Wolbachia modifies thermal preference in Drosophila melanogaster

Wolbachia pipientis modulates germline stem cells and gene expression associated with ubiquitination and histone lysine trimethylation to rescue fertility defects in Drosophila

Wolbachia pipientis are maternally transmitted endosymbiotic bacteria commonly found in arthropods and nematodes. These bacteria manipulate reproduction of the host to increase their transmission using mechanisms, such as cytoplasmic incompatibility, that favor infected female offspring. The underlying mechanisms of reproductive manipulation by W. pipientis remain unresolved. Interestingly, W. pipientis infection partially rescues female fertility in flies containing hypomorphic mutations of bag of marbles (bam) in Drosophila melanogaster, which plays a key role in germline stem cell daughter differentiation. Using RNA-seq, we find that W. pipientis infection in bam hypomorphic females results in differential expression of many of bam's genetic and physical interactors and enrichment of ubiquitination and histone lysine methylation genes. We find that W. pipientis also rescues the fertility and germline stem cell functions of a subset of these genes when knocked down with RNAi in a wild-type bam genotype. Our results show that W. pipientis interacts with ubiquitination and histone lysine methylation genes which could be integral to the mechanism by which W. pipientis modulates germline stem cell gene function.

Symbiosis of intracellular bacteria Wolbachia with insects: a hundred years of study summarized

Wolbachia pipientis is an α-proteobacterium, which is a widespread intracellular symbiont in a number of Arthropoda and some Nematoda species. With insects, W. pipientis forms a symbiont-host system characterized by very close interactions between its components. The mutual effects of Wolbachia on the host and the host on Wolbachia are important biotic factors for both components of this symbiotic system. Wolbachia is able to affect both host reproduction and somatic organ function. Due to its prevalence among insects and a wide variety of both negative (cytoplasmic incompatibility and androcide are among the most well-known examples) and positive (increasing resistance to biotic and abiotic factors, providing vitamins and metabolites) effects on the host organism, Wolbachia is of great interest for both entomologists and microbiologists. The diversity of host phenotypes induced by Wolbachia provides a broad choice of evolutionary strategies (such as reproductive parasitism or mutually beneficial symbiont-host relationships) that it utilizes. The influence of Wolbachia is to be considered in the design of any experiment conducted on insects. The application of sequencing technologies has led to new approaches being created to study the existing relationships within the Wolbachia-insect system, but interpretation of the data obtained is challenging. Nevertheless, the prospects for the use of the whole-genome analysis data to study Wolbachia-host coevolution are beyond doubt. Ongoing projects to introduce Wolbachia strains, which provide antiviral host defense, into insect populations to control the spread of RNA-viruses are actively pursued, which could result in saving many human lives. The aim of this brief review is to summarize the data collected by scientists over the past hundred years of Wolbachia studies and the current understanding of its genetic diversity and mechanisms of interaction with the host, including those based on transcriptome analysis.

Symbiotic microbial population composition of Apolygus lucorum under temperature and pesticide pressures

Insect population control using pesticides faces new challenges as global temperatures change. Symbiotic bacteria of insects play a key role in insect resistance to pesticides, and these symbiotic bacteria themselves are sensitive to the effects of temperature changes. Apolygus lucorum, a sucking pest, survives in a wide range of temperatures (15°C-35°C), and is presently controlled predominantly using the pesticide imidacloprid. Here, we investigated the effects of temperature and imidacloprid on A. lucorum microbial population composition using 16S rRNA sequencing. We found that the application of imidacloprid in high-temperature environments led to an increase in the species diversity of bacteria in the body of A. lucorum. High temperatures may disrupt the symbiotic relationship between certain bacteria and A. lucorum, such as Cedecea neteri. High temperatures led to a decrease in the abundance of Cedecea neteri. Agathobaculum butyriciproducens, Advenella migardefenensis, and Akkermansia muciniphila were very sensitive to temperature and were strongly affected by temperature changes. Microorganisms that were greatly affected by the concentration of imidacloprid in the community include Aeromonas caviae and Akkermansia muciniphila. The aim of this study is to reveal the dynamics and diversity of symbiotic bacteria of A. lucorum treated with imidacloprid at a range of temperatures. These results provide insight into new strategies for pest control in a changing climate.

Interaction and effects of temperature preference under a controlled environment on the diversity and abundance of the microbiome in Lutzomyia longipalpis (Diptera: Psychodidae)

Characterization of the temperature effects on the abundance and richness of the microbiota of Lutzomyia longipalpis, insect vector of Leishmania infantum in America, is an aspect of pivotal importance to understand the interactions between temperature, bacteria, and Leishmania infection. We developed and used a customized device with a temperature gradient (21-34 °C) to assess the temperature preferences of wild females of Lu. longipalpis collected in a rural area (Ricaurte, Cundinamarca, Colombia). Each replicate consisted of 50 females exposed to the gradient for an hour. At the end of the exposure time, insects were collected and separated by the temperature ranges selected varying from 21 °C to 34 °C. They were organized in 17 pools from which total DNA extracts were obtained, and samples were subjected to 16S rRNA amplicon sequencing analyzes. The most abundant phyla across the different temperature ranges were Proteobacteria (17.22-90.73 %), Firmicutes (5.99-77.21 %) and Actinobacteria (1.56-59.85 %). Results also showed an abundance (30 % to 57.36 %) of Pseudomonas (mainly at temperatures of 21-29 °C and 34 °C) that decreased to 6.55 %-13.20 % at temperatures of 31-33 °C, while Bacillus increase its abundance to 67.24 % at 29-33 °C. Serratia also had a greater representation (49.79 %), specifically in sand flies recovered at 25-27 °C. No significant differences were found at α-diversity level when comparing richness using the Shannon-Wiener, Simpson, and Chao1 indices, while β-diversity differences were found using the Bray-Curtis index (F-value of 3.5073, p-value < 0.013, R-squared of 0,4889), especially in the groups of Lu. longipalpis associated at higher temperatures (29-33 °C). It was also possible to detect the presence of endosymbionts such as Spiroplasma and Arsenophonus in the range of 29-33 °C. Rickettsia was only detected in Lu. longipalpis sand flies recovered between 25-27 °C. It was possible to characterize Lu. longipalpis microbiota in response to intraspecific temperature preferences and observe changes in bacterial communities and endosymbionts at different ranges of said environmental variable, which may be important in its vector competence and environmental plasticity to adapt to new climate change scenarios.

Impact of transinfection of Wolbachia from the planthopper Laodelphax striatellus on reproductive fitness and transcriptome of the whitefly Bemisia tabaci

The whitefly Bemisia tabaci is critical global pest threatening crops and leading to agricultural losses. Wolbachia is an intracellular symbiotic bacterium in insects, which can regulate the growth and development of the host through various ways. In a prior study, Wolbachia was found to be transferred to whitefly and induce fitness changes. However, little is known about the underlying mechanisms of host-Wolbachia interactions in B. tabaci. In this study, a Wolbachia strain wStri was isolated from the small brown planthopper, Laodelphex striatellus, and transferred to B. tabaci. The distribution of Wolbachia in whiteflies was determined using fluorescence in situ hybridization. Reciprocal crossing experiments demonstrated that wStri did not induce cytoplasmic incompatibility phenotypes in B. tabaci, but prolonged the developmental duration of the offspring. We performed transcriptomic analysis of Wolbachia-infected female and male adults using Illumina-based RNA-Seq. A total of 843 differentially expressed genes (DEGs) were identified in infected females, among them 141 were significantly up-regulated and 702 were down-regulated by Wolbachia infection. In infected males, of 511 gene sets, 279 host genes were significantly up-regulated, and 232 were down-regulated by Wolbachia infection. KEGG analysis of DEGs demonstrated significant differences in gene pathway distribution between up-regulated and down-regulated genes. These genes are involved in various biological processes, including, but not limited to, detoxification, oxidation-reduction, metabolic processes, and immunity. The transcriptomic profiling of this study offers valuable information on the differential expression of genes in whiteflies following Wolbachia infection, and enhances our understanding of this host-symbiotic interaction.

