A Nightmare for Males? A Maternally Transmitted Male-Killing Bacterium and Strong Female Bias in a Green Lacewing Population

High-quality genome of the zoophytophagous stink bug, Nesidiocoris tenuis, informs their food habit adaptation

The zoophytophagous stink bug, Nesidiocoris tenuis, is a promising natural enemy of micro-pests such as whiteflies and thrips. This bug possesses both phytophagous and entomophagous food habits, enabling it to obtain nutrition from both plants and insects. This trait allows us to maintain its population density in agricultural fields by introducing insectary plants, even when the pest prey density is extremely low. However, if the bugs' population becomes too dense, they can sometimes damage crop plants. This dual character seems to arise from the food preferences and chemosensation of this predator. To understand the genomic landscape of N. tenuis, we examined the whole genome sequence of a commercially available Japanese strain. We used long-read sequencing and Hi-C analysis to assemble the genome at the chromosomal level. We then conducted a comparative analysis of the genome with previously reported genomes of phytophagous and hematophagous stink bugs to focus on the genetic factors contributing to this species' herbivorous and carnivorous tendencies. Our findings suggest that the gustatory gene set plays a pivotal role in adapting to food habits, making it a promising target for selective breeding. Furthermore, we identified the whole genomes of microorganisms symbiotic with this species through genomic analysis. We believe that our results shed light on the food habit adaptations of N. tenuis and will accelerate breeding efforts based on new breeding techniques for natural enemy insects, including genomics and genome editing.

Genomic insights into Spiroplasma endosymbionts that induce male-killing and protective phenotypes in the pea aphid

The endosymbiotic bacteria Spiroplasma (Mollicutes) infect diverse plants and arthropods, and some of which induce male killing, where male hosts are killed during development. Male-killing Spiroplasma strains belong to either the phylogenetically distant Citri-Poulsonii or Ixodetis groups. In Drosophila flies, Spiroplasma poulsonii induces male killing via the Spaid toxin. While Spiroplasma ixodetis infects a wide range of insects and arachnids, little is known about the genetic basis of S. ixodetis-induced male killing. Here, we analyzed the genome of S. ixodetis strains in the pea aphid Acyrthosiphon pisum (Aphididae, Hemiptera). Genome sequencing constructed a complete genome of a male-killing strain, sAp269, consisting of a 1.5 Mb circular chromosome and an 80 Kb plasmid. sAp269 encoded putative virulence factors containing either ankyrin repeat, ovarian tumor-like deubiquitinase, or ribosome inactivating protein domains, but lacked the Spaid toxin. Further comparative genomics of Spiroplasma strains in A. pisum biotypes adapted to different host plants revealed their phylogenetic associations and the diversity of putative virulence factors. Although the mechanisms of S. ixodetis-induced male killing in pea aphids remain elusive, this study underlines the dynamic genome evolution of S. ixodetis and proposes independent acquisition events of male-killing mechanisms in insects.

'Candidatus Tisiphia' is a widespread Rickettsiaceae symbiont in the mosquito Anopheles plumbeus (Diptera: Culicidae)

Symbiotic bacteria can alter host biology by providing protection from natural enemies, or alter reproduction or vectoral competence. Symbiont-linked control of vector-borne disease in Anopheles has been hampered by a lack of symbioses that can establish stable vertical transmission in the host. Previous screening found the symbiont 'Candidatus Tisiphia' in Anopheles plumbeus, an aggressive biter and potential secondary vector of malaria parasites and West Nile virus. We screened samples collected over 10-years across Germany and used climate databases to assess environmental influence on incidence. We observed a 95% infection rate, and that the frequency of infection did not fluctuate with broad environmental factors. Maternal inheritance is indicated by presence in the ovaries through FISH microscopy. Finally, we assembled a high-quality 1.6 Mbp draft genome of 'Ca. Tisiphia' to explore its phylogeny and potential metabolic competence. The infection is closely related to strains found in Culicoides biting midges and shows similar patterns of metabolism, providing no evidence of the capacity to synthesize B-vitamins. This infection offers avenues for onward research in anopheline mosquito symbioses. Additionally, it provides future opportunity to study the impact of 'Ca. Tisiphia' on natural and transinfected hosts, especially in relation to reproductive fitness and vectorial competence and capacity.

