Uncovering Wolbachia diversity upon artificial host transfer

Infection Dynamics of Cotransmitted Reproductive Symbionts Are Mediated by Sex, Tissue, and Development

One of the most prevalent intracellular infections on earth is with Wolbachia, a bacterium in the Rickettsiales that infects a range of insects, crustaceans, chelicerates, and nematodes. Wolbachia is maternally transmitted to offspring and has profound effects on the reproduction and physiology of its hosts, which can result in reproductive isolation, altered vectorial capacity, mitochondrial sweeps, and even host speciation. Some populations stably harbor multiple Wolbachia strains, which can further contribute to reproductive isolation and altered host physiology. However, almost nothing is known about the requirements for multiple intracellular microbes to be stably maintained across generations while they likely compete for space and resources. Here, we use a coinfection of two Wolbachia strains (“wHa” and “wNo”) in Drosophila simulans to define the infection and transmission dynamics of an evolutionarily stable double infection. We find that a combination of sex, tissue, and host development contributes to the infection dynamics of the two microbes and that these infections exhibit a degree of niche partitioning across host tissues. wHa is present at a significantly higher titer than wNo in most tissues and developmental stages, but wNo is uniquely dominant in ovaries. Unexpectedly, the ratio of wHa to wNo in embryos does not reflect those observed in the ovaries, indicative of strain-specific transmission dynamics. Understanding how Wolbachia strains interact to establish and maintain stable infections has important implications for the development and effective implementation of Wolbachia-based vector biocontrol strategies, as well as more broadly defining how cooperation and conflict shape intracellular communities.

IMPORTANCEWolbachia is a maternally transmitted intracellular bacterium that manipulates the reproduction and physiology of arthropods, resulting in drastic effects on the fitness, evolution, and even speciation of its hosts. Some hosts naturally harbor multiple strains of Wolbachia that are stably transmitted across generations, but almost nothing is known about the factors that limit or promote these coinfections, which can have profound effects on the host’s biology and evolution and are under consideration as an insect-management tool. Here, we define the infection dynamics of a known stably transmitted double infection in Drosophila simulans with an eye toward understanding the patterns of infection that might facilitate compatibility between the two microbes. We find that a combination of sex, tissue, and development all contributes to infection dynamics of the coinfection.

Analyses of locomotion, wing morphology, and microbiome in Drosophila nigrosparsa after recovery from antibiotics

Antibiotics, such as tetracycline, have been frequently used to cure arthropods of Wolbachia endosymbionts. After the symbionts have been removed, the hosts must recover for some generations from the side effects of the antibiotics. However, most studies do not assess the direct and indirect longer‐term effects of antibiotics used to remove Wolbachia, which may question the exact contribution of this endosymbiont to the effects observed. Here, we used the fly Drosophila nigrosparsa treated or not with tetracycline for three generations followed by two generations of recovery to investigate the effects of this antibiotic on the fly locomotion, wing morphology, and the gut microbiome. We found that antibiotic treatment did not affect fly locomotion two generations after being treated with the antibiotic. In addition, gut‐microbiome restoration was tested as a more efficient solution to reduce the potential side effects of tetracycline on the microbiome. There was no significant difference in alpha diversity between gut restoration and other treatments, but the abundance of some bacterial taxa differed significantly between the gut‐restoration treatment and the control. We conclude that in D. nigrosparsa the recovery period of two generations after being treated with the antibiotic is sufficient for locomotion, and suggest a general assessment of direct and indirect effects of antibiotics after a particular recovery time.

