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

A strong priority effect in the assembly of a specialized insect-microbe symbiosis

Specialized host-microbe symbioses are ecological communities, whose composition is shaped by various processes. Microbial community assembly in these symbioses is determined in part by interactions between taxa that colonize ecological niches available within habitat patches. The outcomes of these interactions, and by extension the trajectory of community assembly, can display priority effects-dependency on the order in which taxa first occupy these niches. The underlying mechanisms of these phenomena vary from system to system and are often not well resolved. Here, we characterize priority effects in colonization of the squash bug (Anasa tristis) by bacterial symbionts from the genus Caballeronia, using pairs of strains that are known to strongly compete during host colonization, as well as strains that are isogenic and thus functionally identical. By introducing symbiont strains into individual bugs in a sequential manner, we show that within-host populations established by the first colonist are extremely resistant to invasion, regardless of strain identity and competitive interactions. By knocking down the population of an initial colonist with antibiotics, we further show that colonization success by the second symbiont is still diminished even when space in the symbiotic organ is available and ostensibly accessible for colonization. We speculate that resident symbionts exclude subsequent infections by manipulating the host environment, partially but not exclusively by eliciting tissue remodeling of the symbiont organ.

IMPORTANCE

Host-associated microbial communities underpin critical ecosystem processes and human health, and their ability to do so is determined in turn by the various processes that shape their composition. While selection deterministically acts on competing genotypes and species during community assembly, the manner by which selection determines the trajectory of community assembly can differ depending on the sequence by which taxa are established within that community. We document this phenomenon, known as a priority effect, during experimental colonization of a North American insect pest, the squash bug Anasa tristis, by its betaproteobacterial symbionts in the genus Caballeronia. Our study demonstrates how stark, strain-level variation can emerge in specialized host-microbe symbioses simply through differences in the order by which strains colonize the host. Understanding the mechanistic drivers of community structure in host-associated microbiomes can highlight both pitfalls and opportunities for the engineering of these communities and their constituent taxa for societal benefit.

Large-Scale Sampling Reveals the Strain-Level Diversity of Burkholderia Symbionts in Riptortus pedestris and R. linearis (Hemiptera: Alydidae)

Burkholderia (sensu lato) is a diverse group of β-Proteobacteria that exists worldwide in various environments. The SBE clade of this group was thought to be mutualistic with stinkbugs. Riptortus-Burkholderia was suggested as an ideal model system for studying insect-microbe symbiosis. To explore the strain-level diversity of Burkholderia at the individual and population levels of Riptortus stinkbugs (Hemiptera: Alydidae), and to uncover the factors affecting the Burkholderia community, large-scale sampling of two Riptortus species and deep sequencing data (16S amplicon) were used in the present study. Our results showed that: (1) the proportions of facultative symbiotic bacteria Burkholderia were very high, with an average proportion of 87.1% in the samples; (2) only six out of 1373 Burkholderia amplicon sequence variants (ASVs) did not belong to the SBE clade, accounting for only 0.03% of Burkholderia; (3) a relatively small number of Burkholderia ASVs had a large number of sequences, with 22, 54, and 107 ASVs accounting for more than 1.0%, 0.1%, and 0.01% of the total Burkholderia sequences, respectively; (4) multiple Burkholderia ASVs were present in most Riptortus individuals, but there was one dominant or two codominant ASVs, and codominance was more likely to occur when the genetic distance between the two codominant ASVs was small; and (5) the beta diversity of Burkholderia was significantly different between the two host species (PerMANOVA: both Jaccard and Bray-Curtis, p < 0.001) and among localities (PerMANOVA: both Jaccard and Bray-Curtis, p < 0.001). Two-way PerMANOVA also indicated that both the host (Bray-Curtis, p = 0.020; Jaccard, p = 0.001) and geographical location (Bray-Curtis, p = 0.041; Jaccard, p = 0.045) influence Burkholderia communities; furthermore, Mantel tests showed that the Burkholderia communities were significantly correlated with the geographical distance of sample locations (R = 0.056, p = 0.001). Together, our findings demonstrate the fine-scale diversity of Burkholderia symbionts and suggest a region- and host-dependent pattern of Burkholderia in Riptortus stinkbugs.

