Wolbachia transmission dynamics in Formica wood ants

Evidence of phylosymbiosis in Formica ants

Introduction

Insects share intimate relationships with microbes that play important roles in their biology. Yet our understanding of how host-bound microbial communities assemble and perpetuate over evolutionary time is limited. Ants host a wide range of microbes with diverse functions and are an emerging model for studying the evolution of insect microbiomes. Here, we ask whether phylogenetically related ant species have formed distinct and stable microbiomes.

Methods

To answer this question, we investigated the microbial communities associated with queens of 14 Formica species from five clades, using deep coverage 16S rRNA amplicon sequencing.

Results

We reveal that Formica species and clades harbor highly defined microbial communities that are dominated by four bacteria genera: Wolbachia, Lactobacillus, Liliensternia, and Spiroplasma. Our analysis reveals that the composition of Formica microbiomes mirrors the phylogeny of the host, i.e., phylosymbiosis, in that related hosts harbor more similar microbial communities. In addition, we find there are significant correlations between microbe co-occurrences.

Discussion

Our results demonstrate Formica ants carry microbial communities that recapitulate the phylogeny of their hosts. Our data suggests that the co-occurrence of different bacteria genera may at least in part be due to synergistic and antagonistic interactions between microbes. Additional factors potentially contributing to the phylosymbiotic signal are discussed, including host phylogenetic relatedness, host-microbe genetic compatibility, modes of transmission, and similarities in host ecologies (e.g., diets). Overall, our results support the growing body of evidence that microbial community composition closely depends on the phylogeny of their hosts, despite bacteria having diverse modes of transmission and localization within the host.

Assembly of a hybrid Formica aquilonia × F. polyctena ant genome from a haploid male

Formica red wood ants are a keystone species of boreal forest ecosystems and an emerging model system in the study of speciation and hybridization. Here, we performed a standard DNA extraction from a single, field-collected Formica aquilonia × Formica polyctena haploid male and assembled its genome using ~60× of PacBio long reads. After polishing and contaminant removal, the final assembly was 272 Mb (4687 contigs, N50 = 1.16 Mb). Our reference genome contains 98.5% of the core Hymenopteran BUSCOs and was pseudo-scaffolded using the assembly of a related species, F. selysi (28 scaffolds, N50 = 8.49 Mb). Around one-third of the genome consists of repeats, and 17 426 gene models were annotated using both protein and RNAseq data (97.4% BUSCO completeness). This resource is of comparable quality to the few other single individual insect genomes assembled to date and paves the way to genomic studies of admixture in natural populations and comparative genomic approaches in Formica wood ants.

New Strains of Wolbachia Unveiling the Complexity of This Symbiotic Interaction in Solenopsis (Hymenoptera: Formicidae)

Bacteria of the genus Wolbachia are widely distributed in arthropods, particularly in ants; nevertheless, it is still little explored with the Multilocus Sequence Typing (MLST) methodology, especially in the genus Solenopsis, which includes species native to South America. Ants from this genus have species distributed in a cosmopolitan way with some of them being native to South America. In Brazil, they are widely spread and preferentially associated with areas of human activity. This study aimed to investigate the diversity of Wolbachia in ants of the genus Solenopsis through the MLST approach and their phylogenetic relationship, including the relationship between mtDNA from the host and the related Wolbachia strain. We also tested the geographic correlation between the strains to infer transmission and distributional patterns. Fifteen new strains and eleven previously unknown alleles were obtained by sequencing and analyzing the five genes that make up the MLST. The phylogenetic relationship between the strains showed a polyphyletic pattern, indicative of the complexity of the evolutionary history of these bacteria in the analyzed species. We detected the correlation of host’s mitochondrial DNA with Wolbachia diversity which imply that related strains exist in related hosts, strongly suggesting the occurrence of vertical transfer. We found no specificity of the Wolbachia strain for a given geographic region that could indicate either that there is no horizontal transfer of the strain from the environment for the host or that the human action could be shuffling the distribution of the Solenopsis ants and the endosymbiont Wolbachia, as well. Our study highlights the complexity and novelty of Wolbachia diversity with this specific group of ants and the need for further studies that focus on understanding of this intricate relationship.

