Effects of Wolbachia on mtDNA variation and evolution in natural populations of Tetranychus urticae Koch

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.

Genetic diversity of Haemaphysalis longicornis from China and molecular detection of Rickettsia

The tick Haemaphysalis longicornis (Neumann) (Acari, Ixodidae) is distributed throughout China and is the most notorious blood feeding ectoparasite of livestock. Haemaphysalis longicornis can transmit a large number of pathogens that cause human infectious diseases, such as Rickettsia spp. Here, we characterized the genetic structure of H. longicornis and tested for the presence of Rickettsia spp. from five regions in China. Analysis of the two mitochondrial marker sequences (16S rRNA and COI) and the nuclear sequence (ITS2) showed that the overall level of nucleotide diversity was low and the variability did not differ among the five regions. From the five locations, the infection rates of Rickettsia species ranged from 0 to 65%. The nucleotide diversities of the high-infected group were lower than those of the low- and uninfected group. And in neutrality tests for the high-infected group based on COI sequences, the Tajima's D and Fu's FS were coincidentally < 0 and significant, whereas they were closer to zero and non-significant in low- and uninfected groups.

Effects of Wolbachia/Cardinium Infection on the Mitochondrial Phylogeny of Oligonychus castaneae (Acari: Tetranychidae)

A wide range of invertebrates harbor intracellular endosymbiotic bacteria. Within these endosymbionts, Wolbachia and Cardinium, have been attracting particular attention because these bacteria frequently affect the genetic structure and genetic diversity of their hosts. They cause various reproductive alterations such as cytoplasmic incompatibility, parthenogenesis induction, male-killing, and feminization. Through these alterations, they also affect the maternally inherited organelles of their hosts. Mitochondrial DNA (mtDNA) can be used for molecular phylogenetic analysis of invertebrates. However, in Wolbachia- or Cardinium-infected invertebrates, phylogenetic trees based on mtDNA are often inconsistent with those based on nuclear DNA. In the present study, we determined the Wolbachia/Cardinium infection status of 45 populations of the mite, Oligonychus castaneae Ehara & Gotoh (Acari: Tetranychidae), collected throughout Japan. Then, we compared phylogenetic trees of O. castaneae based on both the cytochrome c oxidase subunit I (COI) gene of mtDNA and the 28S rRNA gene of nuclear DNA to clarify the effects of Wolbachia and/or Cardinium infection. We found 106 Wolbachia-infected individuals and 250 Cardinium-infected individuals in a total of 450 individuals, indicating an infection rate of 79%. No double-infected individuals were observed. In the 28S tree, almost all populations formed a single group. In the COI tree, O. castaneae formed four separate groups that more closely followed Wolbachia/Cardinium infection than geographic distribution. These results strongly suggest that the endosymbionts affected mitochondrial variation of O. castaneae.

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

Bacterial endosymbionts are known as important players of the evolutionary ecology of their hosts. However, their distribution, prevalence and diversity are still largely unexplored. To this aim, we investigated infections by the most common bacterial reproductive manipulators in herbivorous spider mites of South-Western Europe. Across 16 populations belonging to three Tetranychus species, Wolbachia was the most prevalent (ca. 61%), followed by Cardinium (12%-15%), while only few individuals were infected by Rickettsia (0.9%-3%), and none carried Arsenophonus or Spiroplasma. These endosymbionts are here reported for the first time in Tetranychus evansi and Tetranychus ludeni, and showed variable infection frequencies between and within species, with several cases of coinfections. Moreover, Cardinium was more prevalent in Wolbachia-infected individuals, which suggests facilitation between these symbionts. Finally, sequence comparisons revealed no variation of the Wolbachia wsp and Rickettsia gtlA genes, but some diversity of the Cardinium 16S rRNA, both between and within populations of the three mite species. Some of the Cardinium sequences identified belonged to distantly-related clades, and the lack of association between these sequences and spider mite mitotypes suggests repeated host switching of Cardinium. Overall, our results reveal a complex community of symbionts in this system, opening the path for future studies.

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.

A Comparison of Wolbachia Infection Frequencies in Varroa With Prevalence of Deformed Wing Virus

Wolbachia are widely distributed bacterial endosymbionts of arthropods and filarial nematodes. These bacteria can affect host fitness in a variety of ways, such as protecting hosts against viruses and other pathogens. Here, we investigate the possible role of Wolbachia in the prevalence of the deformed wing virus (DWV), a highly virulent pathogen of honey bees (Apis mellifera) that is transmitted by parasitic Varroa mites (Varroa destructor). About 180 Varroa mites from 18 beehives were tested for infection with Wolbachia and DWV. We first screened for Wolbachia using two standard primers (wsp and 16S rDNA), and found 26% of the mites to be positive for Wolbachia using the wsp primer and 64% of the mites to be positive using the 16S rDNA primer. Using these intermediate Wolbachia frequencies, we then tested for statistical correlations with virus infection frequencies. The analysis revealed a significant positive correlation between DWV and Wolbachia using the wsp primer, but no significant association between DWV and Wolbachia using the 16S rDNA primer. In conclusion, there is no evidence for an anti-pathogenic effect of Wolbachia in V. destructor, but weak evidence for a pro-pathogenic effect. These results encourage further examination of Wolbachia-virus interactions in Varroa mites since an increased vector competence of the mites may significantly impact disease outbreaks in honey bees.

