. The results showed that the expression levels of DNMT1 and DNMT2 increased in the bacteria-injected larvae. Injection of the heat-killed bacteria also induced the expression of the DNMTs, but lower than that of the live bacteria. To determine whether these genes function during bacterial infection, we injected the inhibitor of DNMTs, 5-azacytidine (5-AZA), into the larvae and 24 h later, the bacterial cells were also injected into the larvae. Bacterial replication and larval mortality were analyzed in the treated and control insects. We found that 5-AZA reduced bacterial replication and also mortality of the bacterial-injected larvae regardless of the pathogenic bacterial species. Interestingly, the expression levels of antimicrobial peptides (AMPs) were also modulated following 5-AZA treatment. In conclusion, we showed that upregulation of the DNMTs in H. armigera following bacterial infections modulates AMPs and thereby affects the insect-bacteria interactions.

Heritable Gammaproteobacterial Symbiont Improves the Fitness of Brachynema germari Kolenati (Hemiptera: Pentatomidae)

The pistachio green stink bug, Brachynema germari Kolenati, is an abundant and economic insect pest in most pistachio-growing regions. Some physiological and ecological features of this pest have been studied, but the microbiological nature of symbiotic bacteria and biological aspects of this host-symbiont interaction have been poorly understood. In the present study, we explored the host-associated environment, phylogeny, and acquisition features of the bacterial symbiont of the insect. Furthermore, the importance of the symbiont on the biological (i.e., lifespan, stage composition, and body weight) and behavioral characteristics (i.e., resting/wandering behaviors of the newborn nymphs) of the host were investigated. We found that a rod-shaped gammaproteobacterium was persistently colonized the fourth midgut region of the insect. Molecular phylogenetic and fluorescence in situ hybridization analyses strongly suggest that this symbiont should be placed in the genus Pantoea of the Enterobacteriales. Egg surface sterilization resulted in the aposymbiotic insects suggesting the vertical transmission of symbiont via egg surface smearing upon oviposition. Symbiotic and aposymbiotic B. germari showed no significant differences in the wandering behaviors of the first nymphal stages, whereas the symbiont-free insects exhibited retarded growth, lower longevity, and adult body weight. Taken together, these data provide a better understanding of the relationship between the bacterial symbiont and B. germari and demonstrate that the insect is heavily affected by the deprival of its gut symbionts.

Cladogenesis and Genomic Streamlining in Extracellular Endosymbionts of Tropical Stink Bugs

Phytophagous stink bugs are globally distributed and many harbor vertically inherited bacterial symbionts that are extracellular, yet little is known about how the symbiont’s genomes have evolved under this transmission strategy. Genome reduction is common in insect intracellular symbionts but limited genome sampling of the extracellular symbionts of distantly related stink bugs has precluded inferring patterns of extracellular symbiont genome evolution. To address this knowledge gap, we completely sequenced the genomes of the uncultivable bacterial symbionts of four neotropical stink bugs of the Edessa genus. Phylogenetic and comparative analyses indicated that the symbionts form a clade within the Pantoea genus and their genomes are highly reduced (∼0.8 Mb). Furthermore, genome synteny analysis and a jackknife approach for phylogenetic reconstruction, which corrected for long branch attraction artifacts, indicated that the Edessa symbionts were the result of a single symbiotic event that was distinct from the symbiosis event giving rise to Candidatus “Pantoea carbekii,” the extracellular symbiont of the invasive pentatomid stink bug, Halyomorpha halys. Metabolic functions inferred from the Edessa symbiont genomes suggests a shift in genomic composition characteristic of its lifestyle in that they retained many host-supportive functions while undergoing dramatic gene loss and establishing a stable relationship with their host insects. Given the undersampled nature of extracellular insect symbionts, this study is the first comparative analysis of these symbiont genomes from four distinct Edessa stink bug species. Finally, we propose the candidate name “Candidatus Pantoea edessiphila” for the species of these symbionts with strain designations according to their host species.