First Insight into Microbiome Profiles of Myrmecophilous Beetles and Their Host, Red Wood Ant Formica polyctena (Hymenoptera: Formicidae)-A Case Study

Formica polyctena belongs to the red wood ant species group. Its nests provide a stable, food rich, and temperature and humidity controlled environment, utilized by a wide range of species, called myrmecophiles. Here, we used the high-throughput sequencing of the 16S rRNA gene on the Illumina platform for identification of the microbiome profiles of six selected myrmecophilous beetles (Dendrophilus pygmaeus, Leptacinus formicetorum, Monotoma angusticollis, Myrmechixenus subterraneus, Ptenidium formicetorum and Thiasophila angulata) and their host F. polyctena. Analyzed bacterial communities consisted of a total of 23 phyla, among which Proteobacteria, Actinobacteria, and Firmicutes were the most abundant. Two known endosymbionts-Wolbachia and Rickettsia-were found in the analyzed microbiome profiles and Wolbachia was dominant in bacterial communities associated with F. polyctena, M. subterraneus, L. formicetorum and P. formicetorum (>90% of reads). In turn, M. angusticollis was co-infected with both Wolbachia and Rickettsia, while in the microbiome of T. angulata, the dominance of Rickettsia has been observed. The relationships among the microbiome profiles were complex, and no relative abundance pattern common to all myrmecophilous beetles tested was observed. However, some subtle, species-specific patterns have been observed for bacterial communities associated with D. pygmaeus, M. angusticollis, and T. angulata.

Wolbachia Horizontal Transmission Events in Ants: What Do We Know and What Can We Learn?

While strict vertical transmission insures the durability of intracellular symbioses, phylogenetic incongruences between hosts and endosymbionts suggest horizontal transmission must also occur. These horizontal acquisitions can have important implications for the biology of the host. Wolbachia is one of the most ecologically successful prokaryotes in arthropods, infecting an estimated 50–70% of all insect species. Much of this success is likely due to the fact that, in arthropods, Wolbachia is notorious for manipulating host reproduction to favor transmission through the female germline. However, its natural potential for horizontal transmission remains poorly understood. Here we evaluate the fundamental prerequisites for successful horizontal transfer, including necessary environmental conditions, genetic potential of bacterial strains, and means of mediating transfers. Furthermore, we revisit the relatedness of Wolbachia strains infecting the Panamanian leaf-cutting ant, Acromyrmex echinatior, and its inquiline social parasite, Acromyrmex insinuator, and compare our results to a study published more than 15 years ago by Van Borm et al. (2003). The results of this pilot study prompt us to reevaluate previous notions that obligate social parasitism reliably facilitates horizontal transfer and suggest that not all Wolbachia strains associated with ants have the same genetic potential for horizontal transmission.

Symbiotic bacterial communities in ants are modified by invasion pathway bottlenecks and alter host behavior

