Endosymbiont diversity and prevalence in herbivorous spider mite populations in South-Western Europe

Fake news? The impact of information mismatch on mating behaviour

Multiple cues are often used for mate choice in complex environments, potentially entailing mismatches in the information conveyed by different sources. We address the consequences of this information mismatch for receivers using the spider mite Tetranychus urticae, in which virgin females are highly valuable mates compared to mated females, given first male sperm precedence. Accordingly, males are known to prefer virgins and distinguish them using cues from the females themselves and that they leave on the substrate. Whereas cues from females are highly reliable, those left on the substrate may not reflect the real female mating status if females move and/or mate. Here, we tested the consequences of such mismatch by exposing males to mated or virgin females on patches previously impregnated with cues deposited by females of either mating status. Male mating attempts were solely affected by substrate cues while female acceptance and the number of mating events were independently affected by both cues. Copulation duration, in contrast, depended mainly on the mating status of the female, with the number of copulations and the total time spent mating being intermediate in environments with mismatched information. We also show that males incur mating costs, reflected in reduced survival in environments with virgin cues. These results suggest that substrate cues left by females are instrumental for males to find their mates. However, in environments with mismatched information, males may pay survival costs without the associated benefit of mating with virgins, or they may lose opportunities to mate with virgins by responding to substrate cues from mated females. The benefit of using multiple cues will then hinge upon the frequency of information mismatch, which itself should vary with the dynamics of populations.

Detection and Analysis of Wolbachia in Plant-Parasitic Nematodes and Insights into Wolbachia Evolution

Since the discovery of Wolbachia in plant-parasitic nematodes (PPNs), there has been increased interest in this earliest branching clade that may hold important clues to early transitions in Wolbachia function in the Ecdysozoa. However, due to the specialized skills and equipment of nematology and the difficulty in culturing most PPNs, these PPN-type Wolbachia remain undersampled and poorly understood. To date, there are few established laboratory methods for working with PPN-type Wolbachia strains, and most research has relied on chance discovery and comparative genomics. Here, we address this challenge by providing detailed methods to assist researchers with more efficiently collecting PPNs and screen these communities, populations, or single nematodes with a newly developed PPN-type Wolbachia-specific PCR assay. We provide an overview of the typical yields and outcomes of these methods, to facilitate further targeted cultivation or experimental methods, and finally we provide a short introduction to some of the specific challenges and solutions in following through with comparative or population genomics on PPN-type Wolbachia strains.

Environmental Factors and the Symbiont Cardinium Influence the Bacterial Microbiome of Spider Mites Across the Landscape

Microbes play a key role in the biology, ecology, and evolution of arthropods. Despite accumulating data on microbial communities in arthropods that feed on plants using piercing-sucking mouthparts, we still lack a comprehensive understanding of the composition and assembly factors of the microbiota, particularly in field-collected spider mites. Here, we applied 16S rRNA amplicon sequencing to investigate the characters of the bacterial community in 140 samples representing 420 mite individuals, belonging to eight Tetranychus species (Acari: Tetranychidae) collected from 26 sites in China. The results showed that the bacterial composition of spider mites varied significantly among different species, locations, and plants. The environment showed a significant influence on the bacterial community of spider mites, with different relative contributions. Latitude and precipitation were found to be the main factors influencing the bacterial community composition. The dissimilarity of bacterial community and geographical distance between mite locations were significantly correlated. The assembly of spider mite bacterial communities seemed to be mainly influenced by stochastic processes. Furthermore, the symbiont Cardinium was found to be important in shaping the microbiota of many Tetranychus species. The relative abundance of Cardinium was > 50% in T. viennensis, T. urticae G, T. urticae R, and T. turkestani. Removing Cardinium reads from our analysis significantly changed Shannon diversity index and weighted beta diversity in these species. Altogether, this study provides novel insights into bacterial diversity patterns that contribute to our knowledge of the symbiotic relationships between arthropods and their bacterial communities.

Concurrent herbivory and metal accumulation: The outcome for plants and herbivores

The effects of metals on plants and herbivores, as well as the interaction among the latter, are well documented. However, the effects of simultaneous herbivory and metal accumulation remain poorly studied. Here, we shed light on this topic by infesting cadmium-accumulating tomato plants (Solanum lycopersicum), either exposed to cadmium or not, with herbivorous spider mites, Tetranychus urticae or T. evansi during 14 days. Whereas on plants without cadmium T. evansi had higher growth rate than T. urticae, on plants with cadmium both mite species had similar growth rates, which were lower than on plants without metal. Plants were affected by both cadmium toxicity and by herbivory, as shown by leaf reflectance, but not on the same wavelengths. Moreover, changes in leaf reflectance on the wavelength affected by herbivores were similar on plants with and without cadmium, and vice versa. Long-term effects of cadmium and herbivory did not affect H₂O₂ concentrations in the plant. Finally, plants infested with spider mites did not accumulate more cadmium, suggesting that metal accumulation is not induced by herbivory. We thus conclude that cadmium accumulation affects two congeneric herbivore species differently and that the effects of herbivory and cadmium toxicity on plants may be disentangled, via leaf reflectance, even during simultaneous exposure.

