High and low temperatures differently affect infection density and vertical transmission of male-killing Spiroplasma symbionts in Drosophila hosts

Warm temperature inhibits cytoplasmic incompatibility induced by endosymbiotic Rickettsiella in spider hosts

Bacterial endosymbionts manipulate reproduction in arthropods to increase their prevalence in the host population. One such manipulation is cytoplasmic incompatibility (CI), wherein the bacteria sabotage sperm in infected males to reduce the hatch rate when mated with uninfected females, but zygotes are 'rescued' when that male mates with an infected female. In the spider Mermessus fradeorum (Linyphiidae), Rickettsiella symbionts cause variable levels of CI. We hypothesised that temperature affects the strength of CI and its rescue in M. fradeorum, potentially mediated by bacterial titre. We reared Rickettsiella-infected spiders in two temperature conditions (26°C vs. 20°C) and tested CI induction in males and rescue in females. In incompatible crosses between infected males and uninfected females, the hatch rate from warm males was doubled (mean ± standard error = 0.687 ± 0.052) relative to cool males (0.348 ± 0.046), indicating that CI induction is weaker in warm males. In rescue crosses between infected females and infected males, female rearing temperature had a marginal effect on CI rescue, but the hatch rate remained high for both warm (0.960 ± 0.023) and cool females (0.994 ± 0.004). Bacterial titre, as measured by quantitative polymerase chain reaction, was lower in warm than cool spiders, particularly in females, suggesting that bacterial titre may play a role in causing the temperature-mediated changes in CI.

Clines of resistance to parasitoids: the multifarious effects of temperature on defensive symbioses in insects

Insects are frequently infected with heritable bacterial endosymbionts. Some of them confer resistance to parasitoids. Such defensive symbionts are sensitive to variation in temperature. Drawing predominantly from the literature on aphids and flies, we show that temperature can affect the reliability of maternal transmission and the strength of protection provided by defensive symbionts. Costs of infection with defensive symbionts can also be temperature-dependent and may even turn into benefits under extreme temperatures, for example, when defensive symbionts increase heat tolerance. Alone or in combination, these mechanisms can drive temperature-associated (latitudinal) clines of infection prevalence with defensive symbionts. This has important consequences for host-parasitoid coevolution, as the relative importance of host-encoded vs. symbiont-provided defenses will shift along such clines.

Sex ratio distorting microbes exacerbate arthropod extinction risk in variable environments

Maternally-inherited sex ratio distorting microbes (SRDMs) are common among arthropod species. Typically, these microbes cause female-biased sex ratios in host broods, either by; killing male offspring, feminising male offspring, or inducing parthenogenesis. As a result, infected populations can experience drastic ecological and evolutionary change. The mechanism by which SRDMs operate is likely to alter their impact on host evolutionary ecology; despite this, the current literature is heavily biased towards a single mechanism of sex ratio distortion, male-killing. Furthermore, amidst the growing concerns surrounding the loss of arthropod diversity, research into the impact of SRDMs on the viability of arthropod populations is generally lacking. In this study, using a theoretical approach, we model the epidemiology of an understudied mechanism of microbially-induced sex ratio distortion-feminisation-to ask an understudied question-how do SRDMs impact extinction risk in a changing environment? We constructed an individual-based model and measured host population extinction risk under various environmental and epidemiological scenarios. We also used our model to identify the precise mechanism modulating extinction. We find that the presence of feminisers increases host population extinction risk, an effect that is exacerbated in highly variable environments. We also identified transmission rate as the dominant epidemiological trait responsible for driving extinction. Finally, our model shows that sex ratio skew is the mechanism driving extinction. We highlight feminisers and, more broadly, SRDMs as important determinants of the resilience of arthropod populations to environmental change.

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.

Preliminary Characterization of Phage-Like Particles from the Male-Killing Mollicute Spiroplasma poulsonii (an Endosymbiont of Drosophila)

Bacteriophages are vastly abundant, diverse, and influential, but with few exceptions (e.g. the Proteobacteria genera Wolbachia and Hamiltonella), the role of phages in heritable bacteria-arthropod interactions, which are ubiquitous and diverse, remains largely unexplored. Despite prior studies documenting phage-like particles in the mollicute Spiroplasma associated with Drosophila flies, genomic sequences of such phage are lacking, and their effects on the Spiroplasma-Drosophila interaction have not been comprehensively characterized. We used a density step gradient to isolate phage-like particles from the male-killing bacterium Spiroplasma poulsonii (strains NSRO and MSRO-Br) harbored by Drosophila melanogaster. Isolated particles were subjected to DNA sequencing, assembly, and annotation. Several lines of evidence suggest that we recovered phage-like particles of similar features (shape, size, DNA content) to those previously reported in Drosophila-associated Spiroplasma strains. We recovered three ~ 19 kb phage-like contigs (two in NSRO and one in MSRO-Br) containing 21-24 open reading frames, a read-alignment pattern consistent with circular permutation, and terminal redundancy (at least in NSRO). Although our results do not allow us to distinguish whether these phage-like contigs represent infective phage-like particles capable of transmitting their DNA to new hosts, their encoding of several typical phage genes suggests that they are at least remnants of functional phage. We also recovered two smaller non-phage-like contigs encoding a known Spiroplasma toxin (Ribosome Inactivating Protein; RIP), and an insertion element, suggesting that they are packaged into particles. Substantial homology of our particle-derived contigs was found in the genome assemblies of members of the Spiroplasma poulsonii clade.

Temperature effects on cellular host-microbe interactions explain continent-wide endosymbiont prevalence

Endosymbioses influence host physiology, reproduction, and fitness, but these relationships require efficient microbe transmission between host generations to persist. Maternally transmitted Wolbachia are the most common known endosymbionts,¹ but their frequencies vary widely within and among host populations for unknown reasons.^2,3 Here, we integrate genomic, cellular, and phenotypic analyses with mathematical models to provide an unexpectedly simple explanation for global wMel Wolbachia prevalence in Drosophila melanogaster. Cooling temperatures decrease wMel cellular abundance at a key stage of host oogenesis, producing temperature-dependent variation in maternal transmission that plausibly explains latitudinal clines of wMel frequencies on multiple continents. wMel sampled from a temperate climate targets the germline more efficiently in the cold than a recently differentiated tropical variant (∼2,200 years ago), indicative of rapid wMel adaptation to climate. Genomic analyses identify a very narrow list of wMel alleles-most notably, a derived stop codon in the major Wolbachia surface protein WspB-that underlie thermal sensitivity of cellular Wolbachia abundance and covary with temperature globally. Decoupling temperate wMel and host genomes further reduces transmission in the cold, a pattern that is characteristic of host-microbe co-adaptation to a temperate climate. Complex interactions among Wolbachia, hosts, and the environment (GxGxE) mediate wMel cellular abundance and maternal transmission, implicating temperature as a key determinant of Wolbachia spread and equilibrium frequencies, in conjunction with Wolbachia effects on host fitness and reproduction.^4,5 Our results motivate the strategic use of locally selected wMel variants for Wolbachia-based biocontrol efforts, which protect millions of individuals from arboviruses that cause human disease.⁶.