Social wasp-associated Tsukamurella sp. strains showed promising biosynthetic and bioactive potential for discovery of novel compounds

In the face of escalating antibiotic resistance, the quest for novel antimicrobial compounds is critical. Actinobacteria is known for producing a substantial fraction of bioactive molecules from microorganisms, nonetheless there is the challenge of metabolic redundancy in bioprospecting. New sources of natural products are needed to overcome these current challenges. Our present work proposes an unexplored potential of Neotropical social wasp-associated microbes as reservoirs of novel bioactive compounds. Using social wasp-associated Tsukamurella sp. strains 8F and 8J, we aimed to determine their biosynthetic potential for producing novel antibiotics and evaluated phylogenetic and genomic traits related to environmental and ecological factors that might be associated with promising bioactivity and evolutionary specialization. These strains were isolated from the cuticle of social wasps and subjected to comprehensive genome sequencing. Our genome mining efforts, employing antiSMASH and ARTS, highlight the presence of BGCs with minimal similarity to known compounds, suggesting the novelty of the molecules they may produce. Previous, bioactivity assays of these strains against bacterial species which harbor known human pathogens, revealed inhibitory potential. Further, our study focuses into the phylogenetic and functional landscape of the Tsukamurella genus, employing a throughout phylogenetic analysis that situates strains 8F and 8J within a distinct evolutionary pathway, matching with the environmental and ecological context of the strains reported for this genus. Our findings emphasize the importance of bioprospecting in uncharted biological territories, such as insect-associated microbes as reservoirs of novel bioactive compounds. As such, we posit that Tsukamurella sp. strains 8F and 8J represent promising candidates for the development of new antimicrobials.

The plant-sucking insect selects assembly of the gut microbiota from environment to enhance host reproduction

Plant-sucking insects have intricate associations with a diverse array of microorganisms to facilitate their adaptation to specific ecological niches. The midgut of phytophagous true bugs is generally structured into four distinct compartments to accommodate their microbiota. Nevertheless, there is limited understanding regarding the origins of these gut microbiomes, the mechanisms behind microbial community assembly, and the interactions between gut microbiomes and their insect hosts. In this study, we conducted a comprehensive survey of microbial communities within the midgut compartments of a bean bug Riptortus pedestris, soybean plant, and bulk soil across 12 distinct geographical fields in China, utilizing high-throughput sequencing of the 16 S rRNA gene. Our findings illuminated that gut microbiota of the plant-sucking insects predominantly originated from the surrounding soil environment, and plants also play a subordinate role in mediating microbial acquisition for the insects. Furthermore, our investigation suggested that the composition of the insect gut microbiome was probably shaped by host selection and/or microbe-microbe interactions at the gut compartment level, with marginal influence from soil and geographical factors. Additionally, we had unveiled a noteworthy dynamic in the acquisition of core bacterial taxa, particularly Burkholderia, which were initially sourced from the environment and subsequently enriched within the insect midgut compartments. This bacterial enrichment played a significant role in enhancing insect host reproduction. These findings contribute to our evolving understanding of microbiomes within the insect-plant-soil ecosystem, shedding additional light on the intricate interactions between insects and their microbiomes that underpin the ecological significance of microbial partnerships in host adaptation.

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.

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.

Intercontinental Diversity of Caballeronia Gut Symbionts in the Conifer Pest Bug Leptoglossus occidentalis

Many stinkbugs in the superfamily Coreoidea (Hemiptera: Heteroptera) develop crypts in the posterior midgut, harboring Caballeronia (Burkholderia) symbionts. These symbionts form a monophyletic group in Burkholderia sensu lato, called the “stinkbug-associated beneficial and environmental (SBE)” group, recently reclassified as the new genus Caballeronia. SBE symbionts are separated into the subclades SBE-α and SBE-β. Previous studies suggested a regional effect on the symbiont infection pattern; Japanese and American bug species are more likely to be associated with SBE-α, while European bug species are almost exclusively associated with SBE-β. However, since only a few insect species have been investigated, it remains unclear whether region-specific infection is general. We herein investigated Caballeronia gut symbionts in diverse Japanese, European, and North American populations of a cosmopolitan species, the Western conifer seed bug Leptoglossus occidentalis (Coreoidea: Coreidae). A mole­cular phylogenetic ana­lysis of the 16S rRNA gene demonstrated that SBE-β was the most dominant in all populations. Notably, SBE-α was rarely detected in any region, while a third clade, the “Coreoidea clade” occupied one fourth of the tested populations. Although aposymbiotic bugs showed high mortality, SBE-α- and SBE-β-inoculated insects both showed high survival rates; however, a competition assay demonstrated that SBE-β outcompeted SBE-α in the midgut crypts of L. occidentalis. These results strongly suggest that symbiont specificity in the Leptoglossus-Caballeronia symbiotic association is influenced by the host rather than geography, while the geographic distribution of symbionts may be more important in other bugs.