Biological invasions are a threat to global biodiversity and provide unique opportunities to study ecological processes. Population bottlenecks are a common feature of biological invasions and the severity of these bottlenecks is likely to be compounded as an invasive species spreads from initial invasion sites to additional locations. Despite extensive work on the genetic consequences of bottlenecks, we know little about how they influence microbial communities of the invaders themselves. Due to serial bottlenecks, invasive species may lose microbial symbionts including pathogenic taxa (the enemy release hypothesis) and/or may accumulate natural enemies with increasing time after invasion (the pathogen accumulation and invasive decline hypothesis). We tested these alternate hypotheses by surveying bacterial communities of Argentine ants (Linepithema humile). We found evidence for serial symbiont bottlenecks: the bacterial community richness declined over the invasion pathway from Argentina to New Zealand. The abundance of some genera, such as Lactobacillus, also significantly declined over the invasion pathway. Argentine ants from populations in the United States shared the most genera with ants from their native range in Argentina, while New Zealand shared the least (120 vs. 57, respectively). Nine genera were present in all sites around the globe possibly indicating a core group of obligate microbes. In accordance with the pathogen accumulation and invasive decline hypothesis, Argentine ants acquired genera unique to each specific invaded country. The United States had the most unique genera, though even within New Zealand these ants acquired symbionts. In addition to our biogeographic sampling, we administered antibiotics to Argentine ants to determine if changes in the micro-symbiont community could influence behavior and survival in interspecific interactions. Treatment with the antibiotics spectinomycin and kanamycin only slightly increased Argentine ant interspecific aggression, but this increase significantly decreased survival in interspecific interactions. The survival of the native ant species also decreased when the symbiotic microbial community within Argentine ants was modified by antibiotics. Our work offers support for both the enemy release hypothesis and that invasive species accumulate novel microbial taxa within their invaded range. These changes appear likely to influence invader behavior and survival.

Diversity and Transmission of Gut Bacteria in Atta and Acromyrmex Leaf-Cutting Ants during Development

The social Hymenoptera have distinct larval and adult stages separated by metamorphosis, which implies striking remodeling of external and internal body structures during the pupal stage. This imposes challenges to gut symbionts as existing cultures are lost and may or may not need to be replaced. To elucidate the extent to which metamorphosis interrupts associations between bacteria and hosts, we analyzed changes in gut microbiota during development and traced the transmission routes of dominant symbionts from the egg to adult stage in the leaf-cutting ants Acromyrmex echinatior and Atta cephalotes, which are both important functional herbivores in the New World tropics. Bacterial density remained similar across the developmental stages of Acromyrmex, but Atta brood had very low bacterial prevalences suggesting that bacterial gut symbionts are not actively maintained. We found that Wolbachia was the absolute dominant bacterial species across developmental stages in Acromyrmex and we confirmed that Atta lacks Wolbachia also in the immature stages, and had mostly Mollicutes bacteria in the adult worker guts. Wolbachia in Acromyrmex appeared to be transovarially transmitted similar to transmission in solitary insects. In contrast, Mollicutes were socially transmitted from old workers to newly emerged callows. We found that larval and pupal guts of both ant species contained Pseudomonas and Enterobacter bacteria that are also found in fungus gardens, but hardly or not in adult workers, suggesting they are beneficial only for larval growth and development. Our results reveal that transmission pathways for bacterial symbionts may be very different both between developmental stages and between sister genera and that identifying the mechanisms of bacterial acquisition and loss will be important to clarify their putative mutualistic functions.

Intracellular Symbiotic Bacteria of Camponotus textor, Forel (Hymenoptera, Formicidae)

This study focuses on the weaver ant, Camponotus textor, Forel which occurs in some areas of the Brazilian Cerrado and Atlantic Forest, and its symbionts: Blochmannia, an obligate symbiont of Camponotus, and Wolbachia, known for causing reproductive alterations in their hosts. The main goal of this study was to investigate the presence, frequency of occurrence, and diversity of Wolbachia and Blochmannia strains in C. textor colonies. We found high infection rates (100%) and the occurrence of at least two distinct strains of Blochmannia (H_1 or H_7) in the same species. The observed haplotype variation within a single species may result from the high mutation rate of the symbiont. Similarly, the Wolbachia was found in all colonies with different rates of infections and a new strain (supergroup A) was deposited in the MLST database. The diversity found in the present study shows that there is still much to explore to understand about these symbiotic interactions.