No Evidence for Wolbachia Effects on the Thermal Preference of the Invasive Pest Liriomyza huidobrensis

Heritable endosymbiont Wolbachia is prevalent among arthropods, serving multiple functions for their hosts. However, the role of Wolbachia in mediating thermal preference selection remains largely unexplored. In this study, we utilized a custom-built thermal gradient to evaluate the thermal preference (Tp) of 1367 individuals of the invasive leaf-miner Liriomyza huidobrensis with or without Wolbachia wLhui from Yunnan and Xinjiang populations. Under meticulously controlled conditions and with a vast sample size, we found no significant difference in the mean Tp between wLhui-infected and uninfected leaf miners from either population when host age and sex were not considered. Furthermore, generalized linear model (GLM) analysis revealed no significant correlation between average Tp and age, sex, or Wolbachia infection, nor interactions among these factors, except in the Xinjiang population, where Tp was strongly associated with host age. Finally, we discuss the ecological implications of these findings and propose future research directions on Wolbachia-mediated host Tp in the leaf miner. Overall, our findings do not provide evidence that Wolbachia significantly affects the thermal preference of L. huidobrensis. Further studies across different systems are needed to investigate the complex interactions between Wolbachia and insect thermal behavior.

Wolbachia modify host cell metabolite profiles in response to short-term temperature stress

Wolbachia are common heritable endosymbionts that influence many aspects of ecology and evolution in various insects, yet Wolbachia-mediated intracellular metabolic responses to temperature stress have been largely overlooked. Here, we introduced the Wolbachia strain wLhui from the invasive Liriomyza huidobrensis (Blanchard) into a Drosophila Schneider 2 cell line (S2) and investigated the metabolite profile of wLhui-infected (S2_wLhui) and uninfected cell lines (S2_wu) under short-term exposure to either high (37°C), moderate (27°C), or low (7 and 17°C) temperatures. We find that Wolbachia infection, temperature stress, and their interactions significantly affect cellular metabolic profiles. Most significantly, when comparing the changes in metabolites between S2_wLhui and S2_wu, glycerophospholipids, amino acids, and fatty acids associated with metabolic pathways, microbial metabolism in diverse environments, and other pathways were significantly accumulated at either low or high temperatures. Our findings suggest Wolbachia-induced cellular physiological responses to short-term temperature stress, which may in turn affect the fitness and adaptive ability of its host as an invasive species.

Non-ovarian Wolbachia pipientis titer correlates with fertility rescue of a Drosophila melanogaster bag of marbles hypomorph

Bag of marbles ( bam ) is an essential gene that regulates germline stem cell maintenance and germline stem cell daughter cell differentiation in Drosophila melanogaster . When bam is partially functional (hypomorphic), the introduction of Wolbachia pipientis rescues the mutant fertility phenotype that would otherwise result in partial sterility. Infection by different W. pipientis variants results in differential rescue of the bam hypomorph fertility phenotype. We were intrigued by the varying degrees of rescue exhibited in the bam hypomorph when exposed to different W. pipientis variants, prompting us to investigate whether this phenomenon is attributable to variations in the titers of W. pipientis variants. We found no significant difference in ovarian titer between two W. pipientis variant groups, w Mel-like (low bam hypomorph fertility rescue) and w MelCS-like variants (higher bam hypomorph fertility rescue), at bam hypomorph peak fertility. However, carcass (whole flies without the ovaries) titer between w Mel-like and w MelCS-like infected bam hypomorph differed during peak fertility rescue. A positive correlation emerged between the combined titers of ovarian and carcass infections and fertility, implying a more extensive influence that extends beyond ovarian infection alone.

Wolbachia infection negatively impacts Drosophila simulans heat tolerance in a strain- and trait-specific manner

The susceptibility of insects to rising temperatures has largely been measured by their ability to survive thermal extremes. However, the capacity for maternally inherited endosymbionts to influence insect heat tolerance has been overlooked. Further, while some studies have addressed the impact of heat on traits like fertility, which can decline at temperatures below lethal thermal limits, none have considered the impact of endosymbionts. Here, we assess the impact of three Wolbachia strains (wRi, wAu and wNo) on the survival and fertility of Drosophila simulans exposed to heat stress during development or as adults. The effect of Wolbachia infection on heat tolerance was generally small and trait/strain specific. Only the wNo infection significantly reduced the survival of adult males after a heat shock. When exposed to fluctuating heat stress during development, the wRi and wAu strains reduced egg-to-adult survival but only the wNo infection reduced male fertility. Wolbachia densities of all three strains decreased under developmental heat stress, but reductions occurred at temperatures above those that reduced host fertility. These findings emphasize the necessity to account for endosymbionts and their effect on both survival and fertility when investigating insect responses to heat stress.

Wolbachia effects on thermal preference of natural Drosophila melanogaster are influenced by host genetic background, Wolbachia type, and bacterial titer

Temperature plays a fundamental role in the fitness of all organisms. In particular, it strongly affects metabolism and reproduction in ectotherms that have limited physiological capabilities to regulate their body temperature. The influence of temperature variation on the physiology and behaviour of ectotherms is well studied but we still know little about the influence of symbiotic interactions on thermal preference (Tp ) of the host. A growing number of studies focusing on the Wolbachia-Drosophila host-symbiont system found that Wolbachia can influence Tp in Drosophila laboratory strains. Here, we investigated the effect of Wolbachia on Tp in wild-type D. melanogaster flies recently collected from nature. Consistent with previous data, we found reduced Tp compared to an uninfected control in one of two fly strains infected with the wMelCS Wolbachia type. Additionally, we, for the first time, found that Wolbachia titer variation influences the thermal preference of the host fly. These data indicate that the interaction of Wolbachia and Drosophila resulting in behavioural variation is strongly influenced by the genetic background of the host and symbiont. More studies are needed to better understand the evolutionary significance of Tp variation influenced by Wolbachia in natural Drosophila populations.

The effects of temperature on prevalence of facultative insect heritable symbionts across spatial and seasonal scales

Facultative inheritable endosymbionts are common and diverse in insects and are often found at intermediate frequencies in insect host populations. The literature assessing the relationship between environment and facultative endosymbiont frequency in natural host populations points to temperature as a major component shaping the interaction. However, a synthesis describing its patterns and mechanistic basis is lacking. This mini-review aims to bridge this gap by, following an evolutionary model, hypothesizing that temperature increases endosymbiont frequencies by modulating key phenotypes mediating the interaction. Field studies mainly present positive correlations between temperature and endosymbiont frequency at spatial and seasonal scales; and unexpectedly, temperature is predominantly negatively correlated with the key phenotypes. Higher temperatures generally reduce the efficiency of maternal transmission, reproductive parasitism, endosymbiont influence on host fitness and the ability to protect against natural enemies. From the endosymbiont perspective alone, higher temperatures reduce titer and both high and low temperatures modulate their ability to promote host physiological acclimation and behavior. It is necessary to promote research programs that integrate field and laboratory approaches to pinpoint which processes are responsible for the temperature correlated patterns of endosymbiont prevalence in natural populations.