Plant secondary metabolite and temperature determine the prevalence of Arsenophonus endosymbionts in aphid populations

Transmission rate and role in hosts contribute to the prevalence of an endosymbiont. However, factors affecting transmission and role of facultative endosymbionts are still not well understood. Here, we illustrated that host plants and environmental temperatures affected the transmission, relative abundance, and role of Arsenophonus in the cotton aphid Aphis gossypii. The transmission rate of this endosymbiont from mother aphids to offspring was relatively lower. High temperatures impeded the transmission, and infection rates declined as aphids were exposed to 30 °C. Contents of amino acids and secondary metabolites were remarkable different among host plants. Aphids feeding on zucchini leaves containing a higher titer of amino acids and lower secondary metabolites harbored a relatively lower abundance of Arsenophonus. Concentrations of an amino acid and a plant secondary metabolite, cucurbitacin B, in aphid diet were not associated with Arsenophonus abundance. However, gossypol, another plant secondary metabolite, was strongly related with the abundance. Arsenophonus imparted a fitness benefit to aphids, and the benefit was dependent on host plants and gossypol concentration. In sum, plant secondary metabolite and environmental temperature affect transmission, relative abundance, and role of Arsenophonus, which determine the endosymbiont prevalence in aphid populations. This article is protected by copyright. All rights reserved.

Endosymbiotic male-killing Spiroplasma affect the physiological and behavioural ecology of Macrocheles-Drosophila interactions

While many arthropod endosymbionts are vertically transmitted, phylogenetic studies reveal repeated introductions of hemolymph-dwelling Spiroplasma into Drosophila. Introductions are often attributed to horizontal transmission via ectoparasite vectors. Here, we test if mites prefer to infect Spiroplasma poulsonii MSRO infected flies, and if MSRO infection impairs fly resistance against secondary mite (Macrocheles subbadius) attack. First we tested if mites prefer MSRO+ or MSRO- flies using pair-wise-choice tests across fly ages. We then tested whether mite preferences are explained by changes in fly physiology, specifically increased metabolic rate (measured as CO₂ production). We hypothesize that this preference is due in part to MSRO+ flies expressing higher metabolic rates. However, our results showed mite preference depended on an interaction between fly age and MSRO status: mites avoided 14-days old MSRO+ flies relative to MSRO- flies (31% infection), but prefered MSRO+ flies (64% infection) among 26-day old flies. Using flow-through respirometry, we found 14 day-old MSRO+ flies had higher CO₂ emissions than MSRO- flies (32% greater), whereas at 26 days old the CO₂ production among MSRO+ flies was 20% lower than MSRO- flies. Thus, mite preferences for high metabolic rate hosts did not explain the infection biases in this study. To assess changes in susceptibility to infection, we measured fly endurance using geotaxis assays. Older flies had lower endurance consistent with fly senescence, and this effect was magnified among MSRO+ flies. Given the biological importance of male-killing Spiroplasma, potential changes in the interactions of hosts and potential vectors could impact the ecology and evolution of host species. Importance Male-killing endosymbionts are transmitted mother to daughter and kill male offspring. Despite these major ecological effects, how these endosymbionts colonize new host species is not always clear. Mites are sometimes hypothesized to transfer these bacteria between hosts/host species. Here we test if 1) if mites prefer to infect flies that harbour Spiroplasma poulisoni MSRO and 2) if flies infected with MSRO are less able to resist mite infection. Our results show that flies infected with MSRO have weaker anti-mite resistance but the mite preference/aversion for MSRO+ flies varied with fly age. Given the fitness and population impacts of male-killing Spiroplasma, changes in fly-mite interactions have implications for the ecology and evolution of these symbioses.

Bacterial diversity of Leptocybe invasa Fisher & La Salle (Hymenoptera: Eulophidae) from different geographical conditions in China

Insects harbor numerous endosymbionts, including bacteria, fungi, yeast, and viruses, which could affect the ecology and behavior of their hosts. However, data regarding the effect of environmental factors on endosymbiotic bacteria of Leptocybe invasa (Hymenoptera: Eulophidae) are quite rare. In this study, we assessed the diversity of endosymbiotic bacteria of L. invasa from 10 different geographic populations collected across China through the Illumina MiSeq platform. A total of 547 OTUs were generated, which were annotated into 19 phyla, 33 classes, 75 orders, 137 families, and 274 genera. The dominant bacteria detected in L. invasa were Rickettsia, and Pantoea, Enterobacter, Pseudomonas, Acinetobacter, and Bacillus were also annotated among each population. Nevertheless, the endosymbiotic bacterial abundance and diversity varied among different populations, which was related to the local climate (annual mean high temperature). The bacterial function prediction analysis showed that these endosymbiotic bacteria were concentrated in metabolism, such as carbohydrate, amino acid, and energy metabolism. Overall, the results provide a comprehensive description of the endosymbiotic bacteria in 10 different populations of an important eucalyptus pest L. invasa, and help to understand the endosymbiotic bacterial diversity and adaptation of various conditions.

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.

Direct and indirect effects of altered temperature regimes and phenological mismatches on insect populations

Climate change is transforming ecosystems by altering species ranges, the composition of communities, and trophic interactions. Here, we synthesize recent reviews and subsequent developments to provide an overview of insect ecological and evolutionary responses to altered temperature regimes. We discuss both direct responses to thermal stress and indirect responses arising from phenological mismatches, altered host quality, and changes in natural enemy activity. Altered temperature regimes can increase exposure to both cold and heat stress and result in phenological and morphological mismatches with adjacent trophic levels. Host plant quality varies in a heterogenous way in response to altered temperatures with both increases and decreases observed. Density-dependent effects, spatial heterogeneity, and rapid evolutionary change provide some resilience to these threats.

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.

Does getting defensive get you anywhere?-Seasonal balancing selection, temperature, and parasitoids shape real-world, protective endosymbiont dynamics in the pea aphid

Facultative, heritable endosymbionts are found at intermediate prevalence within most insect species, playing frequent roles in their hosts' defence against environmental pressures. Focusing on Hamiltonella defensa, a common bacterial endosymbiont of aphids, we tested the hypothesis that such pressures impose seasonal balancing selection, shaping a widespread infection polymorphism. In our studied pea aphid (Acyrthosiphon pisum) population, Hamiltonella frequencies ranged from 23.2% to 68.1% across a six-month longitudinal survey. Rapid spikes and declines were often consistent across fields, and we estimated that selection coefficients for Hamiltonella-infected aphids changed sign within this field season. Prior laboratory research suggested antiparasitoid defence as the major Hamiltonella benefit, and costs under parasitoid absence. While a prior field study suggested these forces can sometimes act as counter-weights in a regime of seasonal balancing selection, our present survey showed no significant relationship between parasitoid wasps and Hamiltonella prevalence. Field cage experiments provided some explanation: parasitoids drove modest ~10% boosts to Hamiltonella frequencies that would be hard to detect under less controlled conditions. They also showed that Hamiltonella was not always costly under parasitoid exclusion, contradicting another prediction. Instead, our longitudinal survey - and two overwintering studies - showed temperature to be the strongest predictor of Hamiltonella prevalence. Matching some prior lab discoveries, this suggested that thermally sensitive costs and benefits, unrelated to parasitism, can shape Hamiltonella dynamics. These results add to a growing body of evidence for rapid, seasonal adaptation in multivoltine organisms, suggesting that such adaptation can be mediated through the diverse impacts of heritable bacterial endosymbionts.