Modeling the indirect effect of Wolbachia on the infection dynamics of horizontally transmitted viruses

Intracellular bacteria of the genus Wolbachia are widely distributed in arthropods. There is growing empirical evidence that Wolbachia directly interacts with viruses and other parasites inside the arthropod host, sometimes resulting in low or no pathogen replication. Previous theoretical studies showed that this direct effect of Wolbachia can result in a reduced virus prevalence (within the population), suggesting that Wolbachia could be used in the biological control of vector-borne diseases (e.g., dengue fever). However, Wolbachia might also indirectly affect virus dynamics because Wolbachia-induced reproductive phenotypes (cytoplasmic incompatibility or male killing) increase the larval mortality of hosts and thus alter the age structure of populations. We investigated this indirect effect using mathematical models with overlapping generations, and found the results to depend strongly on the host's life history. In general, the indirect effect can result in two different outcomes: (1) reduced virus prevalence and virus invasion ability, and (2) increased virus prevalence and virus invasion ability. The former occurs for host species with larval competition and undercompensation, the latter for hosts with either adult competition or larval competition and overcompensation. These findings suggest that the effect of Wolbachia on a specific virus is sensitive to the host's life history. We discuss the results with respect to biocontrol programs using Wolbachia.

The distribution of Wolbachia in Cubitermes (Termitidae, Termitinae) castes and colonies: a modelling approach

Wolbachia are endosymbiotic bacteria of arthropods and nematodes that are able to manipulate host reproduction. Although vertically transmitted via the cytoplasm in eggs, horizontal transmission of Wolbachia among and within arthropod species has been shown to be common. Eusocial insects represent interesting models for studying Wolbachia transmission due to colonial organization and close interaction between nestmates. Here we conducted a detailed screening of Wolbachia infection for 15 colonies of the very common soil-feeding termites Cubitermes spp. affinis subarquatus (Termitidae, Termitinae) that consist of four distinct phylogenetic species in the Lopé forest Reserve, Gabon. Infection tests showed that 50% of the individuals were Wolbachia positive (N = 555) with 90% of reproductives and 48% of offspring infected. White soldiers, which are transitional stages preceding mature soldiers, had a significantly higher mean infection rate (74%) than the other castes and stages (63%, 33% and 39% for larvae, workers and mature soldiers, respectively). We used a maximum likelihood method and Akaike’s Information Criterion in order to explain the non-expected high rate of Wolbachia infection in white soldiers. The best model included a species effect for the stochastic loss of Wolbachia and a caste effect for the rate of gain. After fitting, the best model selected was for a species-specific rate of loss with a null rate of new gain for larvae, workers and soldiers and a probability of 0.72 whatever the species, that a white soldier becomes newly contaminated during that stage. The mean expected infection rate in white soldiers without a new gain was estimated to 17% instead of the 74% observed. Here we discuss the possible explanations to the high infection rate observed in white soldiers.

Population genetics of bisexual and unisexual populations of the scaly-winged bark louse Echmepteryx hageni (Insecta: Psocoptera)

The scaly-winged bark louse, Echmepteryx hageni, exhibits a unique pattern of co-existence of apparently differnt reproductive modes. Unisexuality is widespread in eastern North America, while sexual populations are restricted to isolated rock out-croppings in southern Illinois and eastern Kentucky. Three of the four nuclear loci examined show greater genetic diversity in the unisexual form compared to the sexual form of E. hageni, in accordance with the pattern previously shown in mitochondrial genetic data. Neutrality tests of the nuclear loci indicate a consistent signal of demographic expansion in asexual populations, but not in sexual populations. There was evidence of inbreeding in the isolated sexual populations at three of the nuclear loci, and one locus had signs of gene specific balancing selection. However, there is no significant genetic differentiation between bisexual and unisexual populations, possibly due to the greater effective population size of nuclear loci relative to mitochondrial loci. The mitochondrial differentiation of E. hageni populations in the northwestern part of their range (Minnesota and Wisconsin) was also not reflected in the nuclear data. We present three hypotheses that may explain the disparity in observed nuclear and mitochondrial genetic diversity between the reproductive forms of E. hageni.

