Bacterial symbionts of the giant jewel stinkbug Eucorysses grandis (Hemiptera: Scutelleridae)

Exploring bycatch diversity of organisms in whole genome sequencing of Erebidae moths (Lepidoptera)

Models estimate that up to 80% of all butterfly and moth species host vertically transmitted endosymbiotic microorganisms, which can affect the host fitness, metabolism, reproduction, population dynamics, and genetic diversity, among others. The supporting empirical data are however currently highly biased towards the generally more colourful butterflies, and include less information about moths. Additionally, studies of symbiotic partners of Lepidoptera predominantly focus on the common bacterium Wolbachia pipientis, while infections by other inherited microbial partners have more rarely been investigated. Here, we mine the whole genome sequence data of 47 species of Erebidae moths, with the aims to both inform on the diversity of symbionts potentially associated with this Lepidoptera group, and discuss the potential of metagenomic approaches to inform on host associated microbiome diversity. Based on the result of Kraken2 and MetaPhlAn2 analyses, we found clear evidence of the presence of Wolbachia in four species. Our result also suggests the presence of three other bacterial symbionts (Burkholderia spp., Sodalis spp. and Arsenophonus spp.) in three other moth species. Additionally, we recovered genomic material from bracovirus in about half of our samples. The detection of the latter, usually found in mutualistic association to braconid parasitoid wasps, may inform on host-parasite interactions that take place in the natural habitat of the Erebidae moths, suggesting either contamination with material from species of the host community network, or horizontal transfer of members of the microbiome between interacting species.

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.

The Gut Microbiota of the Insect Infraorder Pentatomomorpha (Hemiptera: Heteroptera) for the Light of Ecology and Evolution

The stinkbugs of the infraorder Pentatomomorpha are a group of important plant sap-feeding insects, which host diverse microorganisms. Some are located in their complex morphological midgut compartments, while some within the specialized bacteriomes of insect hosts. This perpetuation of symbioses through host generations is reinforced via the diverse routes of vertical transmission or environmental acquisition of the symbionts. These symbiotic partners, reside either through the extracellular associations in midgut or intracellular associations in specialized cells, not only have contributed nutritional benefits to the insect hosts but also shaped their ecological and evolutionary basis. The stinkbugs and gut microbe symbioses present a valuable model that provides insights into symbiotic interactions between agricultural insects and microorganisms and may become potential agents for insect pest management.

Quorum sensing sets the stage for the establishment and vertical transmission of Sodalis praecaptivus in tsetse flies

Bacterial virulence factors facilitate host colonization and set the stage for the evolution of parasitic and mutualistic interactions. The Sodalis-allied clade of bacteria exhibit striking diversity in the range of both plant and animal feeding insects they inhabit, suggesting the appropriation of universal molecular mechanisms that facilitate establishment. Here, we report on the infection of the tsetse fly by free-living Sodalis praecaptivus, a close relative of many Sodalis-allied symbionts. Key genes involved in quorum sensing, including the homoserine lactone synthase (ypeI) and response regulators (yenR and ypeR) are integral for the benign colonization of S. praecaptivus. Mutants lacking ypeI, yenR and ypeR compromised tsetse survival as a consequence of their inability to repress virulence. Genes under quorum sensing, including homologs of the binary insecticidal toxin PirAB and a putative symbiosis-promoting factor CpmAJ, demonstrated negative and positive impacts, respectively, on tsetse survival. Taken together with results obtained from experiments involving weevils, this work shows that quorum sensing virulence suppression plays an integral role in facilitating the establishment of Sodalis-allied symbionts in diverse insect hosts. This knowledge contributes to the understanding of the early evolutionary steps involved in the formation of insect-bacterial symbiosis. Further, despite having no established history of interaction with tsetse, S. praecaptivus can infect reproductive tissues, enabling vertical transmission through adenotrophic viviparity within a single host generation. This creates an option for the use of S. praecaptivus in the biocontrol of insect disease vectors via paratransgenesis.

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.

A symbiont's guide to the germline

Microbial symbioses exhibit astounding adaptations, yet all symbionts face the problem of how to reliably associate with host offspring every generation. A common strategy is vertical transmission, in which symbionts are directly transmitted from the female to her offspring. The diversity of symbionts and vertical transmission mechanisms is as expansive as the diversity of eukaryotic host taxa that house them. However, there are several common themes among these mechanisms based on the degree to which symbionts associate with the host germline during transmission. In this review, we detail three distinct vertical transmission strategies, starting with associations that are transmitted from host somatic cells to offspring somatic cells, either due to lacking a germline or avoiding it. A second strategy involves somatically-localized symbionts that migrate into the germline during host development. The third strategy we discuss is one in which the symbiont maintains continuous association with the germline throughout development. Unexpectedly, the vast majority of documented vertically inherited symbionts rely on the second strategy: soma-to-germline migration. Given that not all eukaryotes contain a sequestered germline and instead produce offspring from somatic stem cell lineages, this soma-to-germline migration is discussed in the context of multicellular evolution. Lastly, as recent genomics data have revealed an abundance of horizontal gene transfer events from symbiotic and non-symbiotic bacteria to host genomes, we discuss their impact on eukaryotic host evolution.