Microbiome Heritability and Its Role in Adaptation of Hosts to Novel Resources

Microbiomes are involved in most vital processes, such as immune response, detoxification, and digestion and are thereby elementary to organismal functioning and ultimately the host’s fitness. In turn, the microbiome may be influenced by the host and by the host’s environment. To understand microbiome dynamics during the process of adaptation to new resources, we performed an evolutionary experiment with the two-spotted spider mite, Tetranychus urticae. We generated genetically depleted strains of the two-spotted spider mite and reared them on their ancestral host plant and two novel host plants for approximately 12 generations. The use of genetically depleted strains reduced the magnitude of genetic adaptation of the spider mite host to the new resource and, hence, allowed for better detection of signals of adaptation via the microbiome. During the course of adaptation, we tested spider mite performance (number of eggs laid and longevity) and characterized the bacterial component of its microbiome (16S rRNA gene sequencing) to determine: (1) whether the bacterial communities were shaped by mite ancestry or plant environment and (2) whether the spider mites’ performance and microbiome composition were related. We found that spider mite performance on the novel host plants was clearly correlated with microbiome composition. Because our results show that only little of the total variation in the microbiome can be explained by the properties of the host (spider mite) and the environment (plant species) we studied, we argue that the bacterial community within hosts could be valuable for understanding a species’ performance on multiple resources.

Specific sequence of arrival promotes coexistence via spatial niche pre-emption by the weak competitor

Historical contingency, such as the order of species arrival, can modify competitive outcomes via niche modification or pre-emption. However, how these mechanisms ultimately modify stabilising niche and average fitness differences remains largely unknown. By experimentally assembling two congeneric spider mite species feeding on tomato plants during two generations, we show that order of arrival affects species' competitive ability and changes the outcome of competition. Contrary to expectations, order of arrival did not cause positive frequency dependent priority effects. Instead, coexistence was predicted when the inferior competitor (Tetranychus urticae) arrived first. In that case, T. urticae colonised the preferred feeding stratum (leaves) of T. evansi leading to spatial niche pre-emption, which equalised fitness and reduced niche differences, driving community assembly to a close-to-neutrality scenario. Our study demonstrates how the order of species arrival and the spatial context of competitive interactions may jointly determine whether species can coexist.

Interacting host modifier systems control Wolbachia ‐induced cytoplasmic incompatibility in a haplodiploid mite

Reproductive parasites such as Wolbachia spread within host populations by inducing cytoplasmic incompatibility (CI). CI occurs when parasite‐modified sperm fertilizes uninfected eggs and is typified by great variation in strength across biological systems. In haplodiploid hosts, CI has different phenotypic outcomes depending on whether the fertilized eggs die or develop into males. Genetic conflict theories predict the evolution of host modulation of CI, which in turn influences the stability of reproductive parasitism. However, despite the ubiquity of CI‐inducing parasites in nature, there is scarce evidence for intraspecific host modulation of CI strength and phenotype. Here, we tested for intraspecific host modulation of Wolbachia‐induced CI in haplodiploid Tetranychus urticae mites. Using a single CI‐inducing Wolbachia variant and mitochondrion, a nuclear panel was created that consisted of infected and cured near‐isogenic lines. We performed a highly replicated age‐synchronized full diallel cross composed of incompatible and compatible control crosses. We uncovered host modifier systems that cause striking variation in CI strength when carried by infected T. urticae males. We observed a continuum of CI phenotypes in our crosses and identified strong intraspecific female modulation of the CI phenotype. Crosses established a recessive genetic basis for the maternal effect and were consistent with polygenic Mendelian inheritance. Both male and female modulation interacted with the genotype of the mating partner. Our findings identify spermatogenesis as an important target of selection for host modulation of CI strength and underscore the importance of maternal genetic effects for the CI phenotype. Our findings reveal that intraspecific host modulation of CI is underpinned by complex genetic architectures and confirm that the evolution of reproductive parasitism is contingent on host genetics.

Are ecological communities the seat of endosymbiont horizontal transfer and diversification? A case study with soil arthropod community

Maternally inherited endosymbionts of arthropods are one of the most abundant and diverse group of bacteria. These bacterial endosymbionts also show extensive horizontal transfer to taxonomically unrelated hosts and widespread recombination in their genomes. Such horizontal transfers can be enhanced when different arthropod hosts come in contact like in an ecological community. Higher rates of horizontal transfer can also increase the probability of recombination between endosymbionts, as they now share the same host cytoplasm. However, reports of community-wide endosymbiont data are rare as most studies choose few host taxa and specific ecological interactions among the hosts. To better understand endosymbiont spread within host populations, we investigated the incidence, diversity, extent of horizontal transfer, and recombination of three endosymbionts (Wolbachia, Cardinium, and Arsenophonus) in a specific soil arthropod community. Wolbachia strains were characterized with MLST genes whereas 16S rRNA gene was used for Cardinium and Arsenophonus. Among 3,509 individual host arthropods, belonging to 390 morphospecies, 12.05% were infected with Wolbachia, 2.82% with Cardinium and 2.05% with Arsenophonus. Phylogenetic incongruence between host and endosymbiont indicated extensive horizontal transfer of endosymbionts within this community. Three cases of recombination between Wolbachia supergroups and eight incidences of within-supergroup recombination were also found. Statistical tests of similarity indicated supergroup A Wolbachia and Cardinium show a pattern consistent with extensive horizontal transfer within the community but not for supergroup B Wolbachia and Arsenophonus. We highlight the importance of extensive community-wide studies for a better understanding of the spread of endosymbionts across global arthropod communities.