Morphogenesis and development of midgut symbiotic organ of the stinkbug Plautia stali (Hemiptera: Pentatomidae)

Diverse insects are intimately associated with microbial symbionts, which play a variety of biological roles in their adaptation to and survival in the natural environment. Such insects often possess specialized organs for hosting the microbial symbionts. What developmental processes and mechanisms underlie the formation of the host organs for microbial symbiosis is of fundamental biological interest but poorly understood. Here we investigate the morphogenesis of the midgut symbiotic organ and the process of symbiont colonization therein during the developmental course of the stinkbug Plautia stali. Upon hatching, the midgut is a simple and smooth tube. Subsequently, symbiont colonization to the posterior midgut occurs, and thickening and folding of the midgut epithelium proceed during the first instar period. By the second instar, rudimentary crypts have formed, and their inner cavities are colonized by the symbiotic bacteria. From the second instar to the fourth instar, while the alimentary tract grows and the posterior midgut is established as the symbiotic organ with numerous crypts, the anterior midgut and the posterior midgut are structurally and functionally isolated by a strong constriction in the middle. By the early fifth instar, the midgut symbiotic organ attains the maximal length, but toward the mid fifth instar, the basal region of each crypt starts to constrict and narrow, which deforms the midgut symbiotic organ as a whole into a shorter, thicker and twisted shape. By the late fifth instar to adulthood, the crypts are constricted off, by which the symbiotic bacteria are confined in the crypt cavities and isolated from the midgut main tract, and concurrently, the strong midgut constriction in the middle becomes loose and open, by which the food flow from the anterior midgut to the posterior midgut recovers. This study provides the most detailed and comprehensive descriptions ever reported on the morphogenesis of the symbiotic organ and the process of symbiont colonization in an obligatory insect-bacterium gut symbiotic system. Considering that P. stali is recently emerging as a useful model system for experimentally studying the intimate insect-microbe gut symbiosis, the knowledge obtained in this study establishes the foundation for the further development of this research field.

Interfaces between microbes and membranes of host epithelial cells in hemipteran midguts

Certain families of plant-feeding insects in the order Hemiptera (infraorder Pentatomomorpha) have established symbiotic relationships with microbes that inhabit specific pouches (caeca) of their midgut epithelium. The placement of these caeca in a well-delineated region at the most posterior end of the midgut bordering the hindgut is conserved in these families; in situ the convoluted midgut is predictably folded so that this caecal region lies adjacent to the anterior-most region of the midgut. Depending on the hemipteran family, caeca vary in their number and configuration at a given anterior-posterior location. At the host-microbe interface, epithelial plasma membranes of midgut epithelial cells interact with nonself antigens of microbial surfaces. In the different hemipteran species examined, a continuum of interactions is observed between microbes and host membranes. Bacteria can exist as free living cells within the midgut lumen without contacting host membranes while other host cells physically interact extensively with microbial surfaces by extending numerous processes that interdigitate with microbes; and, in many instances, processes completely envelope the microbes. The host cells can embrace the foreign microbes, completely enveloping each with a single host membrane or sometimes enveloping each with the two additional host membranes of a phagosome.

Upregulation of Helicoverpa armigera core RNA interference genes by bacterial infections and its effect on the insect-bacteria interaction

RNA interference (RNAi) is an extremely conserved defence mechanism. The antiviral role of the RNAi pathway in insects is well documented; however, the relevance of this pathway in other aspects of insect immunity is largely unknown. In this study, we questioned whether RNAi has any function during insect-bacteria interactions. For this, we assessed induction of the RNAi pathway in response to bacterial infections by monitoring the expression of dicer1/argonaute1 and dicer2/argonaute2, which are important genes in the microRNA and short interfering RNA sub-pathways respectively. Bacterial cells of Bacillus thuringiensis and Serratia marcescens were injected into the haemocoel of fifth-instar larvae of Helicoverpa armigera, whereas double-distilled water was injected into control insects. Expression levels of the RNAi-related genes increased in the bacteria-injected larvae compared with controls. Transcript knockdown of dicer1 reduced the replication of B. thuringiensis; as a consequence, larval mortality decreased compared with the control. However, replication of S. marcescens increased following dicer1 silencing, which led to higher rates of larval mortality when compared with the control. RNAi of dicer2 promoted replication of both bacteria in the larvae and also enhanced larval mortality. Therefore, dicer1 and dicer2 affected larval survival and the replication rates of the pathogenic bacteria, suggesting their roles in the interactions.