Parallel Sequencing of Wolbachia wCer2 from Donor and Novel Hosts Reveals Multiple Incompatibility Factors and Genome Stability after Host Transfers

The application of Wolbachia in insect pest and vector control requires the establishment of genotypically stable host associations. The cytoplasmic incompatibility (CI) inducing Wolbachia strain wCer2 naturally occurs in the cherry fruit fly Rhagoletis cerasi as co-infection with other strains and was transferred to other fruit fly species by embryonic microinjections. We obtained wCer2 genome data from its native and three novel hosts, Drosophila simulans, Drosophila melanogaster, and Ceratitis capitata and assessed its genome stability, characteristics, and CI factor (cif) genes. De novo assembly was successful from Wolbachia cell-enriched singly infected D. simulans embryos, with minimal host and other bacterial genome traces. The low yield of Wolbachia sequence reads from total genomic extracts of one multiply infected R. cerasi pupa and one singly infected C. capitata adult limited de novo assemblies but was sufficient for comparative analyses. Across hosts wCer2 was stable in genome synteny and content. Polymorphic nucleotide sites were found in wCer2 of each host; however, only one nucleotide was different between R. cerasi and C. capitata, and none between replicated D. simulans lines. The wCer2 genome is highly similar to wAu (D. simulans), wMel (D. melanogaster), and wRec (Drosophila recens). In contrast to wMel and wRec (each with one cif gene pair) and wAu (without any cif genes), wCer2 has three pairs of Type I cif genes, and one Type V cifB gene without a cifA complement. This may explain previously reported CI patterns of wCer2, including incomplete rescue of its own CI modification in three novel host species.

Comparative genome sequencing reveals insights into the dynamics of Wolbachia in native and invasive cherry fruit flies

Wolbachia is a maternally inherited obligate endosymbiont that can induce a wide spectrum of effects in its host, ranging from mutualism to reproductive parasitism. At the genomic level, recombination within and between strains, transposable elements, and horizontal transfer of strains between host species make Wolbachia an evolutionarily dynamic bacterial system. The invasive cherry fruit fly Rhagoletis cingulata arrived in Europe from North America ~40 years ago, where it now co‐occurs with the native cherry pest R. cerasi. This shared distribution has been proposed to have led to the horizontal transfer of different Wolbachia strains between the two species. To better understand transmission dynamics, we performed a comparative genome study of the strain wCin2 in its native United States and invasive European populations of R. cingulata with wCer2 in European R. cerasi. Previous multilocus sequence genotyping (MLST) of six genes implied that the source of wCer2 in R. cerasi was wCin2 from R. cingulata. However, we report genomic evidence discounting the recent horizontal transfer hypothesis for the origin of wCer2. Despite near identical sequences for the MLST markers, substantial sequence differences for other loci were found between wCer2 and wCin2, as well as structural rearrangements, and differences in prophage, repetitive element, gene content, and cytoplasmic incompatibility inducing genes. Our study highlights the need for whole‐genome sequencing rather than relying on MLST markers for resolving Wolbachia strains and assessing their evolutionary dynamics.

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.

Old residents and new arrivals of Rhagoletis species in Europe

The genus Rhagoletis (Diptera: Tephritidae) comprises more than 65 species distributed throughout Europe, Asia and America, including many species of high economic importance. Currently, there are three Rhagoletis species that infest fruits and nuts in Europe. The European cherry fruit fly, Rhagoletis cerasi (may have invaded Europe a long time ago from the Caucasian area of West Asia), and two invasive species (recently introduced from North America): the eastern American cherry fruit fly, R. cingulata, and the walnut husk fly, R. completa. The presence of different Rhagoletis species may enhance population dynamics and establish an unpredictable economic risk for several fruit and nut crops in Europe. Despite their excessive economic importance, little is known on population dynamics, genetics and symbiotic associations for making sound pest control decisions in terms of species-specific, environmental friendly pest control methods. To this end, the current paper (a) summarizes recently accumulated genetic and population data for the European Rhagoletis species and their association with the endosymbiont Wolbachia pipientis, and (b) explores the possibility of using the current knowledge for implementing the innovative biological control methods of sterile insect technique and incompatible insect technique.