Microbiome of Zoophytophagous Biological Control Agent Nesidiocoris tenuis

Many insects are associated with endosymbionts that influence the feeding, reproduction, and distribution of their hosts. Although the small green mirid, Nesidiocoris tenuis (Reuter) (Hemiptera: Miridae), a zoophytophagous predator that feeds on plants as well as arthropods, is a globally important biological control agent, its microbiome has not been sufficiently studied. In the present study, we assessed the microbiome variation in 96 N. tenuis individuals from 14 locations throughout Japan, based on amplicon sequencing of the 16S ribosomal RNA gene. Nine major bacteria associated with N. tenuis were identified: Rickettsia, two strains of Wolbachia, Spiroplasma, Providencia, Serratia, Pseudochrobactrum, Lactococcus, and Stenotrophomonas. Additionally, a diagnostic PCR analysis for three typical insect reproductive manipulators, Rickettsia, Wolbachia, and Spiroplasma, was performed on a larger sample size (n = 360) of N. tenuis individuals; the most prevalent symbiont was Rickettsia (69.7%), followed by Wolbachia (39.2%) and Spiroplasma (6.1%). Although some symbionts were co-infected, their prevalence did not exhibit any specific tendency, such as a high frequency in specific infection combinations. The infection frequency of Rickettsia was significantly correlated with latitude and temperature, while that of Wolbachia and Spiroplasma was significantly correlated with host plants. The predominance of these bacteria and the absence of obligate symbionts suggested that the N. tenuis microbiome is typical for predatory arthropods rather than sap-feeding insects. Rickettsia and Wolbachia were vertically transmitted rather than horizontally transmitted from the prey. The functional validation of each symbiont would be warranted to develop N. tenuis as a biological control agent.

Deep data mining reveals variable abundance and distribution of microbial reproductive manipulators within and among diverse host species

Bacterial symbionts that manipulate the reproduction of their hosts are important factors in invertebrate ecology and evolution, and are being leveraged for host biological control. Infection prevalence restricts which biological control strategies are possible and is thought to be strongly influenced by the density of symbiont infection within hosts, termed titer. Current methods to estimate infection prevalence and symbiont titers are low-throughput, biased towards sampling infected species, and rarely measure titer. Here we develop a data mining approach to estimate symbiont infection frequencies within host species and titers within host tissues. We applied this approach to screen ~32,000 publicly available sequence samples from the most common symbiont host taxa, discovering 2,083 arthropod and 119 nematode infected samples. From these data, we estimated that Wolbachia infects approximately 44% of all arthropod and 34% of all nematode species, while other reproductive manipulators only infect 1-8% of arthropod and nematode species. Although relative titers within hosts were highly variable within and between arthropod species, a combination of arthropod host species and Wolbachia strain explained approximately 36% of variation in Wolbachia titer across the dataset. To explore potential mechanisms for host control of symbiont titer, we leveraged population genomic data from the model system Drosophila melanogaster. In this host, we found a number of SNPs associated with titer in candidate genes potentially relevant to host interactions with Wolbachia. Our study demonstrates that data mining is a powerful tool to detect bacterial infections and quantify infection intensities, thus opening an array of previously inaccessible data for further analysis in host-symbiont evolution.

Cell-based analysis reveals that sex-determining gene signals in Ostrinia are pivotally changed by male-killing Wolbachia

Wolbachia, a maternally transmitted bacterium, shows male-killing, an adaptive phenotype for cytoplasmic elements, in various arthropod species during the early developmental stages. In lepidopteran insects, lethality of males is accounted for by improper dosage compensation in sex-linked genes owing to Wolbachia-induced feminization. Herein, we established Ostrinia scapulalis cell lines that retained sex specificity per the splicing pattern of the sex-determining gene doublesex (Osdsx). We found that Wolbachia transinfection in male cell lines enhanced the female-specific splice variant of Osdsx (Osdsx^F ) while suppressing the male-specific variant (Osdsx^M ), indicating that Wolbachia affects sex-determining gene signals even in vitro. Comparative transcriptome analysis isolated only two genes that behave differently upon Wolbachia infection. The two genes were respectively homologous to Masculinizer (BmMasc) and zinc finger-2 (Bmznf-2), male-specifically expressed sex-determining genes of the silkworm Bombyx mori that encode CCCH-type zinc finger motif proteins. By using cultured cells and organismal samples, OsMasc and Osznf-2 were found to be sex-determining genes of O. scapulalis that are subjected to sex-specific alternative splicing depending upon the chromosomal sex, developmental stage, and infection status. Overall, our findings expound the cellular autonomy in insect sex determination and the mechanism through which sex is manipulated by intracellular selfish microbes.

