Superinfection of Laodelphax striatellus with Wolbachia from Drosophila simulans

Wolbachia are maternally inherited, intracellular alpha-proteobacteria that infect a wide range of arthropods. They manipulate the reproduction of hosts to facilitate their spread into host populations, through ways such as cytoplasmic incompatibility (CI), parthenogenesis, feminization and male killing. The influence of Wolbachia infection on host populations has attracted considerable interest in their possible role in speciation and as a potential agent of biological control. In this study, we used both microinjection and nested PCR to show that the Wolbachia naturally infecting Drosophila simulans can be transferred into a naturally Wolbachia-infected strain of the small brown planthopper Laodelphax striatellus, with up to 30% superinfection frequency in the F(12) generation. The superinfected males of L. striatellus showed unidirectional CI when mated with the original single-infected females, while superinfected females of L. striatellus were compatible with superinfected or single-infected males. These results are, to our knowledge, the first to establish a superinfected horizontal transfer route for Wolbachia between phylogenetically distant insects. The segregation of Wolbachia from superinfected L. striatellus was observed during the spreading process, which suggests that Wolbachia could adapt to a phylogenetically distant host with increased infection frequency in the new host population; however, it would take a long time to establish a high-frequency superinfection line. This study implies a novel way to generate insect lines capable of driving desired genes into Wolbachia-infected populations to start population replacement.     

转染从灰飞虱提取的Wolbachia对豆叶螨繁殖适合度和寿命的影响

【目的】Wolbachia是广泛存在于节肢动物体内的一类母系遗传的共生菌, 能够通过多种机制调节节肢动物的生殖。近年来, 为了更进一步地探究Wolbachia与寄主之间的互作机制, 许多研究者展开了Wolbachia的人工转染研究。【方法】我们在实验室条件下将灰飞虱Laodelphax striatellus (Fallén)感染的Wolbachia提取纯化后, 利用显微注射的方法导入豆叶螨Tetranychus phaselus Ehara体内。研究了注入从灰飞虱提取的Wolbachia和豆叶螨自然感染Wolbachia对豆叶螨繁殖适合度和寿命的影响, 并测定了两种Wolbachia的密度随豆叶螨日龄增长的变化情况。【结果】结果显示, 外源Wolbachia在豆叶螨体内的拷贝数极低, 仅为自然感染豆叶螨体内Wolbachia拷贝数的0.5%左右。与自然感染的Wolbachia不同, 外源Wolbachia在豆叶螨种群中不能引起胞质不亲和, 但能够显著降低雌螨的产卵量。【结论】本研究表明, 来自灰飞虱的Wolbachia具有抑制豆叶螨种群扩张的潜在能力, 对豆叶螨生物防治具有一定的应用价值。     

Mutualistic Wolbachia infection in Aedes albopictus: accelerating cytoplasmic drive

Maternally inherited rickettsial symbionts of the genus Wolbachia occur commonly in arthropods, often behaving as reproductive parasites by manipulating host reproduction to enhance the vertical transmission of infections. One manipulation is cytoplasmic incompatibility (CI), which causes a significant reduction in brood hatch and promotes the spread of the maternally inherited Wolbachia infection into the host population (i.e., cytoplasmic drive). Here, we have examined a Wolbachia superinfection in the mosquito Aedes albopictus and found the infection to be associated with both cytoplasmic incompatibility and increased host fecundity. Relative to uninfected females, infected females live longer, produce more eggs, and have higher hatching rates in compatible crosses. A model describing Wolbachia infection dynamics predicts that increased fecundity will accelerate cytoplasmic drive rates. To test this hypothesis, we used population cages to examine the rate at which Wolbachia invades an uninfected Ae. albopictus population. The observed cytoplasmic drive rates were consistent with model predictions for a CI-inducing Wolbachia infection that increases host fecundity. We discuss the relevance of these results to both the evolution of Wolbachia symbioses and proposed applied strategies for the use of Wolbachia infections to drive desired transgenes through natural populations (i.e., population replacement strategies).     

