Dynamics of double and single Wolbachia infections in Drosophila simulans from New Caledonia

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

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

Wolbachia infection and genetic diversity of Italian populations of Philaenus spumarius, the main vector of Xylella fastidiosa in Europe

Philaenus spumarius is a cosmopolitan species that has become a major threat to European agriculture being recognized as the main vector of the introduced plant pathogen Xylella fastidiosa, the agent of the “olive quick decline syndrome”, a disease which is devastating olive orchards in southern Italy. Wolbachia are bacterial symbionts of many insects, frequently as reproductive parasites, sometime by establishing mutualistic relationships, able to spread within host populations. Philaenus spumarius harbors Wolbachia, but the role played by this symbiont is unknown and data on the infection prevalence within host populations are limited. Here, the Wolbachia infection rate was analyzed in relation to the geographic distribution and the genetic diversity of the Italian populations of P. spumarius. Analysis of the COI gene sequences revealed a geographically structured distribution of the three main mitochondrial lineages of P. spumarius. Wolbachia was detected in half of the populations sampled in northern Italy where most individuals belonged to the western-Mediterranean lineage. All populations sampled in southern and central Italy, where the individuals of the eastern-Mediterranean lineage were largely prevalent, were uninfected. Individuals of the north-eastern lineage were found only in populations from the Alps in the northernmost part of Italy, at high altitudes. In this area, Wolbachia infection reached the highest prevalence, with no difference between north-eastern and western-Mediterranean lineage. Analysis of molecular diversity of COI sequences suggested no significant effect of Wolbachia on population genetics of P. spumarius. Using the MLST approach, six new Wolbachia sequence types were identified. Using FISH, Wolbachia were observed within the host’s reproductive tissues and salivary glands. Results obtained led us to discuss the role of Wolbachia in P. spumarius, the factors influencing the geographic distribution of the infection, and the exploitation of Wolbachia for the control of the vector insect to reduce the spread of X. fastidiosa.

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.

Pervasive effects of Wolbachia on host activity

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

Pervasive Effects of Wolbachia on Host Temperature Preference

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

Loss of cytoplasmic incompatibility and minimal fecundity effects explain relatively low Wolbachia frequencies in Drosophila mauritiana

Maternally transmitted Wolbachia bacteria infect about half of all insect species. Many Wolbachia cause cytoplasmic incompatibility (CI) and reduced egg hatch when uninfected females mate with infected males. Although CI produces a frequency-dependent fitness advantage that leads to high equilibrium Wolbachia frequencies, it does not aid Wolbachia spread from low frequencies. Indeed, the fitness advantages that produce initial Wolbachia spread and maintain non-CI Wolbachia remain elusive. wMau Wolbachia infecting Drosophila mauritiana do not cause CI, despite being very similar to CI-causing wNo from Drosophila simulans (0.068% sequence divergence over 682,494 bp), suggesting recent CI loss. Using draft wMau genomes, we identify a deletion in a CI-associated gene, consistent with theory predicting that selection within host lineages does not act to increase or maintain CI. In the laboratory, wMau shows near-perfect maternal transmission; but we find no significant effect on host fecundity, in contrast to published data. Intermediate wMau frequencies on the island of Mauritius are consistent with a balance between unidentified small, positive fitness effects and imperfect maternal transmission. Our phylogenomic analyses suggest that group-B Wolbachia, including wMau and wPip, diverged from group-A Wolbachia, such as wMel and wRi, 6-46 million years ago, more recently than previously estimated.

wsp-based analysis of Wolbachia strains associated with Phlebotomus papatasi and P. sergenti (Diptera: Psychodidae) main cutaneous leishmaniasis vectors, introduction of a new subgroup wSerg

Sand flies of Phlebotomus papatasi and P. sergenti are the main vectors of cutaneous leishmanisis (CL) in the old world. We aimed to screen Iranian P. papatasi and P. sergenti for their natural infections with Wolbachia and to determine their phylogenetic association with other species. Wolbachia surface protein (wsp) gene was PCR amplified from DNA extracted from Phlebotomus species, sequenced, and were analysed in combination with wsp sequences related to Phelebtominae and other insects. All Wolbachia-infecting Iranian sand flies of P. papatasi and P. sergenti were classified in the Supergroup A., Wolbachia isolated from P. sergenti were clustered in a new subgroup within Supergroup A so-called wSreg. The Wolbachia strains identified from the P. papatasi clustered mainly in the subgroup wPap and partly in wSerg. Multiple Wholbachia infection within a single population of P.papatasi warrants investigation on existence and intensity of cytoplasmic incompatibility between the wPap and wSerg subgroups.