Till evolution do us part: The diversity of symbiotic associations across populations of Philaenus spittlebugs

Symbiotic bacteria have played crucial roles in the evolution of sap-feeding insects and can strongly affect host function. However, their diversity and distribution within species are not well understood; we do not know to what extent environmental factors or associations with other species may affect microbial community profiles. We addressed this question in Philaenus spittlebugs by surveying both insect and bacterial marker gene amplicons across multiple host populations. Host mitochondrial sequence data confirmed morphology-based identification of six species and revealed two divergent clades of Philaenus spumarius. All of them hosted the primary symbiont Sulcia that was almost always accompanied by Sodalis. Interestingly, populations and individuals often differed in the presence of Sodalis sequence variants, suggestive of intra-genome 16S rRNA variant polymorphism combined with rapid genome evolution and/or recent additional infections or replacements of the co-primary symbiont. The prevalence of facultative endosymbionts, including Wolbachia, Rickettsia, and Spiroplasma, varied among populations. Notably, cytochrome I oxidase (COI) amplicon data also showed that nearly a quarter of P. spumarius were infected by parasitoid flies (Verralia aucta). One of the Wolbachia operational taxonomic units (OTUs) was exclusively present in Verralia-parasitized specimens, suggestive of parasitoids as their source and highlighting the utility of host gene amplicon sequencing in microbiome studies.

Wolbachia has subtle effects on thermal preference in highly inbred Drosophila melanogaster which vary with life stage and environmental conditions

Temperature fluctuations are challenging for ectotherms which are not able to regulate body temperature by physiological means and thus have to adjust their thermal environment via behavior. However, little is yet known about whether microbial symbionts influence thermal preference (Tp) in ectotherms by modulating their physiology. Several recent studies have demonstrated substantial effects of Wolbachia infections on host Tp in different Drosophila species. These data indicate that the direction and strength of thermal preference variation is strongly dependent on host and symbiont genotypes and highly variable among studies. By employing highly controlled experiments, we investigated the impact of several environmental factors including humidity, food quality, light exposure, and experimental setup that may influence Tp measurements in adult Drosophila melanogaster flies. Additionally, we assessed the effects of Wolbachia infection on Tp of Drosophila at different developmental stages, which has not been done before. We find only subtle effects of Wolbachia on host Tp which are strongly affected by experimental variation in adult, but not during juvenile life stages. Our in-depth analyses show that environmental variation has a substantial influence on Tp which demonstrates the necessity of careful experimental design and cautious interpretations of Tp measurements together with a thorough description of the methods and equipment used to conduct behavioral studies.

wMel replacement of dengue-competent mosquitoes is robust to near-term change

Rising temperatures are impacting the range and prevalence of mosquito-borne diseases. A promising biocontrol technology replaces wild mosquitoes with those carrying the virus-blocking Wolbachia bacterium. Because the most widely used strain, wMel, is adversely affected by heat stress, we examined how global warming may influence wMel-based replacement. We simulated interventions in two locations with successful field trials using Coupled Model Intercomparison Project Phase 5 climate projections and historical temperature records, integrating empirical data on wMel's thermal sensitivity into a model of Aedes aegypti population dynamics to evaluate introgression and persistence over one year. We show that in Cairns, Australia, climatic futures necessitate operational adaptations for heatwaves exceeding two weeks. In Nha Trang, Vietnam, projected heatwaves of three weeks and longer eliminate wMel under the most stringent assumptions of that symbiont's thermal limits. We conclude that this technology is generally robust to near-term (2030s) climate change. Accelerated warming may challenge this in the 2050s and beyond.

Evolutionary genetics of the mitochondrial genome: insights from Drosophila

Mitochondria are key to energy conversion in virtually all eukaryotes. Intriguingly, despite billions of years of evolution inside the eukaryote, mitochondria have retained their own small set of genes involved in the regulation of oxidative phosphorylation (OXPHOS) and protein translation. Although there was a long-standing assumption that the genetic variation found within the mitochondria would be selectively neutral, research over the past 3 decades has challenged this assumption. This research has provided novel insight into the genetic and evolutionary forces that shape mitochondrial evolution and broader implications for evolutionary ecological processes. Many of the seminal studies in this field, from the inception of the research field to current studies, have been conducted using Drosophila flies, thus establishing the species as a model system for studies in mitochondrial evolutionary biology. In this review, we comprehensively review these studies, from those focusing on genetic processes shaping evolution within the mitochondrial genome, to those examining the evolutionary implications of interactions between genes spanning mitochondrial and nuclear genomes, and to those investigating the dynamics of mitochondrial heteroplasmy. We synthesize the contribution of these studies to shaping our understanding of the evolutionary and ecological implications of mitochondrial genetic variation.

Response to thermal and infection stresses in an American vector of visceral leishmaniasis

Lutzomyia longipalpis is known as one of the primary insect vectors of visceral leishmaniasis. For such ectothermic organisms, the ambient temperature is a critical life factor. However, the impact of temperature has been ignored in many induced-stress situations of the vector life. Therefore, this study explored the interaction of Lu. longipalpis with temperature by evaluating its behaviour across a thermal gradient, thermographic recordings during blood-feeding on mice, and the gene expression of heat shock proteins (HSP) when insects were exposed to extreme temperature or infected. The results showed that 72 h after blood ingestion, Lu. longipalpis became less active and preferred relatively low temperatures. However, at later stages of blood digestion, females increased their activity and remained at higher temperatures. Real-time imaging showed that the body temperature of females can adjust rapidly to the host and remain constant until the end of blood-feeding. Insects also increased the expression of HSP90(83) during blood-feeding. Our findings suggest that Lu. longipalpis interacts with temperature by using its behaviour to avoid temperature-induced physiological damage during the gonotrophic cycle. However, the expression of certain HSP might be triggered to mitigate thermal stress in situations where a behavioural response is not the best option.

Food provisioning to Pardosa spiders decreases the levels of tissue-resident endosymbiotic bacteria

The diversity, host specificity, and physiological effects of endosymbiotic bacteria in spiders (Araneae) are poorly characterized. We used 16S rDNA sequencing to evaluate endosymbionts in the cephalothorax and legs of a wolf spider Pardosa agrestis. We tested the effects of feeding once or twice daily with fruit flies, aphids, or starved and compared them to those of syntopically occurring Pardosa palustris. The feeding increased traveled distance up to five times in some of the groups provisioned with food relative to the starved control. The Shannon diversity t-test revealed significant differences between these component communities of the two spider species. The increased frequency of feeding with fruit flies, but not aphids, increased the dominance and decreased the alpha diversity of OTUs. The obligate or facultative endosymbionts were present in all analyzed spider individuals and were represented mostly by Rickettsiella, Rhabdochlamydia, Spiroplasma, and the facultative intracellular parasite Legionella. Vertically transmitted endosymbionts were less common, represented by Wolbachia pipientis and Rickettsia sp. H820. The relative abundance of Mycoplasma spp. was negatively correlated with provisioned or killed aphids. In conclusion, the tissues of Pardosa spiders host tremendously diverse assemblages of bacteria, including obligate or facultative endosymbionts, with yet unknown phenotypic effects.

Wolbachia endosymbiotic bacteria alter the gut microbiome in the fly Drosophila nigrosparsa

Wolbachia are known to cause reproductive manipulations and in some arthropod species, Wolbachia were reported to cause changes in gut microbiome. However, the effects of Wolbachia bacteria on the microbiomes of their hosts, including Drosophila flies, have not been fully accessed. Here, we checked the bacterial microbiome in guts of Wolbachia-uninfected and of Wolbachia-infected Drosophila nigrosparsa, both separated into a bleach-only (embryos bleached) and a gnotobiotic (embryos bleached and inoculated with bacteria) treatment. We observed a clear separation between the Wolbachia-infected and the Wolbachia-uninfected samples, and the infected samples had higher variation in alpha diversity than the uninfected ones. There were reductions in the abundances of Proteobacteria (Pseudomonadota), especially Acetobacter, in the infected samples of both treatments. These findings highlight that Wolbachia change the gut microbiome in D. nigrosparsa as well as that the interactions between Wolbachia and bacteria like Acetobacter need to be investigated.