Cardinium inhibits Wolbachia in its mite host, Tyrophagus putrescentiae, and affects host fitness

Interactions among endosymbiotic bacteria inside their eukaryotic hosts are poorly understood, particularly in mites. The mite Tyrophagus putrescentiae is a common, medically important generalist species that has many intracellular and gut bacterial symbionts. In the experiments, we examined bacterial abundances and composition in mite populations obtained by controlled mixing of stock mite populations that differed in the presence/absence of the major intracellular bacteria Wolbachia and Cardinium. Changes in microbial communities were characterized using 16S ribosomal RNA high-throughput sequencing (pooled mite individuals) and quantitative PCR for key microbial taxa (individual mites). Mite fitness was estimated as a parameter of population growth. We detected that in mixed mite populations, Cardinium and Wolbachia can co-occur in the same mite individual. The presence of Cardinium was negatively correlated with the presence of Wolbachia and Bartonella, while the Bartonella and Wolbachia were positively correlated in individual level samples. Since mixed populations had lower abundances of Wolbachia, while the abundance of Cardinium did not change, we suggest that the presence of Cardinium inhibits the growth of Wolbachia. The mixed mite populations had lower population growth than parental populations. The possible effect of symbionts on the fitness of mixed population is discussed.

Wolbachia and host intrinsic reproductive barriers contribute additively to postmating isolation in spider mites

Wolbachia are maternally-inherited bacteria that induce cytoplasmic incompatibility in many arthropod species. However, the ubiquity of this isolation mechanism for host speciation processes remains elusive, as only few studies have examined Wolbachia-induced incompatibilities when host populations are not genetically compatible. Here, we used three populations of two genetically differentiated colour forms of the haplodiploid spider mite Tetranychus urticae to dissect the interaction between Wolbachia-induced and host-associated incompatibilities, and their relative contribution to postmating isolation. We found that these two sources of incompatibility act through different mechanisms in an additive fashion. Host-associated incompatibility contributes 1.5 times more than Wolbachia-induced incompatibility in reducing hybrid production, the former through an overproduction of haploid sons at the expense of diploid daughters (ca. 75% decrease) and the latter by increasing the embryonic mortality of daughters (by ca. 49%). Furthermore, regardless of cross direction, we observed near-complete F1 hybrid sterility and complete F2 hybrid breakdown between populations of the two forms, but Wolbachia did not contribute to this outcome. We thus show mechanistic independence and an additive nature of host-intrinsic and Wolbachia-induced sources of isolation. Wolbachia may contribute to reproductive isolation in this system, thereby potentially affecting host differentiation and distribution in the field.

Newly introduced Cardinium endosymbiont reduces microbial diversity in the rice brown planthopper Nilaparvata lugens

Symbiotic microorganisms in invertebrates play vital roles in host ecology and evolution. Cardinium, a common intracellular symbiont, is transinfected into the important agricultural pest Nilaparvata lugens (rice brown planthopper) to regulate its reproduction, but how this impacts its microbial community is unknown. Here, we characterized the bacterial microbiota from N. lugens, with or without Cardinium, at different developmental stages and in various adult tissues using 16S ribosomal ribonucleic acid (16S rRNA) gene sequencing. Upon infection with Cardinium, we found that microbial diversity in the different developmental stages of N. lugens (especially females), and in female midguts and male testes, was lower than that in the uninfected control. There was a negative correlation between Cardinium and most related genera and between Bacteroidetes and Proteobacteria. Although the microbial structure varied during Cardinium infection, Acinetobacter spp. were a core microbiome genus. The Cardinium infection enhanced the relative density of midgut-associated Acinetobacter spp., with both bacteria exhibiting tissue-specific tropism. In addition, this infection caused the changes of main microbial functions in N. lugens. These results offer insights into the effects of alien (i.e. newly introduced from other organism) Cardinium infection on N. lugens-associated microbiotas, aiding in the development of transinfected endosymbionts for pest control.