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

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

The Wolbachia WO bacteriophage proteome in the Aedes albopictus C/wStr1 cell line: evidence for lytic activity?

Wolbachia pipientis (Rickettsiales), an obligate intracellular alphaproteobacterium in insects, manipulates host reproduction to maximize invasion of uninfected insect populations. Modification of host population structure has potential applications for control of pest species, particularly if Wolbachia can be maintained, manipulated, and genetically engineered in vitro. Although Wolbachia maintains an obligate mutualism with genome stability in nematodes, arthropods can be co-infected with distinct Wolbachia strains, and horizontal gene transfer between strains is potentially mediated by WO phages encoded within Wolbachia genomes. Proteomic analysis of a robust, persistent infection of a mosquito cell line with wStr from the planthopper, Laodelphax striatellus, revealed expression of a full array of WO phage genes, as well as nine of ten non-phage genes that occur between two distinct clusters of WOMelB genes in the genome of wMel, which infects Drosophila melanogaster. These non-phage genes encode potential host-adaptive proteins and are expressed in wStr at higher levels than phage structural proteins. A subset of seven of the non-phage genes is flanked by highly conserved non-coding sequences, including a putative promoter element, that are not present in a syntenically arranged array of homologs in plasmids from three tick-associated Rickettsia spp. These studies expand our understanding of wStr in a host cell line derived from the mosquito, Aedes albopictus, and provide a basis for investigating conditions that favor the lytic phase of the WO phage life cycle and recovery of infectious phage particles.

Mosaic composition of ribA and wspB genes flanking the virB8-D4 operon in the Wolbachia supergroup B-strain, wStr

The obligate intracellular bacterium, Wolbachia pipientis (Rickettsiales), is a widespread, vertically transmitted endosymbiont of filarial nematodes and arthropods. In insects, Wolbachia modifies reproduction, and in mosquitoes, infection interferes with replication of arboviruses, bacteria and plasmodia. Development of Wolbachia as a tool to control pest insects will be facilitated by an understanding of molecular events that underlie genetic exchange between Wolbachia strains. Here, we used nucleotide sequence, transcriptional and proteomic analyses to evaluate expression levels and establish the mosaic nature of genes flanking the T4SS virB8-D4 operon from wStr, a supergroup B-strain from a planthopper (Hemiptera) that maintains a robust, persistent infection in an Aedes albopictus mosquito cell line. Based on protein abundance, ribA, which contains promoter elements at the 5′-end of the operon, is weakly expressed. The 3′-end of the operon encodes an intact wspB, which encodes an outer membrane protein and is co-transcribed with the vir genes. WspB and vir proteins are expressed at similar, above average abundance levels. In wStr, both ribA and wspB are mosaics of conserved sequence motifs from Wolbachia supergroup A- and B-strains, and wspB is nearly identical to its homolog from wCobU4-2, an A-strain from weevils (Coleoptera). We describe conserved repeated sequence elements that map within or near pseudogene lesions and transitions between A- and B-strain motifs. These studies contribute to ongoing efforts to explore interactions between Wolbachia and its host cell in an in vitro system.

Comparative genomics of Wolbachia and the bacterial species concept

The importance of host-specialization to speciation processes in obligate host-associated bacteria is well known, as is also the ability of recombination to generate cohesion in bacterial populations. However, whether divergent strains of highly recombining intracellular bacteria, such as Wolbachia, can maintain their genetic distinctness when infecting the same host is not known. We first developed a protocol for the genome sequencing of uncultivable endosymbionts. Using this method, we have sequenced the complete genomes of the Wolbachia strains wHa and wNo, which occur as natural double infections in Drosophila simulans populations on the Seychelles and in New Caledonia. Taxonomically, wHa belong to supergroup A and wNo to supergroup B. A comparative genomics study including additional strains supported the supergroup classification scheme and revealed 24 and 33 group-specific genes, putatively involved in host-adaptation processes. Recombination frequencies were high for strains of the same supergroup despite different host-preference patterns, leading to genomic cohesion. The inferred recombination fragments for strains of different supergroups were of short sizes, and the genomes of the co-infecting Wolbachia strains wHa and wNo were not more similar to each other and did not share more genes than other A- and B-group strains that infect different hosts. We conclude that Wolbachia strains of supergroup A and B represent genetically distinct clades, and that strains of different supergroups can co-exist in the same arthropod host without converging into the same species. This suggests that the supergroups are irreversibly separated and that barriers other than host-specialization are able to maintain distinct clades in recombining endosymbiont populations. Acquiring a good knowledge of the barriers to genetic exchange in Wolbachia will advance our understanding of how endosymbiont communities are constructed from vertically and horizontally transmitted genes.