Effects of Wolbachia on Transposable Element Expression Vary Between Drosophila melanogaster Host Genotypes

Transposable elements (TEs) are repetitive DNA sequences capable of changing position in host genomes, thereby causing mutations. TE insertions typically have deleterious effects but they can also be beneficial. Increasing evidence of the contribution of TEs to adaptive evolution further raises interest in understanding what factors impact TE activity. Based on previous studies associating the bacterial endosymbiont Wolbachia with changes in the abundance of piRNAs, a mechanism for TE repression, and to transposition of specific TEs, we hypothesized that Wolbachia infection would interfere with TE activity. We tested this hypothesis by studying the expression of 14 TEs in a panel of 25 Drosophila melanogaster host genotypes, naturally infected with Wolbachia and annotated for TE insertions. The host genotypes differed significantly in Wolbachia titers inside individual flies, with broad-sense heritability around 20%, and in the number of TE insertions, which depended greatly on TE identity. By removing Wolbachia from the target host genotypes, we generated a panel of 25 pairs of Wolbachia-positive and Wolbachia-negative lines in which we quantified transcription levels for our target TEs. We found variation in TE expression that was dependent on Wolbachia status, TE identity, and host genotype. Comparing between pairs of Wolbachia-positive and Wolbachia-negative flies, we found that Wolbachia removal affected TE expression in 21.1% of the TE-genotype combinations tested, with up to 2.3 times differences in the median level of transcript. Our data show that Wolbachia can impact TE activity in host genomes, underscoring the importance this endosymbiont can have in the generation of genetic novelty in hosts.

Transcription dynamics of heat-shock proteins (Hsps) and endosymbiont titres in response to thermal stress in whitefly, Bemisia tabaci (Asia-I)

The sweet potato whitefly, Bemisia tabaci (Gennadius), is one of the several species complexes of whitefly that are currently significant agricultural pests. Bemisia tabaci infests more than 600 plant species and thrives under a wide range of temperature conditions. In addition to the direct damage caused by sucking plant sap, it vectors several plant viruses. Heat-shock proteins play a pivotal role in enabling the insect to extend its geographical location, survival, and reproduction under different stress conditions. B. tabaci harbours several endosymbionts under the genera Portiera, Rickettsia, Hamiltonella, Wolbachia, Arsenophonus, Cardinium, and Fritschea that directly or indirectly affect its fitness. By accelerating cuticle biosynthesis and sclerotisation, symbiotic microbes can reduce or enhance tolerance to extreme temperatures and detoxify heavy metals. Thus, symbionts or microbial communities can expand or constrain the abiotic niche space of their host and affect its ability to adapt to changing conditions. The present study delineates the effect of thermal stress on the expression of heat-shock genes and endosymbionts in B. tabaci. Studies of the expression level of heat-shock proteins with the help of quantitative real-time polymerase chain reaction (qRT-PCR) showed that heat- and cold-shock treatment fuels the increased expression of heat-shock proteins (Hsp40 and Hsp70). However, Hsp90 was not induced by a heat- and cold-shock treatment. A significant decrease in the relative titre of secondary endosymbionts, such as Rickettsia, Arsenophonus, and Wolbachia, were recorded in B. tabaci upon heat treatment. However, the titre of the primary symbiont, C. Portiera, was relatively unaffected by both cold and heat treatments. These results are indicative of the fact that Hsp genes and endosymbionts in B. tabaci are modulated in response to thermal stress, and this might be responsible for the adaptation of whitefly under changing climatic scenario.

Plasticity in Chemical Host Plant Recognition in Herbivorous Insects and Its Implication for Pest Control

Chemical communication is very important in herbivorous insects, with many species being important agricultural pests. They often use olfactory cues to find their host plants at a distance and evaluate their suitability upon contact with non-volatile cues. Responses to such cues are modulated through interactions between various stimuli of biotic and abiotic origin. In addition, the response to the same stimulus can vary as a function of, for example, previous experience, age, mating state, sex, and morph. Here we summarize recent advances in the understanding of plant localization and recognition in herbivorous insects with a focus on the interplay between long- and short-range signals in a complex environment. We then describe recent findings illustrating different types of plasticity in insect plant choice behavior and the underlying neuronal mechanisms at different levels of the chemosensory pathway. In the context of strong efforts to replace synthetic insecticides with alternative pest control methods, understanding combined effects between long- and close-range chemical cues in herbivore-plant interactions and their complex environment in host choice are crucial to develop effective plant protection methods. Furthermore, plasticity of behavioral and neuronal responses to chemical cues needs to be taken into account to develop effective sustainable pest insect control through behavioral manipulation.

Rare Wolbachia genotypes in laboratory Drosophila melanogaster strains

Symbiotic bacteria of the genus Wolbachia are widespread in Drosophila melanogaster populations. Based on the polymorphism of the Wolbachia genome, the symbionts’ diversity in D. melanogaster is presented by two groups: MEL (wMel, wMel2, wMel3 and wMel4) and CS (wMelCS and wMelCS2). The wMel genotype is predominant in natural D. melanogaster populations and is distributed all over the world. The CS genotypes, on the other hand, are of particular interest because it is unclear how they are maintained in the fruit f ly populations since they should have been eliminated from them due to their low frequency and genetic drift or been replaced by the wMel genotype. However, this is not what is really observed, which means these genotypes are supported by selection. It is known that the wMelPlus strain of the wMelCS genotype can increase the lifespan of infected f lies at high temperatures. The same genotype also increases the intensity of dopamine metabolism in Drosophila compared to the MEL-group genotypes. In the present study, we searched for the rare Wolbachia wMelCS and wMelCS2 genotypes, as well as for new genotypes in wild-type D. melanogaster strains and in several mutant laboratory strains. The symbiont was found in all populations, in 200 out of 385 wild-type strains and in 83 out of 170 mutant strains. Wolbachia diversity in D. melanogaster wild-type strains was represented by the wMel, wMelCS and wMelCS2 genotypes. More than 90 % of the infected strains carried wMel; 9 %, wMelCS2; and only two strains were found to carry wMelCS. No new Wolbachia genotypes were found. The northernmost point reported for the wMelCS2 genotype was Izhevsk city (Udmurtia, Russia). For the f irst time the wMelCS2 genotype was detected in D. melanogaster from the Sakhalin Island, and wMelCS, in the f lies from Nalchik (the North Caucasus). A comparison of Wolbachia genetic diversity between the wild-type laboratory strains and previously obtained data on mutant laboratory strains demonstrated differences in the frequencies of rare CS genotypes, which were more prevalent in mutant strains, apparently due to the breeding history of these Drosophila strains

Complex effects of environment and Wolbachia infections on the life history of Drosophila melanogaster hosts

Wolbachia bacteria are common endosymbionts of many arthropods found in gonads and various somatic tissues. They manipulate host reproduction to enhance their transmission and confer complex effects on fitness-related traits. Some of these effects can serve to increase the survival and transmission efficiency of Wolbachia in the host population. The Wolbachia-Drosophila melanogaster system represents a powerful model to study the evolutionary dynamics of host-microbe interactions and infections. Over the past decades, there has been a replacement of the ancestral wMelCS Wolbachia variant by the more recent wMel variant in worldwide D. melanogaster populations, but the reasons remain unknown. To investigate how environmental change and genetic variation of the symbiont affect host developmental and adult life-history traits, we compared effects of both Wolbachia variants and uninfected controls in wild-caught D. melanogaster strains at three developmental temperatures. While Wolbachia did not influence any developmental life-history traits, we found that both lifespan and fecundity of host females were increased without apparent fitness trade-offs. Interestingly, wMelCS-infected flies were more fecund than uninfected and wMel-infected flies. By contrast, males infected with wMel died sooner, indicating sex-specific effects of infection that are specific to the Wolbachia variant. Our study uncovered complex temperature-specific effects of Wolbachia infections, which suggests that symbiont-host interactions in nature are strongly dependent on the genotypes of both partners and the thermal environment.