Wolbachia prevalence, diversity, and ability to induce cytoplasmic incompatibility in mosquitoes

To protect humans and domestic animals from mosquito borne diseases, alternative methods to chemical insecticides have to be found. Pilot studies using the vertically transmitted bacterial endosymbiont Wolbachia were already launched in different parts of the world. Wolbachia can be used either in Incompatible Insect Technique (IIT), to decrease mosquito population, or to decrease the ability of mosquitoes to transmit pathogens. Not all mosquito species are naturally infected with Wolbachia: while in Culex pipiens and Aedes albopictus almost all individuals harbor Wolbachia, putative infections have to be further investigated in Anopheles species and in Aedes aegypti. All Wolbachia-based control methods rely on the ability of Wolbachia to induce cytoplasmic incompatibility (CI) resulting in embryonic death in incompatible crossings. Knowledge on CI diversity in mosquito is required to find the better Wolbachia-mosquito associations to optimize the success of both 'sterile insect' and 'pathogen blocking' Wolbachia-based methods.

Molecular analyses identify hybridization-mediated nuclear evolution in newly discovered fungal hybrids

Hybridization may be a major driver in the evolution of plant pathogens. In a high elevation Alpine larch stand in Montana, a novel hybrid fungal pathogen of trees originating from the mating of Heterobasidion irregulare with H. occidentale has been recently discovered. In this study, sequence analyses of one mitochondrial and four nuclear loci from 11 Heterobasidion genotypes collected in the same Alpine larch stand indicated that hybridization has increased allelic diversity by generating novel polymorphisms unreported in either parental species. Sequence data and ploidy analysis through flow cytometry confirmed that heterokaryotic (n + n) genotypes were not first-generation hybrids, but were the result of multiple backcrosses, indicating hybrids are fertile. Additionally, all admixed genotypes possessed the H. occidentale mitochondrion, indicating that the hybrid progeny may have been backcrossing mostly with H. occidentale. Based on reticulate phylogenetic network analysis by PhyloNet, Bayesian assignment, and ordination tests, alleles can be defined as H. irregulare-like or H. occidentale-like. H. irregulare-like alleles are clearly distinct from all known H. irregulare alleles and are derived from the admixing of both Heterobasidion species. Instead, all but one H. occidentale alleles found in hybrids, although novel, were not clearly distinct from alleles found in the parental H. occidentale population. This discovery demonstrates that Alpine larch can be a universal host favouring the interspecific hybridization between H. irregulare and H. occidentale and the hybridization-mediated evolution of a nucleus, derived from H. irregulare parental species but clearly distinct from it.

Symbiont-Driven Male Mating Success in the Neotropical Drosophila paulistorum Superspecies

Microbial symbionts are ubiquitous associates of living organisms but their role in mediating reproductive isolation (RI) remains controversial. We addressed this knowledge gap by employing the Drosophila paulistorum-Wolbachia model system. Semispecies in the D. paulistorum species complex exhibit strong RI between each other and knockdown of obligate mutualistic Wolbachia bacteria in female D. paulistorum flies triggers loss of assortative mating behavior against males carrying incompatible Wolbachia strains. Here we set out to determine whether de novo RI can be introduced by Wolbachia-knockdown in D. paulistorum males. We show that Wolbachia-knockdown D. paulistorum males (i) are rejected as mates by wild type females, (ii) express altered sexual pheromone profiles, and (iii) are devoid of the endosymbiont in pheromone producing cells. Our findings suggest that changes in Wolbachia titer and tissue tropism can induce de novo premating isolation by directly or indirectly modulating sexual behavior of their native D. paulistorum hosts.