Antagonistic interaction between male-killing and cytoplasmic incompatibility induced by Cardinium and Wolbachia in the whitefly, Bemisia tabaci

Cardinium and Wolbachia are maternally inherited bacterial symbionts of arthropods that can manipulate host reproduction by increasing the fitness of infected females. Here, we report that Cardinium and Wolbachia coinfection induced male-killing and cytoplasmic incompatibility (CI) when they coexisted in a cryptic species of whitefly, Bemisia tabaci Asia II7. Cardinium and Wolbachia symbionts were either singly or simultaneously localized in the bacteriocytes placed in the abdomen of B. tabaci nymphs and adults. Cardinium-Wolbachia coinfection induced male-killing and resulted in a higher female sex ratio in the intraspecific amphigenetic progeny of Asia II7 I_CWH and I_CWL lines; interestingly, male-killing induction was enhanced with increased Cardinium titer. Moreover, single infection of Wolbachia induced partial CI in the Asia II7 I_W line and resulted in reduced fecundity, higher embryonic mortality, and lower female sex ratio. The uninfected Asia II7 I_U line had significantly higher fecundity, lower embryonic and nymphal mortalities, and a lower level of CI than both the Wolbachia-infected Asia II7 I_W line and the Cardinium-Wolbachia-coinfected Asia II7 I_CWH line. Our findings indicate that Cardinium-Wolbachia coinfection induced male-killing, which may have had antagonistic effects on Wolbachia-induced CI in the Asia II7 whiteflies. For the first time, our study revealed that B. tabaci Asia II7 reproduction is co-manipulated by Cardinium and Wolbachia endosymbionts.

Microbial Communities of Stored Product Mites: Variation by Species and Population

Arthropod-associated microorganisms are important because they affect host fitness, protect hosts from pathogens, and influence the host's ability to vector pathogens. Stored product mites (Astigmata) often establish large populations in various types of food items, damaging the food by direct feeding and introducing contaminants, including their own bodies, allergen-containing feces, and associated microorganisms. Here we access the microbial structure and abundance in rearing diets, eggs, feces fraction, and mite bodies of 16 mite populations belonging to three species (Carpoglyphus lactis, Acarus siro, and Tyrophagus putrescentiae) using quantitative PCR and 16S ribosomal RNA (rRNA) gene amplicon sequencing. The mite microbiomes had a complex structure dominated by the following bacterial taxa (OTUs): (a) intracellular symbionts of the genera Cardinium and Wolbachia in the mite bodies and eggs; (b) putative gut symbionts of the genera Solitalea, Bartonella, and Sodalis abundant in mite bodies and also present in mite feces; (c) feces-associated or environmental bacteria of the genera Bacillus, Staphylococcus, and Kocuria in the diet, mite bodies, and feces. Interestingly and counterintuitively, the differences between microbial communities in various conspecific mite populations were higher than those between different mite species. To explain some of these differences, we hypothesize that the intracellular bacterial symbionts can affect microbiome composition in mite bodies, causing differences between microbial profiles. Microbial profiles differed between various sample types, such as mite eggs, bodies, and the environment (spent growth medium-SPGM). Low bacterial abundances in eggs may result in stochastic effects in parent-offspring microbial transmission, except for the intracellular symbionts. Bacteria in the rearing diet had little effect on the microbial community structure in SPGM and mite bodies. Mite fitness was positively correlated with bacterial abundance in SPGM and negatively correlated with bacterial abundances in mite bodies. Our study demonstrates critical host-microbe interactions, affecting all stages of mite growth and leading to alteration of the environmental microbiome. Correlational evidence based on absolute quantitation of bacterial 16S rRNA gene copies suggests that mite-associated microorganisms are critical for modulating important pest properties of mites by altering population growth.

Consequences of population structure for sex allocation and sexual conflict

Both sex allocation and sexual conflict can be modulated by spatial structure. However, how the interplay between the type of dispersal and the scale of competition simultaneously affects these traits in sub-divided populations is rarely considered. We investigated sex allocation and sexual conflict evolution in meta-populations of the spider mite Tetranychus urticae evolving under budding (pairing females from the same patch) or random (pairing females from different patches) dispersal and either local (fixed sampling from each subpopulation) or global (sampling as a function of subpopulation productivity) competition. Females evolving under budding dispersal produced less female-biased offspring sex ratios than those from the random dispersal selection regimes, contradicting theoretical predictions. In contrast, the scale of competition did not strongly affect sex allocation. Offspring sex ratio and female fecundity were unaffected by the number of mates, but female fecundity was highest when their mates evolved under budding dispersal, suggesting these males inflict less harm than those evolving under random dispersal. This work highlights that population structure can impact the evolution of sex allocation and sexual conflict. Moreover, selection on either trait may reciprocally affect the evolution of the other, for example via effects on fecundity.

The microbiota in spider mite feces potentially reflects intestinal bacterial communities in the host

Microorganisms provide many physiological functions to herbivorous hosts. Spider mites (genus Tetranychus) are important agricultural pests throughout the world; however, the composition of the spider mite microbial community, especially gut microbiome, remains unclear. Here, we investigated the bacterial community in five spider mite species and their associated feces by deep sequencing of the 16S rRNA gene. The composition of the bacterial community was significantly different among the five prevalent spider mite species, and some bacterial symbionts showed host-species specificity. Moreover, the abundance of the bacterial community in spider mite feces was significantly higher than that in the corresponding spider mite samples. However, Flavobacterium was detected in all samples, and represent a "core microbiome". Remarkably, the maternally inherited endosymbiont Wolbachia was detected in both spider mite and feces. Overall, these results offer insight into the complex community of symbionts in spider mites, and give a new direction for future studies.