Male-killing mechanisms vary between Spiroplasma species

Male-killing, a male-specific death of arthropod hosts during development, is induced by Spiroplasma (Mollicutes) endosymbionts of the Citri-Poulsonii and the Ixodetis groups, which are phylogenetically distant groups. Spiroplasma poulsonii induces male-killing in Drosophila melanogaster (Diptera) using the Spaid toxin that harbors ankyrin repeats, whereas little is known about the origin and mechanisms of male-killing induced by Spiroplasma ixodetis. Here, we analyzed the genome and the biological characteristics of a male-killing S. ixodetis strain sHm in the moth Homona magnanima (Tortricidae, Lepidoptera). Strain sHm harbored a 2.1 Mb chromosome and two potential plasmids encoding Type IV effectors, putatively involved in virulence and host-symbiont interactions. Moreover, sHm did not harbor the spaid gene but harbored 10 ankyrin genes that were homologous to those in other S. ixodetis strains. In contrast to the predominant existence of S. poulsonii in hemolymph, our quantitative PCR assays revealed a systemic distribution of strain sHm in H. magnanima, with particularly high titers in Malpighian tubules but low titers in hemolymph. Furthermore, transinfection assays confirmed that strain sHm can infect cultured cells derived from distantly related insects, namely Aedes albopictus (Diptera) and Bombyx mori (Lepidoptera). These results suggest different origins and characteristics of S. ixodetis- and S. poulsonii-induced male-killing.

Sex determination systems as the interface between male-killing bacteria and their hosts

Arthropods host a range of sex-ratio-distorting selfish elements, including diverse maternally inherited endosymbionts that solely kill infected males. Male-killing heritable microbes are common, reach high frequency, but until recently have been poorly understood in terms of the host–microbe interaction. Additionally, while male killing should generate strong selection for host resistance, evidence of this has been scant. The interface of the microbe with host sex determination is integral to the understanding of how death is sex limited and how hosts can evolve evasion of male killing. We first review current knowledge of the mechanisms diverse endosymbionts use to induce male-specific death. We then examine recent evidence that these agents do produce intense selection for host nuclear suppressor elements. We argue, from our understanding of male-killing mechanisms, that suppression will commonly involve evolution of the host sex determination pathways and that the host's response to male-killing microbes thus represents an unrecognized driver of the diversity of arthropod sex determination. Further work is required to identify the genes and mechanisms responsible for male-killing suppression, which will both determine the components of sex determination (or other) systems associated with suppressor evolution, and allow insight into the mechanism of male killing itself.

Cytoplasmic incompatibility in the semivoltine longicorn beetle Acalolepta fraudatrix (Coleoptera: Cerambycidae) double infected with Wolbachia

Wolbachia are obligatory endosymbiotic α-proteobacteria found in many arthropods. They are maternally inherited, and can induce reproductive alterations in the hosts. Despite considerable recent progress in studies on the associations between Wolbachia and various taxonomic groups of insects, none of the researches have revealed the effects of Wolbachia on longicorn beetles as the host insect. Acalolepta fraudatrix is a forest longicorn beetle that is distributed in East Asia. In this study, the relationship between Wolbachia and A. fraudatrix was investigated. Out of two populations of A. fraudatrix screened for Wolbachia using the genes ftsZ, wsp, and 16S rRNA, only one of the populations showed detection of all three genes indicating the presence of Wolbachia. Electron microscopy and fluorescent in situ hybridization also confirmed that the A. fraudatrix population was infected with Wolbachia. Sequencing the wsp genes derived from single insects revealed that two strains of Wolbachia coexisted in the insects based on the detection of two different sequences of the wsp gene. We designated these strains as wFra1 and wFra2. The bacterial titers of wFra1 were nearly 2-fold and 3-fold higher than wFra2 in the testes and ovaries, respectively. The two strains of Wolbachia in the insects were completely eliminated by rearing the insects on artificial diets containing 1% concentration of tetracycline for 1 generation. Reciprocal crosses between Wolbachia-infected and Wolbachia-uninfected A. fraudatrix demonstrated that only eggs produced by the crosses between Wolbachia-infected males and Wolbachia-uninfected females did not hatch, indicating that Wolbachia infecting A. fraudatrix causes cytoplasmic incompatibility in the host insect. This is the first report showing the effect of Wolbachia on reproductive function in a longicorn beetle, A. fraudatrix.