Geography has a greater effect than Wolbachia infection on population genetic structure in the spider mite, Tetranychus pueraricola

Wolbachia is an intracellular symbiotic bacterium that infects various spider mite species and is associated with alterations in host reproduction, which indicates the potential role in mite evolution. However, studies of Wolbachia infections in the spider mite Tetranychus pueraricola, a major agricultural pest, are limited. Here, we used multilocus sequence typing to determine Wolbachia infection status and examined the relationship between Wolbachia infection status and mitochondrial diversity in T. pueraricola from 12 populations in China. The prevalence of Wolbachia ranged from 2.8 to 50%, and three strains (wTpue1, wTpue2, and wTpue3) were identified. We also found double infections (wTpue1 + wTpue3) within the same individuals. Furthermore, the wTpue1 strain caused weak cytoplasmic incompatibility (CI) (egg hatchability ~55%), whereas another widespread strain, wTpue3, did not induce CI. There was no reduction in mitochondrial DNA (mtDNA) or nuclear DNA diversity among infected individuals, and mtDNA haplotypes did not correspond to specific Wolbachia strains. Phylogenetic analysis and analysis of molecular variance revealed that the distribution of mtDNA and nuclear DNA haplotypes were significantly associated with geography. These findings indicate that Wolbachia infection in T. pueraricola is complex, but T. pueraricola genetic differentiation likely resulted from substantial geographic isolation.

The genetic diversity of SMLS (Sitobion miscanthi L type symbiont) and its effect on the fitness, mitochondrial DNA diversity and Buchnera aphidicola dynamic of wheat aphid, Sitobion miscanthi (Hemiptera: Aphididae)

SMLS (Sitobion miscanthi L type symbiont) is a recently discovered aphid secondary symbiont. Using evidence extracted from 16S rRNA sequences, previous studies indicate that SMLS is the most widely distributed and most recently transferred secondary symbiont in Chinese Sitobion miscanthi populations. Here, we further investigated genetic diversity among SMLS geographic strains with multiloci data. Furthermore, the influence of SMLS on S. miscanthi was uncovered with ecological and evolutionary evidence. The results indicated that there was limited influence of infection with SMLS on variation and evolutionary patterns of S. miscanthi mitochondrial DNA. By hemolymph injection, the SMLS-infected and SMLS-uninfected S. miscanthi clones with the identical genetic background were built in this study. Although similar Buchnera aphidicola dynamics were observed between SMLS-infected and SMLS-uninfected S. miscanthi population, B. aphidicola density of SMLS-infected S. miscanthi population was always significantly higher than SMLS-uninfected ones. The results of fitness measurements indicated that under laboratory rearing conditions, transfection of SMLS could confer modest advantages to some fitness components of S. miscanthi, that is, total number of offspring, longevity, age of first reproduction and weight of adult. However, as SMLS is not strictly associated with S. miscanthi, further investigations are needed to uncover the mechanisms responsible for this inconceivable association.

Symbiosis in an overlooked microcosm: a systematic review of the bacterial flora of mites

A dataset of bacterial diversity found in mites was compiled from 193 publications (from 1964 to January 2015). A total of 143 mite species belonging to the 3 orders (Mesostigmata, Sarcoptiformes and Trombidiformes) were recorded and found to be associated with approximately 150 bacteria species (in 85 genera, 51 families, 25 orders and 7 phyla). From the literature, the intracellular symbiont Cardinium, the scrub typhus agent Orientia, and Wolbachia (the most prevalent symbiont of arthropods) were the dominant mite-associated bacteria, with approximately 30 mite species infected each. Moreover, a number of bacteria of medical and veterinary importance were also reported from mites, including species from the genera Rickettsia, Anaplasma, Bartonella, Francisella, Coxiella, Borrelia, Salmonella, Erysipelothrix and Serratia. Significant differences in bacterial infection patterns among mite taxa were identified. These data will not only be useful for raising awareness of the potential for mites to transmit disease, but also enable a deeper understanding of the relationship of symbionts with their arthropod hosts, and may facilitate the development of intervention tools for disease vector control. This review provides a comprehensive overview of mite-associated bacteria and is a valuable reference database for future research on mites of agricultural, veterinary and/or medical importance.

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.

Inconsistent genetic structure among members of a multitrophic system: did bruchid parasitoids (Horismenus spp.) escape the effects of bean domestication?