The All-Rounder Sodalis: A New Bacteriome-Associated Endosymbiont of the Lygaeoid Bug Henestaris halophilus (Heteroptera: Henestarinae) and a Critical Examination of Its Evolution

Hemipteran insects are well-known in their ability to establish symbiotic relationships with bacteria. Among them, heteropteran insects present an array of symbiotic systems, ranging from the most common gut crypt symbiosis to the more restricted bacteriome-associated endosymbiosis, which have only been detected in members of the superfamily Lygaeoidea and the family Cimicidae so far. Genomic data of heteropteran endosymbionts are scarce and have merely been analyzed from the Wolbachia endosymbiont in bed bug and a few gut crypt-associated symbionts in pentatomoid bugs. In this study, we present the first detailed genomic analysis of a bacteriome-associated endosymbiont of a phytophagous heteropteran, present in the seed bug Henestaris halophilus (Hemiptera: Heteroptera: Lygaeoidea). Using phylogenomics and genomics approaches, we have assigned the newly characterized endosymbiont to the Sodalis genus, named as Candidatus Sodalis baculum sp. nov. strain kilmister. In addition, our findings support the reunification of the Sodalis genus, currently divided into six different genera. We have also conducted comparative analyses between 15 Sodalis species that present different genome sizes and symbiotic relationships. These analyses suggest that Ca. Sodalis baculum is a mutualistic endosymbiont capable of supplying the amino acids tyrosine, lysine, and some cofactors to its host. It has a small genome with pseudogenes but no mobile elements, which indicates middle-stage reductive evolution. Most of the genes in Ca. Sodalis baculum are likely to be evolving under purifying selection with several signals pointing to the retention of the lysine/tyrosine biosynthetic pathways compared with other Sodalis.

Culturable symbionts associated with the reproductive and digestive tissues of the Neotropical brown stinkbug Euschistus heros

Symbionts are widely distributed in eukaryotes, and potentially affect the physiology, ecology and evolution of their host. Most insects harbour free-living bacteria in their haemocoel and gut lumen, intracellular-living bacteria in a range of tissues or bacteria in host-derived specialized cells. Stinkbugs, as do many arthropods, harbour extracellular bacteria in the gut that may affect the fitness of their host. This study identified the culturable symbionts associated with the ovaries, spermatheca, seminal vesicle and posterior midgut region (V4) of males and females of Euschistus heros (F.) (Hemiptera: Pentatomidae). Several culture media were used to isolate the bacteria associated with these structures. The selected colonies (morphotypes) were cultured in liquid medium, subjected to genomic DNA extraction, 16S rRNA gene amplification, and restriction fragment length polymorphism (RFLP) analyses. Morphotypes with distinct RFLP patterns were purified and sequenced, and the sequences obtained were used for putative identification and phylogenetic analysis. Comparison of the sequences with those available in the EzTaxon-e database and the use of a matrix of paired distances grouped the isolates in phylotypes belonging to the Phylum Proteobacteria. Proteobacteria was represented by γ-Proteobacteria phylotypes belonging to Enterobacteriaceae, while Firmicutes had Bacilli phylotypes distributed in Enterococcaceae and Staphylococcaceae. Some of the phylotypes identified were associated exclusively with single structures, such as ovaries, spermatheca and the V4 midgut region of males and females. All culturable bacteria associated with the seminal vesicle were also associated with other tissues.

Symbiotic microorganisms in Puto superbus (Leonardi, 1907) (Insecta, Hemiptera, Coccomorpha: Putoidae)

The scale insect Puto superbus (Putoidae) lives in mutualistic symbiotic association with bacteria. Molecular phylogenetic analyses have revealed that symbionts of P. superbus belong to the gammaproteobacterial genus Sodalis. In the adult females, symbionts occur both in the bacteriocytes constituting compact bacteriomes and in individual bacteriocytes, which are dispersed among ovarioles. The bacteriocytes also house a few small, rod-shaped Wolbachia bacteria in addition to the numerous large, elongated Sodalis-allied bacteria. The symbiotic microorganisms are transovarially transmitted from generation to generation. In adult females which have choriogenic oocytes in the ovarioles, the bacteriocytes gather around the basal part of the tropharium. Next, the entire bacteriocytes pass through the follicular epithelium surrounding the neck region of the ovariole and enter the space between oocyte and follicular epithelium (perivitelline space). In the perivitelline space, the bacteriocytes assemble extracellularly in the deep depression of the oolemma at the anterior pole of the oocyte, forming a "symbiont ball".

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.

Phylogenetically Diverse Burkholderia Associated with Midgut Crypts of Spurge Bugs, Dicranocephalus spp. (Heteroptera: Stenocephalidae)

Diverse phytophagous heteropteran insects, commonly known as stinkbugs, are associated with specific gut symbiotic bacteria, which have been found in midgut cryptic spaces. Recent studies have revealed that members of the stinkbug families Coreidae and Alydidae of the superfamily Coreoidea are consistently associated with a specific group of the betaproteobacterial genus Burkholderia, called the "stinkbug-associated beneficial and environmental (SBE)" group, and horizontally acquire specific symbionts from the environment every generation. However, the symbiotic system of another coreoid family, Stenocephalidae remains undetermined. We herein investigated four species of the stenocephalid genus Dicranocephalus. Examinations via fluorescence in situ hybridization (FISH) and transmission electron microscopy (TEM) revealed the typical arrangement and ultrastructures of midgut crypts and gut symbionts. Cloning and molecular phylogenetic analyses of bacterial genes showed that the midgut crypts of all species are colonized by Burkholderia strains, which were further assigned to different subgroups of the genus Burkholderia. In addition to the SBE-group Burkholderia, a number of stenocephalid symbionts belonged to a novel clade containing B. sordidicola and B. udeis, suggesting a specific symbiont clade for the Stenocephalidae. The symbiotic systems of stenocephalid bugs may provide a unique opportunity to study the ongoing evolution of symbiont associations in the stinkbug-Burkholderia interaction.