Discovery of wing imaginal discs in the penultimate instar of the lacewing Mallada desjardinsi (Insecta: Neuroptera: Chrysopidae) with histological notes on postembryonic imaginal disc development

Holometabolous insects are alternatively named "Endopterygota" because, in the larvae of many taxa, the wing primordia in the lateral regions of the meso- and metathoracic segments form more or less invaginated structures called wing imaginal discs. Holometabolous insects exhibit differential developmental timing of the wing during ontogeny. The condition in which wing growth is deferred until the end of larval life has been considered ancestral, whereas early disc formation has been recognized as the derived condition. Even though wing disc development in holometabolous insects has been studied extensively in select groups, many questions remain about the development of the wing imaginal disc in the orders Raphidioptera, Megaloptera, Neuroptera, and Mecoptera. To clarify whether the wing imaginal disc of Neuroptera is typical of the derived condition, we examined the ontogeny of the wing imaginal discs in the lacewing Mallada desjardinsi histologically. Using both light microscopy and transmission electron microscopy, we were able to recognize wing imaginal discs in the penultimate larval instar (prefinal larval instar) of this species. To date, neuropteran insects have been characterized as having late-forming wing imaginal discs. However, our findings show that the developmental pattern of the wing imaginal discs within the Neuroptera represents a more derived pattern of development in the Holometabola than was assumed previously.

Rapid molecular evolution of Spiroplasma symbionts of Drosophila

Spiroplasma is a genus of Mollicutes whose members include plant pathogens, insect pathogens and endosymbionts of animals. Spiroplasma phenotypes have been repeatedly observed to be spontaneously lost in Drosophila cultures, and several studies have documented a high genomic turnover in Spiroplasma symbionts and plant pathogens. These observations suggest that Spiroplasma evolves quickly in comparison to other insect symbionts. Here, we systematically assess evolutionary rates and patterns of Spiroplasma poulsonii, a natural symbiont of Drosophila. We analysed genomic evolution of sHy within flies, and sMel within in vitro culture over several years. We observed that S. poulsonii substitution rates are among the highest reported for any bacteria, and around two orders of magnitude higher compared with other inherited arthropod endosymbionts. The absence of mismatch repair loci mutS and mutL is conserved across Spiroplasma, and likely contributes to elevated substitution rates. Further, the closely related strains sMel and sHy (>99.5 % sequence identity in shared loci) show extensive structural genomic differences, which potentially indicates a higher degree of host adaptation in sHy, a protective symbiont of Drosophila hydei. Finally, comparison across diverse Spiroplasma lineages confirms previous reports of dynamic evolution of toxins, and identifies loci similar to the male-killing toxin Spaid in several Spiroplasma lineages and other endosymbionts. Overall, our results highlight the peculiar nature of Spiroplasma genome evolution, which may explain unusual features of its evolutionary ecology.

Silence of the killers: discovery of male-killing suppression in a rearing strain of the small brown planthopper, Laodelphax striatellus

According to evolutionary theory, sex ratio distortions caused by reproductive parasites such as Wolbachia and Spiroplasma are predicted to be rapidly normalized by the emergence of host nuclear suppressors. However, such processes in the evolutionary arms race are difficult to observe because sex ratio biases will be promptly hidden and become superficially unrecognizable. The evolution of genetic suppressors has been reported in just two insect species so far. In the small brown planthopper, Laodelphax striatellus, female-biases caused by Spiroplasma, which is a 'late' male-killer, have been found in some populations. During the continuous rearing of L. striatellus, we noted that a rearing strain had a 1 : 1 sex ratio even though it harboured Spiroplasma. Through introgression crossing experiments with a strain lacking suppressors, we revealed that the L. striatellus strain had the zygotic male-killing suppressor acting as a dominant trait. The male-killing phenotype was hidden by the suppressor even though Spiroplasma retained its male-killing ability. This is the first study to demonstrate the existence of a late male-killing suppressor and its mode of inheritance. Our results, together with those of previous studies, suggest that the inheritance modes of male-killing suppressors are similar regardless of insect order or early or late male killing.