Speciation in sexual organisms is defined as the inability of two populations to get viable offspring. Speciation in asexual, obligate endosymbionts is thought to be an indirect consequence of host-specialization. An important question is if divergent endosymbionts would start blending if the host barrier isolating them were removed. Here, we have studied Wolbachia, an abundant group of bacteria in the insect world. Wolbachia is classified into supergroups based on multi-locus sequence typing. We have sequenced the genomes from the Wolbachia strains wNo and wHa. These are particularly interesting since they belong to different supergroups yet co-occur as a double-infection in natural populations of Drosophila simulans. A comparative genomics study showed that wHa and wNo contain no uniquely shared genes. Instead, each strain shares unique gene functions with members of the same supergroup that infect other hosts. This unexpected finding suggests an alternative means of ecological speciation, indicating that speciation is not restricted to host-specialization but rather that related endosymbionts can coexist as separate species in the same host. Our study sheds light on the genomic divergence between different partners inhabiting the intracellular niche of the same host organism.

Draft genome sequence of the male-killing Wolbachia strain wBol1 reveals recent horizontal gene transfers from diverse sources

Background

The endosymbiont Wolbachia pipientis causes diverse and sometimes dramatic phenotypes in its invertebrate hosts. Four Wolbachia strains sequenced to date indicate that the constitution of the genome is dynamic, but these strains are quite divergent and do not allow resolution of genome diversification over shorter time periods. We have sequenced the genome of the strain wBol1-b, found in the butterfly Hypolimnas bolina, which kills the male offspring of infected hosts during embyronic development and is closely related to the non-male-killing strain wPip from Culex pipiens.

Results

The genomes of wBol1-b and wPip are similar in genomic organisation, sequence and gene content, but show substantial differences at some rapidly evolving regions of the genome, primarily associated with prophage and repetitive elements. We identified 44 genes in wBol1-b that do not have homologs in any previously sequenced strains, indicating that Wolbachia’s non-core genome diversifies rapidly. These wBol1-b specific genes include a number that have been recently horizontally transferred from phylogenetically distant bacterial taxa. We further report a second possible case of horizontal gene transfer from a eukaryote into Wolbachia.

Conclusions

Our analyses support the developing view that many endosymbiotic genomes are highly dynamic, and are exposed and receptive to exogenous genetic material from a wide range of sources. These data also suggest either that this bacterial species is particularly permissive for eukaryote-to-prokaryote gene transfers, or that these transfers may be more common than previously believed. The wBol1-b-specific genes we have identified provide candidates for further investigations of the genomic bases of phenotypic differences between closely-related Wolbachia strains.

Wolbachia in Anastrepha fruit flies (Diptera: Tephritidae)

Endosymbiotic bacteria of the genus Wolbachia are widespread among arthropods and cause a variety of reproductive abnormalities, such as cytoplasmic incompatibility, thelytokous parthenogenesis, male-killing, and host feminization. In this study, we used three sets of Wolbachia-specific primers (16S rDNA, ftsZ, and wsp) in conjunction with the polymerase chain reaction (PCR), cloning and sequencing to study the infection of fruit flies (Anastrepha spp. and Ceratitis capitata) by Wolbachia. The flies were collected at several localities in Brazil and at Guayaquil, Ecuador. All of the fruit flies studied were infected with Wolbachia supergroup A, in agreement with the high prevalence of this group in South America. Phylogenetic analysis showed that the wsp gene was the most sensitive gene for studying the relationships among Wolbachia strains. The Wolbachia sequences detected in these fruit flies were similar to those such as wMel reported for other fruit flies. These results show that the infection of Anastrepha fruit flies by Wolbachia is much more widespread than previously thought.