Variable fitness effects of Wolbachia infection in Drosophila melanogaster

Maternally inherited Wolbachia bacteria are extremely widespread among insects and their presence is usually associated with parasitic modifications of host fitness. Wolbachia pipientis infects Drosophila melanogaster populations from all continents, but their persistence in this species occurs despite any strong parasitic effects. Here, we have investigated the symbiosis between Wolbachia and D. melanogaster and found that Wolbachia infection can have significant survival and fecundity effects. Relative to uninfected flies, infected females from three fly strains showed enhanced survival or fecundity associated with Wolbachia infection, one strain showed both and one strain responded positively to Wolbachia removal. We found no difference in egg hatch rates (cytoplasmic incompatibility) for crosses between infected males and uninfected females, although there were fecundity differences. Females from this cross consistently produced fewer eggs than infected females and these fecundity differences could promote the spread of infection just like cytoplasmic incompatibility. More surprising, we found that infected females often had the greatest fecundity when mated to uninfected males. This could also promote the spread of Wolbachia infection, though here the fitness benefits would also help to spread infection when Wolbachia are rare. We suggest that variable fitness effects, in both sexes, and which interact strongly with the genetic background of the host, could increase cytoplasmic drive rates in some genotypes and help explain the widespread persistence of Wolbachia bacteria in D. melanogaster populations. These interactions may further explain why many D. melanogaster populations are polymorphic for Wolbachia infection. We discuss our results in the context of host-symbiont co-evolution.     

Spatially explicit models of Turelli-Hoffmann Wolbachia invasive wave fronts

This paper examines different mathematical models of insect dispersal and infection spread and compares these with field data. Reaction-diffusion and integro-difference equation models are used to model the spatio-temporal spread of Wolbachia in Drosophila simulans populations. The models include cytoplasmic incompatibility between infected females and uninfected males that creates a threshold density, similar to an Allee effect, preventing increase from low incidence of infection in the host population. The model builds on an earlier model (Turelli & Hoffmann, 1991) by incorporating imperfect maternal transmission. The results of simulations of the models using the same parameter values produce different dynamics for each model. These differences become very marked in the integro-difference equation models when insect dispersal patterns are assumed to be non-Gaussian. The success or failure of invasion by Wolbachia in the simulations may be attributed to the insect dispersal mechanism used in the model rather than the parameter values. As the models predict very different outcomes for the integro-difference models depending on the underlying assumptions of insect dispersal patterns, this emphasizes that good field data on real (rather than idealized) dispersal patterns need to be collected before models such as these can be used for predictive purposes.     

Wolbachia and cytoplasmic incompatibility in mosquitoes

Wolbachia are maternally inherited bacteria that induce cytoplasmic incompatibility in mosquitoes, and are able to use these patterns of sterility to spread themselves through populations. For this reason they have been proposed as a gene drive system for mosquito genetic replacement, as well as for the reduction of population size or for modulating population age structure in order to reduce disease transmission. Here, recent progress in the study of mosquito Wolbachia is reviewed. We now have much more comprehensive estimates of the parameters that can affect the spread of Wolbachia through natural populations from low starting frequencies, and for waves of spread to be maintained in the face of partial barriers to gene flow. In Aedes albopictus these dynamics are extremely favourable, with very high maternal transmission fidelity and levels of incompatibility recorded. Correspondence between measurements taken in the lab and field is much better than in the Drosophila simulans model system. Important research goals are also discussed, including Wolbachia transformation, interspecific transfer and the elucidation of the mechanisms of incompatibility and rescue; all will be aided by a wealth of new Wolbachia genome information.     

Can cytoplasmic incompatibility inducing Wolbachia promote the evolution of mate preferences?

The maternally inherited bacterium, Wolbachia pipientis, manipulates host reproduction by rendering uninfected females reproductively incompatible with infected males (cytoplasmic incompatibility, CI). Hosts may evolve mechanisms, such as mate preferences, to avoid fitness costs of Wolbachia infection. Despite the potential importance of mate choice for Wolbachia population dynamics, this possibility remains largely unexplored. Here we model the spread of an allele encoding female mate preference for uninfected males alongside the spread of CI inducing Wolbachia. Mate preferences can evolve but the spread of the preference allele depends on factors associated with both Wolbachia infection and the preference allele itself. Incomplete maternal transmission of Wolbachia, fitness costs and low CI, improve the spread of the preference allele and impact on the population dynamics of Wolbachia. In addition, mate preferences are found in infected individuals. These results have important consequences for the fate of Wolbachia and studies addressing mate preferences in infected populations.     

