Multiple endosymbiont infections and reproductive manipulations in a linyphiid spider population

In many arthropods, maternally inherited endosymbiotic bacteria can increase infection frequency by manipulating host reproduction. Multiple infections of different bacteria in a single host population are common, yet few studies have documented concurrent endosymbiont phenotypes or explored their potential interactions. We hypothesized that spiders might be a particularly useful taxon for investigating endosymbiont interactions, because they are host to a plethora of endosymbiotic bacteria and frequently exhibit multiple infections. We established two matrilines from the same population of the linyphiid spider Mermessus fradeorum and then used antibiotic curing and controlled mating assays to demonstrate that each matriline was subject to a distinct endosymbiotic reproductive manipulation. One matriline was co-infected with Rickettsia and Wolbachia and produced offspring with a radical female bias. Antibiotic treatment eliminated both endosymbionts and restored an even sex ratio to subsequent generations. Chromosomal and fecundity observations suggest a feminization mechanism. In the other matriline, a separate factorial mating assay of cured and infected spiders demonstrated strong cytoplasmic incompatibility (CI) induced by a different strain of Wolbachia. However, males with this Wolbachia induced only mild CI when mated with the Rickettsia–Wolbachia females. In a subsequent survey of a field population of M. fradeorum, we detected these same three endosymbionts infecting 55% of the spiders in almost all possible combinations, with nearly half of the infected spiders exhibiting multiple infection. Our results suggest that a dynamic network of endosymbionts may interact both within multiply infected hosts and within a population subject to multiple strong reproductive manipulations.     

Evolution and invasion dynamics of multiple infections with Wolbachia investigated using matrix based models

Endosymbiotic bacteria are often transmitted vertically from one host generation to the next via oocytes cytoplasm. The generally small number of colonizing bacteria in the oocytes leads to a bottleneck at each generation, resulting in genetic homogenization of the symbiotic population. Nevertheless, in many of the species infected by Wolbachia (maternally transmitted bacteria), individuals do sometimes simultaneously harbor several bacterial strains, owing to the fact that Wolbachia induces cytoplasmic incompatibility (CI) that maintains multiple infections. CI occurs in crosses in which the male is infected by at least one Wolbachia strain that the female lacks, and consequently it favors individuals with the greatest symbiotic diversity. CI results in death of offspring in diploid species. In haplodiploid individuals, unfertilized eggs hatch normally into males and fertilized ones, which would lead to females, either die (female mortality type: FM) or develop into males (male development type: MD). Until now, only one theoretical study, restricted to diploid species, has investigated the associations where multiple CI-inducing Wolbachia co-exist, and explored the conditions under which multiple infections can spread. The consequences of double infections on Wolbachia maintenance in host populations, and the selective pressures to which it is subjected have not yet been analysed. Here, we have re-written a model previously developed for single infection in matrix form, which allows easy extension to multiple infections and introduction of mutant strains. We show that (i) the CI type has a strong influence on invasiveness and maintenance of multiple infections; (ii) double infection lowers the invasion threshold of less competitive strains that hitch-hike with their companion strain; (iii) when multiple infections occur, as in single infections, the strains selected are those which maximize the production of infected offspring; and (iv) for the MD CI type, invasion of mutant strains can carry the whole infection to extinction.     

Endosymbionts moderate constrained sex allocation in a haplodiploid thrips species in a temperature-sensitive way

Maternally inherited bacterial endosymbionts that affect host fitness are common in nature. Some endosymbionts colonise host populations by reproductive manipulations (such as cytoplasmic incompatibility; CI) that increase the reproductive fitness of infected over uninfected females. Theory predicts that CI-inducing endosymbionts in haplodiploid hosts may also influence sex allocation, including in compatible crosses, however, empirical evidence for this is scarce. We examined the role of two common CI-inducing endosymbionts, Cardinium and Wolbachia, in the sex allocation of Pezothrips kellyanus, a haplodiploid thrips species with a split sex ratio. In this species, irrespective of infection status, some mated females are constrained to produce extremely male-biased broods, whereas other females produce extremely female-biased broods. We analysed brood sex ratio of females mated with males of the same infection status at two temperatures. We found that at 20 °C the frequency of constrained sex allocation in coinfected pairs was reduced by 27% when compared to uninfected pairs. However, at 25 °C the constrained sex allocation frequency increased and became similar between coinfected and uninfected pairs, resulting in more male-biased population sex ratios at the higher temperature. This temperature-dependent pattern occurred without changes in endosymbiont densities and compatibility. Our findings indicate that endosymbionts affect sex ratios of haplodiploid hosts beyond the commonly recognised reproductive manipulations by causing female-biased sex allocation in a temperature-dependent fashion. This may contribute to a higher transmission efficiency of CI-inducing endosymbionts and is consistent with previous models that predict that CI by itself is less efficient in driving endosymbiont invasions in haplodiploid hosts.Subject terms: Evolutionary genetics, Evolutionary ecology, Parasitology     