Infection Patterns and Fitness Effects of Rickettsia and Sodalis Symbionts in the Green Lacewing Chrysoperla carnea

Simple Summary

Bacteria have occupied a wide range of habitats including insect hosts. There they can strongly affect host physiology and ecology in a positive or negative way. Bacteria living exclusively inside other organisms are called endosymbionts. They often establish a long-term and stable association with their host. Although more and more studies focus on endosymbiont–insect interactions, the group of Neuroptera is largely neglected in such studies. We were interested in the common green lacewing (Chrysoperla carnea), a representative of Neuroptera, which is mainly known for its use in biological pest control. We asked ourselves which endosymbionts are present in these lacewings. By screening natural and laboratory populations, we found that the endosymbiont Rickettsia is present in all populations but the symbiont Sodalis only occurred in laboratory populations. We were curious whether both endosymbionts affect reproduction success. Through establishing and studying green lacewing lines carrying different endosymbionts, we found that Rickettsia had no effect on the insect reproduction, while Sodalis reduced the number of eggs laid by lacewings, alone and in co-infections with Rickettsia. The economic and ecological importance of green lacewings in biological pest control warrants a more profound understanding of its biology, which might be strongly influenced by symbionts.

Abstract

Endosymbionts are widely distributed in insects and can strongly affect their host ecology. The common green lacewing (Chrysoperla carnea) is a neuropteran insect which is widely used in biological pest control. However, their endosymbionts and their interactions with their hosts have not been very well studied. Therefore, we screened for endosymbionts in natural and laboratory populations of Ch. carnea using diagnostic PCR amplicons. We found the endosymbiont Rickettsia to be very common in all screened natural and laboratory populations, while a hitherto uncharacterized Sodalis strain was found only in laboratory populations. By establishing lacewing lines with no, single or co-infections of Sodalis and Rickettsia, we found a high vertical transmission rate for both endosymbionts (>89%). However, we were only able to estimate these numbers for co-infected lacewings. Sodalis negatively affected the reproductive success in single and co-infected Ch. carnea, while Rickettsia showed no effect. We hypothesize that the fitness costs accrued by Sodalis infections might be more tolerable in the laboratory than in natural populations, as the latter are also prone to fluctuating environmental conditions and natural enemies. The economic and ecological importance of lacewings in biological pest control warrants a more profound understanding of its biology, which might be influenced by symbionts.

Corn Stunt Disease: An Ideal Insect-Microbial-Plant Pathosystem for Comprehensive Studies of Vector-Borne Plant Diseases of Corn

Over 700 plant diseases identified as vector-borne negatively impact plant health and food security globally. The pest control of vector-borne diseases in agricultural settings is in urgent need of more effective tools. Ongoing research in genetics, molecular biology, physiology, and vector behavior has begun to unravel new insights into the transmission of phytopathogens by their insect vectors. However, the intricate mechanisms involved in phytopathogen transmission for certain pathosystems warrant further investigation. In this review, we propose the corn stunt pathosystem (Zea mays-Spiroplasma kunkelii-Dalbulus maidis) as an ideal model for dissecting the molecular determinants and mechanisms underpinning the persistent transmission of a mollicute by its specialist insect vector to an economically important monocotyledonous crop. Corn stunt is the most important disease of corn in the Americas and the Caribbean, where it causes the severe stunting of corn plants and can result in up to 100% yield loss. A comprehensive study of the corn stunt disease system will pave the way for the discovery of novel molecular targets for genetic pest control targeting either the insect vector or the phytopathogen.

Laboratory Hybridization Between the Green Lacewings Chrysoperla comanche (Neuroptera: Chrysopidae) and Chrysoperla rufilabris, Predators of the Asian Citrus Psyllid

Chrysoperla comanche (Banks) and its sibling species Chrysoperla rufilabris (Burmeister) are voracious predators of the Asian citrus psyllid, Diaphorina citri Kuwayama (Hemiptera: Liviidae), an invasive pest that vectors a bacterium responsible for the lethal and incurable citrus greening disease. The Comanche lacewing naturally occurs in Southern California, whereas C. rufilabris is currently one of only two commercially available green lacewing in the United States. These sister species can be separated by larval morphological traits, by differences in courtship songs, and possibly by three nuclear genes wingless, PepCK, ATPase, yet they are not distinguishable based on the mitochondrial barcode gene (COI). Releasing in a new area a biological control agent capable of hybridizing with a resident species may pose risks that range from local displacement to irreversible loss of genetic identity. Therefore, we performed no-choice laboratory crosses to assess pre- and postzygotic isolation. We show that fertile and viable hybrid progeny could be readily obtained in interspecific crosses and backcrosses and, although there is a trend toward lower hybrid fitness, postzygotic isolation is overall weak and might not prevent loss of genetic identity under natural conditions. It remains to be determined if differences in courtship songs will prevent hybridization in the wild, as shown for other green lacewings. We also report a low prevalence of Rickettsia infection in both species.