Multiple rescue factors within a Wolbachia strain

Wolbachia-induced cytoplasmic incompatibility (CI) is expressed when infected males are crossed with either uninfected females or females infected with Wolbachia of different CI specificity. In diploid insects, CI results in embryonic mortality, apparently due to the the loss of the paternal set of chromosomes, usually during the first mitotic division. The molecular basis of CI has not been determined yet; however, several lines of evidence suggest that Wolbachia exhibits two distinct sex-dependent functions: in males, Wolbachia somehow "imprints" the paternal chromosomes during spermatogenesis (mod function), whereas in females, the presence of the same Wolbachia strain(s) is able to restore embryonic viability (resc function). On the basis of the ability of Wolbachia to induce the modification and/or rescue functions in a given host, each bacterial strain can be classified as belonging in one of the four following categories: mod(+) resc(+), mod(-) resc(+), mod(-) resc(-), and mod(+) resc(-). A so-called "suicide" mod(+) resc(-) strain has not been found in nature yet. Here, a combination of embryonic cytoplasmic injections and introgression experiments was used to transfer nine evolutionary, distantly related Wolbachia strains (wYak, wTei, wSan, wRi, wMel, wHa, wAu, wNo, and wMa) into the same host background, that of Drosophila simulans (STCP strain), a highly permissive host for CI expression. We initially characterized the modification and rescue properties of the Wolbachia strains wYak, wTei, and wSan, naturally present in the yakuba complex, upon their transfer into D. simulans. Confocal microscopy and multilocus sequencing typing (MLST) analysis were also employed for the evaluation of the CI properties. We also tested the compatibility relationships of wYak, wTei, and wSan with all other Wolbachia infections. So far, the cytoplasmic incompatibility properties of different Wolbachia variants are explained assuming a single pair of modification and rescue factors specific to each variant. This study shows that a given Wolbachia variant can possess multiple rescue determinants corresponding to different CI systems. In addition, our results: (a) suggest that wTei appears to behave in D. simulans as a suicide mod(+) resc(-) strain, (b) unravel unique CI properties, and (c) provide a framework to understand the diversity and the evolution of new CI-compatibility types.

Male development time influences the strength of Wolbachia-induced cytoplasmic incompatibility expression in Drosophila melanogaster

Cytoplasmic incompatibility (CI) is the most widespread reproductive modification induced in insects by the maternally inherited intracellular bacteria, Wolbachia. Expression of CI in Drosophila melanogaster is quite variable. Published papers typically show that CI expression is weak and often varies between different Drosophila lines and different labs reporting the results. The basis for this variability is not well understood but is often considered to be due to unspecified host genotype interactions with Wolbachia. Here, we show that male development time can greatly influence CI expression in D. melanogaster. In a given family, males that develop fastest express very strong CI. The "younger brothers" of these males (males that take longer to undergo larval development) quickly lose their ability to express the CI phenotype as a function of development time. This effect is independent of male age effects and is enhanced when flies are reared under crowded conditions. No correlation is seen between this effect and Wolbachia densities in testes, suggesting that a more subtle interaction between host and symbiont is responsible. The observed younger brother effect may explain much of the reported variability in CI expression in this species. When male development time is controlled, it is possible to obtain consistently high levels of CI expression, which will benefit future studies that wish to use D. melanogaster as a model host to unravel CI mechanisms.

Supergroup F Wolbachia bacteria parasitise lice (Insecta: Phthiraptera)

We studied six species of lice from three of the four suborders of lice. These lice were infected with Wolbachia bacteria from supergroups A and F. This is the first report of an infection of supergroup F Wolbachia in lice. To date, Wolbachia from supergroup F have been found in filarial nematodes, Mansonella spp., and, rarely, in insects. We inferred the phylogeny of the Wolbachia from lice and representatives of all Wolbachia supergroups, with nucleotide sequences from the small subunit ribosomal RNA gene (SSU rRNA). There was no evidence of congruence between the taxon of louse and the Wolbachia bacteria that infect lice. There is no evidence that Wolbachia and their louse hosts co-evolved at least at the level of Wolbachia supergroups. We propose a novel mechanism for the horizontal transfer of Wolbachia between different species of lice from birds: transfer of Wolbachia during phoresis by hippoboscid flies.