Symbiont-mediated fly survival is independent of defensive symbiont genotype in the Drosophila melanogaster-Spiroplasma-wasp interaction

When a parasite attacks an insect, the outcome is commonly modulated by the presence of defensive heritable symbionts residing within the insect host. Previous studies noted markedly different strengths of Spiroplasma-mediated fly survival following attack by the same strain of wasp. One difference between the two studies was the strain of Spiroplasma used. We therefore performed a laboratory experiment to assess whether Spiroplasma-mediated protection depends upon the strain of Spiroplasma. We perform this analysis using the two strains of male-killing Spiroplasma used previously, and examined response to challenge by two strains of Leptopilina boulardi and two strains of Leptopilina heterotoma wasp. We found no evidence Spiroplasma strain affected fly survival following wasp attack. In contrast, analysis of the overall level of protection, including the fecundity of survivors of wasp attack, did indicate the two Spiroplasma strains tested varied in protective efficiency against three of the four wasp strains tested. These data highlight the sensitivity of symbiont-mediated protection phenotypes to laboratory conditions, and the importance of common garden comparison. Our results also indicate that Spiroplasma strains can vary in protective capacity in Drosophila, but these differences may exist in the relative performance of survivors of wasp attack, rather than in survival of attack per se.

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.

Symbiont-mediated protection varies with wasp genotype in the Drosophila melanogaster-Spiroplasma interaction

The ability of an insect to survive attack by natural enemies can be modulated by the presence of defensive symbionts. Study of aphid-symbiont-enemy interactions has indicated that protection may depend on the interplay of symbiont, host and attacking parasite genotypes. However, the importance of these interactions is poorly understood outside of this model system. Here, we study interactions within a Drosophila model system, in which Spiroplasma protect their host against parasitoid wasps and nematodes. We examine whether the strength of protection conferred by Spiroplasma to its host, Drosophila melanogaster varies with strain of attacking Leptopilina heterotoma wasp. We perform this analysis in the presence and absence of ethanol, an environmental factor that also impacts the outcome of parasitism. We observed that Spiroplasma killed all strains of wasp. However, the protection produced by Spiroplasma following wasp attack depended on wasp strain. A composite measure of protection, including both the chance of the fly surviving attack and the relative fecundity/fertility of the survivors, varied from a <4% positive effect of the symbiont following attack of the fly host by the Lh14 strain of wasp to 21% for the Lh-Fr strain in the absence of ethanol. We also observed that environmental ethanol altered the pattern of protection against wasp strains. These data indicate that the dynamics of the Spiroplasma-Drosophila-wasp tripartite interaction depend upon the genetic diversity within the attacking wasp population, and that prediction of symbiont dynamics in natural systems will thus require analysis across natural enemy genotypes and levels of environmental ethanol.

Exposure to opposing temperature extremes causes comparable effects on Cardinium density but contrasting effects on Cardinium-induced cytoplasmic incompatibility

Terrestrial arthropods, including insects, commonly harbor maternally inherited intracellular symbionts that confer benefits to the host or manipulate host reproduction to favor infected female progeny. These symbionts may be especially vulnerable to thermal stress, potentially leading to destabilization of the symbiosis and imposing costs to the host. For example, increased temperatures can reduce the density of a common reproductive manipulator, Wolbachia, and the strength of its crossing incompatibility (cytoplasmic incompatibility, or CI) phenotype. Another manipulative symbiont, Cardinium hertigii, infects ~ 6–10% of Arthropods, and also can induce CI, but there is little homology between the molecular mechanisms of CI induced by Cardinium and Wolbachia. Here we investigated whether temperature disrupts the CI phenotype of Cardinium in a parasitic wasp host, Encarsia suzannae. We examined the effects of both warm (32°C day/ 29°C night) and cool (20°C day/ 17°C night) temperatures on Cardinium CI and found that both types of temperature stress modified aspects of this symbiosis. Warm temperatures reduced symbiont density, pupal developmental time, vertical transmission rate, and the strength of both CI modification and rescue. Cool temperatures also reduced symbiont density, however this resulted in stronger CI, likely due to cool temperatures prolonging the host pupal stage. The opposing effects of cool and warm-mediated reductions in symbiont density on the resulting CI phenotype indicates that CI strength may be independent of density in this system. Temperature stress also modified the CI phenotype only if it occurred during the pupal stage, highlighting the likely importance of this stage for CI induction in this symbiosis.

Insects often harbor heritable symbiotic bacteria that infect their cells and/or bodily fluids. These heritable bacteria are passed from mother to offspring and can have substantial effects on host insect biology, and include bacteria like Cardinium that cause mating incompatibilities between symbiont-infected and uninfected individuals. Often, the extent of these symbiont-conferred modifications correlates with the bacterial density in the host. The appearance of these phenotypes is also affected by temperature stress, which often reduces bacterial density. However, here we find that temperature-altered strength of Cardinium-induced mating incompatibility in a whitefly parasitoid wasp can be independent of Cardinium density. While heat treatment reduced the symbiont density and the phenotype, as expected, cold treatment also reduced symbiont density but increased the degree of mating incompatibility. Here, the prolonged duration of the host pupal development in the cold treatments appeared to be more important for phenotype strength. These results suggest that the connection between bacterial density and phenotype strength may not be as general as previously thought. Furthermore, the modification of this manipulative phenotype has implications for the effectiveness of the host, Encarsia suzannae, as a biological control agent.

Diversity and Phylogenetic Analyses Reveal Horizontal Transmission of Endosymbionts Between Whiteflies and Their Parasitoids

Endosymbionts are widely distributed among insects via intraspecific vertical transmission and interspecific horizontal transmission. Parasitoids have attracted considerable interest due to their possible role in the horizontal transmission of endosymbionts. Horizontal transmission of endosymbionts between whiteflies via parasitoids has been revealed in the laboratory. However, whether this occurs under field conditions remains unknown. Here, the diversity and phylogenetic relationships of endosymbionts in 1,350 whiteflies and 36 parasitoids that emerged from whitefly nymphs collected from three locations in Jiangsu Province of China were investigated. Only Rickettsia and Wolbachia were identified in both whiteflies and parasitoids, with an overall infection frequency of 22.67% in whiteflies and 16.67% in parasitoids for Wolbachia and of 12.15% in whiteflies and 25% in parasitoids for Rickettsia. Despite the distant relationship between whiteflies and their parasitoids, phylogenetic analyses revealed that the Rickettsia and Wolbachia individuals collected from the two types of organisms were grouped together. Furthermore, shared haplotypes were also identified, which was consistent with the horizontal transmission of endosymbionts between parasitoids and whiteflies. In addition, a parasitoid resistance-related symbiont, Hamiltonella, was detected in whiteflies at a 100% infection frequency, probably accounting for the relatively low parasitism of the whiteflies in the field. The factors affecting the infection frequency of the four secondary endosymbionts in whiteflies were also examined.