Enhancing vector refractoriness to trypanosome infection: achievements, challenges and perspectives

With the absence of effective prophylactic vaccines and drugs against African trypanosomosis, control of this group of zoonotic neglected tropical diseases depends the control of the tsetse fly vector. When applied in an area-wide insect pest management approach, the sterile insect technique (SIT) is effective in eliminating single tsetse species from isolated populations. The need to enhance the effectiveness of SIT led to the concept of investigating tsetse-trypanosome interactions by a consortium of researchers in a five-year (2013-2018) Coordinated Research Project (CRP) organized by the Joint Division of FAO/IAEA. The goal of this CRP was to elucidate tsetse-symbiome-pathogen molecular interactions to improve SIT and SIT-compatible interventions for trypanosomoses control by enhancing vector refractoriness. This would allow extension of SIT into areas with potential disease transmission. This paper highlights the CRP's major achievements and discusses the science-based perspectives for successful mitigation or eradication of African trypanosomosis.

Host and symbiont genetic contributions to fitness in a Trichogramma-Wolbachia symbiosis

The fitness effects associated with Wolbachia infection have wide-ranging ecological and evolutionary consequences for host species. How these effects are modulated by the relative influence of host and Wolbachia genomes has been described as a balancing act of genomic cooperation and conflict. For vertically transmitted symbionts, like cytoplasmic Wolbachia, concordant host-symbiont fitness interests would seem to select for genomic cooperation. However, Wolbachia's ability to manipulate host reproductive systems and distort offspring sex ratios presents an evolutionary conflict of interest with infected hosts. In the parthenogenesis-inducing (PI) form of Wolbachia found in many haplodiploid insects, Wolbachia fitness is realized through females and is enhanced by their feminization of male embryos and subsequent parthenogenetic reproduction. In contrast, as long as Wolbachia is not fixed in a population and sexual reproduction persists, fitness for the host species is realized through both male and female offspring production. How these cooperating and competing interests interact and the relative influence of host and Wolbachia genomes were investigated in the egg parasitoid Trichogramma kaykai, where Wolbachia infection has remained at a low frequency in the field. A factorial design in which laboratory cultures of Wolbachia-infected T. kaykai were cured and re-infected with alternative Wolbachia strains was used to determine the relative influence of host and Wolbachia genomes on host fitness values. Our results suggest fitness variation is largely a function of host genetic background, except in the case of offspring sex ratio where a significant interaction between host and Wolbachia genomes was found. We also find a significant effect associated with the horizontal transfer of Wolbachia strains, which we discuss in terms of the potential for coadaptation in PI-Wolbachia symbioses.

Current state of knowledge on Wolbachia infection among Coleoptera: a systematic review

Background

Despite great progress in studies on Wolbachia infection in insects, the knowledge about its relations with beetle species, populations and individuals, and the effects of bacteria on these hosts, is still unsatisfactory. In this review we summarize the current state of knowledge about Wolbachia occurrence and interactions with Coleopteran hosts.

Methods

An intensive search of the available literature resulted in the selection of 86 publications that describe the relevant details about Wolbachia presence among beetles. These publications were then examined with respect to the distribution and taxonomy of infected hosts and diversity of Wolbachia found in beetles. Sequences of Wolbachia genes (16S rDNA, ftsZ) were used for the phylogenetic analyses.

Results

The collected publications revealed that Wolbachia has been confirmed in 204 beetle species and that the estimated average prevalence of this bacteria across beetle species is 38.3% and varies greatly across families and genera (0–88% infected members) and is much lower (c. 13%) in geographic studies. The majority of the examined and infected beetles were from Europe and East Asia. The most intensively studied have been two groups of herbivorous beetles: Curculionidae and Chrysomelidae. Coleoptera harbor Wolbachia belonging to three supergroups: F found in only three species, and A and B found in similar numbers of beetles (including some doubly infected); however the latter two were most prevalent in different families. A total of 59% of species with precise data were found to be totally infected. Single infections were found in 69% of species and others were doubly- or multiply-infected. Wolbachia caused numerous effects on its beetle hosts, including selective sweep with host mtDNA (found in 3% of species), cytoplasmic incompatibility (detected in c. 6% of beetles) and other effects related to reproduction or development (like male-killing, possible parthenogenesis or haplodiploidy induction, and egg development). Phylogenetic reconstructions for Wolbachia genes rejected cospeciation between these bacteria and Coleoptera, with minor exceptions found in some Hydraenidae, Curculionidae and Chrysomelidae. In contrast, horizontal transmission of bacteria has been suspected or proven in numerous cases (e.g., among beetles sharing habitats and/or host plants).