Characteristics, phenotype, and transmission of Wolbachia in the sweet potato whitefly, Bemisia tabaci (Hemiptera: Aleyrodidae), and its parasitoid Eretmocerus sp. nr. emiratus (Hymenoptera: Aphelinidae)

Wolbachia is a common intracellular bacterial endosymbiont of insects, causing a variety of effects including reproductive manipulations such as cytoplasmic incompatibility (CI). In this study, we characterized Wolbachia in the whitefly Bemisia tabaci and in the whitefly parasitoid Eretmocerus sp. nr. emiratus. We also tested for horizontal transmission of Wolbachia between and within trophic levels, and we determined the phenotype of Wolbachia in E. sp. nr. emiratus. Using multilocus sequence typing and phylogenetic analyses, we found that B. tabaci and E. sp. nr. emiratus each harbor a different and unique strain of Wolbachia. Both strains belong to the phylogenetic supergroup B. No evidence for horizontal transmission of Wolbachia between and within trophic levels was found in our study system. Finally, crossing results were consistent with a CI phenotype; when Wolbachia-infected E. sp. nr. emiratus males mate with uninfected females, wasp progeny survival dropped significantly, and the number of females was halved. This is the first description of CI caused by Wolbachia in the economically important genus Eretmocerus. Our study underscores the expectation that horizontal transmission events occur rarely in the dynamics of secondary symbionts such as Wolbachia, and highlights the importance of understanding the effects of symbionts on the biology of natural enemies.

Bacterial endosymbionts in field-collected samples of Trialeurodes sp. nr. abutiloneus (Hemiptera: Aleyrodidae)

Facultative bacterial endosymbionts are common, influential associates of arthropods, yet their movement among host species has not been well documented. Plant-mediated transmission of Rickettsia has been shown for the whitefly Bemisia tabaci. Bemisia tabaci in USA cotton fields harbors the secondary symbionts Rickettsia and Hamiltonella, and co-occurs with Trialeurodes sp. nr. abutiloneus whiteflies. To determine whether symbionts may be shared, the microbial diversity of these whiteflies on cotton across the USA was analyzed. Trialeurodes sp. nr. abutiloneus bore Portiera, Pseudomonas, Serratia, Arsenophonus and Wolbachia. No Rickettsia or Hamiltonella were detected. These results provide no evidence for horizontal transmission of symbionts between these whitefly genera.

Presence of Wolbachia in three hymenopteran species: Diprion pini (Hymenoptera: Diprionidae), Neodiprion sertifer (Hymenoptera: Diprionidae), and Dahlbominus fuscipennis (Hymenoptera: Eulophidae)

Sawflies are important pests of various plant species. Diprion pini (L.) and Neodiprion sertifer (Geoffroy) (Hymenoptera: Diprionidae) are two of the most important sawfly pests in Italy, and both species are parasitized by the hymenopteran parasitoid Dahlbominus fuscipennis (Zetterstedt). Bacterial endosymbionts are currently studied for their high potential in strategies of biocontrol in a number of insect species. In this study, we investigated the presence of symbiotic bacteria (Wolbachia and Cardinium) in the three species of hymenoptera mentioned earlier, both in wild and laboratory populations. Although all samples were negative for the presence of Cardinium, 100% prevalence for Wolbachia was detected, as all examined individuals resulted to be PCR positive. Furthermore, 16S rDNA and ftsZ gene sequencing indicated that all individuals from the three hymenopteran species are infected by a single Wolbachia strain. Additionally, we report the presence of gynandromorphic individuals in D. pini, both in wild and laboratory-reared populations. Heat treatments on D. pini colonies removed the Wolbachia symbionts, but they also prevented the development of adults.

A metatranscriptomic approach to the identification of microbiota associated with the ant Formica exsecta

Social insects live in cooperative colonies, often in high densities and with closely related individuals, and interact using social contact behaviours. Compared to solitary insects, social insects have evolved multi-level immunity that includes immune responses common to holometabolous insects, and social immunity, which is exclusive to social taxa. This suggests that social insects may be subject to high pathogen pressure, yet relatively little is known about the range of symbiotic and pathogenic microbial communities that associate with social insects. In this study we examined transcriptome data generated from the ant Formica exsecta for sequences identifying as microbes (or other organisms potentially of non-ant origin). Sequences showing homology to two viruses and several other potentially or obligate intracellular organisms, such as Wolbachia, Arsenophonus, Entomoplasmatales and Microsporidia, were present in the transcriptome data. These homologous sequence matches correspond to genera/species that have previously been associated with a variety of insects, including social insects. There were also sequences with identity to several other microbes such as common moulds and soil bacteria. We conclude that this sequence data provides a starting point for a deeper understanding of the biological interactions between a species of ant and the micro- and macrobiotic communities that it potentially encounters.