沃尔巴克氏体在中国三种稻飞虱中的感染

用PCR方法检测了采集于不同地域稻田的3种稻飞虱共生菌沃尔巴克氏体(Wolbachia)的感染,发现灰飞虱Laodelphax striatellus、褐飞虱Nilaparvata lugens、白背飞虱Sogatella furcifera为沃尔巴克氏体所感染。克隆了编码沃尔巴克氏体外膜蛋白质的wsp基因并进行了序列测定。对wsp的RFLP分析证实了这些飞虱为单一沃尔巴克氏体感染。研究了灰飞虱中沃尔巴克氏体所诱导的胞质不相容性及其在不同地域灰飞虱中的分布。还发现能寄生于上述3种飞虱的稻虱红螯蜂也受同种沃尔巴克氏体感染。沃尔巴克氏体可能通过这种寄生蜂在不同昆虫间横向传播。     

Genetic Diversity and Spatial Genetic Structure of the Grassland Perennial Saxifraga granulata along Two River Systems

Due to changes in land use, the natural habitats of an increasing number of plant species have become more and more fragmented. In landscapes that consist of patches of suitable habitat, the frequency and extent of long-distance seed dispersal can be expected to be an important factor determining local genetic diversity and regional population structure of the remaining populations. In plant species that are restricted to riparian habitats, rivers can be expected to have a strong impact on the dynamics and spatial genetic structure of populations as they may enable long-distance seed dispersal and thus maintain gene flow between fragmented populations. In this study, we used polymorphic microsatellite markers to investigate the genetic diversity and the spatial genetic structure of 28 populations of Saxifraga granulata along two rivers in central Belgium. We hypothesized that rivers might be essential for gene flow among increasingly isolated populations of this species. Genetic diversity was high (H_S = 0.68), which to a certain extent can be explained by the octoploid nature of S. granulata in the study area. Populations along the Dijle and Demer rivers were also highly differentiated (G” _ST = 0.269 and 0.164 and D _EST = 0.190 and 0.124, respectively) and showed significant isolation-by-distance, indicating moderate levels of gene flow primarily between populations that are geographically close to each other. Along the river Demer population genetic diversity was higher upstream than downstream, suggesting that seed dispersal via the water was not the primary mode of dispersal. Overall, these results indicate that despite increasing fragmentation populations along both rivers were highly genetically diverse. The high ploidy level and longevity of S. granulata have most likely buffered negative effects of fragmentation on genetic diversity and the spatial genetic structure of populations in riparian grasslands.     

From parasite to mutualist: rapid evolution of Wolbachia in natural populations of Drosophila

Wolbachia are maternally inherited bacteria that commonly spread through host populations by causing cytoplasmic incompatibility, often expressed as reduced egg hatch when uninfected females mate with infected males. Infected females are frequently less fecund as a consequence of Wolbachia infection. However, theory predicts that because of maternal transmission, these "parasites" will tend to evolve towards a more mutualistic association with their hosts. Drosophila simulans in California provided the classic case of a Wolbachia infection spreading in nature. Cytoplasmic incompatibility allowed the infection to spread through individual populations within a few years and from southern to northern California (more than 700 km) within a decade, despite reducing the fecundity of infected females by 15%-20% under laboratory conditions. Here we show that the Wolbachia in California D. simulans have changed over the last 20 y so that infected females now exhibit an average 10% fecundity advantage over uninfected females in the laboratory. Our data suggest smaller but qualitatively similar changes in relative fecundity in nature and demonstrate that fecundity-increasing Wolbachia variants are currently polymorphic in natural populations.     

Population genetic structure of the round goby in Lake Michigan: implications for dispersal of invasive species