Anthropogenic range expansion and cultural practices have modified the distribution, abundance and genetic diversity of domesticated organisms, thereby altering multitrophic assemblages through space and time. The putative Mesoamerican domestication centre of the common bean, Phaseolus vulgaris L., in Mexico allows investigating the effects of plant domestication on the genetic structure of members of a multitrophic system. The aim of this study was to compare the evolutionary history of Horismenus parasitoids (Hymenoptera: Eulophidae) to those of their bruchid beetle hosts (Coleoptera: Bruchidae) and their domesticated host plant (P. vulgaris), in the context of traditional agriculture in Mexico. We analyzed the population genetic structure of four Horismenus species in Mexico using mitochondrial COI haplotype data. The two most abundant parasitoid species were Horismenus depressus and Horismenus missouriensis. Horismenus missouriensis were infected by Wolbachia endosymbionts and had little to no population differentiation (F(ST) = 0.06). We suspect the mitochondrial history of H. missouriensis to be blurred by Wolbachia, because differentiation among infected vs. non-infected individuals exists (F(ST) = 0.11). Populations of H. depressus were found to be highly differentiated (F(ST) = 0.34), but the genetic structuring could not be explained by tested spatial components. We then compared the genetic structure observed in this parasitoid species to previously published studies on bruchid beetles and their host plants. Despite extensive human-mediated migration and likely population homogenization of its two Acanthoscelides bruchid beetle hosts, H. depressus populations are structured like its host plant, by a recent dispersal from a diverse ancestral gene pool. Distinct evolutionary dynamics may explain inconsistent patterns among trophic levels. Parasitoids likely migrate from wild bean populations and are poorly adapted to bean storage conditions similar to their bruchid beetle hosts. Integrating several trophic levels to the study of evolutionary history has proven to be fruitful in detecting different ecological responses to human-mediated disturbances and host parasite interactions.

Male-killing Wolbachia and mitochondrial selective sweep in a migratory African insect

Background

Numerous recent studies have shown that resident symbiotic microorganisms of insects play a fundamental role in host ecology and evolution. The lepidopteran pest, African armyworm (Spodoptera exempta), is a highly migratory and destructive species found throughout sub-Saharan Africa, that can experience eruptive outbreaks within the space of a single generation, making predicting population dynamics and pest control forecasting extremely difficult. Three strains of Wolbachia have recently been identified infecting this species in populations sampled from Tanzania. In this study, we examined the interaction between Wolbachia pipiensis infections and the co-inherited marker, mtDNA, within populations of armyworm, as a means to investigate the population biology and evolutionary history of Wolbachia and its host.

Results

A Wolbachia-infected isofemale line was established in the laboratory. Phenotypic studies confirmed the strain wExe1 as a male-killer. Partial sequencing of the mitochondrial COI gene from 164 individual field-collected armyworm of known infection status revealed 17 different haplotypes. There was a strong association between Wolbachia infection status and mtDNA haplotype, with a single dominant haplotype, haplo1 (90.2% prevalence), harbouring the endosymbiont. All three Wolbachia strains were associated with this haplotype. This indicates that Wolbachia may be driving a selective sweep on armyworm haplotype diversity. Despite very strong biological and molecular evidence that the samples represent a single species (including from nuclear 28S gene markers), the 17 haplotypes did not fall into a monophyletic clade within the Spodoptera genus; with six haplotypes (2 each from 3 geographically separate populations) differing by >11% in their nucleotide sequence to the other eleven.

Conclusions

This study suggests that three strains of Wolbachia may be driving a selective sweep on armyworm haplotype diversity, and that based on COI sequence data, S. exempta is not a monophyletic group within the Spodoptera genus. This has clear implications for the use of mtDNA as neutral genetic markers in insects, and also demonstrates the impact of Wolbachia infections on host evolutionary genetics.

Population genetics of two asexually and sexually reproducing psocids species inferred by the analysis of mitochondrial and nuclear DNA sequences

Background

The psocids Liposcelis bostrychophila and L. entomophila (Psocoptera: Liposcelididae) are found throughout the world and are often associated with humans, food stores and habitations. These insects have developed high levels of resistance to various insecticides in grain storage systems. However, the population genetic structure and gene flow of psocids has not been well categorized, which is helpful to plan appropriate strategies for the control of these pests.

Methodology/Principal Findings

The two species were sampled from 15 localities in China and analyzed for polymorphisms at the mitochondrial DNA (Cytb) and ITS (ITS1-5.8S-ITS2) regions. In total, 177 individual L. bostrychophila and 272 individual L. entomophila were analysed. Both Cytb and ITS sequences showed high genetic diversity for the two species with haplotype diversities ranged from 0.154±0.126 to 1.000±0.045, and significant population differentiation (mean F_ST = 0.358 for L. bostrychophila; mean F_ST = 0.336 for L. entomophila) was also detected among populations investigated. A Mantel test indicated that for both species there was no evidence for isolation-by-distance (IBD). The neutrality test and mismatch distribution statistics revealed that the two species might have undergone population expansions in the past.

Conclusion

Both L. bostrychophila and L. entomophila displayed high genetic diversity and widespread population genetic differentiation within and between populations. The significant population differentiation detected for both psocids may be mainly due to other factors, such as genetic drift, inbreeding or control practices, and less by geographic distance since an IBD effect was not found.