Chaos of Wolbachia sequences inside the compact fig syconia of Ficus benjamina (Ficus: moraceae)

Figs and fig wasps form a peculiar closed community in which the Ficus tree provides a compact syconium (inflorescence) habitat for the lives of a complex assemblage of Chalcidoid insects. These diverse fig wasp species have intimate ecological relationships within the closed world of the fig syconia. Previous surveys of Wolbachia, maternally inherited endosymbiotic bacteria that infect vast numbers of arthropod hosts, showed that fig wasps have some of the highest known incidences of Wolbachia amongst all insects. We ask whether the evolutionary patterns of Wolbachia sequences in this closed syconium community are different from those in the outside world. In the present study, we sampled all 17 fig wasp species living on Ficus benjamina, covering 4 families, 6 subfamilies, and 8 genera of wasps. We made a thorough survey of Wolbachia infection patterns and studied evolutionary patterns in wsp (Wolbachia Surface Protein) sequences. We find evidence for high infection incidences, frequent recombination between Wolbachia strains, and considerable horizontal transfer, suggesting rapid evolution of Wolbachia sequences within the syconium community. Though the fig wasps have relatively limited contact with outside world, Wolbachia may be introduced to the syconium community via horizontal transmission by fig wasps species that have winged males and visit the syconia earlier.

Congruent genetic structure in the lichen-forming fungus Lobaria pulmonaria and its green-algal photobiont

The extent of codispersal of symbionts is one of the key factors shaping genetic structures of symbiotic organisms. Concordant patterns of genetic structure are expected in vertically transmitted symbioses, whereas horizontal transmission generally uncouples genetic structures unless the partners are coadapted. Here, we compared the genetic structures of mutualists, the lichen-forming fungus Lobaria pulmonaria and its primary green-algal photobiont, Dictyochloropsis reticulata. We performed analysis of molecular variance and variogram analysis to compare genetic structures between symbiosis partners. We simulated the expected number of multilocus-genotype recurrences to reveal whether the distribution of multilocus genotypes of either species was concordant with panmixia. Simulations and tests of linkage disequilibrium provided compelling evidence for the codispersal of mutualists. To test whether genotype associations between symbionts were consistent with randomness, as expected under horizontal transmission, we simulated the recurrence of fungal-algal multilocus genotype associations expected by chance. Our data showed nonrandom associations of fungal and algal genotypes. Either vertical transmission or horizontal transmission coupled with coadaptation between symbiont genotypes may have created these nonrandom associations. This study is among the first to show codispersal and highly congruent genetic structures in the partners of a lichen mutualism.

Comparisons of host mitochondrial, nuclear and endosymbiont bacterial genes reveal cryptic fig wasp species and the effects of Wolbachia on host mtDNA evolution and diversity

Background

Figs and fig-pollinating wasp species usually display a highly specific one-to-one association. However, more and more studies have revealed that the "one-to-one" rule has been broken. Co-pollinators have been reported, but we do not yet know how they evolve. They may evolve from insect speciation induced or facilitated by Wolbachia which can manipulate host reproduction and induce reproductive isolation. In addition, Wolbachia can affect host mitochondrial DNA evolution, because of the linkage between Wolbachia and associated mitochondrial haplotypes, and thus confound host phylogeny based on mtDNA. Previous research has shown that fig wasps have the highest incidence of Wolbachia infection in all insect taxa, and Wolbachia may have great influence on fig wasp biology. Therefore, we look forward to understanding the influence of Wolbachia on mitochondrial DNA evolution and speciation in fig wasps.