Impact of heat stress on the fitness outcomes of symbiotic infection in aphids: a meta-analysis

Beneficial microorganisms shape the evolutionary trajectories of their hosts, facilitating or constraining the colonization of new ecological niches. One convincing example entails the responses of insect-microbe associations to rising temperatures. Indeed, insect resilience to stressful high temperatures depends on the genetic identity of the obligate symbiont and the presence of heat-protective facultative symbionts. As extensively studied organisms, aphids and their endosymbiotic bacteria represent valuable models to address eco-evolutionary questions about the thermal ecology of insect-microbe partnerships, with broad relevance to various biological systems and insect models. This meta-analysis aims to quantify the context-dependent impacts of symbionts on host phenotype in benign or stressful heat conditions, across fitness traits, types of heat stress and symbiont species. We found that warming lowered the benefits (resistance to parasitoids) and costs (development, fecundity) of infection by facultative symbionts, which was overall mostly beneficial to the hosts under short-term heat stress (heat shock) rather than extended warming. Heat-tolerant genotypes of the obligate symbiont Buchnera aphidicola and some facultative symbionts (Rickettsia sp., Serratia symbiotica) improved or maintained aphid fitness under heat stress. We discuss the implications of these findings for the general understanding of the cost-benefit balance of insect-microbe associations across multiple traits and their eco-evolutionary dynamics faced with climate change.

A wMel Wolbachia variant in Aedes aegypti from field-collected Drosophila melanogaster with increased phenotypic stability under heat stress

Mosquito‐borne diseases remain a major cause of morbidity and mortality. Population replacement strategies involving the wMel strain of Wolbachia are being used widely to control mosquito‐borne diseases. However, these strategies may be influenced by temperature because wMel is vulnerable to heat. wMel infections in Drosophila melanogaster are genetically diverse, but few transinfections of wMel variants have been generated in Aedes aegypti. Here, we successfully transferred a wMel variant (termed wMelM) originating from a field‐collected D. melanogaster into Ae. aegypti. The new wMelM variant (clade I) is genetically distinct from the original wMel transinfection (clade III), and there are no genomic differences between wMelM in its original and transinfected host. We compared wMelM with wMel in its effects on host fitness, temperature tolerance, Wolbachia density, vector competence, cytoplasmic incompatibility and maternal transmission under heat stress in a controlled background. wMelM showed a higher heat tolerance than wMel, likely due to higher overall densities within the mosquito. Both wMel variants had minimal host fitness costs, complete cytoplasmic incompatibility and maternal transmission, and dengue virus blocking under laboratory conditions. Our results highlight phenotypic differences between Wolbachia variants and wMelM shows potential as an alternative strain in areas with strong seasonal temperature fluctuations.

Diet-induced changes in titer support a discrete response of Wolbachia-associated plastic recombination in Drosophila melanogaster

Plastic recombination in Drosophila melanogaster has been associated with a variety of extrinsic and intrinsic factors such as temperature, starvation, and parasite infection. The bacterial endosymbiont Wolbachia pipientis has also been associated with plastic recombination in D. melanogaster. Wolbachia infection is pervasive in arthropods and this infection induces a variety of phenotypes in its hosts, the strength of which can depend on bacterial titer. Here, we test the hypothesis that the magnitude of Wolbachia-associated plastic recombination in D. melanogaster depends on titer. To manipulate titer, we raised Wolbachia-infected and uninfected flies on diets that have previously been shown to increase or decrease Wolbachia titer relative to controls. We measured recombination in treated and control individuals using a standard backcrossing scheme with two X-linked visible markers. Our results recapitulate previous findings that Wolbachia infection is associated with increased recombination rate across the yellow-vermillion interval of the X chromosome. Our data show no significant effect of diet or diet by Wolbachia interactions on recombination, suggesting that diet-induced changes in Wolbachia titer have no effect on the magnitude of plastic recombination. These findings represent one of the first steps toward investigating Wolbachia-associated plastic recombination and demonstrate that the phenotype is a discrete response rather than a continuous one.

Behavior Individuality: A Focus on Drosophila melanogaster

Among individuals, behavioral differences result from the well-known interplay of nature and nurture. Minute differences in the genetic code can lead to differential gene expression and function, dramatically affecting developmental processes and adult behavior. Environmental factors, epigenetic modifications, and gene expression and function are responsible for generating stochastic behaviors. In the last decade, the advent of high-throughput sequencing has facilitated studying the genetic basis of behavior and individuality. We can now study the genomes of multiple individuals and infer which genetic variations might be responsible for the observed behavior. In addition, the development of high-throughput behavioral paradigms, where multiple isogenic animals can be analyzed in various environmental conditions, has again facilitated the study of the influence of genetic and environmental variations in animal personality. Mainly, Drosophila melanogaster has been the focus of a great effort to understand how inter-individual behavioral differences emerge. The possibility of using large numbers of animals, isogenic populations, and the possibility of modifying neuronal function has made it an ideal model to search for the origins of individuality. In the present review, we will focus on the recent findings that try to shed light on the emergence of individuality with a particular interest in D. melanogaster.

A perspective on insect-microbe holobionts facing thermal fluctuations in a climate-change context

Temperature influences the ecology and evolution of insects and their symbionts by impacting each partner independently and their interactions, considering the holobiont as a primary unit of selection. There are sound data about the responses of these partnerships to constant temperatures and sporadic thermal stress (mostly heat shock). However, the current understanding of the thermal ecology of insect-microbe holobionts remains patchy because the complex thermal fluctuations (at different spatial and temporal scales) experienced by these organisms in nature have often been overlooked experimentally. This may drastically constrain our ability to predict the fate of mutualistic interactions under climate change, which will alter both mean temperatures and thermal variability. Here, we tackle down these issues by focusing on the effects of temperature fluctuations on the evolutionary ecology of insect-microbe holobionts. We propose potentially worth-investigating research avenues to (i) evaluate the relevance of theoretical concepts used to predict the biological impacts of temperature fluctuations when applied to holobionts; (ii) acknowledge the plastic (behavioural thermoregulation, physiological acclimation) and genetic responses (evolution) expressed by holobionts in fluctuating thermal environments; and (iii) explore the potential impacts of previously unconsidered patterns of temperature fluctuations on the outcomes and the dynamic of these insect-microbe associations.

Wolbachia-Conferred Antiviral Protection Is Determined by Developmental Temperature

Wolbachia is a maternally transmitted bacterium that is widespread in arthropods and filarial nematodes and confers strong antiviral protection in Drosophila melanogaster and other arthropods. Wolbachia-transinfected Aedes aegypti mosquitoes are currently being deployed to fight transmission of dengue and Zika viruses. However, the mechanism of antiviral protection and the factors influencing are still not fully understood. Here, we show that temperature modulates Wolbachia-conferred protection in Drosophila melanogaster. Temperature after infection directly impacts Drosophila C virus (DCV) replication and modulates Wolbachia protection. At higher temperatures, viruses proliferate more and are more lethal, while Wolbachia confers lower protection. Strikingly, host developmental temperature is a determinant of Wolbachia-conferred antiviral protection. While there is strong protection when flies develop from egg to adult at 25°C, the protection is highly reduced or abolished when flies develop at 18°C. However, Wolbachia-induced changes during development are not sufficient to limit virus-induced mortality, as Wolbachia is still required to be present in adults at the time of infection. This developmental effect is general, since it was present in different host genotypes, Wolbachia variants, and upon infection with different viruses. Overall, we show that Wolbachia-conferred antiviral protection is temperature dependent, being present or absent depending on the environmental conditions. This interaction likely impacts Wolbachia-host interactions in nature and, as a result, frequencies of host and symbionts in different climates. Dependence of Wolbachia-mediated pathogen blocking on developmental temperature could be used to dissect the mechanistic bases of protection and influence the deployment of Wolbachia to prevent transmission of arboviruses. IMPORTANCE Insects are often infected with beneficial intracellular bacteria. The bacterium Wolbachia is extremely common in insects and can protect them from pathogenic viruses. This effect is being used to prevent transmission of dengue and Zika viruses by Wolbachia-infected mosquitoes. To understand the biology of insects in the wild, we need to discover which factors affect Wolbachia-conferred antiviral protection. Here, we show that the temperature at which insects develop from eggs to adults can determine the presence or absence of antiviral protection. The environment, therefore, strongly influences this insect-bacterium interaction. Our work may help to provide insights into the mechanism of viral blocking by Wolbachia, deepen our understanding of the geographical distribution of host and symbiont, and incentivize further research on the temperature dependence of Wolbachia-conferred protection for control of mosquito-borne disease.