Double infection of Wolbachia and Spiroplasma alters induced plant defense and spider mite fecundity

BACKGROUND

Herbivore-associated bacterial symbionts can change plant physiology and influence herbivore fitness. The spider mite Tetranychus truncatus is a notorious pest harboring various bacterial symbionts; however, the effect of bacterial symbionts on host plant physiology remains unclear. Here, we investigated whether infection with the endosymbionts Wolbachia and Spiroplasma altered spider mite performance on tomato plants and affected plant-induced defenses.

RESULTS

Wolbachia and Spiroplasma were mainly located in the gnathosoma and ovaries of their spider mite hosts. Wolbachia and Spiroplasma significantly improved spider mite reproductive performance in cultivated and wild-type tomato. However, in plants deficient in jasmonic acid (JA) and salicylic acid (SA), there were no significant differences in reproduction between spider mites infected with Wolbachia and Spiroplasma and uninfected mites. The results indicated that the reproduction benefits conferred by endosymbionts may relate to plant defenses. Both spider mites infected with Wolbachia and Spiroplasma and uninfected mites induced similar levels of JA and SA accumulation in tomato, whereas tomato plants damaged by spider mites infected with both Wolbachia and Spiroplasma showed lower expression levels of JA- and SA-responsive genes than those damaged by uninfected spider mites. In addition, mites infected with Wolbachia and Spiroplasma mites consumed more tomato amino acids compared to uninfected spider mites, which may have contributed to host fecundity.

CONCLUSIONS

Our results suggest that the reproduction benefits conferred by endosymbionts may be associated with changes in plant defense parameters and the concentrations of plant amino acids. The results highlight the importance of endosymbionts in interactions between spider mites and their host plants. © 2020 Society of Chemical Industry.

Variation in the microbiome of the spider mite Tetranychus truncatus with sex, instar and endosymbiont infection

Most arthropod-associated bacterial communities play a crucial role in host functional traits, whose structure could be dominated by endosymbionts. The spider mite Tetranychus truncatus is a notorious agricultural pest harboring various endosymbionts, yet the effects of endosymbionts on spider mite microbiota remain largely unknown. Here, using deep sequencing of the 16S rRNA gene, we characterized the microbiota of male and female T. truncatus with different endosymbionts (Wolbachia and Spiroplasma) across different developmental stages. Although the spider mite microbiota composition varied across the different developmental stages, Proteobacteria were the most dominant bacteria harbored in all samples. Positive relationships among related operational taxonomic units dominated the significant coassociation networks among bacteria. Moreover, the spider mites coinfected with Wolbachia and Spiroplasma had a significantly higher daily fecundity and juvenile survival rate than the singly infected or uninfected spider mites. The possible function of spider-mite associated bacteria was discussed. Our results highlight the dynamics of spider mite microbiotas across different life stages, and the potential role of endosymbionts in shaping the microbiota of spider mites and improving host fitness.

The distribution of herbivores between leaves matches their performance only in the absence of competitors

Few studies have tested how plant quality and the presence of competitors interact in determining how herbivores choose between different leaves within a plant. We investigated this in two herbivorous spider mites sharing tomato plants: Tetranychus urticae, which generally induces plant defenses, and Tetranychus evansi, which suppresses them, creating asymmetrical effects on coinfesting competitors. On uninfested plants, both herbivore species preferred young leaves, coinciding with increased mite performance. On plants with heterospecifics, the mites did not prefer leaves on which they had a better performance. In particular, T. urticae avoided leaves infested with T. evansi, which is in agreement with T. urticae being outcompeted by T. evansi. In contrast, T. evansi did not avoid leaves with the other species, but distributed itself evenly over plants infested with heterospecifics. We hypothesize that this behavior of T. evansi may prevent further spread of T. urticae over the shared plant. Our results indicate that leaf age determines within-plant distribution of herbivores only in absence of competitors. Moreover, they show that this distribution depends on the order of arrival of competitors and on their effects on each other, with herbivores showing differences in behavior within the plant as a possible response to the outcome of those interactions.

Molecular characterization of Cardinium, Rickettsia, Spiroplasma and Wolbachia in mite species from citrus orchards

Tetranychidae spider mites are considered key citrus pests in some production areas, especially Tetranychus urticae Koch. Over the past decades, pesticide overuse seems to have promoted T. urticae population selection in citrus orchards. However, the microbiota has also been pointed out as a plausible explanation for population structure or plant host specialisation observed in several arthropod species. In this work, we have determined the incidence of Cardinium, Rickettsia, Spiroplasma and Wolbachia as representatives of major distorter bacteria genera in Aplonobia histricina (Berlese), Eutetranychus banksi (McGregor), Eutetranychus orientalis (Klein), Panonychus citri (McGregor), Tetranychus evansi Baker and Pritchard, Tetranychus turkestani Ugarov and Nikolskii, and T. urticae populations from Spanish citrus orchards. Only Wolbachia was detected by PCR. The multilocus alignment approach and phylogenetic inference indicated that all detected Wolbachia belong to supergroup B. The deep analysis of each 16S rDNA, ftsZ and wsp gene sequences allowed identifying several phylogenetically different Wolbachia sequences. It probably indicates the presence of several different races or strains, all of them belonging to supergroup B. The wsp sequence typing analysis unveiled the presence of the two already identified alleles (61 and 370) and allowed to contribute with five new alleles, supporting the presence of different but related B-races in the studied mite populations. The results are discussed and related to T. urticae population structure, previously observed in Spanish citrus orchards.