Grandeur Alliances: Symbiont Metabolic Integration and Obligate Arthropod Hematophagy

Several arthropod taxa live exclusively on vertebrate blood. This food source lacks essential metabolites required for the maintenance of metabolic homeostasis, and as such, these arthropods have formed symbioses with nutrient-supplementing microbes that facilitate their host's 'hematophagous' feeding ecology. Herein we highlight metabolic contributions of bacterial symbionts that reside within tsetse flies, bed bugs, lice, reduviid bugs, and ticks, with specific emphasis on B vitamin and cofactor biosynthesis. Importantly, these arthropods can transmit pathogens of medical and veterinary relevance and/or cause infestations that induce psychological and dermatological distress. Microbial metabolites, and the biochemical pathways that generate them, can serve as specific targets of novel control mechanisms aimed at disrupting the metabolism of hematophagous arthropods, thus combatting pest invasion and vector-borne pathogen transmission.

Female-specific specialization of a posterior end region of the midgut symbiotic organ in Plautia splendens and allied stinkbugs

Many stinkbugs (Insecta: Hemiptera: Heteroptera) are associated with bacterial symbionts in a posterior region of the midgut. In these stinkbugs, adult females excrete symbiont-containing materials from the anus for transmission of the beneficial symbionts to their offspring. For ensuring the vertical symbiont transmission, a variety of female-specific elaborate traits at the cellular, morphological, developmental, and behavioral levels have been reported from diverse stinkbugs of the families Plataspidae, Urostylididae, Parastrachiidae, etc. Meanwhile, such elaborate female-specific traits for vertical symbiont transmission have been poorly characterized for the largest and economically important stinkbug family Pentatomidae. Here, we investigated the midgut symbiotic system of a pentatomid stinkbug, Plautia splendens. A specific gammaproteobacterial symbiont was consistently present extracellularly in the cavity of numerous crypts arranged in four rows on the midgut fourth section. The symbiont was smeared on the egg surface upon oviposition by adult females, orally acquired by newborn nymphs, and thereby transmitted vertically to the next generation and important for growth and survival of the host insects. We found that, specifically in adult females, several rows of crypts at the posterior end region of the symbiotic midgut were morphologically differentiated and conspicuously enlarged, often discharging the symbiotic bacteria from the crypt cavity to the main tract of the symbiotic midgut. The female-specific enlarged end crypts were also found in other pentatomid stinkbugs Plautia stali and Carbula crassiventris. These results suggest that the enlarged end crypts represent a female-specific specialized morphological trait for vertical symbiont transmission commonly found among stinkbugs of the family Pentatomidae.

Infection prevalence of Sodalis symbionts among stinkbugs

INTRODUCTION

Diverse insects and other organisms are associated with microbial symbionts, which often significantly contribute to growth and survival of their hosts and/or drastically affect phenotypes of their hosts in a variety of ways. Sodalis glossinidius was first identified as a facultative bacterial symbiont of tsetse flies, and recent studies revealed that Sodalis-allied bacteria encompass diverse ecological niches ranging from free-living bacteria through facultative symbionts to obligate symbionts associated with a diverse array of insects. Despite potential ecological and evolutionary relevance of the Sodalis symbionts, their infection prevalence in natural insect populations has been poorly investigated.

RESULTS

Here we surveyed diverse stinkbugs and allied terrestrial heteropteran bugs, which represented 17 families, 77 genera, 108 species, 310 populations and 960 individuals, for infection with the Sodalis symbionts. Diagnostic PCR detected relatively low infection frequencies of the Sodalis symbionts: 13.6% (14/103) of the species, 7.5% (22/295) of the populations, and 4.3% (35/822) of the individuals of the stinkbugs except for those belonging to the family Urostylididae. Among the urostylidid stinkbugs, strikingly, the Sodalis symbionts exhibited very high infection frequencies: 100% (5/5) of the species, 100% (15/15) of the populations, and 94.2% (130/138) of the individuals we examined. Molecular phylogenetic analysis based on bacterial 16S rRNA gene sequences revealed that all the symbionts were placed in the clade of Sodalis-allied bacteria while the symbiont phylogeny did not reflect the systematics of their stinkbug hosts. Notably, the Sodalis symbionts of the urostylidid stinkbugs were not clustered with the Sodalis symbionts of the other stinkbug groups on the phylogeny, suggesting their distinct evolutionary trajectories.

CONCLUSIONS

The relatively low infection frequency and the overall host-symbiont phylogenetic incongruence suggest that the Sodalis symbionts are, in general, facultative symbiotic associates in the majority of the stinkbug groups. On the other hand, it is conceivable, although speculative, that the Sodalis symbionts may play some substantial biological roles for their host stinkbugs of the Urostylididae.