Diverse wMel variants of Wolbachia pipientis differentially rescue fertility and cytological defects of the bag of marbles partial loss of function mutation in Drosophila melanogaster

In Drosophila melanogaster, the maternally inherited endosymbiont Wolbachia pipientis interacts with germline stem cell genes during oogenesis. One such gene, bag of marbles (bam) is the key switch for differentiation and also shows signals of adaptive evolution for protein diversification. These observations have led us to hypothesize that W. pipientis could be driving the adaptive evolution of bam for control of oogenesis. To test this hypothesis, we must understand the specificity of the genetic interaction between bam and W. pipientis. Previously, we documented that the W. pipientis variant, wMel, rescued the fertility of the bamBW hypomorphic mutant as a transheterozygote over a bam null. However, bamBW was generated more than 20 years ago in an uncontrolled genetic background and maintained over a balancer chromosome. Consequently, the chromosome carrying bamBW accumulated mutations that have prevented controlled experiments to further assess the interaction. Here, we used CRISPR/Cas9 to engineer the same single amino acid bam hypomorphic mutation (bamL255F) and a new bam null disruption mutation into the w1118 isogenic background. We assess the fertility of wildtype bam, bamL255F/bamnull hypomorphic, and bamL255F/bamL255F mutant females, each infected individually with 10 W. pipientis wMel variants representing three phylogenetic clades. Overall, we find that all of the W. pipientis variants tested here rescue bam hypomorphic fertility defects with wMelCS-like variants exhibiting the strongest rescue effects. In addition, these variants did not increase wildtype bam female fertility. Therefore, both bam and W. pipientis interact in genotype-specific ways to modulate female fertility, a critical fitness phenotype.

Wolbachia as translational science: controlling mosquito-borne pathogens

In this review we examine how exploiting the Wolbachia-mosquito relationship has become an increasingly popular strategy for controlling arbovirus transmission. Field deployments of Wolbachia-infected mosquitoes have led to significant decreases in dengue virus incidence via high levels of mosquito population suppression and replacement, emphasizing the success of Wolbachia approaches. Here, we examine how improved knowledge of Wolbachia-host interactions has provided key insight into the mechanisms of the essential phenotypes of pathogen blocking and cytoplasmic incompatibility. And we discuss recent studies demonstrating that extrinsic factors, such as ambient temperature, can modulate Wolbachia density and maternal transmission. Finally, we assess the prospects of using Wolbachia to control other vectors and agricultural pest species.

Spider Mites Singly Infected With Either Wolbachia or Spiroplasma Have Reduced Thermal Tolerance

Heritable symbionts play an essential role in many aspects of host ecology in a temperature-dependent manner. However, how temperature impacts the host and their interaction with endosymbionts remains largely unknown. Here, we investigated the impact of moderate (20°C) and high (30 and 35°C) temperatures on symbioses between the spider mite Tetranychus truncatus and two maternally inherited endosymbionts (Wolbachia and Spiroplasma). We found that the thermal tolerance of mites (as measured by survival after heat exposure) was lower for mites that were singly infected with either Wolbachia or Spiroplasma than it was for co-infected or uninfected mites. Although a relatively high temperature (30°C) is thought to promote bacterial replication, rearing at high temperature (35°C) resulted in losses of Wolbachia and particularly Spiroplasma. Exposing the mites to 20°C reduced the density and transmission of Spiroplasma but not Wolbachia. The four spider mite strains tested differed in the numbers of heat shock genes (Hsps) induced under moderate or high temperature exposure. In thermal preference (Tp) assays, the two Wolbachia-infected spider mite strains preferred a lower temperature than strains without Wolbachia. Our results show that endosymbiont-mediated spider mite responses to temperature stress are complex, involving a combination of changing endosymbiont infection patterns, altered thermoregulatory behavior, and transcription responses.

Microbes increase thermal sensitivity in the mosquito Aedes aegypti, with the potential to change disease distributions

The mosquito Aedes aegypti is the primary vector of many disease-causing viruses, including dengue (DENV), Zika, chikungunya, and yellow fever. As consequences of climate change, we expect an increase in both global mean temperatures and extreme climatic events. When temperatures fluctuate, mosquito vectors will be increasingly exposed to temperatures beyond their upper thermal limits. Here, we examine how DENV infection alters Ae. aegypti thermotolerance by using a high-throughput physiological 'knockdown' assay modeled on studies in Drosophila. Such laboratory measures of thermal tolerance have previously been shown to accurately predict an insect's distribution in the field. We show that DENV infection increases thermal sensitivity, an effect that may ultimately limit the geographic range of the virus. We also show that the endosymbiotic bacterium Wolbachia pipientis, which is currently being released globally as a biological control agent, has a similar impact on thermal sensitivity in Ae. aegypti. Surprisingly, in the coinfected state, Wolbachia did not provide protection against DENV-associated effects on thermal tolerance, nor were the effects of the two infections additive. The latter suggests that the microbes may act by similar means, potentially through activation of shared immune pathways or energetic tradeoffs. Models predicting future ranges of both virus transmission and Wolbachia's efficacy following field release may wish to consider the effects these microbes have on host survival.

Sickness behaviors across vertebrate taxa: proximate and ultimate mechanisms

There is nothing like a pandemic to get the world thinking about how infectious diseases affect individual behavior. In this respect, sick animals can behave in ways that are dramatically different from healthy animals: altered social interactions and changes to patterns of eating and drinking are all hallmarks of sickness. As a result, behavioral changes associated with inflammatory responses (i.e. sickness behaviors) have important implications for disease spread by affecting contacts with others and with common resources, including water and/or sleeping sites. In this Review, we summarize the behavioral modifications, including changes to thermoregulatory behaviors, known to occur in vertebrates during infection, with an emphasis on non-mammalian taxa, which have historically received less attention. We then outline and discuss our current understanding of the changes in physiology associated with the production of these behaviors and highlight areas where more research is needed, including an exploration of individual and sex differences in the acute phase response and a greater understanding of the ecophysiological implications of sickness behaviors for disease at the population level.

Pervasive effects of Wolbachia on host activity

Heritable symbionts have diverse effects on the physiology, reproduction and fitness of their hosts. Maternally transmitted Wolbachia are one of the most common endosymbionts in nature, infecting about half of all insect species. We test the hypothesis that Wolbachia alter host behaviour by assessing the effects of 14 different Wolbachia strains on the locomotor activity of nine Drosophila host species. We find that Wolbachia alter the activity of six different host genotypes, including all hosts in our assay infected with wRi-like Wolbachia strains (wRi, wSuz and wAur), which have rapidly spread among Drosophila species in about the last 14 000 years. While Wolbachia effects on host activity were common, the direction of these effects varied unpredictably and sometimes depended on host sex. We hypothesize that the prominent effects of wRi-like Wolbachia may be explained by patterns of Wolbachia titre and localization within host somatic tissues, particularly in the central nervous system. Our findings support the view that Wolbachia have wide-ranging effects on host behaviour. The fitness consequences of these behavioural modifications are important for understanding the evolution of host-symbiont interactions, including how Wolbachia spread within host populations.