Results

We surveyed 76 pollinator wasp specimens from nine Ficus microcarpa trees each growing at a different location in Hainan and Fujian Provinces, China. We found that all wasps were morphologically identified as Eupristina verticillata, but diverged into three clades with 4.22-5.28% mtDNA divergence and 2.29-20.72% nuclear gene divergence. We also found very strong concordance between E. verticillata clades and Wolbachia infection status, and the predicted effects of Wolbachia on both mtDNA diversity and evolution by decreasing mitochondrial haplotypes.

Conclusions

Our study reveals that the pollinating wasp E. verticillata on F. microcarpa has diverged into three cryptic species, and Wolbachia may have a role in this divergence. The results also indicate that Wolbachia strains infecting E. verticillata have likely resulted in selective sweeps on host mitochondrial DNA.

Assessment of bacterial diversity in the cattle tick Rhipicephalus (Boophilus) microplus through tag-encoded pyrosequencing

Background

Ticks are regarded as the most relevant vectors of disease-causing pathogens in domestic and wild animals. The cattle tick, Rhipicephalus (Boophilus) microplus, hinders livestock production in tropical and subtropical parts of the world where it is endemic. Tick microbiomes remain largely unexplored. The objective of this study was to explore the R. microplus microbiome by applying the bacterial 16S tag-encoded FLX-titanium amplicon pyrosequencing (bTEFAP) technique to characterize its bacterial diversity. Pyrosequencing was performed on adult males and females, eggs, and gut and ovary tissues from adult females derived from samples of R. microplus collected during outbreaks in southern Texas.

Results

Raw data from bTEFAP were screened and trimmed based upon quality scores and binned into individual sample collections. Bacteria identified to the species level include Staphylococcus aureus, Staphylococcus chromogenes, Streptococcus dysgalactiae, Staphylococcus sciuri, Serratia marcescens, Corynebacterium glutamicum, and Finegoldia magna. One hundred twenty-one bacterial genera were detected in all the life stages and tissues sampled. The total number of genera identified by tick sample comprised: 53 in adult males, 61 in adult females, 11 in gut tissue, 7 in ovarian tissue, and 54 in the eggs. Notable genera detected in the cattle tick include Wolbachia, Coxiella, and Borrelia. The molecular approach applied in this study allowed us to assess the relative abundance of the microbiota associated with R. microplus.

Conclusions

This report represents the first survey of the bacteriome in the cattle tick using non-culture based molecular approaches. Comparisons of our results with previous bacterial surveys provide an indication of geographic variation in the assemblages of bacteria associated with R. microplus. Additional reports on the identification of new bacterial species maintained in nature by R. microplus that may be pathogenic to its vertebrate hosts are expected as our understanding of its microbiota expands. Increased awareness of the role R. microplus can play in the transmission of pathogenic bacteria will enhance our ability to mitigate its economic impact on animal agriculture globally. This recognition should be included as part of analyses to assess the risk for re-invasion of areas like the United States of America where R. microplus was eradicated.

Pyrosequencing analysis of endosymbiont population structure: co-occurrence of divergent symbiont lineages in a single vesicomyid host clam