Evidence of environmental and vertical transmission of Burkholderia symbionts in the oriental chinch bug, Cavelerius saccharivorus (Heteroptera: Blissidae)

The vertical transmission of symbiotic microorganisms is omnipresent in insects, while the evolutionary process remains totally unclear. The oriental chinch bug, Cavelerius saccharivorus (Heteroptera: Blissidae), is a serious sugarcane pest, in which symbiotic bacteria densely populate the lumen of the numerous tubule-like midgut crypts that the chinch bug develops. Cloning and sequence analyses of the 16S rRNA genes revealed that the crypts were dominated by a specific group of bacteria belonging to the genus Burkholderia of the Betaproteobacteria. The Burkholderia sequences were distributed into three distinct clades: the Burkholderia cepacia complex (BCC), the plant-associated beneficial and environmental (PBE) group, and the stinkbug-associated beneficial and environmental group (SBE). Diagnostic PCR revealed that only one of the three groups of Burkholderia was present in ∼89% of the chinch bug field populations tested, while infections with multiple Burkholderia groups within one insect were observed in only ∼10%. Deep sequencing of the 16S rRNA gene confirmed that the Burkholderia bacteria specifically colonized the crypts and were dominated by one of three Burkholderia groups. The lack of phylogenetic congruence between the symbiont and the host population strongly suggested host-symbiont promiscuity, which is probably caused by environmental acquisition of the symbionts by some hosts. Meanwhile, inspections of eggs and hatchlings by diagnostic PCR and egg surface sterilization demonstrated that almost 30% of the hatchlings vertically acquire symbiotic Burkholderia via symbiont-contaminated egg surfaces. The mixed strategy of symbiont transmission found in the oriental chinch bug might be an intermediate stage in evolution from environmental acquisition to strict vertical transmission in insects.

The crypt-dwelling primary bacterial symbiont of the polyphagous pentatomid pest Halyomorpha halys (Hemiptera: Pentatomidae)

A recent invader of North America, the brown marmorated stink bug (Halyomorpha halys Stål) is a polyphagous pentatomid that harbors a gammaproteobacterial mutualist in the crypts of specialized midgut gastric caeca (region V4). Histological analyses revealed a single rod-shaped morphology abundant in distal V4 midgut caecal crypts. A strong fluorescence signal was detected when thin sections of these tissues were hybridized with a fluorescently-labeled, Enterobacteriaceae-specific oligonucleotide probe. A single operational taxonomic unit (OTU) assigned to the Pantoea genus represented >99% of 3,454 16S rDNA amplicons obtained from midgut V4 tissues and egg samples. Detection of H. halys primary symbiont in DNA extracted from eggs suggested vertical maternal inheritance as the mode of intergenerational transmission. Consistent detection of the bacterial symbiont in geographically distinct H. halys populations strongly supports an intimate association between these two organisms. An inferred phylogeny of gammaproteobacterial symbionts of pentatomids placed the Pantoea-assigned OTU from H. halys within a clade distinct from primary bacterial symbionts of related stink bugs, Nezara viridula (L.) and Eurydema rugosa Motschulsky. Given these data, Candidatus "Pantoea carbekii" is proposed as the name of the primary bacterial symbiont of H. halys.

Analysis of multiple tsetse fly populations in Uganda reveals limited diversity and species-specific gut microbiota

The invertebrate microbiome contributes to multiple aspects of host physiology, including nutrient supplementation and immune maturation processes. We identified and compared gut microbial abundance and diversity in natural tsetse flies from Uganda using five genetically distinct populations of Glossina fuscipes fuscipes and multiple tsetse species (Glossina morsitans morsitans, G. f. fuscipes, and Glossina pallidipes) that occur in sympatry in one location. We used multiple approaches, including deep sequencing of the V4 hypervariable region of the 16S rRNA gene, 16S rRNA gene clone libraries, and bacterium-specific quantitative PCR (qPCR), to investigate the levels and patterns of gut microbial diversity from a total of 151 individuals. Our results show extremely limited diversity in field flies of different tsetse species. The obligate endosymbiont Wigglesworthia dominated all samples (>99%), but we also observed wide prevalence of low-density Sodalis (tsetse's commensal endosymbiont) infections (<0.05%). There were also several individuals (22%) with high Sodalis density, which also carried coinfections with Serratia. Albeit in low density, we noted differences in microbiota composition among the genetically distinct G. f. fuscipes flies and between different sympatric species. Interestingly, Wigglesworthia density varied in different species (10(4) to 10(6) normalized genomes), with G. f. fuscipes having the highest levels. We describe the factors that may be responsible for the reduced diversity of tsetse's gut microbiota compared to those of other insects. Additionally, we discuss the implications of Wigglesworthia and Sodalis density variations as they relate to trypanosome transmission dynamics and vector competence variations associated with different tsetse species.