Creating outbred and inbred populations in haplodiploids to measure adaptive responses in the laboratory

Laboratory studies are often criticized for not being representative of processes occurring in natural populations. One reason for this is the fact that laboratory populations generally do not capture enough of the genetic variation of natural populations. This can be mitigated by mixing the genetic background of several field populations when creating laboratory populations. From these outbred populations, it is possible to generate inbred lines, thereby freezing and partitioning part of their variability, allowing each genotype to be characterized independently. Many studies addressing adaptation of organisms to their environment, such as those involving quantitative genetics or experimental evolution, rely on inbred or outbred populations, but the methodology underlying the generation of such biological resources is usually not explicitly documented. Here, we developed different procedures to circumvent common pitfalls of laboratory studies, and illustrate their application using two haplodiploid species, the spider mites Tetranychus urticae and Tetranychus evansi. First, we present a method that increases the chance of capturing high amounts of variability when creating outbred populations, by performing controlled crosses between individuals from different field-collected populations. Second, we depict the creation of inbred lines derived from such outbred populations, by performing several generations of sib-mating. Third, we outline an experimental evolution protocol that allows the maintenance of a constant population size at the beginning of each generation, thereby preventing bottlenecks and diminishing extinction risks. Finally, we discuss the advantages of these procedures and emphasize that sharing such biological resources and combining them with available genetic tools will allow consistent and comparable studies that greatly contribute to our understanding of ecological and evolutionary processes.

Population-specific effect of Wolbachia on the cost of fungal infection in spider mites

Many studies have revealed the ability of the endosymbiotic bacterium Wolbachia to protect its arthropod hosts against diverse pathogens. However, as Wolbachia may also increase the susceptibility of its host to infection, predicting the outcome of a particular Wolbachia-host-pathogen interaction remains elusive. Yet, understanding such interactions and their eco-evolutionary consequences is crucial for disease and pest control strategies. Moreover, how natural Wolbachia infections affect artificially introduced pathogens for biocontrol has never been studied. Tetranychus urticae spider mites are herbivorous crop pests, causing severe damage on numerous economically important crops. Due to the rapid evolution of pesticide resistance, biological control strategies using entomopathogenic fungi are being developed. However, although spider mites are infected with various Wolbachia strains worldwide, whether this endosymbiont protects them from fungi is as yet unknown. Here, we compared the survival of two populations, treated with antibiotics or naturally harboring different Wolbachia strains, after exposure to the fungal biocontrol agents Metarhizium brunneum and Beauveria bassiana. To control for potential effects of the bacterial community of spider mites, we also compared the susceptibility of two populations naturally uninfected by Wolbachia, treated with antibiotics or not. In one population, Wolbachia-infected mites had a better survival than uninfected ones in absence of fungi but not in their presence, whereas in the other population Wolbachia increased the mortality induced by B. bassiana. In one naturally Wolbachia-uninfected population, the antibiotic treatment increased the susceptibility of spider mites to M. brunneum, but it had no effect in the other treatments. These results suggest that natural Wolbachia infections may not hamper and may even improve the success of biological control using entomopathogenic fungi. However, they also draw caution on the generalization of such effects, given the complexity of within-host-pathogens interaction and the potential eco-evolutionary consequences of the use of biocontrol agents for Wolbachia-host associations.

Endosymbiont diversity in natural populations of Tetranychus mites is rapidly lost under laboratory conditions

Although the diversity of bacterial endosymbionts in arthropods is well documented, whether and how such diversity is maintained remains an open question. We investigated the temporal changes occurring in the prevalence and composition of endosymbionts after transferring natural populations of Tetranychus spider mites from the field to the laboratory. These populations, belonging to three different Tetranychus species (T. urticae, T. ludeni and T. evansi) carried variable infection frequencies of Wolbachia, Cardinium, and Rickettsia. We report a rapid change of the infection status of these populations after only 6 months of laboratory rearing, with an apparent loss of Rickettsia and Cardinium, while Wolbachia apparently either reached fixation or was lost. We show that Wolbachia had variable effects on host longevity and fecundity, and induced variable levels of cytoplasmic incompatibility (CI) in each fully infected population, despite no sequence divergence in the markers used and full CI rescue between all populations. This suggests that such effects are largely dependent upon the host genotype. Subsequently, we used these data to parameterize a theoretical model for the invasion of CI-inducing symbionts in haplodiploids, which shows that symbiont effects are sufficient to explain their dynamics in the laboratory. This further suggests that symbiont diversity and prevalence in the field are likely maintained by environmental heterogeneity, which is reduced in the laboratory. Overall, this study highlights the lability of endosymbiont infections and draws attention to the limitations of laboratory studies to understand host-symbiont interactions in natural populations.