Understanding subsequent dispersal of non-native species following introduction is important for predicting the extent and speed of range expansion and is critical for effective management and risk assessment. Post-introduction dispersal may occur naturally or via human transport, but assessing the relative contribution of each is difficult for many organisms. Here, we use data from seven microsatellite markers to study patterns of dispersal and gene flow among 12 pierhead populations of the round goby (Neogobius melanostomus) in Lake Michigan. We find significant population structure among sampling sites within this single Great Lake: (1) numerous populations exhibited significant pairwise F _ST and (2) a Bayesian assignment analysis revealed three distinct genetic clusters, corresponding to different pierhead locations, and genetic admixture between these clusters in the remaining populations. Genetic differentiation (F _ST) is generally related to geographic distance (i.e., isolation by distance), but is periodically interrupted at the scale of Lake Michigan due to gene flow among geographically distant sites. Moreover, average genetic differentiation among populations exhibit a significant, negative correlation with the amount of shipping cargo at ports. Our results, therefore, provide evidence that genetic structure of the round goby in Lake Michigan results from limited natural dispersal with frequent long-distance dispersal through anthropogenic activities such as commercial shipping. Our study suggests that while round gobies can undoubtedly disperse and found new populations through natural dispersal mechanisms, their spread within and among the Great Lakes is likely aided by transport via ships. We, therefore, recommend that ballast-water treatment and management may limit the spread of non-native species within the Great Lakes after the initial introduction in addition to preventing the introduction of non-native species to the Great Lakes.     

Wolbachia and nuclear-nuclear interactions contribute to reproductive incompatibility in the spider mite Panonychus mori (Acari: Tetranychidae)

Maternally transmitted bacteria of the genus Wolbachia are obligate, intracellular symbionts that are responsible for cytoplasmic incompatibility in a wide range of arthropods such as insects and mites. Spider mites often show uni- and bidirectional incompatibilities among populations with and without Wolbachia. Therefore, we surveyed the presence of Wolbachia by PCR and then conducted crossing experiments among 25 populations of Panonychus mori to determine how Wolbachia are related to the incompatibility in this species. Five out of the 25 populations were infected with Wolbachia. These five populations were treated with an antibiotic (rifampicin) to eliminate Wolbachia. We carried out round-robin crosses among 20 Wolbachia-uninfected populations, five infected populations and five rifampicin-treated populations (30 x 30=900 crosses in total). Incompatibility among P. mori populations was caused by Wolbachia infection, nuclear-cytoplasmic interactions or nuclear-nuclear interactions. Wolbachia-mediated incompatibility was observed in crosses between uninfected females and infected males or between females and males harboring different Wolbachia strains. Nuclear-cytoplasmic interactions may be responsible for the unidirectional incompatibility in crosses between the two northern populations and one of the southern populations. Bidirectional incompatibility caused by nuclear-nuclear interactions was observed in 99 combinations of interpopulation crosses (99/300=0.33). Although no geographical trends were detected in the distribution of bidirectionally compatible populations, the results reveal a genetic divergence among P. mori populations.     

Diversity and phylogenetic relationships of Wolbachia in Drosophila and other native Hawaiian insects

Wolbachia is a genus of parasitic alphaproteobacteria found in arthropods and nematodes, and represents on of the most common, widespread endosymbionts known. Wolbachia affects a variety of reproductive functions in its host (e.g., male killing, cytoplasmic incompatibility, parthenogenesis), which have the potential to dramatically impact host evolution and species formation. Here, we present the first broad-scale study to screen natural populations of native Hawaiian insects for Wolbachia, focusing on the endemic Diptera. Results indicate that Wolbachia infects native Hawaiian taxa, with alleles spanning phylogenetic supergroups, A and B. The overall frequency of Wolbachia incidene in Hawaiian insects was 14%. The incidence of infection in native Hawaiian Diptera was 11% for individuals and 12% for all species screened. Wolbachia was not detected in two large, widespread Hawaiian dipteran families—Dolichopodidae (44 spp screened) and Limoniidae (12 spp screened). Incidence of infection within endemic Hawaiian lineages that carry Wolbachia was 18% in Drosophilidae species, 25% in Caliphoridae species, > 90% in Nesophrosyne species, 20% in Drosophila dasycnemia and 100% in Nesophrosyne craterigena. Twenty unique alleles were recovered in this study, of which 18 are newly recorded. Screening of endemic populations of D. dasycnemia across Hawaii Island revealed 4 unique alleles. Phylogenetic relationships and allele diversity provide evidence for horizontal transfer of Wolbachia among Hawaiian arthropod lineages.     