Infection pattern and negative effects of a facultative endosymbiont on its insect host are environment-dependent

Regiella insecticola is a bacterial endosymbiont in insects that exhibits a negative effect on the fitness of hosts. Thus, it is not clear why this costly endosymbiont can persist in host populations. Here, we tested a hypothesis that the infection pattern and negative roles of the endosymbiont were not constant but environmentally dependent. The grain aphids Sitobion avenae, belonging to different genotypes and infected with Regiella or not, were used in this study. We found that S. avenae populations were infected with Regiella, Hamiltonella defensa, Serratia symbiotica and Rickettsia. The predominant endosymbionts in the aphid populations varied with season. Serratia and Rickettsia were predominant from December to February while Regiella predominated from March to May. The vertical transmission of Regiella was poorer at high temperature, but following conditioning for seven generations, the transmission rate improved. Regiella inhibited the production of winged aphids at 25 °C, but it did not affect winged morph production at the higher temperatures of 28 °C and 31 °C. Regiella infection decreased the intrinsic rate of increase (r_m) of aphids at 25 °C and 28 °C. However, at 31 °C, the effect of Regiella on the r_m varied depending on the aphid genotype and density. Thus, the negative effects of this endosymbiont on its host were environmentally dependent.

Shifting the Balance: Heat Stress Challenges the Symbiotic Interactions of the Asian Citrus Psyllid, Diaphorina citri (Hemiptera, Liviidae)

Global warming may impact biodiversity by disrupting biological interactions, including long-term insect-microbe mutualistic associations. Symbiont-mediated insect tolerance to high temperatures is an ecologically important trait that significantly influences an insect's life history. Disruption of microbial symbionts that are required by insects would substantially impact their pest status. Diaphorina citri, a worldwide citrus pest, is associated with the mutualistic symbionts Candidatus Carsonella ruddii and Candidatus Profftella armatura. Wolbachia is also associated with D. citri, but its contribution to the host is unknown. Symbiont density is dependent on a range of factors, including the thermosensitivity of the host and/or symbiont to heat stress. Here, we predicted that short-term heat stress of D. citri would disrupt the host-symbiont phenological synchrony and differentially affect the growth and density of symbionts. We investigated the effects of exposing D. citri eggs to different temperatures for different periods of time on the growth dynamics of symbionts during the nymphal development of D. citri (first instar to fifth instar) by real-time polymerase chain reaction. Symbiont densities were assessed as the number of gene copies, using specific molecular markers: 16S rRNA for Carsonella and Profftella and ftsZ for Wolbachia. Statistical modeling of the copy numbers of symbionts revealed differences in their growth patterns, particularly in the early instars of heat-shocked insects. Wolbachia was the only symbiont to benefit from heat-shock treatment. Although the symbionts responded differently to heat stress, the lack of differences in symbiont densities between treated and control late nymphs suggests the existence of an adaptive genetic process to restore phenological synchrony during the development of immatures in preparation for adult life. Our findings contribute to the understanding of the potential deleterious effects of high temperatures on host-symbiont interactions. Our data also suggest that the effects of host exposure to high temperatures in symbiont growth are highly variable and dependent on the interactions among members of the community of symbionts harbored by a host. Such dependence points to unpredictable consequences for agroecosystems worldwide due to climate change-related effects on the ecological traits of symbiont-dependent insect pests.

Uncovering the hidden players in Lepidoptera biology: the heritable microbial endosymbionts

The Lepidoptera is one of the most widespread and recognisable insect orders. Due to their remarkable diversity, economic and ecological importance, moths and butterflies have been studied extensively over the last 200 years. More recently, the relationship between Lepidoptera and their heritable microbial endosymbionts has received increasing attention. Heritable endosymbionts reside within the host’s body and are often, but not exclusively, inherited through the female line. Advancements in molecular genetics have revealed that host-associated microbes are both extremely prevalent among arthropods and highly diverse. Furthermore, heritable endosymbionts have been repeatedly demonstrated to play an integral role in many aspects of host biology, particularly host reproduction. Here, we review the major findings of research of heritable microbial endosymbionts of butterflies and moths. We promote the Lepidoptera as important models in the study of reproductive manipulations employed by heritable endosymbionts, with the mechanisms underlying male-killing and feminisation currently being elucidated in moths and butterflies. We also reveal that the vast majority of research undertaken of Lepidopteran endosymbionts concerns Wolbachia. While this highly prevalent bacterium is undoubtedly important, studies should move towards investigating the presence of other, and interacting endosymbionts, and we discuss the merits of examining the microbiome of Lepidoptera to this end. We finally consider the importance of understanding the influence of endosymbionts under global environmental change and when planning conservation management of endangered Lepidoptera species.

Incidence of Facultative Bacterial Endosymbionts in Spider Mites Associated with Local Environments and Host Plants

Spider mites are frequently associated with multiple endosymbionts whose infection patterns often exhibit spatial and temporal variation. However, the association between endosymbiont prevalence and environmental factors remains unclear. Here, we surveyed endosymbionts in natural populations of the spider mite, Tetranychus truncatus, in China, screening 935 spider mites from 21 localities and 12 host plant species. Three facultative endosymbiont lineages, Wolbachia, Cardinium, and Spiroplasma, were detected at different infection frequencies (52.5%, 26.3%, and 8.6%, respectively). Multiple endosymbiont infections were observed in most local populations, and the incidence of individuals with the Wolbachia-Spiroplasma coinfection was higher than expected from the frequency of each infection within a population. Endosymbiont infection frequencies exhibited associations with environmental factors: Wolbachia infection rates increased at localities with higher annual mean temperatures, while Cardinium and Spiroplasma infection rates increased at localities from higher altitudes. Wolbachia was more common in mites from Lycopersicon esculentum and Glycine max compared to those from Zea mays This study highlights that host-endosymbiont interactions may be associated with environmental factors, including climate and other geographically linked factors, as well as the host's food plant.IMPORTANCE The aim of this study was to examine the incidence of endosymbiont distribution and the infection patterns in spider mites. The main findings are that multiple endosymbiont infections were more common than expected and that endosymbiont infection frequencies were associated with environmental factors. This work highlights that host-endosymbiont interactions need to be studied within an environmental and geographic context.

Winding paths to simplicity: genome evolution in facultative insect symbionts

Symbiosis between organisms is an important driving force in evolution. Among the diverse relationships described, extensive progress has been made in insect–bacteria symbiosis, which improved our understanding of the genome evolution in host-associated bacteria. Particularly, investigations on several obligate mutualists have pushed the limits of what we know about the minimal genomes for sustaining cellular life. To bridge the gap between those obligate symbionts with extremely reduced genomes and their non-host-restricted ancestors, this review focuses on the recent progress in genome characterization of facultative insect symbionts. Notable cases representing various types and stages of host associations, including those from multiple genera in the family Enterobacteriaceae (class Gammaproteobacteria), Wolbachia (Alphaproteobacteria) and Spiroplasma (Mollicutes), are discussed. Although several general patterns of genome reduction associated with the adoption of symbiotic relationships could be identified, extensive variation was found among these facultative symbionts. These findings are incorporated into the established conceptual frameworks to develop a more detailed evolutionary model for the discussion of possible trajectories. In summary, transitions from facultative to obligate symbiosis do not appear to be a universal one-way street; switches between hosts and lifestyles (e.g. commensalism, parasitism or mutualism) occur frequently and could be facilitated by horizontal gene transfer.

This review synthesizes the recent progress in genome characterization of insect-symbiotic bacteria, the emphases include (i) patterns of genome organization, (ii) evolutionary models and trajectories, and (iii) comparisons between facultative and obligate symbionts.