Inter- and intraspecific variation of spider mite susceptibility to fungal infections: Implications for the long-term success of biological control

Spider mites are severe pests of several annual and perennial crops worldwide, often causing important economic damages. As rapid evolution of pesticide resistance in this group hampers the efficiency of chemical control, alternative control strategies, such as the use of entomopathogenic fungi, are being developed. However, while several studies have focused on the evaluation of the control potential of different fungal species and/or isolates as well as their compatibility with other control methods (e.g., predators or chemical pesticides), knowledge on the extent of inter- and intraspecific variation in spider mite susceptibility to fungal infection is as yet incipient. Here, we measured the mortality induced by two generalist fungi, Beauveria bassiana and Metarhizium brunneum, in 12 spider mite populations belonging to different Tetranychus species: T. evansi, T. ludeni, and T. urticae (green and red form), within a full factorial experiment. We found that spider mite species differed in their susceptibility to infection by both fungal species. Moreover, we also found important intraspecific variation for this trait. These results draw caution on the development of single strains as biocontrol agents. Indeed, the high level of intraspecific variation suggests that (a) the one-size-fits-all strategy may fail to control spider mite populations and (b) hosts resistance to infection may evolve at a rapid pace. Finally, we propose future directions to better understand this system and improve the long-term success of spider mite control strategies based on entomopathogenic fungi.

Effect of Cadmium Accumulation on the Performance of Plants and of Herbivores That Cope Differently With Organic Defenses

Some plants are able to accumulate in their shoots metals at levels that are toxic to most other organisms. This ability may serve as a defence against herbivores. Therefore, both metal-based and organic defences may affect herbivores. However, how metal accumulation affects the interaction between herbivores and organic plant defences remains overlooked. To fill this gap, we studied the interactions between tomato (Solanum lycopersicum), a model plant that accumulates cadmium, and two spider-mite species, Tetranychus urticae and Tetranychus evansi that, respectively, induce and suppress organic plant defences, measurable via the activity of trypsin inhibitors. We exposed plants to different concentrations of cadmium and measured its effects on mites and plants. In the plant, despite clear evidence for cadmium accumulation, we did not detect any cadmium effects on traits that reflect the general response of the plant, such as biomass, water content, and carbon/nitrogen ratio. Still, we found effects of cadmium upon the quantity of soluble sugars and on leaf reflectance, where it may indicate structural modifications in the cells. These changes in plant traits affected the performance of spider mites feeding on those plants. Indeed, the oviposition of both spider mite species was higher on plants exposed to low concentrations of cadmium than on control plants, but decreased at concentrations above 0.5 mM. Therefore, herbivores with contrasting responses to organic defences showed a similar hormetic response to metal accumulation by the plants. Additionally, we show that the induction and suppression of plant defences by these spider-mite species was not affected by the amount of cadmium supplied to the plants. Furthermore, the effect of cadmium on the performance of spider mites was not altered by infestation with T. urticae or T. evansi. Together, our results suggest no interaction between cadmium-based and organic plant defences, in our system. This may be useful for plants living in heterogeneous environments, as they may use one or the other defence mechanism, depending on their relative performance in each environment.

Comparative Genomics of Wolbachia-Cardinium Dual Endosymbiosis in a Plant-Parasitic Nematode

Wolbachia and Cardinium are among the most important and widespread of all endosymbionts, occurring in nematodes and more than half of insect and arachnid species, sometimes as coinfections. These symbionts are of significant interest as potential biocontrol agents due to their abilities to cause major effects on host biology and reproduction through cytoplasmic incompatibility, sex ratio distortion, or obligate mutualism. The ecological and metabolic effects of coinfections are not well understood. This study examined a Wolbachia-Cardinium coinfection in the plant-parasitic nematode (PPN), Pratylenchus penetrans, producing the first detailed study of such a coinfection using fluorescence in situ hybridization (FISH), polymerase chain reaction (PCR), and comparative genomic analysis. Results from FISH and single-nematode PCR showed 123/127 individuals in a focal population carried Cardinium (denoted strain cPpe), and 48% were coinfected with Wolbachia strain wPpe. Both endosymbionts showed dispersed tissue distribution with highest densities in the anterior intestinal walls and gonads. Phylogenomic analyses confirmed an early place of cPpe and long distance from a sister strain in another PPN, Heterodera glycines, supporting a long history of both Cardinium and Wolbachia in PPNs. The genome of cPpe was 1.36 Mbp with 35.8% GC content, 1,131 predicted genes, 41% having no known function, and missing biotin and lipoate synthetic capacity and a plasmid present in other strains, despite having a slightly larger genome compared to other sequenced Cardinium. The larger genome revealed expansions of gene families likely involved in host-cellular interactions. More than 2% of the genes of cPpe and wPpe were identified as candidate horizontally transferred genes, with some of these from eukaryotes, including nematodes. A model of the possible Wolbachia-Cardinium interaction is proposed with possible complementation in function for pathways such as methionine and fatty acid biosynthesis and biotin transport.