Elevated temperature and toxic Microcystis reduce Daphnia fitness and modulate gut microbiota

The gut microbiota has been increasingly recognized to regulate host fitness, which in turn is dependent on stability of community structure and composition. Many biotic and abiotic factors have been demonstrated to shape gut microbiota of cladocerans. However, the interactive effects of these variables on cladocerans fitness due to alteration of gut microbiota and their linkage with life history parameters are poorly understood. Here, we investigated the responses of Daphnia magna gut microbiota to the combined effects of toxic Microcystis aeruginosa and high temperature and its associations with fitness. We found that under good food regime, the temperature has no effect on the composition of the gut microbiota, whereas under high proportion of toxic M. aeruginosa and high temperature conditions, D. magna lost their symbionts. High proportion of toxic M. aeruginosa and high temperature had synergistically negative effects on D. magna performance due to altered gut microbiota. The high abundance of symbiotic Comamonadaceae and good food increased D. magna fitness. The present study illustrates that understanding life history strategies in response to multiple stressors related to changes in the gut microbiota diversity and composition requires integrated approaches that incorporate multiple linked traits and tether them to one another.

Pervasive Effects of Wolbachia on Host Temperature Preference

Heritable symbionts can modify a range of ecologically important host traits, including behavior. About half of all insect species are infected with maternally transmitted Wolbachia, a bacterial endosymbiont known to alter host reproduction, nutrient acquisition, and virus susceptibility. Here, we broadly test the hypothesis that Wolbachia modifies host behavior by assessing the effects of eight different Wolbachia strains on the temperature preference of six Drosophila melanogaster subgroup species. Four of the seven host genotypes infected with A-group Wolbachia strains (wRi in Drosophila simulans, wHa in D. simulans, wSh in Drosophila sechellia, and wTei in Drosophila teissieri) prefer significantly cooler temperatures relative to uninfected genotypes. Contrastingly, when infected with divergent B-group wMau, Drosophila mauritiana prefers a warmer temperature. For most strains, changes to host temperature preference do not alter Wolbachia titer. However, males infected with wSh and wTei tend to experience an increase in titer when shifted to a cooler temperature for 24 h, suggesting that Wolbachia-induced changes to host behavior may promote bacterial replication. Our results indicate that Wolbachia modifications to host temperature preference are likely widespread, which has important implications for insect thermoregulation and physiology. Understanding the fitness consequences of these Wolbachia effects is crucial for predicting evolutionary outcomes of host-symbiont interactions, including how Wolbachia spreads to become common.IMPORTANCE Microbes infect a diversity of species, influencing the performance and fitness of their hosts. Maternally transmitted Wolbachia bacteria infect most insects and other arthropods, making these bacteria some of the most common endosymbionts in nature. Despite their global prevalence, it remains mostly unknown how Wolbachia influence host physiology and behavior to proliferate. We demonstrate pervasive effects of Wolbachia on Drosophila temperature preference. Most hosts infected with A-group Wolbachia prefer cooler temperatures, whereas the one host species infected with divergent B-group Wolbachia prefers warmer temperatures, relative to uninfected genotypes. Changes to host temperature preference generally do not alter Wolbachia abundance in host tissues, but for some A-group strains, adult males have increased Wolbachia titer when shifted to a cooler temperature. This suggests that Wolbachia-induced changes to host behavior may promote bacterial replication. Our results help elucidate the impact of endosymbionts on their hosts amid the global Wolbachia pandemic.

Impacts of Low Temperatures on Wolbachia (Rickettsiales: Rickettsiaceae)-Infected Aedes aegypti (Diptera: Culicidae)

In recent decades, the occurrence and distribution of arboviral diseases transmitted by Aedes aegypti mosquitoes has increased. In a new control strategy, populations of mosquitoes infected with Wolbachia are being released to replace existing populations and suppress arboviral disease transmission. The success of this strategy can be affected by high temperature exposure, but the impact of low temperatures on Wolbachia-infected Ae. aegypti is unclear, even though low temperatures restrict the abundance and distribution of this species. In this study, we considered low temperature cycles relevant to the spring season that are close to the distribution limits of Ae. aegypti, and tested the effects of these temperature cycles on Ae. aegypti, Wolbachia strains wMel and wAlbB, and Wolbachia phage WO. Low temperatures influenced Ae. aegypti life-history traits, including pupation, adult eclosion, and fertility. The Wolbachia-infected mosquitoes, especially wAlbB, performed better than uninfected mosquitoes. Temperature shift experiments revealed that low temperature effects on life history and Wolbachia density depended on the life stage of exposure. Wolbachia density was suppressed at low temperatures but densities recovered with adult age. In wMel Wolbachia there were no low temperature effects specific to Wolbachia phage WO. The findings suggest that Wolbachia-infected Ae. aegypti are not adversely affected by low temperatures, indicating that the Wolbachia replacement strategy is suitable for areas experiencing cool temperatures seasonally.

Microbial symbionts expanding or constraining abiotic niche space in insects

In addition to their well-studied contributions to their host's nutrition, digestion, and defense, microbial symbionts of insects are increasingly found to affect their host's response toward abiotic stressors. In particular, symbiotic microbes can reduce or enhance tolerance to temperature extremes, improve desiccation resistance by aiding cuticle biosynthesis and sclerotization, and detoxify heavy metals. As such, individual symbionts or microbial communities can expand or constrain the abiotic niche space of their host and determine its adaptability to fluctuating environments. In light of the increasing impact of humans on climate and environment, a better understanding of host-microbe interactions is necessary to predict how different insect species will respond to changes in abiotic conditions.

The Intracellular Symbiont Wolbachia pipientis Enhances Recombination in a Dose-Dependent Manner

Wolbachia pipientis is an intracellular alphaproteobacterium that infects 40%-60% of insect species and is well known for host reproductive manipulations. Although Wolbachia are primarily maternally transmitted, evidence of horizontal transmission can be found in incongruent host-symbiont phylogenies and recent acquisitions of the same Wolbachia strain by distantly related species. Parasitoids and predator-prey interactions may indeed facilitate the transfer of Wolbachia between insect lineages, but it is likely that Wolbachia are acquired via introgression in many cases. Many hypotheses exist to explain Wolbachia prevalence and penetrance, such as nutritional supplementation, protection from parasites, protection from viruses, or direct reproductive parasitism. Using classical genetics, we show that Wolbachia increase recombination in infected lineages across two genomic intervals. This increase in recombination is titer-dependent as the wMelPop variant, which infects at higher load in Drosophila melanogaster, increases recombination 5% more than the wMel variant. In addition, we also show that Spiroplasma poulsonii, another bacterial intracellular symbiont of D. melanogaster, does not induce an increase in recombination. Our results suggest that Wolbachia infection specifically alters its host's recombination landscape in a dose-dependent manner.

Wolbachia affect behavior and possibly reproductive compatibility but not thermoresistance, fecundity, and morphology in a novel transinfected host, Drosophila nigrosparsa

Wolbachia, intracellular endosymbionts, are estimated to infect about half of all arthropod species. These bacteria manipulate their hosts in various ways for their maximum benefits. The rising global temperature may accelerate species migration, and thus, horizontal transfer of Wolbachia may occur across species previously not in contact. We transinfected and then cured the alpine fly Drosophila nigrosparsa with Wolbachia strain wMel to study its effects on this species. We found low Wolbachia titer, possibly cytoplasmic incompatibility, and an increase in locomotion of both infected larvae and adults compared with cured ones. However, no change in fecundity, no impact on heat and cold tolerance, and no change in wing morphology were observed. Although Wolbachia increased locomotor activities in this species, we conclude that D. nigrosparsa may not benefit from the infection. Still, D. nigrosparsa can serve as a host for Wolbachia because vertical transmission is possible but may not be as high as in the native host of wMel, Drosophila melanogaster.