A Wolbachia-associated fitness benefit depends on genetic background in Drosophila simulans

The alpha-proteobacteria Wolbachia infect a number of insect species and influence host reproduction to favour the spread of infected females through a population. The fitness effect of this infection is important in understanding the spread and maintenance of Wolbachia within and among host populations. However, a full elucidation of fitness effect requires careful control of host genetic background. Here, I transferred a single clone of Wolbachia (the wHa strain) into three genetically distinct isofemale lines of the fly Drosophila simulans using microinjection methodology. These lines carried one of the three described mitochondrial haplogroups (siI, siII or siIII) and differ in nuclear genome as well. Population cage assays showed that wHa-infected siIII flies enjoyed a dramatic fitness benefit compared to uninfected siIII. In contrast, wHa did not affect the fitness of siI or siII flies. This study points to the importance of host-by-symbiont interaction terms that may play an important role in organismal-fitness.

Decreased diversity but increased substitution rate in host mtDNA as a consequence of Wolbachia endosymbiont infection

A substantial fraction of insects and other terrestrial arthropods are infected with parasitic, maternally transmitted endosymbiotic bacteria that manipulate host reproduction. In addition to imposing direct selection on the host to resist these effects, endosymbionts may also have indirect effects on the evolution of the mtDNA with which they are cotransmitted. Patterns of mtDNA diversity and evolution were examined in Drosophila recens, which is infected with the endosymbiont Wolbachia, and its uninfected sister species D. subquinaria. The level of mitochondrial, but not nuclear, DNA diversity is much lower in D. recens than in D. subquinaria, consistent with the hypothesized diversity-purging effects of an evolutionarily recent Wolbachia sweep. The d(N)/d(S) ratio in mtDNA is significantly greater in D. recens, suggesting that Muller's ratchet has brought about an increased rate of substitution of slightly deleterious mutations. The data also reveal elevated rates of synonymous substitutions in D. recens, suggesting that these sites may experience weak selection. These findings show that maternally transmitted endosymbionts can severely depress levels of mtDNA diversity within an infected host species, while accelerating the rate of divergence among mtDNA lineages in different species.

Evolutionary history of Wolbachia infections in the fire ant Solenopsis invicta

Background

Wolbachia are endosymbiotic bacteria that commonly infect numerous arthropods. Despite their broad taxonomic distribution, the transmission patterns of these bacteria within and among host species are not well understood. We sequenced a portion of the wsp gene from the Wolbachia genome infecting 138 individuals from eleven geographically distributed native populations of the fire ant Solenopsis invicta. We then compared these wsp sequence data to patterns of mitochondrial DNA (mtDNA) variation of both infected and uninfected host individuals to infer the transmission patterns of Wolbachia in S. invicta.

Results

Three different Wolbachia (wsp) variants occur within S. invicta, all of which are identical to previously described strains in fire ants. A comparison of the distribution of Wolbachia variants within S. invicta to a phylogeny of mtDNA haplotypes suggests S. invicta has acquired Wolbachia infections on at least three independent occasions. One common Wolbachia variant in S. invicta (wSinvictaB) is associated with two divergent mtDNA haplotype clades. Further, within each of these clades, Wolbachia-infected and uninfected individuals possess virtually identical subsets of mtDNA haplotypes, including both putative derived and ancestral mtDNA haplotypes. The same pattern also holds for wSinvictaA, where at least one and as many as three invasions into S. invicta have occurred. These data suggest that the initial invasions of Wolbachia into host ant populations may be relatively ancient and have been followed by multiple secondary losses of Wolbachia in different infected lineages over time. Finally, our data also provide additional insights into the factors responsible for previously reported variation in Wolbachia prevalence among S. invicta populations.

Conclusion

The history of Wolbachia infections in S. invicta is rather complex and involves multiple invasions or horizontal transmission events of Wolbachia into this species. Although these Wolbachia infections apparently have been present for relatively long time periods, these data clearly indicate that Wolbachia infections frequently have been secondarily lost within different lineages. Importantly, the uncoupled transmission of the Wolbachia and mtDNA genomes suggests that the presumed effects of Wolbachia on mtDNA evolution within S. invicta are less severe than originally predicted. Thus, the common concern that use of mtDNA markers for studying the evolutionary history of insects is confounded by maternally inherited endosymbionts such as Wolbachia may be somewhat unwarranted in the case of S. invicta.