Thermal sensitivity of bacteriocytes constrains the persistence of intracellular bacteria in whitefly symbiosis under heat stress

Temperature affects the persistence of diverse symbionts of insects. Our previous study indicates that the whitefly symbionts confined within bacteriocytes or scattered throughout the body cavity outside bacteriocytes may have differential thermal sensitivity. However, the underlying mechanisms remain largely unknown. Here, we report that following continuous heat stress, Portiera and Hamiltonella were almost completely depleted in two species of Middle East-Asia Minor 1 (MEAM1) and Mediterranean (MED) of the Bemisia tabaci whitefly cryptic species complex. Meanwhile, proliferation of bacteriocytes was severely inhibited and approximately 50% of the nymphs had lost one of the two bacteriomes. While cell size of bacteriocytes was increased, cell number was severely decreased leading to reduction of total volume of bacteriocytes. Moreover, bacteriocyte organelles and associated symbionts were lysed, and huge amount of electron-dense inclusions accumulated. Eventually, Portiera and Hamiltonella failed to be transmitted to the next generation. In contrast, Rickettsia could be detected although at a reduced level, and successfully transmitted to eggs. The results suggest that the thermal sensitivity of bacteriocytes may limit thermal tolerance and vertical transmission of the associated symbionts, and consequently different patterns of distribution of symbionts may affect their capacity to tolerate unfavourable temperatures and persistence in the host.

Relative Abundance and Strain Diversity in the Bacterial Endosymbiont Community of a Sap-Feeding Insect Across Its Native and Introduced Geographic Range

Most insects are associated with bacterial symbionts. The bacterial diversity and community composition within hosts may play an important role in shaping insect population biology, ecology and evolution. We focussed on the bacterial microbiome of the Australian fig homotomid Mycopsylla fici (Hemiptera: Psylloidea), which can cause defoliation of its only host tree, Ficus macrophylla. This sap-feeding insect is native to mainland Australia and Lord Howe Island (LHI) but also occurs where its host has been planted, notably in New Zealand. By using a high-throughput 16S rDNA amplicon sequencing approach, we compared the bacterial diversity and community composition in individual adult males of four host populations, Sydney, Brisbane, LHI and Auckland. We also compared males, females and nymphs of the Sydney population. The microbiome of M. fici was simple and consisted mostly of the following three maternally inherited endosymbiont species: the primary endosymbiont Carsonella, a secondary (S-) endosymbiont and Wolbachia. However, the relative abundance of their sequence reads varied between host populations, except for similarities between Sydney and Auckland. In addition, insects from Sydney and Auckland had identical bacterial strains supporting the hypothesis that Sydney is the source population for Auckland. In contrast, mainland and LHI populations harboured the same S-endosymbiont, co-diverged Carsonella but different Wolbachia strains. Besides detecting endosymbiont-specific patterns of either co-evolution or horizontal acquisition, our study highlights that relative abundance of maternally inherited endosymbionts should also be taken into account when studying bacterial communities across host populations, as variations in bacterial density may impact host biology and ecology.

Effects of different temperature regimes on survival of Diaphorina citri and its endosymbiotic bacterial communities

The Asian citrus psyllid, Diaphorina citri, is a major pest of citrus and vector of citrus greening (huanglongbing) in Asian. In our field-collected psyllid samples, we discovered that Fuzhou (China) and Faisalabad (Pakistan), populations harbored an obligate primary endosymbiont Candidatus Carsonella (gen. nov.) with a single species, Candidatus Carsonella ruddii (sp. nov.) and a secondary endosymbiont, Wolbachia surface proteins (WSP) which are intracellular endosymbionts residing in the bacteriomes. Responses of these symbionts to different temperatures were examined and their host survival assessed. Diagnostic PCR assays showed that the endosymbionts infection rates were not significantly reduced in both D. citri populations after 24 h exposure to cold or heat treatments. Although quantitative PCR assays showed significant reduction of WSP relative densities at 40°C for 24 h, a substantial decrease occurred as the exposure duration increased beyond 3 days. Under the same temperature regimes, Ca. C. ruddii density was initially less affected during the first exposure day, but rapidly reduced at 3-5 days compared to WSP. However, the mortality of the psyllids increased rapidly as exposure time to heat treatment increased. The responses of the two symbionts to unfavorable temperature regimes highlight the complex host-symbionts interactions between D. citri and its associated endosymbionts.

Heritable symbionts in a world of varying temperature

Heritable microbes represent an important component of the biology, ecology and evolution of many plants, animals and fungi, acting as both parasites and partners. In this review, we examine how heritable symbiont-host interactions may alter host thermal tolerance, and how the dynamics of these interactions may more generally be altered by thermal environment. Obligate symbionts, those required by their host, are considered to represent a thermally sensitive weak point for their host, associated with accumulation of deleterious mutations. As such, these symbionts may represent an important determinant of host thermal envelope and spatial distribution. We then examine the varied relationship between thermal environment and the frequency of facultative symbionts that provide ecologically contingent benefits or act as parasites. We note that some facultative symbionts directly alter host thermotolerance. We outline how thermal environment will alter the benefits/costs of infection more widely, and additionally modulate vertical transmission efficiency. Multiple patterns are observed, with symbionts being cold sensitive in some species and heat sensitive in others, with varying and non-coincident thresholds at which phenotype and transmission are ablated. Nevertheless, it is clear that studies aiming to predict ecological and evolutionary dynamics of symbiont-host interactions need to examine the interaction across a range of thermal environments. Finally, we discuss the importance of thermal sensitivity in predicting the success/failure of symbionts to spread into novel species following natural/engineered introduction.

Host Plant Determines the Population Size of an Obligate Symbiont (Buchnera aphidicola) in Aphids

Buchnera aphidicolais an obligate endosymbiont that provides aphids with several essential nutrients. Though much is known about aphid-Buchnera interactions, the effect of the host plant on Buchnera population size remains unclear. Here we used quantitative PCR (qPCR) techniques to explore the effects of the host plant on Buchnera densities in the cotton-melon aphid, Aphis gossypii Buchneratiters were significantly higher in populations that had been reared on cucumber for over 10 years than in populations maintained on cotton for a similar length of time. Aphids collected in the wild from hibiscus and zucchini harbored more Buchnera symbionts than those collected from cucumber and cotton. The effect of aphid genotype on the population size of Buchnera depended on the host plant upon which they fed. When aphids from populations maintained on cucumber or cotton were transferred to novel host plants, host survival and Buchnera population size fluctuated markedly for the first two generations before becoming relatively stable in the third and later generations. Host plant extracts from cucumber, pumpkin, zucchini, and cowpea added to artificial diets led to a significant increase in Buchnera titers in the aphids from the population reared on cotton, while plant extracts from cotton and zucchini led to a decrease in Buchnera titers in the aphids reared on cucumber. Gossypol, a secondary metabolite from cotton, suppressed Buchnera populations in populations from both cotton and cucumber, while cucurbitacin from cucurbit plants led to higher densities. Together, the results suggest that host plants influence Buchnera population processes and that this may provide phenotypic plasticity in host plant use for clonal aphids.