Influences of two coexisting endosymbionts, CI-inducing Wolbachia and male-killing Spiroplasma, on the performance of their host Laodelphax striatellus (Hemiptera: Delphacidae)

The small brown planthopper Laodelphax striatellus (Hemiptera: Delphacidae) is reported to have the endosymbiont Wolbachia, which shows a strong cytoplasmic incompatibility (CI) between infected males and uninfected females. In the 2000s, female-biased L. striatellus populations were found in Taiwan, and this sex ratio distortion was the result of male-killing induced by the infection of another endosymbiont, Spiroplasma. Spiroplasma infection is considered to negatively affect both L. striatellus and Wolbachia because the male-killing halves the offspring of L. striatellus and hinders the spread of Wolbachia infection via CI. Spiroplasma could have traits that increase the fitness of infected L. striatellus and/or coexisting organisms because the coinfection rates of Wolbachia and Spiroplasma were rather high in some areas. In this study, we investigated the influences of the infection of these two endosymbionts on the development, reproduction, and insecticide resistance of L. striatellus in the laboratory. Our results show that the single-infection state of Spiroplasma had a negative influence on the fertility of L. striatellus, while the double-infection state had no significant influence. At late nymphal and adult stages, the abundance of Spiroplasma was lower in the double-infection state than in the single-infection state. In the double-infection state, the reduction of Spiroplasma density may be caused by competition between the two endosymbionts, and the negative influence of Spiroplasma on the fertility of host may be relieved. The resistance of L. striatellus to four insecticides was compared among different infection states of endosymbionts, but Spiroplasma infection did not contribute to increase insecticide resistance. Because positive influences of Spiroplasma infection were not found in terms of the development, reproduction, and insecticide resistance of L. striatellus, other factors improving the fitness of Spiroplasma-infected L. striatellus may be related to the high frequency of double infection in some L. striatellus populations.

Rapid comeback of males: evolution of male-killer suppression in a green lacewing population

Evolutionary theory predicts that the spread of cytoplasmic sex ratio distorters leads to the evolution of host nuclear suppressors, although there are extremely few empirical observations of this phenomenon. Here, we demonstrate that a nuclear suppressor of a cytoplasmic male killer has spread rapidly in a population of the green lacewing Mallada desjardinsi An M. desjardinsi population, which was strongly female-biased in 2011 because of a high prevalence of the male-killing Spiroplasma endosymbiont, had a sex ratio near parity in 2016, despite a consistent Spiroplasma prevalence. Most of the offspring derived from individuals collected in 2016 had 1 : 1 sex ratios in subsequent generations. Contrastingly, all-female or female-biased broods appeared frequently from crossings of these female offspring with males derived from a laboratory line founded by individuals collected in 2011. These results suggest near-fixation of a nuclear suppressor against male killing in 2016 and reject the notion that a non-male-killing Spiroplasma variant has spread in the population. Consistently, no significant difference was detected in mitochondrial haplotype variation between 2011 and 2016. These findings, and earlier findings in the butterfly Hypolimnas bolina in Samoa, suggest that these quick events of male recovery occur more commonly than is generally appreciated.

The microbiota of hematophagous ectoparasites collected from migratory birds

Arthropod vectors are responsible for the transmission of human pathogens worldwide. Several arthropod species are bird ectoparasites, however, no study to date has characterized their microbiota as a whole. We sampled hematophagous ectoparasites that feed on migratory birds and performed 16S rRNA gene metabarcoding to characterize their microbial community. A total of 194 ectoparasites were collected from 115 avian hosts and classified into three groups: a) Hippoboscidae diptera; b) ticks; c) other arthropods. Metabarcoding showed that endosymbionts were the most abundant genera of the microbial community, including Wolbachia for Hippoboscidae diptera, Candidatus Midichloria for ticks, Wolbachia and Arsenophonus for the other arthropod group. Genera including pathogenic species were: Rickettsia, Borrelia, Coxiella, Francisella, Bartonella, Anaplasma. Co-infection with Borrelia-Rickettsia and Anaplasma-Rickettsia was also observed. A global overview of the microbiota of ectoparasites sampled from migratory birds was obtained with the use of 16S rRNA gene metabarcoding. A novel finding is the first identification of Rickettsia in the common swift louse fly, Crataerina pallida. Given their possible interaction with pathogenic viruses and bacteria, the presence of endosymbionts in arthropods merits attention. Finally, molecular characterization of genera, including both pathogenic and symbiont species, plays a pivotal role in the design of targeted molecular diagnostics.