Nonrandom Wolbachia infection status of Drosophila melanogaster strains with different mtDNA haplotypes

Wolbachia are maternally inherited bacteria, which typically spread in the host population by inducing cytoplasmic incompatibility (CI). In Drosophila melanogaster, Wolbachia is quite common but CI is variable, with most of the studies reporting low levels of CI. Surveying mitochondrial DNA (mtDNA) variation and infection status in a worldwide D. melanogaster collection, we found that the Wolbachia infection was not randomly distributed among flies with different mtDNA haplotypes. This preferential infection of some mtDNA haplotypes could be caused by a recent spread of mtDNA haplotypes associated with the infection. The comparison of contemporary D. melanogaster samples with lines collected more than 50 years ago shows that indeed one haplotype with a high incidence of Wolbachia infection has increased in frequency. Consistent with this observation, we found that the acquisition of a Wolbachia infection in a population from Crete was accompanied with an almost complete mtDNA replacement, with the Wolbachia-associated haplotype becoming abundant. Although it is difficult to identify the evolutionary forces causing the global increase of wMel, the parallel sweep of Wolbachia and an mtDNA haplotype suggests a fitness advantage of the Wolbachia infection.     

Coupled population dynamics of endosymbionts within and between hosts

Many insect species are infected with maternally transmitted endosymbionts, the most widely documented being Wolbachia . The rate of spread and equilibrium of prevalence of these infections depend on two parameters – maternal transmission fidelity and relative fitness of infected cytoplasmic lineages. Both transmission fidelity and the phenotypic effect of endosymbionts often increase with endosymbiont density within hosts. Thus, the dynamics of infection prevalence in host populations depends on processes affecting within-host density of endosymbionts. In theory, the equilibrium prevalence of infection by male-killing endosymbionts is much more sensitive to changes in transmission fidelity and relative fitness than is that of endosymbionts that cause cytoplasmic incompatibility. In natural populations, male-killers exhibit much greater temporal and spatial variation in the prevalence of infection than do endosymbionts that cause cytoplasmic incompatibility. Thus, the population dynamics of endosymbiont infections, especially those that cause male-killing, is likely to be governed by environmental and genetic variables that affect within-host density of these infections.     

Wolbachia distribution and cytoplasmic incompatibility based on a survey of 42 spider mite species (Acari: Tetranychidae) in Japan

Wolbachia are a group of maternally inherited bacteria that infect a wide range of arthropods. Wolbachia infections are known to result in the expression of various abnormal reproductive phenotypes, the best known being cytoplasmic incompatibility. The first systematic survey of 42 spider mite species in Japan revealed that seven species (16.7%) were infected with Wolbachia. Wolbachia in the spider mites were grouped into three subgroups in supergroup B by phylogenetic analyses of the wsp gene. Most spider mites did not show cytoplasmic incompatibility when infected males were crossed with uninfected females. However, all infected populations of Panonychus mori and Oligonychus gotohi (five and four populations, respectively) possessed modification-positive strains of Wolbachia, and the cytoplasmic incompatibility decreased egg hatchability and female ratio of the spider mites. Thus, some Wolbachia strains cause sex ratio distortion in their hosts.     

Fitness advantage and cytoplasmic incompatibility in Wolbachia single- and superinfected Aedes albopictus

Wolbachia are obligate, maternally inherited, intracellular bacteria that infect numerous insects and other invertebrates. Wolbachia infections have evolved multiple mechanisms to manipulate host reproduction and facilitate invasion of naive host populations. One such mechanism is cytoplasmic incompatibility (CI) that occurs in many insect species, including Aedes albopictus (Asian tiger mosquito). The multiple Wolbachia infections that occur naturally in A. albopictus make this mosquito a useful system in which to study CI. Here, experiments employ mosquito strains that have been introgressed to provide genetically similar strains that harbor differing Wolbachia infection types. Cytoplasmic incompatibility levels, host longevity, egg hatch rates, and fecundity are examined. Crossing results demonstrate a pattern of additive unidirectional cytoplasmic incompatibility. Furthermore, relative to uninfected females, infected females are at a reproductive advantage due to both cytoplasmic incompatibility and a fitness increase associated with Wolbachia infection. In contrast, no fitness difference was observed in comparisons of single- and superinfected females. We discuss the observed results in regard to the evolution of the Wolbachia/A. albopictus symbiosis and the observed pattern of Wolbachia infection in natural populations.     