Discussion

The present knowledge about Wolbachia infection across beetle species and populations is very uneven. Even the basic data about infection status in species and frequency of infected species across genera and families is very superficial, as only c. 0.15% of all beetle species have been tested so far. Future studies on Wolbachia diversity in Coleoptera should still be based on the Multi-locus Sequence Typing system, and next-generation sequencing technologies will be important for uncovering Wolbachia relations with host evolution and ecology, as well as with other, co-occurring endosymbiotic bacteria.

Phenotypic shift in Wolbachia virulence towards its native host across serial horizontal passages

Vertical transmission mode is predicted to decrease the virulence of symbionts. However, Wolbachia, a widespread vertically transmitted endosymbiont, exhibits both negative and beneficial effects on arthropod fitness. This 'Jekyll and Hyde' behaviour, as well as its ability to live transiently outside host cells and to establish new infections via horizontal transmission, may reflect the capacity of Wolbachia to exhibit various phenotypes depending on the prevailing environmental constraints. To study the ability of Wolbachia to readily cope with new constraints, we forced this endosymbiont to spread only via horizontal transmission. To achieve this, we performed serial horizontal transfers of haemolymph from Wolbachia-infected to naive individuals of the isopod Armadillidium vulgare. Across passages, we observed phenotypic changes in the symbiotic relationship: (i) The Wolbachia titre increased in both haemolymph and nerve cord but remained stable in ovaries; (ii) Wolbachia infection was benign at the beginning of the experiment, but highly virulent, killing most hosts after only a few passages. Such a phenotypic shift after recurrent horizontal passages demonstrates that Wolbachia can rapidly change its virulence when facing new environmental constraints. We thoroughly discuss the potential mechanism(s) underlying this phenotypic change, which are likely to be crucial for the ongoing radiation of Wolbachia in arthropods.

Insertion sequence polymorphism and genomic rearrangements uncover hidden Wolbachia diversity in Drosophila suzukii and D. subpulchrella

Ability to distinguish between closely related Wolbachia strains is crucial for understanding the evolution of Wolbachia-host interactions and the diversity of Wolbachia-induced phenotypes. A useful model to tackle these issues is the Drosophila suzukii - Wolbachia association. D. suzukii, a destructive insect pest, harbor a non-CI inducing Wolbachia 'wSuz' closely related to the strong CI-inducing wRi strain. Multi locus sequence typing (MLST) suggests presence of genetic homogeneity across wSuz strains infecting European and American D. suzukii populations, although different Wolbachia infection frequencies and host fecundity levels have been observed in both populations. Currently, it is not clear if these differences are due to cryptic wSuz polymorphism, host background, geographical factors or a combination of all of them. Here, we have identified geographical diversity in wSuz in D. suzukii populations from different continents using a highly diagnostic set of markers based on insertion sequence (IS) site polymorphism and genomic rearrangements (GR). We further identified inter-strain diversity between Wolbachia infecting D. suzukii and its sister species D. subpulchrella (wSpc). Based on our results, we speculate that discernible wSuz variants may associate with different observed host phenotypes, a hypothesis that demands future investigation. More generally, our results demonstrate the utility of IS and GRs in discriminating closely related Wolbachia strains.