On the ubiquity and phylogeny of Wolbachia in lice

Wolbachia are intracellular bacteria that occur in an estimated 20% of arthropod species. They are of broad interest because they profoundly affect the reproductive fitness of diverse host taxa. Here we document the apparent ubiquity and diversity of Wolbachia in the insect orders Anoplura (sucking lice) and Mallophaga (chewing lice), by detecting single or multiple infections in each of 25 tested populations of lice, representing 19 species from 15 genera spanning eight taxonomic families. Phylogenetic analyses indicate a high diversity of Wolbachia in lice, as evidenced by the identification of 39 unique strains. Some of these strains are apparently unique to lice, whereas others are similar to strains that infect other insect taxa. Wolbachia are transmitted from infected females to their offspring via egg cytoplasm, such that similar species of lice are predicted to have similar strains of Wolbachia. This predicted pattern is not supported in the current study and may reflect multiple events of recent horizontal transmission between host species. At present, there is no known mechanism that would allow for this latter mode of transmission to and within species of lice.

Evolution of Wolbachia cytoplasmic incompatibility types

The success of obligate endosymbiotic Wolbachia infections in insects is due in part to cytoplasmic incompatibility (CI), whereby Wolbachia bacteria manipulate host reproduction to promote their invasion and persistence within insect populations. The observed diversity of CI types raises the question of what the evolutionary pathways are by which a new CI type can evolve from an ancestral type. Prior evolutionary models assume that Wolbachia exists within a host individual as a clonal infection. While endosymbiotic theory predicts a general trend toward clonality, Wolbachia provides an exception in which there is selection to maintain diversity. Here, evolutionary trajectories are discussed that assume that a novel Wolbachia variant will co-exist with the original infection type within a host individual as a superinfection. Relative to prior models, this assumption relaxes requirements and allows additional pathways for the evolution of novel CI types. In addition to describing changes in the Wolbachia infection frequency associated with the hypothesized evolutionary events, the predicted impact of novel CI variants on the host population is also described. This impact, resulting from discordant evolutionary interests of symbiont and host, is discussed as a possible cause of Wolbachia loss from the host population or host population extinction. The latter is also discussed as the basis for an applied strategy for the suppression of insect pest populations. Model predictions are discussed relative to a recently published Wolbachia genome sequence and prior characterization of CI in naturally and artificially infected insects.

The incomplete natural history of mitochondria

Mitochondrial DNA (mtDNA) has been used to study molecular ecology and phylogeography for 25 years. Much important information has been gained in this way, but it is time to reflect on the biology of the mitochondrion itself and consider opportunities for evolutionary studies of the organelle itself and its ecology, biochemistry and physiology. This review has four sections. First, we review aspects of the natural history of mitochondria and their DNA to show that it is a unique molecule with specific characteristics that differ from nuclear DNA. We do not attempt to cover the plethora of differences between mitochondrial and nuclear DNA; rather we spotlight differences that can cause significant bias when inferring demographic properties of populations and/or the evolutionary history of species. We focus on recombination, effective population size and mutation rate. Second, we explore some of the difficulties in interpreting phylogeographical data from mtDNA data alone and suggest a broader use of multiple nuclear markers. We argue that mtDNA is not a sufficient marker for phylogeographical studies if the focus of the investigation is the species and not the organelle. We focus on the potential bias caused by introgression. Third, we show that it is not safe to assume a priori that mtDNA evolves as a strictly neutral marker because both direct and indirect selection influence mitochondria. We outline some of the statistical tests of neutrality that can, and should, be applied to mtDNA sequence data prior to making any global statements concerning the history of the organism. We conclude with a critical examination of the neglected biology of mitochondria and point out several surprising gaps in the state of our knowledge about this important organelle. Here we limelight mitochondrial ecology, sexually antagonistic selection, life-history evolution including ageing and disease, and the evolution of mitochondrial inheritance.