Bacterial symbionts of a devastating coffee plant pest, the stinkbug Antestiopsis thunbergii (Hemiptera: Pentatomidae)

Stinkbugs of the genus Antestiopsis, so-called antestia bugs or variegated coffee bugs, are notorious pests of coffee plants in Africa. We investigated the symbiotic bacteria associated with Antestiopsis thunbergii, a major coffee plant pest in Rwanda. PCR, cloning, sequencing, and phylogenetic analysis of bacterial genes identified four distinct bacterial lineages associated with A. thunbergii: a gammaproteobacterial gut symbiont and symbionts representing the genera Sodalis, Spiroplasma, and Rickettsia. In situ hybridization showed that the gut symbiont densely occupied the lumen of midgut crypts, whereas the Sodalis symbiont, the Spiroplasma symbiont, and the Rickettsia symbiont sparsely and sporadically infected various cells and tissues. Diagnostic PCR survey of 154 A. thunbergii individuals collected at 8 localities in Rwanda revealed high infection frequencies (100% for the gut symbiont, 51.3% for the Sodalis symbiont, 52.6% for the Spiroplasma symbiont, and 24.0% for the Rickettsia symbiont). These results suggest that the gut symbiont is the primary symbiotic associate of obligate nature for A. thunbergii, whereas the Sodalis symbiont, the Spiroplasma symbiont, and the Rickettsia symbiont are the secondary symbiotic associates of facultative nature. We observed high coinfection frequencies, i.e., 7.8% of individuals with quadruple infection with all the symbionts, 32.5% with triple infections with the gut symbiont and two of the secondary symbionts, and 39.6% with double infections with the gut symbiont and any of the three secondary symbionts, which were statistically not different from the expected coinfection frequencies and probably reflected random associations. The knowledge of symbiotic microbiota in A. thunbergii will provide useful background information for controlling this devastating coffee plant pest.

The importance of gut symbionts in the development of the brown marmorated stink bug, Halyomorpha halys (Stål)

The invasive brown marmorated stink bug, Halyomorpha halys (Stål), has become a severe agricultural pest and nuisance problem since its introduction in the U.S. Research is being conducted to understand its biology and to find management solutions. Its symbiotic relationship with gut symbionts is one aspect of its biology that is not understood. In the family Pentatomidae, the reliance on gut symbionts for successful development seems to vary depending on the species of stink bug. This research assessed the role of gut symbionts in the development, survivorship, and fecundity of H. halys. We compared various fitness parameters of nymphs and adults reared from surface sterilized and untreated egg masses during two consecutive generations under laboratory conditions. Results provided direct evidence that H. halys is negatively impacted by the prevention of vertical transmission of its gut symbionts and that this impact is significant in the first generation and manifests dramatically in the subsequent generation. Developmental time and survivorship of treated cohorts in the first generation were significantly affected during third instar development through to the adult stage. Adults from the sterilized treatment group exhibited longer pre-oviposition periods, produced fewer egg masses, had significantly smaller clutch sizes, and the hatch rate and survivorship of those eggs were significantly reduced. Observations following hatch of surface sterilized eggs also revealed significant effects on wandering behavior of the first instars. The second generation progeny from adults of the sterilized cohorts showed significantly lower survival to adulthood, averaging only 0.3% compared to 20.8% for the control cohorts. Taken together, results demonstrate that H. halys is heavily impacted by deprival of its gut symbionts. Given the economic status of this invasive pest, further investigations may lead to management tactics that disrupt this close symbiotic relationship in the biology of H. halys.

Interwoven biology of the tsetse holobiont

Microbial symbionts can be instrumental to the evolutionary success of their hosts. Here, we discuss medically significant tsetse flies (Diptera: Glossinidae), a group comprised of over 30 species, and their use as a valuable model system to study the evolution of the holobiont (i.e., the host and associated microbes). We first describe the tsetse microbiota, which, despite its simplicity, harbors a diverse range of associations. The maternally transmitted microbes consistently include two Gammaproteobacteria, the obligate mutualists Wigglesworthia spp. and the commensal Sodalis glossinidius, along with the parasitic Alphaproteobacteria Wolbachia. These associations differ in their establishment times, making them unique and distinct from previously characterized symbioses, where multiple microbial partners have associated with their host for a significant portion of its evolution. We then expand into discussing the functional roles and intracommunity dynamics within this holobiont, which enhances our understanding of tsetse biology to encompass the vital functions and interactions of the microbial community. Potential disturbances influencing the tsetse microbiome, including salivary gland hypertrophy virus and trypanosome infections, are highlighted. While previous studies have described evolutionary consequences of host association for symbionts, the initial steps facilitating their incorporation into a holobiont and integration of partner biology have only begun to be explored. Research on the tsetse holobiont will contribute to the understanding of how microbial metabolic integration and interdependency initially may develop within hosts, elucidating mechanisms driving adaptations leading to cooperation and coresidence within the microbial community. Lastly, increased knowledge of the tsetse holobiont may also contribute to generating novel African trypanosomiasis disease control strategies.

Phylogenetic analysis of symbionts in feather-feeding lice of the genus Columbicola: evidence for repeated symbiont replacements

BACKGROUND

Many groups of insects have obligate bacterial symbionts that are vertically transmitted. Such associations are typically characterized by the presence of a monophyletic group of bacteria living in a well-defined host clade. In addition the phylogeny of the symbiotic bacteria is typically congruent with that of the host, signifying co-speciation. Here we show that bacteria living in a single genus of feather lice, Columbicola (Insecta: Phthiraptera), present an exception to this typical pattern.