Extracellular Osmotic Stress Reduces the Vesicle Size while Keeping a Constant Neurotransmitter Concentration

Secretory cells respond to hypertonic stress by cell shrinking, which causes a reduction in exocytosis activity and the amount of signaling molecules released from single exocytosis events. These changes in exocytosis have been suggested to result from alterations in biophysical properties of cell cytoplasm and plasma membrane, based on the assumption that osmotic stress does not affect the secretory vesicle content and size prior to exocytosis. To further investigate whether vesicles in secretory cells are affected by the osmolality of the extracellular environment, we used intracellular electrochemical cytometry together with transmission electron microscopy imaging to quantify and determine the catecholamine concentration of dense core vesicles in situ before and after cell exposure to osmotic stress. In addition, single cell amperometry recordings of exocytosis at chromaffin cells were used to monitor the effect on exocytosis activity and quantal release when cells were exposed to osmotic stress. Here we show that hypertonic stress hampers exocytosis secretion after the first pool of readily releasable vesicles have been fused and that extracellular osmotic stress causes catecholamine filled vesicles to shrink, mainly by reducing the volume of the halo solution surrounding the protein matrix in dense core vesicles. In addition, the vesicles demonstrate the ability to perform adjustments in neurotransmitter content during shrinking, and intracellular amperometry measurements in situ suggest that vesicles reduce the catecholamine content to maintain a constant concentration within the vesicle compartment. Hence, the secretory vesicles in the cell cytoplasm are highly affected and respond to extracellular osmotic stress, which gives a new perspective to the cause of reduction in quantal size by these vesicles when undergoing exocytosis.

Wolbachia pseudogenes and low prevalence infections in tropical but not temperate Australian tephritid fruit flies: manifestations of lateral gene transfer and endosymbiont spillover?

Background

Maternally inherited Wolbachia bacteria infect many insect species. They can also be transferred horizontally into uninfected host lineages. A Wolbachia spillover from an infected source population must occur prior to the establishment of heritable infections, but this spillover may be transient. In a previous study of tephritid fruit fly species of tropical Australia we detected a high incidence of identical Wolbachia strains in several species as well as Wolbachia pseudogenes. Here, we have investigated this further by analysing field specimens of 24 species collected along a 3,000 km climate gradient of eastern Australia.

Results

Wolbachia sequences were detected in individuals of nine of the 24 (37 %) species. Seven (29 %) species displayed four distinct Wolbachia strains based on characterisation of full multi locus sequencing (MLST) profiles; the strains occurred as single and double infections in a small number of individuals (2–17 %). For the two remaining species all individuals had incomplete MLST profiles and Wolbachia pseudogenes that may be indicative of lateral gene transfer into host genomes. The detection of Wolbachia was restricted to northern Australia, including in five species that only occur in the tropics. Within the more widely distributed Bactrocera tryoni and Bactrocera neohumeralis, Wolbachia also only occurred in the north, and was not linked to any particular mitochondrial haplotypes.

Conclusions

The presence of Wolbachia pseudogenes at high prevalence in two species in absence of complete MLST profiles may represent footprints of historic infections that have been lost. The detection of identical low prevalence strains in a small number of individuals of seven species may question their role as reproductive manipulator and their vertical inheritance. Instead, the findings may be indicative of transient infections that result from spillover events from a yet unknown source. These spillover events appear to be restricted to northern Australia, without proliferation in host lineages further south. Our study highlights that tropical fruit fly communities contain Wolbachia pseudogenes and may be exposed to frequent horizontal Wolbachia transfer. It also emphasises that global estimates of Wolbachia frequencies may need to consider lateral gene transfer and Wolbachia spillover that may be regionally restricted, transient and not inherited.

Electronic supplementary material

The online version of this article (doi:10.1186/s12862-015-0474-2) contains supplementary material, which is available to authorized users.