Titer regulation in arthropod-Wolbachia symbioses

Symbiosis between intracellular bacteria (endosymbionts) and animals are widespread. The alphaproteobacterium Wolbachia pipientis is known to maintain a variety of symbiotic associations, ranging from mutualism to parasitism, with a wide range of invertebrates. Wolbachia infection might deeply affect host fitness (e.g. reproductive manipulation and antiviral protection), which is thought to explain its high prevalence in nature. Bacterial loads significantly influence both the infection dynamics and the extent of bacteria-induced host phenotypes. Hence, fine regulation of bacterial titers is considered as a milestone in host-endosymbiont interplay. Here, we review both environmental and biological factors modulating Wolbachia titers in arthropods.

Microorganisms in the reproductive tissues of arthropods

Microorganisms that reside within or transmit through arthropod reproductive tissues have profound impacts on host reproduction, health and evolution. In this Review, we discuss select principles of the biology of microorganisms in arthropod reproductive tissues, including bacteria, viruses, protists and fungi. We review models of specific symbionts, routes of transmission, and the physiological and evolutionary outcomes for both hosts and microorganisms. We also identify areas in need of continuing research, to answer the fundamental questions that remain in fields within and beyond arthropod-microorganism associations. New opportunities for research in this area will drive a broader understanding of major concepts as well as the biodiversity, mechanisms and translational applications of microorganisms that interact with host reproductive tissues.

Spatio-temporal distribution of Spiroplasma infections in the tsetse fly (Glossina fuscipes fuscipes) in northern Uganda

Tsetse flies (Glossina spp.) are vectors of parasitic trypanosomes, which cause human (HAT) and animal African trypanosomiasis (AAT) in sub-Saharan Africa. In Uganda, Glossina fuscipes fuscipes (Gff) is the main vector of HAT, where it transmits Gambiense disease in the northwest and Rhodesiense disease in central, southeast and western regions. Endosymbionts can influence transmission efficiency of parasites through their insect vectors via conferring a protective effect against the parasite. It is known that the bacterium Spiroplasma is capable of protecting its Drosophila host from infection with a parasitic nematode. This endosymbiont can also impact its host's population structure via altering host reproductive traits. Here, we used field collections across 26 different Gff sampling sites in northern and western Uganda to investigate the association of Spiroplasma with geographic origin, seasonal conditions, Gff genetic background and sex, and trypanosome infection status. We also investigated the influence of Spiroplasma on Gff vector competence to trypanosome infections under laboratory conditions. Generalized linear models (GLM) showed that Spiroplasma probability was correlated with the geographic origin of Gff host and with the season of collection, with higher prevalence found in flies within the Albert Nile (0.42 vs 0.16) and Achwa River (0.36 vs 0.08) watersheds and with higher prevalence detected in flies collected in the intermediate than wet season. In contrast, there was no significant correlation of Spiroplasma prevalence with Gff host genetic background or sex once geographic origin was accounted for in generalized linear models. Additionally, we found a potential negative correlation of Spiroplasma with trypanosome infection, with only 2% of Spiroplasma infected flies harboring trypanosome co-infections. We also found that in a laboratory line of Gff, parasitic trypanosomes are less likely to colonize the midgut in individuals that harbor Spiroplasma infection. These results indicate that Spiroplasma infections in tsetse may be maintained by not only maternal but also via horizontal transmission routes, and Spiroplasma infections may also have important effects on trypanosome transmission efficiency of the host tsetse. Potential functional effects of Spiroplasma infection in Gff could have impacts on vector control approaches to reduce trypanosome infections.

Loss of cytoplasmic incompatibility in Wolbachia-infected Aedes aegypti under field conditions

Wolbachia bacteria are now being introduced into Aedes aegypti mosquito populations for dengue control. When Wolbachia infections are at a high frequency, they influence the local transmission of dengue by direct virus blocking as well as deleterious effects on vector mosquito populations. However, the effectiveness of this strategy could be influenced by environmental temperatures that decrease Wolbachia density, thereby reducing the ability of Wolbachia to invade and persist in the population and block viruses. We reared wMel-infected Ae. aegypti larvae in the field during the wet season in Cairns, North Queensland. Containers placed in the shade produced mosquitoes with a high Wolbachia density and little impact on cytoplasmic incompatibility. However, in 50% shade where temperatures reached 39°C during the day, wMel-infected males partially lost their ability to induce cytoplasmic incompatibility and females had greatly reduced egg hatch when crossed to infected males. In a second experiment under somewhat hotter conditions (>40°C in 50% shade), field-reared wMel-infected females had their egg hatch reduced to 25% when crossed to field-reared wMel-infected males. Wolbachia density was reduced in 50% shade for both sexes in both experiments, with some mosquitoes cleared of their Wolbachia infections entirely. To investigate the critical temperature range for the loss of Wolbachia infections, we held Ae. aegypti eggs in thermocyclers for one week at a range of cyclical temperatures. Adult wMel density declined when eggs were held at 26–36°C or above with complete loss at 30–40°C, while the density of wAlbB remained high until temperatures were lethal. These findings suggest that high temperature effects on Wolbachia are potentially substantial when breeding containers are exposed to partial sunlight but not shade. Heat stress could reduce the ability of Wolbachia infections to invade mosquito populations in some locations and may compromise the ability of Wolbachia to block virus transmission in the field. Temperature effects may also have an ecological impact on mosquito populations given that a proportion of the population becomes self-incompatible.

Aedes aegypti mosquitoes infected with Wolbachia symbionts are being deployed in the tropics as a way of reducing disease transmission. Some Wolbachia strains are vulnerable to high temperatures but these effects have not been evaluated outside of a laboratory setting. We reared Ae. aegypti infected with the wMel strain of Wolbachia in the field during the wet season in Cairns, Australia, where the first releases of Wolbachia-infected Ae. aegypti took place. wMel-infected mosquitoes became partially self-incompatible, with reduced egg hatch, when larvae were reared in partial shade where maximum daily temperatures exceeded 39°C. Under these conditions the amount of Wolbachia in adult mosquitoes was reduced to less than 1% of laboratory-reared mosquitoes on average, while some mosquitoes were cleared of Wolbachia entirely. In contrast, wMel was stable when mosquitoes were reared under cooler conditions in full shade. Field trials with the wMel strain are now underway in over 10 countries, but high temperatures in some locales may constrain the ability of Wolbachia to invade natural mosquito populations and block disease transmission.

Symbiosis Comes of age at the 10th Biennial Meeting of Wolbachia Researchers

Wolbachia pipientis is an alpha-proteobacterial, obligate intracellular microbe and arguably the most successful infection on our planet, colonizing 40-60% of insect species. Wolbachia are also present in most, but not all, filarial nematodes where they are obligate mutualists and are the targets for anti-filarial drug discovery. Although Wolbachia are related to important human pathogens they do not infect mammals, but instead are well known for their reproductive manipulations of insect populations, inducing the following phenotypes: male-killing, feminization, parthenogenesis induction, or cytoplasmic incompatibility (CI). The most common of these, CI, results in a sperm-egg incompatibility and increases the relative fecundity of infected females in a population. In the last decade, Wolbachia have also been shown to provide a benefit to insects, where the infection can inhibit RNA virus replication within the host. Wolbachia cannot be cultivated outside of host cells and no genetic tools are available in the symbiont, limiting approaches available to its study. This means that many questions fundamental to our understanding of Wolbachia basic biology remained unknown for decades. The tenth biennial international Wolbachia conference, "Wolbachia Evolution, Ecology, Genomics and Cell Biology: A Chronicle of the Most Ubiquitous Symbiont", was held on June 17-22, 2018, Salem, MA USA. In the review below we highlight the new science presented at the meeting, link it to prior efforts to answer these questions across the Wolbachia genus, and the importance to the field of symbiosis. The topics covered in this review are based on the presentations at the conference.