RESULTS

The phylogeny of Columbicola spp. symbionts revealed the presence of three candidate clades, with the most species-rich clade having a comb-like topology with very short internodes and long terminal branches. Evolutionary simulations indicate that this topology is characteristic of a process of repeated symbiont replacement over a brief time period. The two remaining candidate clades in our study exhibit high levels of nucleotide substitution, suggesting accelerated molecular evolution due to relaxed purifying selection or smaller effective population size, which is typical of many vertically transmitted insect symbionts. Representatives of the fast-evolving and slow-evolving symbiont lineages exhibit the same localization, migration, and transmission patterns in their hosts, implying direct replacement.

CONCLUSIONS

Our findings suggest that repeated, independent symbiont replacements have taken place over the course of the relatively recent radiation of Columbicola spp. These results are compatible with the notion that lice and other insects have the capability to acquire novel symbionts through the domestication of progenitor strains residing in their local environment.

Diversification of endosymbiosis: replacements, co-speciation and promiscuity of bacteriocyte symbionts in weevils

The processes and mechanisms underlying the diversification of host-microbe endosymbiotic associations are of evolutionary interest. Here we investigated the bacteriocyte-associated primary symbionts of weevils wherein the ancient symbiont Nardonella has experienced two independent replacement events: once by Curculioniphilus symbiont in the lineage of Curculio and allied weevils of the tribe Curculionini, and once by Sodalis-allied symbiont in the lineage of grain weevils of the genus Sitophilus. The Curculioniphilus symbiont was detected from 27 of 36 Curculionini species examined, the symbiont phylogeny was congruent with the host weevil phylogeny, and the symbiont gene sequences exhibited AT-biased nucleotide compositions and accelerated molecular evolution. These results suggest that the Curculioniphilus symbiont was acquired by an ancestor of the tribe Curculionini, replaced the original symbiont Nardonella, and has co-speciated with the host weevils over evolutionary time, but has been occasionally lost in several host lineages. By contrast, the Sodalis-allied symbiont of Sitophilus weevils exhibited no host-symbiont co-speciation, no AT-biased nucleotide compositions and only moderately accelerated molecular evolution. These results suggest that the Sodalis-allied symbiont was certainly acquired by an ancestor of the Sitophilus weevils and replaced the original Nardonella symbiont, but the symbiotic association must have experienced occasional re-associations such as new acquisitions, horizontal transfers, replacements and/or losses. We detected Sodalis-allied facultative symbionts in populations of the Curculionini weevils, which might represent potential evolutionary sources of the Sodalis-allied primary symbionts. Comparison of these newcomer bacteriocyte-associated symbiont lineages highlights potential evolutionary trajectories and consequences of novel symbionts after independent replacements of the same ancient symbiont.

Role of symbiotic bacteria in the growth and development of the Sunn pest, Eurygaster integriceps

The Sunn pest, Eurygaster integriceps Puton (Hemiptera: Scutelleridae), is the most important pest of wheat and barley in wide areas of the world. Different aspects of the insect's life history have been studied, but to date nothing is known about their microbial symbionts. Here, the contribution of symbiotic bacteria to the fitness of the bug was investigated by combining two different approaches to manipulate the host's microbial community: the supplementation of antibiotics into the insects' diet and egg surface sterilization. First, bacteria cultured from gut homogenates were subjected to antibiotic screening tests using 20 different antibiotics. Norfloxacin was the most effective antibiotic, with the greatest inhibition zone among all antibiotics tested. Feeding norfloxacin to adult E. integriceps individuals significantly impaired growth and development of the offspring in a dose-dependent manner, i.e., higher antibiotic doses increased the negative effects on nymphal growth and development. Total developmental time from first nymphal instars to adult emergence in control animals was 30.1 days, but when adults had been offered diets with 10, 20, and 30 µg antibiotic per mg diet, the offspring's developmental time was prolonged to 32.8, 34.0, and 34.8 days, respectively. In the highest two doses of norfloxacin, all of the nymphs died before reaching the fifth nymphal instar. Similar results as for the antibiotic treatment were obtained when egg surface sterilization was used to manipulate the microbial community of E. integriceps. These results indicate that bacterial symbionts play a crucial role in the successful development of the host.

Candidatus Sodalis melophagi sp. nov.: phylogenetically independent comparative model to the tsetse fly symbiont Sodalis glossinidius

Bacteria of the genus Sodalis live in symbiosis with various groups of insects. The best known member of this group, a secondary symbiont of tsetse flies Sodalis glossinidius, has become one of the most important models in investigating establishment and evolution of insect-bacteria symbiosis. It represents a bacterium in the early/intermediate state of the transition towards symbiosis, which allows for exploring such interesting topics as: usage of secretory systems for entering the host cell, tempo of the genome modification, and metabolic interaction with a coexisting primary symbiont. In this study, we describe a new Sodalis species which could provide a useful comparative model to the tsetse symbiont. It lives in association with Melophagus ovinus, an insect related to tsetse flies, and resembles S. glossinidius in several important traits. Similar to S. glossinidius, it cohabits the host with another symbiotic bacterium, the bacteriome-harbored primary symbiont of the genus Arsenophonus. As a typical secondary symbiont, Candidatus Sodalis melophagi infects various host tissues, including bacteriome. We provide basic morphological and molecular characteristics of the symbiont and show that these traits also correspond to the early/intermediate state of the evolution towards symbiosis. Particularly, we demonstrate the ability of the bacterium to live in insect cell culture as well as in cell-free medium. We also provide basic characteristics of type three secretion system and using three reference sequences (16 S rDNA, groEL and spaPQR region) we show that the bacterium branched within the genus Sodalis, but originated independently of the two previously described symbionts of hippoboscoids. We propose the name Candidatus Sodalis melophagi for this new bacterium.