Spiroplasma infection in Harmonia axyridis - Diversity and multiple infection

The heritable endosymbiotic bacterium Spiroplasma is found in the harlequin ladybird Harmonia axyridis. The proportion of beetles infected with Spiroplasma in different native H. axyridis populations varies from 2% to 49%. We investigated the polymorphism of Spiroplasma strains in samples from individual beetles from Kyoto, Vladivostok, Troitsa Bay, Novosibirsk, and Gorno-Altaisk. To identify Spiroplasma strains, we analyzed nucleotide polymorphisms of the 16S rRNA gene and the ribosomal internal transcribed spacer (ITS1). The majority of infected beetles were infected with two or more Spiroplasma strains. We measured Spiroplasma density in beetles with different infection status using quantitative PCR. The abundance of Spiroplasma in samples with a single infection is an order of magnitude lower than in samples with multiple infections. Density dependent biological effects of Spiroplasma are discussed.

Evolution of reproductive parasites with direct fitness benefits

Maternally inherited symbionts such as Wolbachia have long been seen mainly as reproductive parasites, with deleterious effects on host fitness. It is becoming clear, however, that, frequently, these symbionts also have beneficial effects on host fitness, either along with reproductive parasitism or not. Using the examples of cytoplasmic incompatibility (CI) and male-killing (MK), we here analyze the effect of direct fitness benefits on the evolution of reproductive parasites. By means of a simple theoretical framework, we synthesize and extend earlier modeling approaches for CI and MK, which usually ignore fitness benefits. Moreover, our framework is not restricted to a particular mechanism underlying the fitness benefit (e.g., protection against pathogens). We derive invasion conditions and equilibrium frequencies for the different infection scenarios. Our results demonstrate the importance of a symbiont's "effective fecundity" (i.e., the product of the relative fecundity of an infected female and her transmission efficiency) for a symbiont's invasion success. In particular, we adopt the concept of effective fecundity to scenarios where CI and MK co-occur in one host population. We confirm that direct fitness benefits substantially facilitate the invasion and spread of infections (for example, by lowering or removing the invasion threshold) or even make invasion possible in the first place (for example, if reproductive parasitism is weak or absent). Finally, we discuss the role of direct fitness benefits in long-term evolutionary dynamics of reproductive phenotypes and highlight their potential to resolve genetic conflicts between maternally inherited symbionts and their hosts.

Evolution and Diversity of Inherited Spiroplasma Symbionts in Myrmica Ants

Microbial partners play important roles in the biology and ecology of animals. In insects, maternally transmitted symbionts are especially common and can have host effects ranging from reproductive manipulation to nutrient provisioning and defense against natural enemies. In this study, we report a genus-wide association of Myrmica ants with the inherited bacterial symbiont Spiroplasma We screen Myrmica ants collected from the wild, including the invasive European fire ant, Myrmica rubra, and find an extraordinarily high prevalence of this symbiont-8 of 9 species, 42 of 43 colonies, and 250 of 276 individual workers harbored Spiroplasma-only one host species was uninfected. In our screens, each host species carried a distinct Spiroplasma strain, and none were infected with more than one strain. All symbionts belong to the citri clade, allied most closely with pathogenic strains of Spiroplasma infecting corn crops and honeybees, and there is strong evidence of host-symbiont persistence across evolutionary time scales. Genome sequencing of two Spiroplasma symbionts revealed candidate genes that may play a part in the symbiosis, a nutrient transporter absent from other Spiroplasma strains, and a ribosome-inactivating protein previously implicated in parasite defense. These results together suggest long-term, likely mutualistic, relationships atypical of Spiroplasma-insect associations with potential significance for broad ecological interactions with MyrmicaIMPORTANCE Animal-associated microbial symbionts can dramatically affect the biology of their hosts. The identification and characterization of these intimate partnerships remain an essential component of describing and predicting species interactions, especially for invasive host species. Ants perform crucial ecological functions as ecosystem engineers, scavengers, and predators, and ants in the genus Myrmica can be aggressive resource competitors and reach high densities in their native and invaded habitats. In this study, a novel symbiosis is identified between Myrmica ants and the facultative bacterial symbiont Spiroplasma Broad host distribution, high frequencies of infection, and host-symbiont codivergence over evolutionary time scales, an uncommon feature of Spiroplasma associations, suggest an important likely mutualistic interaction. Genome sequencing identified highly divergent gene candidates that may contribute to Spiroplasma's role as a possible defensive or nutritional partner in Myrmica.