Development of common leaf-footed bug pests depends on the presence and identity of their environmentally-acquired symbionts

Many beneficial symbioses between bacteria and their terrestrial arthropod hosts are vertically transmitted from mother to offspring, ensuring the progeny acquire necessary partners. Unusually, in several families of coreoid and lygeoid bugs (Hemiptera), nymphs must instead ingest the beneficial symbiont, Burkholderia (sensu lato), from the environment early in development. We studied the effects of Burkholderia on development of two species of leaf-footed bug (Coreidae) in the genus Leptoglossus, L. zonatus and L. phyllopus. We found no evidence for vertical transmission of the symbiont, but found stark differences in performance between symbiotic and aposymbiotic individuals. Symbiotic nymphs grew more rapidly, were approximately four times more likely to survive to adulthood than aposymbiotic bugs, and were two times larger. These findings suggest that Burkholderia is an obligate symbiont for Leptoglossus species. We also tested for variation in fitness effects conferred by four symbiont isolates representing different species within Burkholderia's insect-associated Stinkbug Beneficial and Environmental (SBE) clade. While three isolates conferred similar benefits to hosts, nymphs associated with the fourth isolate grew more slowly and weighed significantly less as adults. The effects of the four isolates were similar for both Leptoglossus species. This work indicates that both Burkholderia acquisition and isolate identity play critical roles in the growth and development of Leptoglossus. Importance Leptoglossus zonatus and L. phyllopus are important polyphagous pests and both species have been well-studied, but generally without regard to their dependance on a bacterial symbiont. Our results indicate that the central role of Burkholderia in the biology of these insects, as well as in other leaf-footed bugs, should be considered in future studies of coreid life history, ecology and pest management. Our work suggests acquisition of Burkholderia is critical for the growth and development of Leptoglossus species. Further, we found that there was variation in performance outcomes according to symbiont identity, even among members of the Stinkbug Beneficial and Environmental clade. This suggests that although environmental acquisition of a symbiont can provide extraordinary flexibility in partner associations, it also carries a risk if the partner is sub-optimal.

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.

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.

The Role of Bacterial Symbionts in Triatomines: An Evolutionary Perspective

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

Continent-Scale Sampling Reveals Fine-Scale Turnover in a Beneficial Bug Symbiont

Many members of animal-associated microbial communities, including the gut flora, are acquired from their host’s environment. While many of these communities are species rich, some true bugs (Hemiptera) in the superfamilies Lygaeoidea and Coreidae allow only ingested Burkholderia to colonize and reproduce in a large portion of the midgut. We studied the spatial structuring of Burkholderia associated with a widespread omnivorous bug genus, Jalysus (Berytidae). We sampled Wickham’s stilt bug, Jalysus wickhami, across the United States and performed limited sampling of its sister species, the spined stilt bug Jalysus spinosus. We asked: (1) What Burkholderia strains are hosted by Jalysus at different locations? (2) Does host insect species, host plant species, or location influence the strain these insects acquire? (3) How does Burkholderia affect the development and reproductive fitness of J. wickhami? We found: (1) Sixty-one Burkholderia strains were present across a sample of 352 individuals, but one strain dominated, accounting for almost half of all symbiont reads. Most strains were closely related to other hemipteran Burkholderia symbionts. (2) Many individuals hosted more than one strain of Burkholderia. (3) J. wickhami and J. spinosus did not differ in the strains they hosted. (4) Insects that fed on different plant species tended to host different Burkholderia, but this accounted for only 4% of the variation in strains hosted. In contrast, the location at which an insect was collected explained 27% of the variation in symbiont strains. (5) Burkholderia confers important fitness benefits to J. wickhami. In laboratory experiments, aposymbiotic (Burkholderia-free) insects developed more slowly and laid fewer eggs than symbiotic (Burkholderia-colonized) insects. (6) In the lab, nymphs sometimes acquired Burkholderia via indirect exposure to adults, indicating that horizontal symbiont transmission can occur via adult insect-mediated enrichment of Burkholderia in the local environment – a phenomenon not previously reported in bug-Burkholderia relationships. Taken together, the results suggest that for these bugs, critical nutritional requirements are outsourced to a highly diverse and spatially structured collection of Burkholderia strains acquired from the environment and, occasionally, from conspecific adults.