Reproductive effects and geographical distributions of two Wolbachia strains infecting the Neotropical beetle, Chelymorpha alternans Boh. (Chrysomelidae, Cassidinae)

Wolbachia are maternally inherited endocellular bacteria known to alter insect host reproduction to facilitate their own transmission. Multiple Wolbachia infections are more common in tropical than temperate insects but few studies have investigated their dynamics in field populations. The beetle, Chelymorpha alternans, found throughout the Isthmus of Panama, is infected with two strains of Wolbachia, wCalt1 (99.2% of beetles) and wCalt2 (53%). Populations infected solely by the wCalt1 strain were limited to western Pacific Panama, whereas populations outside this region were either polymorphic for single (wCalt1) and double infections (wCalt1 + wCalt2) or consisted entirely of double infections. The wCalt2 strain was not found as a single infection in the wild. Both strains caused cytoplasmic incompatibility (CI). The wCalt1 strain caused weak CI (approximately 20%) and the double infection induced moderate CI (approximately 70-90%) in crosses with uninfected beetles. The wCalt1 strain rescued about 75% of eggs fertilized by sperm from wCalt2 males. Based on the relationships of beetle mtDNA and infection status, maternal transmission, and repeated population sampling we determined that the double infection invaded C. alternans populations about 100,000 years ago and that the wCalt2 strain appears to be declining in some populations, possibly due to environmental factors. This may be the first study to demonstrate an association between widespread strain loss and environmental factors in the field.

Influence of Two Wolbachia Strains on Population Structure of East AfricanDrosophila simulans

Drosophila simulans is hypothesized to have originated in continental East Africa or Madagascar. In this study, we investigated evolutionary forces operating on mitochondrial DNA (mtDNA) in populations of D. simulans from Zimbabwe, Malawi, Tanzania, and Kenya. Variation in mtDNA may be affected by positive selection, background selection, demographic history, and/or any maternally inherited factor such as the bacterial symbiont Wolbachia. In East Africa, the wRi and wMa Wolbachia strains associate with the siII or siIII mitochondrial haplogroups, respectively. To ask how polymorphism relates to Wolbachia infection status, we sequenced 1776 bp of mitochondrial DNA and 1029 bp of the X-linked per locus from 79 lines. The two southern populations were infected with wRi and exhibited significantly reduced mtDNA variation, while Wolbachia-uninfected siII flies from Tanzania and Kenya showed high levels of mtDNA polymorphism. These are the first known populations of D. simulans that do not exhibit reduced mtDNA variation. We observed no mitochondrial variation in the siIII haplogroup regardless of Wolbachia infection status, suggesting positive or background selection. These populations offer a unique opportunity to monitor evolutionary dynamics in ancestral populations that harbor multiple strains of Wolbachia.

Mitochondrial genotype affects fitness in Drosophila simulans

Drosophila simulans is known to harbor three distinct mitochondrial DNA (mtDNA) haplotype groups (siI, -II, and -III) with nearly 3.0% interhaplotypic divergence but <0.06% intrahaplotypic diversity. With the large amount of genetic variation in this system, the potential power to detect intraspecific fitness differences in fly lines that carry distinct haplotypes is great. We test three life-history traits on fly lines with known sequence differences in the mtDNA genome after controlling the nuclear genome by backcrossing. We find that flies with the siI haplotype are fastest developing and have the lowest probability of surviving to three experimental periods (2-6, 12-17, and 34-39 days of age). Wild-type males with siIII mtDNA were more active while disruption of specific coadapted nucleo-mitochondrial complexes caused a significant decrease in activity. These results are discussed in the context of the geographic distribution of each haplotype.

Wolbachia distribution and cytoplasmic incompatibility during sperm development: the cyst as the basic cellular unit of CI expression

The growth and distribution of the intracellular microbe Wolbachia pipientis during spermatogenesis in several different host/symbiont genetic combinations in Drosophila melanogaster and Drosophila simulans is described. Considerable intra- and inter-strain variation in Wolbachia density and tissue distribution was observed. Wolbachia were found inside spermatocytes and spermatids or within the somatic cyst cells surrounding the germ cells. Some strains displayed both tissue distributions. High rates of cytoplasmic incompatibility (CI) are correlated with high levels of Wolbachia only when spermatocytes and/or spermatids harbor the microbe. Wolbachia infection of somatic cyst cells, although sometimes present at high levels, did not result in significant CI expression. CI-inducing Wolbachia strains within D. simulans showed no distinguishable differences in distribution or density within infected spermatids. To dissect the relative contribution of host and symbiont to the expression of CI, Wolbachia from various host strains known to exhibit varying levels of CI were introgressed into new uninfected host genetic backgrounds. These introgression experiments confirm that the mod(+)/mod(-) phenotype is an intrinsic Wolbachia trait and is not determined by host factors. The level of sperm modification in those lines harboring Wolbachia capable of modifying sperm, however, is influenced by host genetic background. These results form the basis of the Wolbachia Infected Spermatocyte/Spermatid Hypothesis (WISSH). According to WISSH, Wolbachia infection in spermatocytes and then spermatids during sperm development is required for CI expression.