The enemy within: phloem-limited pathogens

The growing impact of phloem-limited pathogens on high-value crops has led to a renewed interest in understanding how they cause disease. Although these pathogens cause substantial crop losses, many are poorly characterized. In this review, we present examples of phloem-limited pathogens that include intracellular bacteria with and without cell walls, and viruses. Phloem-limited pathogens have small genomes and lack many genes required for core metabolic processes, which is, in part, an adaptation to the unique phloem environment. For each pathogen class, we present multiple case studies to highlight aspects of disease caused by phloem-limited pathogens. The pathogens presented include Candidatus Liberibacter asiaticus (citrus greening), Arsenophonus bacteria, Serratia marcescens (cucurbit yellow vine disease), Candidatus Phytoplasma asteris (Aster Yellows Witches' Broom), Spiroplasma kunkelii, Potato leafroll virus and Citrus tristeza virus. We focus on commonalities in the virulence strategies of these pathogens, and aim to stimulate new discussions in the hope that widely applicable disease management strategies can be found.

Quantification of Densities of Bacterial Endosymbionts of Insects by Real-time PCR

Increased attention has been paid to the endosymbiotic bacteria of insects. Because most insect endosymbionts are uncultivable, quantitative PCR (qPCR) is a practical and convenient method to quantify endosymbiont titers. Here we report a protocol for real-time qPCR based on SYBR Green I fluorescence as well as some tips to prevent possible pitfalls.

Green lacewings (Neuroptera: Chrysopidae) are commonly associated with a diversity of rickettsial endosymbionts

BACKGROUND

Bacterial symbionts transmitted from mothers to offspring are found in the majority of arthropods. Numerous studies have illustrated their wide impact on host biology, such as reproduction, behavior, and physiology One of the most common inherited symbionts is Rickettsia spp. (Alphaproteobacteria, Rickettsiales), which are found in about one-quarter of terrestrial arthropods, as well as in other invertebrates. In insect populations, Rickettsia spp. have been reported to cause reproductive modifications and fecundity-enhancing effects. Here, we investigated the incidence and genetic diversity of Rickettsia symbionts in green lacewings (Neuroptera, Chrysopidae), which are best known for their use as biological control agents against crop pests.

RESULTS

We screened 18 species of green lacewings and allies for Rickettsia and found the symbiont in 10 species, infecting 20-100% of sampled individuals. Strain characterization based on multiple bacterial loci revealed an unprecedented diversity of Rickettsia associated with lacewings, suggesting multiple independent acquisitions. Further, the detected Rickettsia lineages are restricted to a specific lineage (i.e., species or genus) of investigated lacewings, and these associations are stable across multiple sampled locations and points in time.

CONCLUSIONS

We conclude that Rickettsia-lacewing symbioses are common and evolutionarily stable. The role of these symbionts remains to be identified, but is potentially important to optimizing their use in biological pest control.

Challenging the Wigglesworthia, Sodalis, Wolbachia symbiosis dogma in tsetse flies: Spiroplasma is present in both laboratory and natural populations

Profiling of wild and laboratory tsetse populations using 16S rRNA gene amplicon sequencing allowed us to examine whether the "Wigglesworthia-Sodalis-Wolbachia dogma" operates across species and populations. The most abundant taxa, in wild and laboratory populations, were Wigglesworthia (the primary endosymbiont), Sodalis and Wolbachia as previously characterized. The species richness of the microbiota was greater in wild than laboratory populations. Spiroplasma was identified as a new symbiont exclusively in Glossina fuscipes fuscipes and G. tachinoides, members of the palpalis sub-group, and the infection prevalence in several laboratory and natural populations was surveyed. Multi locus sequencing typing (MLST) analysis identified two strains of tsetse-associated Spiroplasma, present in G. f. fuscipes and G. tachinoides. Spiroplasma density in G. f. fuscipes larva guts was significantly higher than in guts from teneral and 15-day old male and female adults. In gonads of teneral and 15-day old insects, Spiroplasma density was higher in testes than ovaries, and was significantly higher density in live versus prematurely deceased females indicating a potentially mutualistic association. Higher Spiroplasma density in testes than in ovaries was also detected by fluorescent in situ hybridization in G. f. fuscipes.