Genome sequence of the Drosophila melanogaster male-killing Spiroplasma strain MSRO endosymbiont

Spiroplasmas are helical and motile members of a cell wall-less eubacterial group called Mollicutes. Although all spiroplasmas are associated with arthropods, they exhibit great diversity with respect to both their modes of transmission and their effects on their hosts; ranging from horizontally transmitted pathogens and commensals to endosymbionts that are transmitted transovarially (i.e., from mother to offspring). Here we provide the first genome sequence, along with proteomic validation, of an endosymbiotic inherited Spiroplasma bacterium, the Spiroplasma poulsonii MSRO strain harbored by Drosophila melanogaster. Comparison of the genome content of S. poulsonii with that of horizontally transmitted spiroplasmas indicates that S. poulsonii has lost many metabolic pathways and transporters, demonstrating a high level of interdependence with its insect host. Consistent with genome analysis, experimental studies showed that S. poulsonii metabolizes glucose but not trehalose. Notably, trehalose is more abundant than glucose in Drosophila hemolymph, and the inability to metabolize trehalose may prevent S. poulsonii from overproliferating. Our study identifies putative virulence genes, notably, those for a chitinase, the H₂O₂-producing glycerol-3-phosphate oxidase, and enzymes involved in the synthesis of the eukaryote-toxic lipid cardiolipin. S. poulsonii also expresses on the cell membrane one functional adhesion-related protein and two divergent spiralin proteins that have been implicated in insect cell invasion in other spiroplasmas. These lipoproteins may be involved in the colonization of the Drosophila germ line, ensuring S. poulsonii vertical transmission. The S. poulsonii genome is a valuable resource to explore the mechanisms of male killing and symbiont-mediated protection, two cardinal features of many facultative endosymbionts.

Most insect species, including important disease vectors and crop pests, harbor vertically transmitted endosymbiotic bacteria. These endosymbionts play key roles in their hosts’ fitness, including protecting them against natural enemies and manipulating their reproduction in ways that increase the frequency of symbiont infection. Little is known about the molecular mechanisms that underlie these processes. Here, we provide the first genome draft of a vertically transmitted male-killing Spiroplasma bacterium, the S. poulsonii MSRO strain harbored by D. melanogaster. Analysis of the S. poulsonii genome was complemented by proteomics and ex vivo metabolic experiments. Our results indicate that S. poulsonii has reduced metabolic capabilities and expresses divergent membrane lipoproteins and potential virulence factors that likely participate in Spiroplasma-host interactions. This work fills a gap in our knowledge of insect endosymbionts and provides tools with which to decipher the interaction between Spiroplasma bacteria and their well-characterized host D. melanogaster, which is emerging as a model of endosymbiosis.

Invasion of the Q2 mitochondrial variant of Mediterranean Bemisia tabaci in southern Italy: possible role of bacterial endosymbionts

BACKGROUND

The whitefly Bemisia tabaci (Gennadius) is a complex of cryptic species, some of which, namely the Mediterranean (MED) and the Middle East-Asia Minor 1 (MEAM1), are highly invasive and injurious crop pests worldwide and able to displace local genotypes. Invasiveness of B. tabaci may depend on the phenotype of inherited bacterial endosymbionts. Here, the B. tabaci genetic diversity variation that has occurred in recent years in southern Italy was examined. Whitefly was genotyped by restriction fragment length polymorphism analysis of polymerase-chain-reaction-amplified fragments (PCR-RFLP) of the COI gene and molecular identification of endosymbionts. Possible factors leading to the observed genetic diversity were examined.

RESULTS

Q1 and Q2 mitochondrial types of MED, the only species found, coexisted in the field, while MEAM1 disappeared. A large spreading of Q2 (70% of individuals) was observed for the first time in Italy. Q2 showed a significant female-biased sex ratio and largely outnumbered Q1 on solanaceous hosts, in greenhouses and on insecticide-treated plants. Q1, with an even sex ratio, slightly prevailed on non-solanaceous hosts, especially on wild and untreated plants. Endosymbiont composition was associated with the mitochondrial type. Hamiltonella and Rickettsia were found at near fixation in Q1 and Q2 respectively; Arsenophonus, Cardinium and Wolbachia were found in both types, although at different frequencies.

CONCLUSIONS

Q2 invasion seems to have been favoured by the agroecological conditions of southern Italy and by the female-biased sex ratio. Endosymbionts may have a role in Q2 invasiveness, acting as sex-ratio manipulators (e.g. Rickettsia) and possibly by benefiting the host fitness.

Differential responses of the whitefly Bemisia tabaci symbionts to unfavorable low and high temperatures

The whitefly Bemisia tabaci complex contains many cryptic species, of which the Middle East-Asia Minor 1 (MEAM1) and Mediterranean (MED) are notorious invasive pests. In our field-collected whitefly samples, MEAM1 harbors an obligate primary symbiont "Candidatus Portiera aleyrodidarum" and two secondary symbionts, "Candidatus Hamiltonella defensa" and Rickettsia sp., whereas MED has only "Ca. Portiera aleyrodidarum" and "Ca. Hamiltonella defensa." Both "Ca. Portiera aleyrodidarum" and "Ca. Hamiltonella defensa" are intracellular endosymbionts residing in the bacteriomes, whereas Rickettsia sp. has a scattered distribution throughout the host body cavity. We examined responses of these symbionts to adverse temperatures as well as survival of the host insects. After cold treatment at 5 or 10 °C or heat treatment at 35 or 40 °C for 24 h, respectively, the infection rates of all symbionts were not significantly decreased based on diagnosis PCR. However, quantitative PCR assays indicated significant reduction of "Ca. Hamiltonella defensa" at 40 °C, and the reduction became greater as the duration increased. Compared with "Ca. Hamiltonella defensa," "Ca. Portiera aleyrodidarum" was initially less affected in the first day but then showed more rapid reduction at days 3-5. The density of Rickettsia sp. fluctuated but was not reduced significantly at 40 °C. Meanwhile, the mortality rates of the host whiteflies elevated rapidly as the duration of exposure to heat treatment increased. The differential responses of various symbionts to adverse temperatures imply complex interactions among the symbionts inside the same host insect and highlight the importance of taking the whole bacterial community into account in studies of symbioses.

Location of symbionts in the whitefly Bemisia tabaci affects their densities during host development and environmental stress

Bacterial symbionts often enhance the physiological capabilities of their arthropod hosts and enable their hosts to expand into formerly unavailable niches, thus leading to biological diversification. Many arthropods, including the worldwide invasive whitefly Bemisia tabaci, have individuals simultaneously infected with symbionts of multiple genera that occur in different locations in the host. This study examined the population dynamics of symbionts that are located in different areas within B. tabaci. While densities of Portiera and Hamiltonella (which are located in bacteriocytes) appeared to be well-regulated during host development, densities of Rickettsia (which are not located in bacteriocytes) were highly variable among individual hosts during host development. Host mating did not significantly affect symbiont densities. Infection by Tomato yellow leaf curl virus did not affect Portiera and Hamiltonella densities in either sex, but increased Rickettsia densities in females. High and low temperatures did not affect Portiera and Hamiltonella densities, but low temperature (15°C) significantly suppressed Rickettsia densities whereas high temperature (35°C) had little effect on Rickettsia densities. The results are consistent with the view that the population dynamics of bacterial symbionts in B. tabaci are regulated by symbiont location within the host and that the regulation reflects adaptation between the bacteria and insect.