Correlation between the green-island phenotype and Wolbachia infections during the evolutionary diversification of Gracillariidae leaf-mining moths

Internally feeding herbivorous insects such as leaf miners have developed the ability to manipulate the physiology of their host plants in a way to best meet their metabolic needs and compensate for variation in food nutritional composition. For instance, some leaf miners can induce green-islands on yellow leaves in autumn, which are characterized by photosynthetically active green patches in otherwise senescing leaves. It has been shown that endosymbionts, and most likely bacteria of the genus Wolbachia, play an important role in green-island induction in the apple leaf-mining moth Phyllonorycter blancardella. However, it is currently not known how widespread is this moth-Wolbachia-plant interaction. Here, we studied the co-occurrence between Wolbachia and the green-island phenotype in 133 moth specimens belonging to 74 species of Lepidoptera including 60 Gracillariidae leaf miners. Using a combination of molecular phylogenies and ecological data (occurrence of green-islands), we show that the acquisitions of the green-island phenotype and Wolbachia infections have been associated through the evolutionary diversification of Gracillariidae. We also found intraspecific variability in both green-island formation and Wolbachia infection, with some species being able to form green-islands without being infected by Wolbachia. In addition, Wolbachia variants belonging to both A and B supergroups were found to be associated with green-island phenotype suggesting several independent origins of green-island induction. This study opens new prospects and raises new questions about the ecology and evolution of the tripartite association between Wolbachia, leaf miners, and their host plants.

Polyploidy versus endosymbionts in obligately thelytokous thrips

Background

Thelytoky, the parthenogenetic development of females, has independently evolved in several insect orders yet the study of its mechanisms has so far mostly focussed on haplodiploid Hymenoptera, while alternative mechanisms of thelytoky such as polyploidy are far less understood. In haplodiploid insects, thelytoky can be encoded in their genomes, or induced by maternally inherited bacteria such as Wolbachia or Cardinium. Microbially facilitated thelytoky usually results in complete homozygosity due to gamete duplication and can be reverted into arrhenotoky, the parthenogenetic development of males, through treatment with antibiotics. In contrast, genetically encoded thelytoky cannot be removed and may result in conservation of heterozygosity due to gamete fusion. We have probed the obligate thelytoky of the greenhouse thrips, Heliothrips haemorrhoidalis (Bouché), a significant cosmopolitan pest and a model species of thelytoky in the haplodiploid insect order Thysanoptera. Earlier studies suggested terminal fusion as a mechanism for thelytoky in this species, while another study reported presence of Wolbachia; later it was speculated that Wolbachia plays a role in this thrips’ thelytokous reproduction.

Results

By using PCR and sequence analysis, we demonstrated that global population samples of H. haemorrhoidalis were not infected with Wolbachia, Cardinium or any other known bacterial reproductive manipulators. Antibiotic treatment of this thrips did also not result in male production. Some individuals carried two different alleles in two nuclear loci, histone 3 and elongation factor 1 alpha, suggesting heterozygosity. However, the majority of individuals had three different alleles suggesting that they were polyploid. Genetic diversity across both nuclear loci was low in all populations, and absent from mitochondrial cytochrome oxidase I, indicating that this species had experienced genetic bottlenecks, perhaps due to its invasion biology or a switch to thelytoky.

Conclusions

Geographically broad sampling and experimental manipulation revealed low genetic diversity, absence of Wolbachia but presence of three different alleles of nuclear loci in most analysed individuals of obligately thelytokous H. haemorrhoidalis. This suggests that polyploidy may be involved in the thelytokous reproduction of this thrips species, and polyploidy may be a contributing factor in the reproduction of Thysanoptera and other haplodiploid insect orders.

Electronic supplementary material

The online version of this article (doi:10.1186/s12862-015-0304-6) contains supplementary material, which is available to authorized users.