Wolbachia segregation dynamics and levels of cytoplasmic incompatibility in Drosophila sechellia

In Drosophila sechellia, the endocellular bacterium Wolbachia induces cytoplasmic incompatibility (CI): in crosses involving infected males, a partial or complete embryonic mortality occurs unless the female bears the same Wolbachia. D. sechellia is known to harbour two Wolbachia variants, namely wSh and wSn, closely related to wHa and wNo, respectively, two strains infecting the populations of D. simulans from the Seychelles archipelago and New Caledonia. Strikingly, the two species show similar infection patterns: in D. sechellia, wSh can be present on its own or in double infection with wSn, but individuals carrying wSn only do not occur; in D. simulans, wHa can be present on its own or in double infection with wNo, but individuals carrying wNo only do not occur, or occur at very low frequency. Previous experiments on D. simulans showed that lines singly infected by wNo can be obtained by segregation, and stably maintained. Here we investigate this issue in D. sechellia through an 18 generation experiment, and show that wSn and wSh singly infected lines can arise by segregation. Using singly infected lines obtained in this experiment, we estimate the CI intensities of wSh and wSn in D. sechellia, and compare these to the CI intensities of the same Wolbachia injected into D. simulans. Our results do not suggest any consistent effect of the host species on the CI induced by wSh. On the contrary, it seems that wSn expression is repressed by host factors in D. sechellia.

Evolution of Wolbachia-induced cytoplasmic incompatibility in Drosophila simulans and D. sechellia

The intracellular bacterium Wolbachia invades arthropod host populations through various mechanisms, the most common of which being cytoplasmic incompatibility (CI). CI involves elevated embryo mortality when infected males mate with uninfected females or females infected with different, incompatible Wolbachia strains. The present study focuses on this phenomenon in two Drosophila species: D. simulans and D. sechellia. Drosophila simulans populations are infected by several Wolbachia strains, including wHa and wNo. Drosophila sechellia is infected by only two Wolbachia: wSh and wSn. In both Drosophila species, double infections with Wolbachia are found. As indicated by several molecular markers, wHa is closely related to wSh, and wNo to wSn. Furthermore, the double infections in the two host species are associated with closely related mitochondrial haplotypes, namely siI (associated with wHa and wNo in D. simulans) and se (associated with wSh and wSn in D. sechellia). To test the theoretical prediction that Wolbachia compatibility types can diverge rapidly, we injected wSh and wSn into D. simulans, to compare their CI properties to those of their sister strains wHa and wNo, respectively, in the same host genetic background. We found that within each pair of sister strains CI levels were similar and that sister strains were fully compatible. We conclude that the short period for which the Wolbachia sister strains have been evolving separated from each other was not sufficient for their CI properties to diverge significantly.

Divergence of mitochondrial dna is not corroborated by nuclear dna, morphology, or behavior in Drosophila simulans

We ask whether the observed mitochondrial DNA (mtDNA) population subdivision of Drosophila simulans is indicative of organismal structure or of specific processes acting on the mitochondrial genome. Factors either intrinsic or extrinsic to the host genome may influence the evolutionary dynamics of mtDNA. Potential intrinsic factors include adaptation of the mitochondrial genome and of nucleomitochondrial gene complexes specific to the local environment. An extrinsic force that has been shown to influence mtDNA evolution in invertebrates is the bacterial endosymbiont Wolbachia. Evidence presented in this study suggests that mtDNA is not a good indicator of organismal subdivision in D. simulans. Furthermore, there is no evidence to suggest that Wolbachia causes any reduction in nuclear gene flow in this species. The observed differentiation in mtDNA is not corroborated by data from NADH: ubiquinone reductase 75kD subunit precursor or the Alcohol dehydrogenase-related loci, from the shape or size of the male genital arch, or from assortative premating behavior. We discuss these results in relation to a mitochondrial genetic species concept and the potential for Wolbachia-induced incompatibility to be a mechanism of speciation in insects. We conclude with an iterated appeal to include phylogenetic and statistical tests of neutrality as a supplement to phylogenetic and population genetic analyses when using mtDNA as an evolutionary marker.