Male-killing Spiroplasma induces sex-specific cell death via host apoptotic pathway

Some symbiotic bacteria cause remarkable reproductive phenotypes like cytoplasmic incompatibility and male-killing in their host insects. Molecular and cellular mechanisms underlying these symbiont-induced reproductive pathologies are of great interest but poorly understood. In this study, Drosophila melanogaster and its native Spiroplasma symbiont strain MSRO were investigated as to how the host's molecular, cellular and morphogenetic pathways are involved in the symbiont-induced male-killing during embryogenesis. TUNEL (terminal deoxynucleotidyl transferase dUTP nick end labeling) staining, anti-cleaved-Caspase-3 antibody staining, and apoptosis-deficient mutant analysis unequivocally demonstrated that the host's apoptotic pathway is involved in Spiroplasma-induced male-specific embryonic cell death. Double-staining with TUNEL and an antibody recognizing epidermal marker showed that embryonic epithelium is the main target of Spiroplasma-induced male-specific apoptosis. Immunostaining with antibodies against markers of differentiated and precursor neural cells visualized severe neural defects specifically in Spiroplasma-infected male embryos as reported in previous studies. However, few TUNEL signals were detected in the degenerate nervous tissues of male embryos, and the Spiroplasma-induced neural defects in male embryos were not suppressed in an apoptosis-deficient host mutant. These results suggest the possibility that the apoptosis-dependent epidermal cell death and the apoptosis-independent neural malformation may represent different mechanisms underlying the Spiroplasma-induced male-killing. Despite the male-specific progressive embryonic abnormality, Spiroplasma titers remained almost constant throughout the observed stages of embryonic development and across male and female embryos. Strikingly, a few Spiroplasma-infected embryos exhibited gynandromorphism, wherein apoptotic cell death was restricted to male cells. These observations suggest that neither quantity nor proliferation of Spiroplasma cells but some Spiroplasma-derived factor(s) may be responsible for the expression of the male-killing phenotype.

Symbiotic bacteria are ubiquitously associated with diverse insects, and affect their host biology in a variety of ways. In Drosophila fruit flies, infection with Spiroplasma symbionts often causes male-specific embryonic mortality, resulting in the production of all-female offspring. This striking phenotype is called “male-killing”, whose underlying mechanisms are of great interest. Here we investigated Drosophila melanogaster and its native Spiroplasma symbiont strain to understand how the host's molecular, cellular and morphogenetic pathways are involved in the symbiont-induced male-killing. Specifically in Spiroplasma-infected male embryos, pathogenic phenotypes including massive cell death throughout the body and neural malformation were observed. We unequivocally identified that the male-specific cell death preferentially occurs in the embryonic epithelium via the host's apoptotic pathway. Meanwhile, we found that, unexpectedly, the male-specific neural defects occur independently of host's apoptosis, suggesting that at least two different mechanisms may be involved in the Spiroplasma-induced male-killing. Also unexpected was the finding that Spiroplasma titers are almost constant throughout embryogenesis irrespective of sex despite the male-specific severe apoptosis. We serendipitously found Spiroplasma-infected sexual mosaic embryos, wherein apoptosis was associated with male cells, which suggests that some Spiroplasma-derived factor(s) may selectively act on male cells and cause male-killing.

Low temperature reveals genetic variability against male-killing Spiroplasma in Drosophila melanogaster natural populations

Spiroplasma endosymbionts are maternally inherited microorganisms which infect many arthropod species. In some Drosophila species, it acts as a reproductive manipulator, spreading in populations by killing the sons of infected mothers. Distinct Drosophila melanogaster populations from Brazil exhibit variable male-killing Spiroplasma prevalences. In this study, we investigated the presence of variability for the male-killing phenotype among Drosophila and/or Spiroplasma strains and verified if it correlates with the endosymbiont prevalence in natural populations. For that, we analyzed the male-killing expression when Spiroplasma strains from different populations were transferred to a standard D. melanogaster line (Canton-S) and when a common Spiroplasma strain was transferred to different wild-caught D. melanogaster lines, both at optimal and challenging temperatures for the bacteria. No variation was observed in the male-killing phenotype induced by different Spiroplasma strains. No phenotypic variability among fly lines was detected at optimal temperature (23 °C), as well. Conversely, significant variation in the male-killing expression was revealed among D. melanogaster lines at 18.5 °C, probably caused by imperfect transmission of the endosymbiont. Distinct lines differed in their average sex ratios as well as in the pattern of male-killing expression as the infected females aged. Greater variation occurred among lines from one locality, although there was no clear correlation between the male-killing intensity and the endosymbiont prevalence in each population. Imperfect transmission or male killing may also occur in the field, thus helping to explain the low or intermediate prevalences reported in nature. We discuss the implications of our results for the dynamics of male-killing Spiroplasma in natural populations.

Purine biosynthesis-deficient Burkholderia mutants are incapable of symbiotic accommodation in the stinkbug

The Riptortus-Burkholderia symbiotic system represents a promising experimental model to study the molecular mechanisms involved in insect-bacterium symbiosis due to the availability of genetically manipulated Burkholderia symbiont. Using transposon mutagenesis screening, we found a symbiosis-deficient mutant that was able to colonize the host insect but failed to induce normal development of host's symbiotic organ. The disrupted gene was identified as purL involved in purine biosynthesis. In vitro growth impairment of the purL mutant and its growth dependency on adenine and adenosine confirmed the functional disruption of the purine synthesis gene. The purL mutant also showed defects in biofilm formation, and this defect was not rescued by supplementation of purine derivatives. When inoculated to host insects, the purL mutant was initially able to colonize the symbiotic organ but failed to attain a normal infection density. The low level of infection density of the purL mutant attenuated the development of the host's symbiotic organ at early instar stages and reduced the host's fitness throughout the nymphal stages. Another symbiont mutant-deficient in a purine biosynthesis gene, purM, showed phenotypes similar to those of the purL mutant both in vitro and in vivo, confirming that the purL phenotypes are due to disrupted purine biosynthesis. These results demonstrate that the purine biosynthesis genes of the Burkholderia symbiont are critical for the successful accommodation of symbiont within the host, thereby facilitating the development of the host's symbiotic organ and enhancing the host's fitness values.

Density dynamics of diverse Spiroplasma strains naturally infecting different species of Drosophila

Facultative heritable bacterial endosymbionts can have dramatic effects on their hosts, ranging from mutualistic to parasitic. Within-host bacterial endosymbiont density plays a critical role in maintenance of a symbiotic relationship, as it can affect levels of vertical transmission and expression of phenotypic effects, both of which influence the infection prevalence in host populations. Species of genus Drosophila are infected with Spiroplasma, whose characterized phenotypic effects range from that of a male-killing reproductive parasite to beneficial defensive endosymbiont. For many strains of Spiroplasma infecting at least 17 species of Drosophila, however, the phenotypic effects are obscure. The infection prevalence of these Spiroplasma vary within and among Drosophila species, and little is known about the within-host density dynamics of these diverse strains. To characterize the patterns of Spiroplasma density variation among Drosophila we used quantitative PCR to assess bacterial titer at various life stages of three species of Drosophila naturally-infected with two different types of Spiroplasma. For naturally infected Drosophila species we found that non-male-killing infections had consistently lower densities than the male-killing infection. The patterns of Spiroplasma titer change during aging varied among Drosophila species infected with different Spiroplasma strains. Bacterial density varied within and among populations of Drosophila, with individuals from the population with the highest prevalence of infection having the highest density. This density variation underscores the complex interaction of Spiroplasma strain and host genetic background in determining endosymbiont density.