If host is refractory, insistent parasite goes berserk: Trypanosomatid Blastocrithidia raabei in the dock bug Coreus marginatus

Here we characterized the development of the trypanosomatid Blastocrithidia raabei in the dock bug Coreus marginatus using light and electron microscopy. This parasite has been previously reported to occur in the host hemolymph, which is rather typical for dixenous trypanosomatids transmitted to a plant or vertebrate with insect's saliva. In addition, C. marginatus has an unusual organization of the intestine, which makes it refractory to microbial infections: two impassable segments isolate the anterior midgut portion responsible for digestion and absorption from the posterior one containing symbiotic bacteria. Our results refuted the possibility of hemolymph infection, but revealed that the refractory nature of the host provokes very aggressive behavior of the parasite and makes its life cycle more complex, reminiscent of that in some dixenous trypanosomatids. In the pre-barrier midgut portion, the epimastigotes of B. raabei attach to the epithelium and multiply similarly to regular insect trypanosomatids. However, when facing the impassable constricted region, the parasites rampage and either fiercely break through the isolating segments or attack the intestinal epithelium in front of the barrier. The cells of the latter group pass to the basal lamina and accumulate there, causing degradation of the epitheliocytes and thus helping the epimastigotes of the former group to advance posteriorly. In the symbiont-containing post-barrier midgut segment, the parasites either attach to bacterial cells and produce cyst-like amastigotes (CLAs) or infect enterocytes. In the rectum, all epimastigotes attach either to the cuticular lining or to each other and form CLAs. We argue that in addition to the specialized life cycle B. raabei possesses functional cell enhancements important either for the successful passage through the intestinal barriers (enlarged rostrum and well-developed Golgi complex) or as food reserves (vacuoles in the posterior end).

Versatile and Dynamic Symbioses Between Insects and Burkholderia Bacteria

Symbiotic associations with microorganisms represent major sources of ecological and evolutionary innovations in insects. Multiple insect taxa engage in symbioses with bacteria of the genus Burkholderia, a diverse group that is widespread across different environments and whose members can be mutualistic or pathogenic to plants, fungi, and animals. Burkholderia symbionts provide nutritional benefits and resistance against insecticides to stinkbugs, defend Lagria beetle eggs against pathogenic fungi, and may be involved in nitrogen metabolism in ants. In contrast to many other insect symbioses, the known associations with Burkholderia are characterized by environmental symbiont acquisition or mixed-mode transmission, resulting in interesting ecological and evolutionary dynamics of symbiont strain composition. Insect-Burkholderia symbioses present valuable model systems from which to derive insights into general principles governing symbiotic interactions because they are often experimentally and genetically tractable and span a large fraction of the diversity of functions, localizations, and transmission routes represented in insect symbioses.

Host-symbiont specificity determined by microbe-microbe competition in an insect gut

Despite the omnipresence of specific host-symbiont associations with acquisition of the microbial symbiont from the environment, little is known about how the specificity of the interaction evolved and is maintained. The bean bug Riptortus pedestris acquires a specific bacterial symbiont of the genus Burkholderia from environmental soil and harbors it in midgut crypts. The genus Burkholderia consists of over 100 species, showing ecologically diverse lifestyles, and including serious human pathogens, plant pathogens, and nodule-forming plant mutualists, as well as insect mutualists. Through infection tests of 34 Burkholderia species and 18 taxonomically diverse bacterial species, we demonstrate here that nonsymbiotic Burkholderia and even its outgroup Pandoraea could stably colonize the gut symbiotic organ and provide beneficial effects to the bean bug when inoculated on aposymbiotic hosts. However, coinoculation revealed that the native symbiont always outcompeted the nonnative bacteria inside the gut symbiotic organ, explaining the predominance of the native Burkholderia symbiont in natural bean bug populations. Hence, the abilities for colonization and cooperation, usually thought of as specific traits of mutualists, are not unique to the native Burkholderia symbiont but, to the contrary, competitiveness inside the gut is a derived trait of the native symbiont lineage only and was thus critical in the evolution of the insect gut symbiont.

Burkholderia Gut Symbionts Associated with European and Japanese Populations of the Dock Bug Coreus marginatus (Coreoidea: Coreidae)

Insects of the heteropteran superfamilies Coreoidea and Lygaeoidea are consistently associated with symbionts of a specific group of the genus Burkholderia, called the “stinkbug-associated beneficial and environmental (SBE)” group. The symbiosis is maintained by the environmental transmission of symbionts. We investigated European and Japanese populations of the dock bug Coreus marginatus (Coreoidea: Coreidae). High nymphal mortality in reared aposymbiotic insects suggested an obligate host-symbiont association in this species. Molecular phylogenetic analyses based on 16S rRNA gene sequences revealed that all 173 individuals investigated were colonized by Burkholderia, which were further assigned to different subgroups of the SBE in a region-dependent pattern.