Polyphyly of gut symbionts in stinkbugs of the family Cydnidae

Symbiotic bacteria associated with midgut crypts of stinkbugs of the family Cydnidae, representing seven species and 13 populations, were investigated. All of the symbionts were species specific, and constituted at least four distinct lineages in the Gammaproteobacteria, indicating multiple evolutionary origins of the gut symbionts among the burrower bugs.

Novel clade of alphaproteobacterial endosymbionts associated with stinkbugs and other arthropods

Here we report a novel clade of secondary endosymbionts associated with insects and other arthropods. Seed bugs of the genus Nysius (Hemiptera: Lygaeidae) harbor the primary gammaproteobacterial symbiont Schneideria nysicola within a pair of bacteriomes in the abdomen. Our survey of Nysius species for their facultative bacterial associates consistently yielded a novel type of alphaproteobacterial 16S rRNA gene sequence in addition to those of Wolbachia. Diagnostic PCR survey of 343 individuals representing 24 populations of four Nysius species revealed overall detection rates of the alphaproteobacteria at 77.6% in Nysius plebeius, 87.7% in Nysius sp. 1, 81.0% in Nysius sp. 2, and 100% in Nysius expressus. Further survey of diverse stinkbugs representing 24 families, 191 species, and 582 individuals detected the alphaproteobacteria from an additional 12 species representing six families. Molecular phylogenetic analysis showed that the alphaproteobacteria from the stinkbugs form a distinct and coherent monophyletic group in the order Rickettsiales together with several uncharacterized endosymbionts from fleas and ticks. The alphaproteobacterial symbiont clade was allied to bacterial clades such as the endosymbionts of acanthamoebae, the endosymbionts of cnidarians, and Midichloria spp., the mitochondrion-associated endosymbionts of ticks. In situ hybridization and electron microscopy identified small filamentous bacterial cells in various tissues of N. plebeius, including the bacteriome and ovary. The concentrated localization of the symbiont cells at the anterior pole of oocytes indicated its vertical transmission route through host insect generations. The designation "Candidatus Lariskella arthropodarum" is proposed for the endosymbiont clade.

Diversity of symbiotic organs and bacterial endosymbionts of lygaeoid bugs of the families blissidae and lygaeidae (hemiptera: heteroptera: lygaeoidea)

Here we present comparative data on the localization and identity of intracellular symbionts among the superfamily Lygaeoidea (Insecta: Hemiptera: Heteroptera: Pentatomomorpha). Five different lygaeoid species from the families Blissidae and Lygaeidae (sensu stricto; including the subfamilies Lygaeinae and Orsillinae) were analyzed. Fluorescence in situ hybridization (FISH) revealed that all the bugs studied possess paired bacteriomes that are differently shaped in the abdomen and harbor specific endosymbionts therein. The endosymbionts were also detected in female gonads and at the anterior poles of developing eggs, indicating vertical transmission of the endosymbionts via ovarial passage, in contrast to the posthatch symbiont transmission commonly found among pentatomoid bugs (Pentatomomorpha: Pentatomoidea). Phylogenetic analysis based on 16S rRNA and groEL genes showed that the endosymbionts of Ischnodemus sabuleti, Arocatus longiceps, Belonochilus numenius, Orsillus depressus, and Ortholomus punctipennis constitute at least four distinct clades in the Gammaproteobacteria. The endosymbiont phylogeny did not agree with the host phylogeny based on the mitochondrial cytochrome oxidase I (COI) gene, but there was a local cospeciating pattern within the subfamily Orsillinae. Meanwhile, the endosymbiont of Belonochilus numenius (Lygaeidae: Orsillinae), although harbored in paired bacteriomes as in other lygaeoid bugs of the related genera Nysius, Ortholomus, and Orsillus, was phylogenetically close to "Candidatus Rohrkolberia cinguli," the endosymbiont of Chilacis typhae (Lygaeoidea: Artheneidae), suggesting an endosymbiont replacement in this lineage. The diverse endosymbionts and the differently shaped bacteriomes may reflect independent evolutionary origins of the endosymbiotic systems among lygaeoid bugs.

Multiple origins of endosymbiosis within the Enterobacteriaceae (γ-Proteobacteria): convergence of complex phylogenetic approaches

BACKGROUND

The bacterial family Enterobacteriaceae gave rise to a variety of symbiotic forms, from the loosely associated commensals, often designated as secondary (S) symbionts, to obligate mutualists, called primary (P) symbionts. Determination of the evolutionary processes behind this phenomenon has long been hampered by the unreliability of phylogenetic reconstructions within this group of bacteria. The main reasons have been the absence of sufficient data, the highly derived nature of the symbiont genomes and lack of appropriate phylogenetic methods. Due to the extremely aberrant nature of their DNA, the symbiotic lineages within Enterobacteriaceae form long branches and tend to cluster as a monophyletic group. This state of phylogenetic uncertainty is now improving with an increasing number of complete bacterial genomes and development of new methods. In this study, we address the monophyly versus polyphyly of enterobacterial symbionts by exploring a multigene matrix within a complex phylogenetic framework.