Mutualism breakdown by amplification of Wolbachia genes

Most insect species are associated with vertically transmitted endosymbionts. Because of the mode of transmission, the fitness of these symbionts is dependent on the fitness of the hosts. Therefore, these endosymbionts need to control their proliferation in order to minimize their cost for the host. The genetic bases and mechanisms of this regulation remain largely undetermined. The maternally inherited bacteria of the genus Wolbachia are the most common endosymbionts of insects, providing some of them with fitness benefits. In Drosophila melanogaster, WolbachiawMelPop is a unique virulent variant that proliferates massively in the hosts and shortens their lifespan. The genetic bases of wMelPop virulence are unknown, and their identification would allow a better understanding of how Wolbachia levels are regulated. Here we show that amplification of a region containing eight Wolbachia genes, called Octomom, is responsible for wMelPop virulence. Using Drosophila lines selected for carrying Wolbachia with different Octomom copy numbers, we demonstrate that the number of Octomom copies determines Wolbachia titers and the strength of the lethal phenotype. Octomom amplification is unstable, and reversion of copy number to one reverts all the phenotypes. Our results provide a link between genotype and phenotype in Wolbachia and identify a genomic region regulating Wolbachia proliferation. We also prove that these bacteria can evolve rapidly. Rapid evolution by changes in gene copy number may be common in endosymbionts with a high number of mobile elements and other repeated regions. Understanding wMelPop pathogenicity and variability also allows researchers to better control and predict the outcome of releasing mosquitoes transinfected with this variant to block human vector-borne diseases. Our results show that transition from a mutualist to a pathogen may occur because of a single genomic change in the endosymbiont. This implies that there must be constant selection on endosymbionts to control their densities.

An elegant experimental evolution approach reveals that a strain of the symbiotic bacterium Wolbachia that over-replicates and shortens the life of its fruit fly host owes this property to the amplification of a small region of its genome. Read the accompanying Primer.

Insects frequently carry intracellular bacteria that are passed from generation to generation through their eggs. These intracellular symbionts can be beneficial or parasitic, but because of their mode of transmission, they are always dependent on the reproduction of their carriers. They therefore have to control their own growth in order to minimize deleterious effects on the host. Bacteria of the genus Wolbachia are the most common maternally transmitted intracellular bacteria in insects. Most Wolbachia variants that are naturally associated with the fruit fly Drosophila melanogaster are benign to their hosts and provide them with protection against viruses. However, wMelPop is a virulent Wolbachia variant that over-replicates massively and shortens the lifespan of its fruit fly host. Here we show that amplification of a Wolbachia genomic region containing eight genes—called Octomom—is responsible for the pathogenic effects of wMelPop. Our results provide a link between genotype and phenotype in Wolbachia and show that virulence in symbionts can be simply caused by increases in gene copy number. These results also indicate that gene copy number variation may be a common mechanism underlying rapid evolution of intracellular symbionts.

Passage of Wolbachia pipientis through mutant drosophila melanogaster induces phenotypic and genomic changes

Wolbachia pipientis is a nearly ubiquitous, maternally transmitted bacterium that infects the germ line of insect hosts. Estimates are that Wolbachia infects 40 to 60% of insect species on the planet, making it one of the most prevalent infections on Earth. However, we know surprisingly little about the molecular mechanisms used by Wolbachia to infect its hosts. We passaged Wolbachia through normally restrictive Drosophila melanogaster hosts, bottlenecking Wolbachia through stochastic segregation while simultaneously selecting for mutants that could recolonize these previously restrictive hosts. Here, we show that Wolbachia alters its behavior when passaged through heterozygous mutant flies. After only three generations, Wolbachia was able to colonize the previously restrictive hosts at control titers. Additionally, the Wolbachia organisms passaged through heterozygous mutant D. melanogaster alter their pattern of tissue-specific Wsp protein production, suggesting a behavioral response to the host genotype. Using whole-genome resequencing, we identified the mutations accumulated by these lineages of Wolbachia and confirmed the existence and persistence of the mutations through clone library Sanger sequencing. Our results suggest that Wolbachia can quickly adapt to new host contexts, with genomic mutants arising after only two generations.