Comparative cytology, physiology and transcriptomics of Burkholderia insecticola in symbiosis with the bean bug Riptortus pedestris and in culture

In the symbiosis of the bean bug Riptortus pedestris with Burkholderia insecticola, the bacteria occupy an exclusive niche in the insect midgut and favor insect development and reproduction. In order to understand how the symbiotic bacteria stably colonize the midgut crypts and which services they provide to the host, we compared the cytology, physiology, and transcriptomics of free-living and midgut-colonizing B. insecticola. The analyses revealed that midgut-colonizing bacteria were smaller in size and had lower DNA content, they had increased stress sensitivity, lost motility, and an altered cell surface. Transcriptomics revealed what kinds of nutrients are provided by the bean bug to the Burkholderia symbiont. Transporters and metabolic pathways of diverse sugars such as rhamnose and ribose, and sulfur compounds like sulfate and taurine were upregulated in the midgut-colonizing symbionts. Moreover, pathways enabling the assimilation of insect nitrogen wastes, i.e. allantoin and urea, were also upregulated. The data further suggested that the midgut-colonizing symbionts produced all essential amino acids and B vitamins, some of which are scarce in the soybean food of the host insect. Together, these findings suggest that the Burkholderia symbiont is fed with specific nutrients and also recycles host metabolic wastes in the insect gut, and in return, the bacterial symbiont provides the host with essential nutrients limited in the insect food, contributing to the rapid growth and enhanced reproduction of the bean bug host.

Infection dynamics of insecticide-degrading symbionts from soil to insects in response to insecticide spraying

Insecticide resistance is a serious concern in modern agriculture, and an understanding of the underlying evolutionary processes is pivotal to prevent the problem. The bean bug Riptortus pedestris, a notorious pest of leguminous crops, acquires a specific Burkholderia symbiont from the environment every generation, and harbors the symbiont in the midgut crypts. The symbiont's natural role is to promote insect development but the insect host can also obtain resistance against the insecticide fenitrothion (MEP) by acquiring MEP-degrading Burkholderia from the environment. To understand the developing process of the symbiont-mediated MEP resistance in response to the application of the insecticide, we investigated here in parallel the soil bacterial dynamics and the infected gut symbionts under different MEP-spraying conditions by culture-dependent and culture-independent analyses, in conjunction with stinkbug rearing experiments. We demonstrate that MEP application did not affect the total bacterial soil population but significantly decreased its diversity while it dramatically increased the proportion of MEP-degrading bacteria, mostly Burkholderia. Moreover, we found that the infection of stinkbug hosts with MEP-degrading Burkholderia is highly specific and efficient, and is established after only a few times of insecticide spraying at least in a field soil with spraying history, suggesting that insecticide resistance could evolve in a pest bug population more quickly than was thought before.

Expression and Function of Different Guanine-Plus-Cytosine Content 16S rRNA Genes in Haloarcula hispanica at Different Temperatures

The halophilic archaeon Haloarcula hispanica harbors three ribosomal RNA (rRNA) operons (rrnA, rrnB, and rrnC) that contain the 16S rRNA genes rrsA, rrsB, and rrsC, respectively. Although rrsB and rrsC (rrsBC) have almost identical sequences, the rrsA and rrsBC sequences differ by 5.4%, and they differ by 2.5% with respect to guanine-plus-cytosine content (P_GC). The strong correlation between the typical growth temperatures of archaea and P_GC of their 16S rRNA genes suggests that H. hispanica may harbor different 16S rRNA genes having different P_GC to maintain rapid growth in a wide range of temperatures. We therefore performed reverse transcription-coupled quantitative PCR to assess expression levels of rrsA (P_GC, 58.9%) and rrsBC (P_GC, 56.4-56.5%) at various temperatures. The expression ratio of rrsA to rrsBC increased with culture temperature. Mutants with complete deletions of one or two of the three rRNA operons were constructed and their growth rates at different temperatures compared to that of the wild-type. The growth characteristics of the rRNA operon single-mutant strains were indistinguishable from the wild-type. The rRNA operon double-mutant strains maintained the same temperature range as wild-type but displayed reduced growth rates. In particular, the double-mutant strains grew much slower than wild-type at low temperature related to minimum growth temperature of the wild-type. On the other hand, at physiologically high temperatures the wild-type and the double-mutant strain which harbors only rrnA with high-P_GCrrsA grew significantly faster than the double-mutant strain which harbors only rrnC with low-P_GCrrsC. These findings suggest the importance of 16S rRNAs transcribed from rrsA with high-P_GC in maintaining rapid growth of this halophilic archaeon at raised growth temperatures.