RESULTS

We assembled the richest taxon sampling of Enterobacteriaceae to date (50 taxa, 69 orthologous genes with no missing data) and analyzed both nucleic and amino acid data sets using several probabilistic methods. We particularly focused on the long-branch attraction-reducing methods, such as a nucleotide and amino acid data recoding and exclusion (including our new approach and slow-fast analysis), taxa exclusion and usage of complex evolutionary models, such as nonhomogeneous model and models accounting for site-specific features of protein evolution (CAT and CAT+GTR). Our data strongly suggest independent origins of four symbiotic clusters; the first is formed by Hamiltonella and Regiella (S-symbionts) placed as a sister clade to Yersinia, the second comprises Arsenophonus and Riesia (S- and P-symbionts) as a sister clade to Proteus, the third Sodalis, Baumannia, Blochmannia and Wigglesworthia (S- and P-symbionts) as a sister or paraphyletic clade to the Pectobacterium and Dickeya clade and, finally, Buchnera species and Ishikawaella (P-symbionts) clustering with the Erwinia and Pantoea clade.

CONCLUSIONS

The results of this study confirm the efficiency of several artifact-reducing methods and strongly point towards the polyphyly of P-symbionts within Enterobacteriaceae. Interestingly, the model species of symbiotic bacteria research, Buchnera and Wigglesworthia, originated from closely related, but different, ancestors. The possible origins of intracellular symbiotic bacteria from gut-associated or pathogenic bacteria are suggested, as well as the role of facultative secondary symbionts as a source of bacteria that can gradually become obligate maternally transferred symbionts.

Use of the Internal Transcribed Spacer (ITS) Regions to Examine Symbiont Divergence and as a Diagnostic Tool for Sodalis-Related Bacteria

Bacteria excel in most ecological niches, including insect symbioses. A cluster of bacterial symbionts, established within a broad range of insects, share high 16S rRNA similarities with the secondary symbiont of the tsetse fly (Diptera: Glossinidae), Sodalis glossinidius. Although 16S rRNA has proven informative towards characterization of this clade, the gene is insufficient for examining recent divergence due to selective constraints. Here, we assess the application of the internal transcribed spacer (ITS) regions, specifically the ITS(glu) and ITS(ala,ile), used in conjunction with 16S rRNA to enhance the phylogenetic resolution of Sodalis-allied bacteria. The 16S rRNA + ITS regions of Sodalis and allied bacteria demonstrated significant divergence and were robust towards phylogenetic resolution. A monophyletic clade of Sodalis isolates from tsetse species, distinct from other Enterobacteriaceae, was consistently observed suggesting diversification due to host adaptation. In contrast, the phylogenetic distribution of symbionts isolated from hippoboscid flies and various Hemiptera and Coleoptera were intertwined suggesting either horizontal transfer or a recent establishment from an environmental source. Lineage splitting of Sodalis-allied bacteria into symbiotic and free-living sister groups was also observed. Additionally, we propose an ITS region as a diagnostic marker for the identification of additional Sodalis-allied symbionts in the field. These results expand our knowledge of informative genome regions to assess genetic divergence since splitting from the last common ancestor, of this versatile insect symbiont clade that have become increasingly recognized as valuable towards our understanding of the evolution of symbiosis. These facultative and recently associated symbionts may provide a novel source of traits adaptable to the dynamic ecologies encountered by diverse host backgrounds.

Evolution of symbiotic organs and endosymbionts in lygaeid stinkbugs

We investigated seed bugs of the genus Nysius (Insecta: Hemiptera: Lygaeidae) for their symbiotic bacteria. From all the samples representing 4 species, 18 populations and 281 individuals, specific bacterial 16S rRNA gene sequences were consistently identified, which formed a distinct clade in the Gammaproteobacteria. In situ hybridization showed that the bacterium was endocellularly localized in a pair of large bacteriomes that were amorphous in shape, deep red in color, and in association with gonads. In the ovary of adult females, the endosymbiont was also localized in the 'infection zone' in the middle of each germarium and in the 'symbiont ball' at the anterior pole of each oocyte, indicating vertical transmission of the endosymbiont through the ovarial passage. Phylogenetic analyses based on bacterial 16S rRNA, groEL and gyrB genes consistently supported a coherent monophyly of the Nysius endosymbionts. The possibility of a sister relationship to 'Candidatus Kleidoceria schneideri', the bacteriome-associated endosymbiont of a lygaeid bug Kleidocerys resedae, was statistically rejected, indicating independent evolutionary origins of the endosymbionts in the Lygaeidae. The endosymbiont genes consistently exhibited AT-biased nucleotide compositions and accelerated rates of molecular evolution, and the endosymbiont genome was only 0.6 Mb in size. The endosymbiont phylogeny was congruent with the host insect phylogeny, suggesting strict vertical transmission and host-symbiont co-speciation over evolutionary time. Based on these results, we discuss the evolution of bacteriomes and endosymbionts in the Heteroptera, most members of which are associated with gut symbiotic bacteria. The designation 'Candidatus Schneideria nysicola' is proposed for the endosymbiont clade.