Wolbachia induce cytoplasmic incompatibility and affect mate preference in Habrobracon hebetor to increase the chance of its transmission to the next generation

Wolbachia are common intracellular bacteria that are generally found in arthropods, including a high proportion of insects and also some nematodes. This intracellular symbiont can affect sex ratio with a variety of reproductive anomalies in the host, including cytoplasmic incompatibility (CI) in haplodiploids. In this study, we questioned if the parasitoid wasp, Habrobracon hebetor (Hym.: Braconidae), an important biological control agent of many lepidopteran larvae, is infected with Wolbachia. To test this, DNA was extracted from adult insects and subjected to PCR using specific primers to Wolbachia target genes. The results showed a high rate of Wolbachia infection in this parasitoid wasp. To determine the biological function of Wolbachia in H. hebetor, we removed this bacterium from the wasps using antibiotic treatment (cured wasps). Results of crossing experiments revealed that Wolbachia induced CI in H. hebetor in which cured females crossed with infected males produced only males, while both male and female progeny were observed for other crosses. Also, we showed that the presence of Wolbachia in females increased fecundity and female offspring of this parasitoid wasp. The presence of Wolbachia in the males had no significant effect on fecundity and female production, but might have incurred costs. We also investigated the effect of Wolbachia on mate choice and found that Wolbachia affects mating behavior of H. hebetor. Together, we showed that Wolbachia induces CI in H. hebetor and affects host mating behavior in favor of its transmission. Wolbachia utilize these strategies to increase the frequency of infected females in the host population.

Potential Management Tactics for Pistachio Stink Bugs, Brachynema germari, Acrosternum heegeri and Acrosternum arabicum (Hemiptera: Pentatomidae): High Temperature and Chemical Surface Sterilants Leading to Symbiont Suppression

Insect symbionts offer an opportunity to deal with the anticipated elevated demand for novel pest management strategies. One approach is the disruption of essential symbionts required by the pests. In the present study, we examined the effects of symbiont elimination strategies, high temperature and sterilization agents, on the fitness of three stink bugs, Brachynema germari Kolenati, Acrosternum heegeri Fieber, and Acrosternum arabicum Wagner by using demographic approach. In the high-temperature experiments, almost all insects exhibited severe fitness defects, including elevated nymphal mortality and reduced population growth parameters (especially intrinsic rate of increase, r), as well as significant reductions in the gut symbiont titers. In the egg surface sterilization assays, we experimentally assessed the effects of sterilization agents on the bugs and their symbionts and observed similar fitness defects to those observed under the high-temperature condition. According to the results, we concluded that the host's defective phenotypes are attributable not to the heat stress itself but to the suppression of the symbiont titer, which highlights the possibility that global warming and elevated temperature may negatively affect this mutualism. Together, the results suggest the biological importance of the bacterial symbiont for the host that might help us for better management of these important pests in the future.

Recurrent evolution of gut symbiotic bacteria in pentatomid stinkbugs

BACKGROUND

Diverse animals are intimately associated with microbial symbionts. How such host-symbiont associations have evolved is a fundamental biological issue. Recent studies have revealed a variety of evolutionary relationships, such as obligatory, facultative, and free-living, of gut bacterial symbiosis within the stinkbug family Pentatomidae, although the whole evolutionary picture remains elusive.

RESULTS

Here we investigated a comprehensive assembly of Japanese pentatomid stinkbugs representing 28 genera, 35 species, and 143 populations. Polymerase chain reaction (PCR), cloning, and sequencing of bacterial 16S rRNA gene from their midgut symbiotic organ consistently detected a single bacterial species from each of the insect samples, indicating a general tendency toward monosymbiotic gut association. Bacterial sequences detected from different populations of the same species were completely or nearly identical, indicating that the majority of the gut symbiotic associations are stably maintained at the species level. Furthermore, bacterial sequences detected from different species in the same genus tended to form well-supported clades, suggesting that host-symbiont associations are often stable even at the genus level. Meanwhile, when we compared such sequences with published sequences available in DNA databases, we found a number of counter-examples to such stable host-symbiont relationships; i.e., symbionts from different host species in the same genus may be phylogenetically distant, and symbionts from the same host species may be phylogenetically diverse. Likewise, symbionts of diverse pentatomid species may be closely related to symbionts of other stinkbug families, and symbionts of diverse pentatomid species may even be allied to free-living bacteria. Molecular evolutionary analyses revealed that higher molecular evolutionary rates, higher AT nucleotide compositions, and smaller genome sizes tended to be associated with the pentatomid symbionts constituting the stable lineages, whereas these traits were rarely observed in the pentatomid symbionts of promiscuous type.

CONCLUSIONS

These results indicate that gut symbiotic bacteria have evolved repeatedly and dynamically in the stinkbug family Pentatomidae, which have plausibly entailed frequent symbiont acquisitions, losses, replacements and transfers, while establishing a number of relatively stable host-symbiont associations. The diverse host-symbiont relationships observed in the Pentatomidae will provide an ideal arena for investigating the evolution of symbiosis experimentally and theoretically.