Expression of cytoplasmic incompatibility in Drosophila simulans and its impact on infection frequencies and distribution of Wolbachia pipientis

The aim of this study is to examine the expression of cytoplasmic incompatibility and investigate the distribution and population frequencies of Wolbachia pipientis strains in Drosophila simulans. Nucleotide sequence data from 16S rDNA and a Wolbachia surface protein coding sequence and cytoplasmic incompatibility assays identify four distinct Wolbachia strains: wHa, wRi, wMa, and wAu. The levels of cytoplasmic incompatibility between six lines carrying these strains of bacteria and three control lines without bacteria are characterized. Flies infected with wHa and wRi are bidirectionally incompatible, and males that carry either strain can only successfully produce normal numbers of offspring with females carrying the same bacterial strain. Males infected with wAu do not express incompatibility. Males infected with the wMa strain express intermediate incompatibility when mated to females with no bacteria and no incompatibility with females with any other Wolbachia strain. We conduct polymerase chain reaction/restriction fragment length polymorphism assays to distinguish the strain of Wolbachia and the mitochondrial haplotype to survey populations for each type and associations between them. Drosophila simulans is known to have three major mitochondrial haplotypes (siI, sill, and siIII) and two subtypes (siIIA and siIIB). All infected lines of the sil haplotype carry wHa, wNo, or both; wMa and wNo are closely related and it is not clear whether they are distinct strains or variants of the same strain. Infected lines with the silIA haplotype harbor wRi and the siIIB haplotype carries wAu. The wMa infection is found in siIII haplotype lines. The phenotypic expression of cytoplasmic incompatibility and its relation to between-population differences in frequencies of Wolbachia infection are discussed.     

Generation of a novel Wolbachia infection in Aedes albopictus (Asian tiger mosquito) via embryonic microinjection

Genetic strategies that reduce or block pathogen transmission by mosquitoes are being investigated as a means to augment current control measures. Strategies of vector suppression and replacement are based upon intracellular Wolbachia bacteria, which occur naturally in many insect populations. Maternally inherited Wolbachia have evolved diverse mechanisms to manipulate host insect reproduction and promote infection invasion. One mechanism is cytoplasmic incompatibility (CI) through which Wolbachia promotes infection spread by effectively sterilizing uninfected females. In a prior field test, releases of Wolbachia-infected males were used to suppress a field population of Culex pipiens. An additional strategy would employ Wolbachia as a vehicle to drive desired transgenes into vector populations (population replacement). Wolbachia-based population suppression and population replacement strategies require an ability to generate artificial Wolbachia associations in mosquitoes. Here, we demonstrate a technique for transferring Wolbachia (transfection) in a medically important mosquito species: Aedes albopictus (Asian tiger mosquito). Microinjection was used to transfer embryo cytoplasm from a double-infected Ae. albopictus line into an aposymbiotic line. The resulting mosquito line is single-infected with the wAlbB Wolbachia type. The artificially generated infection type is not known to occur naturally and displays a new CI crossing type and the first known example of bidirectional CI in Aedes mosquitoes. We discuss the results in relation to applied mosquito control strategies and the evolution of Wolbachia infections in Ae. albopictus.     

Dynamics of Introduced Populations of Phragmidium violaceum and Implications for Biological Control of European Blackberry in Australia

Phragmidium violaceum causes leaf rust on the European blackberry (Rubus fruticosus L. aggregate). Multiple strains of this pathogen have been introduced into southern Australia for the biological control of at least 15 taxa of European blackberry, a nonindigenous, invasive plant. In climates conducive to leaf rust, the intensity of disease varies within and among infestations of the genetically variable host. Genetic markers developed from the selective amplification of microsatellite polymorphic loci were used to assess the population genetic structure and reproductive biology of P. violaceum within and among four geographically isolated and diseased infestations of the European blackberry in Victoria, Australia. Despite the potential for long-distance aerial dispersal of urediniospores, there was significant genetic differentiation among all populations, which was not associated with geographic separation. An assessment of multilocus linkage disequilibrium revealed temporal and geographic variation in the occurrence of random mating among the four populations. The presence of sexual spore states and the results of genetic analyses indicated that recombination, and potentially random migration and genetic drift, played an important role in maintaining genotypic variation within populations. Recombination and genetic differentiation in P. violaceum, as well as the potential for metapopulation structure, suggest the need to release additional, genetically diverse strains of the biocontrol agent at numerous sites across the distribution of the Australian blackberry infestation for maximum establishment and persistence.