Bacteria-eukaryote endosymbioses are perhaps the most pervasive co-evolutionary associations in nature. Here, intracellular chemosynthetic symbionts of deep-sea clams (Vesicomyidae) were analysed by amplicon pyrosequencing to explore how symbiont transmission mode affects the genetic diversity of the within-host symbiont population. Vesicomyid symbionts (Gammaproteobacteria) are presumed to be obligately intracellular, to undergo nearly strict vertical transmission between host generations, and to be clonal within a host. However, recent data show that vesicomyid symbionts can be acquired laterally via horizontal transfer between hosts or uptake from the environment, potentially creating opportunities for multiple symbiont strains to occupy the same host. Here, genotype-specific PCR and direct sequencing of the bacterial internal transcribed spacer initially demonstrated the co-occurrence of two symbiont strains, symA and symB (93.5% nt identity), in 8 of 118 Vesicomya sp. clams from 3 of 7 hydrothermal vent sites on the Juan de Fuca Ridge. To confirm multiple strains within individual clams, amplicon pyrosequencing of two symbiont loci was used to obtain deep-coverage measurements (mean: approximately 1500x coverage per locus per clam) of symbiont population structure. Pyrosequencing confirmed symA-symB co-occurrence for two individuals, showing the presence of both genotypes in amplicon pools. However, in the majority of clams, the endosymbiont population was remarkably homogenous, with > 99.5% of sequences collapsing into a single symbiont genotype in each clam. These results support the hypothesis that a predominantly vertical transmission strategy leads to the fixation of a single symbiont strain in most hosts. However, mixed symbiont populations do occur in vesicomyids, potentially facilitating the exchange of genetic material between divergent symbiont lineages.

Inherited fungal and bacterial endosymbionts of a parasitic wasp and its cockroach host

Bacterial endosymbionts of insects are increasingly being recognized as common, diverse, and integral to the biology of their hosts. Inherited fungal symbionts have been largely overlooked, however, even though insect guts appear to be a key habitat for an incredible array of fungal diversity. Like bacteria, fungal symbionts also likely play important roles in the ecology and evolution of their insect associates. The objective of this study was to lay the foundations for understanding the roles of the vertically transmitted fungal and bacterial associates of both the brownbanded cockroach, Supella longipalpa, and its parasitic wasp, Comperia merceti. We used culture-dependent and culture-independent molecular methods and phylogenetic analyses in order to identify the symbionts. Two fungal associates of brownbanded cockroaches were found. To our knowledge, this is the first record of vertically transmitted fungal symbionts in the order Blattaria. The wasp was found to house a close relative of one of the cockroach fungi but no bacterial symbionts. Finally, the brownbanded cockroaches also harbored three lineages of bacterial symbionts: Blattabacterium and two lineages of Wolbachia, indicating the number of vertically transmitted symbionts in this insect may be as many as five.

Almost there: transmission routes of bacterial symbionts between trophic levels

Many intracellular microbial symbionts of arthropods are strictly vertically transmitted and manipulate their host's reproduction in ways that enhance their own transmission. Rare horizontal transmission events are nonetheless necessary for symbiont spread to novel host lineages. Horizontal transmission has been mostly inferred from phylogenetic studies but the mechanisms of spread are still largely a mystery. Here, we investigated transmission of two distantly related bacterial symbionts – Rickettsia and Hamiltonella – from their host, the sweet potato whitefly, Bemisia tabaci, to three species of whitefly parasitoids: Eretmocerus emiratus, Eretmocerus eremicus and Encarsia pergandiella. We also examined the potential for vertical transmission of these whitefly symbionts between parasitoid generations. Using florescence in situ hybridization (FISH) and transmission electron microscopy we found that Rickettsia invades Eretmocerus larvae during development in a Rickettsia-infected host, persists in adults and in females, reaches the ovaries. However, Rickettsia does not appear to penetrate the oocytes, but instead is localized in the follicular epithelial cells only. Consequently, Rickettsia is not vertically transmitted in Eretmocerus wasps, a result supported by diagnostic polymerase chain reaction (PCR). In contrast, Rickettsia proved to be merely transient in the digestive tract of Encarsia and was excreted with the meconia before wasp pupation. Adults of all three parasitoid species frequently acquired Rickettsia via contact with infected whiteflies, most likely by feeding on the host hemolymph (host feeding), but the rate of infection declined sharply within a few days of wasps being removed from infected whiteflies. In contrast with Rickettsia, Hamiltonella did not establish in any of the parasitoids tested, and none of the parasitoids acquired Hamiltonella by host feeding. This study demonstrates potential routes and barriers to horizontal transmission of symbionts across trophic levels. The possible mechanisms that lead to the differences in transmission of species of symbionts among species of hosts are discussed.