A change in the bacterial community of spider mites decreases fecundity on multiple host plants

Bacterial symbionts may influence the fitness of their herbivore hosts, but such effects have been poorly studied across most invertebrate groups. The spider mite, Tetranychus truncatus, is a polyphagous agricultural pest harboring various bacterial symbionts whose function is largely unknown. Here, by using a high‐throughput 16S rRNA amplicon sequencing approach, we characterized the bacterial diversity and community composition of spider mites fed on five host plants after communities were modified following tetracycline exposure. We demonstrated that spider mite bacterial diversity and community composition were significantly affected by host plants and antibiotics. In particular, the abundance of the maternally inherited endosymbionts Wolbachia and Spiroplasma significantly differed among spider mites that were reared on different plant species and were completely removed by antibiotics. There was an overall tendency for daily fecundity to be lower in the mites with reduced bacterial diversity following the antibiotic treatment. Our data suggest that host plants and antibiotics can shape spider mite bacterial communities and that bacterial symbionts improve mite performance.

Suppression of Plant Defenses by Herbivorous Mites Is Not Associated with Adaptation to Host Plants

Some herbivores suppress plant defenses, which may be viewed as a result of the coevolutionary arms race between plants and herbivores. However, this ability is usually studied in a one-herbivore-one-plant system, which hampers comparative studies that could corroborate this hypothesis. Here, we extend this paradigm and ask whether the herbivorous spider-mite Tetranychus evansi, which suppresses the jasmonic-acid pathway in tomato plants, is also able to suppress defenses in other host plants at different phylogenetic distances from tomatoes. We test this using different plants from the Solanales order, namely tomato, jimsonweed, tobacco, and morning glory (three Solanaceae and one Convolvulaceae), and bean plants (Fabales). First, we compare the performance of T. evansi to that of the other two most-commonly found species of the same genus, T. urticae and T. ludeni, on several plants. We found that the performance of T. evansi is higher than that of the other species only on tomato plants. We then showed, by measuring trypsin inhibitor activity and life history traits of conspecific mites on either clean or pre-infested plants, that T. evansi can suppress plant defenses on all plants except tobacco. This study suggests that the suppression of plant defenses may occur on host plants other than those to which herbivores are adapted.

Identification of spider-mite species and their endosymbionts using multiplex PCR

Spider mites of the genus Tetranychidae are severe crop pests. In the Mediterranean a few species coexist, but they are difficult to identify based on morphological characters. Additionally, spider mites often harbour several species of endosymbiotic bacteria, which may affect the biology of their hosts. Here, we propose novel, cost-effective, multiplex diagnostic methods allowing a quick identification of spider-mite species as well as of the endosymbionts they carry. First, we developed, and successfully multiplexed in a single PCR, primers to identify Tetranychus urticae, T. evansi and T. ludeni, some of the most common tetranychids found in southwest Europe. Moreover, we demonstrated that this method allows detecting multiple species in a single pool, even at low frequencies (up to 1/100), and can be used on entire mites without DNA extraction. Second, we developed another set of primers to detect spider-mite endosymbionts, namely Wolbachia, Cardinium and Rickettsia in a multiplex PCR, along with a generalist spider-mite primer to control for potential failure of DNA amplification in each PCR. Overall, our method represents a simple, cost-effective and reliable method to identify spider-mite species and their symbionts in natural field populations, as well as to detect contaminations in laboratory rearings. This method may easily be extended to other species.

Herbivore-Associated Bacteria as Potential Mediators and Modifiers of Induced Plant Defense Against Spider Mites and Thrips

Induced plant defense, comprising contact with exogenous stimuli, production of endogenous signals alerting the plant, associated biochemical cascades, and local and/or systemic expression of the defense mechanisms, critically depends on the nature of the inducing agents. At large, bio-trophic pathogenic microorganisms and viruses usually trigger the salicylate (SA)-mediated pathway, whereas necro-trophic pathogens and herbivores usually trigger the jasmonate (JA)-mediated pathway in plants. The SA- and JA-mediated pathways do not operate independently but commonly interfere with each other. Several recent studies revealed abnormal plant responses upon herbivore attack in diverse plant-herbivore systems. Observed abnormalities range from suppression of the common JA-pathway, induction of the SA-pathway to no response, yet the underlying proximate causes and ultimate consequences of these variations are elusive. Strikingly, some studies provide compelling evidence that anti-herbivore plant responses may decisively depend on bacteria associated with the herbivore attacking the plant (HAB for herbivore-associated bacteria). HAB may influence herbivore recognition by the plant and alter the biochemical cascades inside plants. Here, I report cases in point of HAB manipulating induced anti-herbivore plant responses, suggest spatial and temporal categorization of HAB, and point at proximate and ultimate aspects of plant defense manipulation by HAB. Following, I overview the diversity of HAB of spider mites and herbivorous thrips, argue that, considering recently reported phenomena of abnormal plant responses upon spider mite attack, some of these HAB could represent important, but hitherto largely neglected, mediators/modifiers of induced plant defense against spider mites and thrips, and conclude with suggestions for future research.