Vertical transmission of a Drosophila endosymbiont via cooption of the yolk transport and internalization machinery

Spiroplasma is a diverse bacterial clade that includes many vertically transmitted insect endosymbionts, including Spiroplasma poulsonii, a natural endosymbiont of Drosophila melanogaster. These bacteria persist in the hemolymph of their adult host and exhibit efficient vertical transmission from mother to offspring. In this study, we analyzed the mechanism that underlies their vertical transmission, and here we provide strong evidence that these bacteria use the yolk uptake machinery to colonize the germ line. We show that Spiroplasma reaches the oocyte by passing through the intercellular space surrounding the ovarian follicle cells and is then endocytosed into oocytes within yolk granules during the vitellogenic stages of oogenesis. Mutations that disrupt yolk uptake by oocytes inhibit vertical Spiroplasma transmission and lead to an accumulation of these bacteria outside the oocyte. Impairment of yolk secretion by the fat body results in Spiroplasma not reaching the oocyte and a severe reduction of vertical transmission. We propose a model in which Spiroplasma first interacts with yolk in the hemolymph to gain access to the oocyte and then uses the yolk receptor, Yolkless, to be endocytosed into the oocyte. Cooption of the yolk uptake machinery is a powerful strategy for endosymbionts to target the germ line and achieve vertical transmission. This mechanism may apply to other endosymbionts and provides a possible explanation for endosymbiont host specificity.

Most insect species, including important disease vectors and crop pests, harbor vertically transmitted endosymbiotic bacteria. Studies have shown that many facultative endosymbionts, including Spiroplasma, confer protection against different classes of parasites on their hosts and therefore are attractive tools for the control of vector-borne diseases. The ability to be efficiently transmitted from females to their offspring is the key feature shaping associations between insects and their inherited endosymbionts, but to date, little is known about the mechanisms involved. In oviparous animals, yolk accumulates in developing eggs and serves to meet the nutritional demands of embryonic development. Here we show that Spiroplasma coopts the yolk transport and uptake machinery to colonize the germ line and ensure efficient vertical transmission. The uptake of yolk is a female germ line-specific feature and therefore an attractive target for cooption by endosymbionts that need to maintain high-fidelity maternal transmission.

Spiroplasma in Drosophila melanogaster populations: prevalence, male-killing, molecular identification, and no association with Wolbachia

Spiroplasma endosymbionts are maternally transmitted bacteria that may kill infected sons resulting in the production of female-biased broods. The prevalence of male killers varies considerably both between and within species. Here, we evaluate the spatial and temporal status of male-killing and non-male-killing Spiroplasma infection in three Brazilian populations of Drosophila melanogaster, nearly a decade after the first occurrence report for this species. The incidence of the male-killing Spiroplasma ranged from close to 0 to 17.7 % (so far the highest estimate for a Drosophila species) with a suggestion of temporal decline in a population. We also found non-male-killing Spiroplasma coexisting in one population at lower prevalence (3-5 %), and we did not detect it in the other two. This may be taken as a suggestion of a spreading advantage conferred by the male-killing strategy. Sequencing two loci, we identified the phylogenetic position of Spiroplasma strains from the three localities, showing that all strains group closely in the poulsonii clade. Due to intensive sampling effort, we were able to test the association between Spiroplasma infections and another widespread endosymbiont, Wolbachia, whose prevalence ranged from 81.8 to 100 %. The prevalence of Wolbachia did not differ between Spiroplasma-infected and uninfected strains in our largest sample nor were the prevalences of the two endosymbionts associated across localities.

Mutualism meltdown in insects: bacteria constrain thermal adaptation

Predicting whether and how organisms will successfully cope with climate change presents critical questions for biologists and environmental scientists. Models require knowing how organisms interact with their abiotic environment, as well understanding biotic interactions that include a network of symbioses in which all species are embedded. Bacterial symbionts of insects offer valuable models to examine how microbes can facilitate and constrain adaptation to a changing environment. While some symbionts confer plasticity that accelerates adaptation, long-term bacterial mutualists of insects are characterized by tight lifestyle constraints, genome deterioration, and vulnerability to thermal stress. These essential bacterial partners are eliminated at high temperatures, analogous to the loss of zooanthellae during coral bleaching. Recent field-based studies suggest that thermal sensitivity of bacterial mutualists constrains insect responses. In this sense, highly dependent mutualisms may be the Achilles' heel of thermal responses in insects.

Factors affecting population dynamics of maternally transmitted endosymbionts in Bemisia tabaci

While every individual of Bemisia tabaci (Hemiptera: Aleyrodidae) harbors the primary symbiont (P-symbiont) Portiera, the infection frequencies of the six secondary symbionts (S-symbionts) including Hamiltonella, Arsenophonus, Cardinium, Wolbachia, Rickettsia and Fritschea vary greatly among different populations. To characterize the factors influencing the infection dynamics of the six S-symbionts in B. tabaci, gene-specific PCR were conducted to screen for the presence of the P-symbiont Portiera and the six S-symbionts in 61 (17 B and 44 Q biotypes) field populations collected from different plant species and locations in China. All individuals of the 61 populations hosted the P-symbiont Portiera, but none of them harbored Arsenophonus and Fritschea. The presence and infection rates of Hamiltonella, Cardinium, Rickettsia, Wolbachia and their co-infections Rickettsia + Hamiltonella (RH), Rickettsia + Cardinium (RC), Hamiltonella + Cardinium (HC) and Rickettsia + Hamiltonella + Cardinium (RHC) varied significantly among the 61 field populations; and the observed variations can be explained by biotypes, sexes, host plants and geographical locations of these field populations. Taken together, at least three factors including biotype, host plant and geographical location affect the infection dynamics of S-symbionts in B. tabaci.

Temperature affects the tripartite interactions between bacteriophage WO, Wolbachia, and cytoplasmic incompatibility

Wolbachia infections are a model for understanding intracellular, bacterial symbioses. While the symbiosis is often studied from a binary perspective of host and bacteria, it is increasingly apparent that additional trophic levels can influence the symbiosis. For example, Wolbachia in arthropods harbor a widespread temperate bacteriophage, termed WO, that forms virions and rampantly transfers between coinfections. Here we test the hypothesis that temperatures at the extreme edges of an insect's habitable range alter bacteriophage WO inducibility and in turn, Wolbachia densities and the penetrance of cytoplasmic incompatibility. We report four key findings using the model wasp, Nasonia vitripennis: First, both cold treatment at 18 C and heat treatment at 30 C reduce Wolbachia densities by as much as 74% relative to wasps reared at 25 C. Second, in all cases where Wolbachia densities decline due to temperature changes, phage WO densities increase and inversely associate with Wolbachia densities. Heat has a marked effect on phage WO, yielding phage densities that are 552% higher than the room temperature control. Third, there is a significant affect of insect family on phage WO and endoysmbiont densities. Fourth, at extreme temperatures, there was a temperature-mediated adjustment to the density threshold at which Wolbachia cause complete cytoplasmic incompatibility. Taken together, these results demonstrate that temperature simultaneously affects phage WO densities, endosymbiont densities, and the penetrance of cytoplasmic incompatibility. While temperature shock enhances bacteriophage inducibility and the ensuing bacterial mortality in a wide range of medically and industrially-important bacteria, this is the first investigation of the associations in an obligate intracellular bacteria. Implications to a SOS global sensing feedback mechanism in Wolbachia are discussed.