Bacteria Endosymbiont,Wolbachia, Promotes Parasitism of Parasitoid Wasp Asobara japonica

Wolbachia is the most widespread endosymbiotic bacterium that manipulates reproduction of its arthropod hosts to enhance its own spread throughout host populations. Infection with Wolbachia causes complete parthenogenetic reproduction in many Hymenoptera, producing only female offspring. The mechanism of such reproductive manipulation by Wolbachia has been extensively studied. However, the effects of Wolbachia symbiosis on behavioral traits of the hosts are scarcely investigated. The parasitoid wasp Asobara japonica is an ideal insect to investigate this because symbiotic and aposymbiotic strains are available: Wolbachia-infected Tokyo (TK) and noninfected Iriomote (IR) strains originally collected on the main island and southwest islands of Japan, respectively. We compared the oviposition behaviors of the two strains and found that TK strain females parasitized Drosophila melanogaster larvae more actively than the IR strain, especially during the first two days after eclosion. Removing Wolbachia from the TK strain wasps by treatment with tetracycline or rifampicin decreased their parasitism activity to the level of the IR strain. Morphological and behavioral analyses of both strain wasps showed that Wolbachia endosymbionts do not affect development of the host female reproductive tract and eggs, but do enhance host-searching ability of female wasps. These results suggest the possibility that Wolbachia endosymbionts may promote their diffusion and persistence in the host A. japonica population not only at least partly by parthenogenesis but also by enhancement of oviposition frequency of the host females.     

Evidence for Horizontal Transmission of Secondary Endosymbionts in the Bemisia tabaci Cryptic Species Complex

Bemisia tabaci (Hemiptera: Aleyrodidae) is a globally distributed pest composed of at least 34 morphologically indistinguishable cryptic species. At least seven species of endosymbiont have been found infecting some or all members of the complex. The origin(s) of the associations between specific endosymbionts and their whitefly hosts is unknown. Infection is normally vertical, but horizontal transmission does occur and is one way for new infections to be introduced into individuals. The relationships between the different members of the cryptic species complex and the endosymbionts have not been well explored. In this study, the phylogenies of different cryptic species of the host with those of their endosymbionts were compared. Of particular interest was whether there was evidence for both coevolution and horizontal transmission. Congruence was observed for the primary endosymbiont, Portiera aleyrodidarum, and partial incongruence in the case of two secondary endosymbionts, Arsenophonus and Cardinium and incongruence for a third, Wolbachia. The patterns observed for the primary endosymbiont supported cospeciation with the host while the patterns for the secondary endosymbionts, and especially Wolbachia showed evidence of host shifts and extinctions through horizontal transmission rather than cospeciation. Of particular note is the observation of several very recent host shift events in China between exotic invader and indigenous members of the complex. These shifts were from indigenous members of the complex to the invader as well as from the invader to indigenous relatives.     

Extranuclear DNA and the Endosymbionts of <Emphasis Type="Italic">Paramecium aurelia</Emphasis>

THE basis of cytoplasmic inheritance in the killer system of Paramecium aurelia has been located to endosymbionts* in the cytoplasm. Breeding experiments have shown the maintenance and replication of some of the endosymbionts in their cellular environment to depend on nuclear genes of the Paramecium host cell¹. There are indications of a specific adaptation between the endosymbionts and certain strains or syngens (≡sibling species)².     

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.     

The effect of Wolbachia versus genetic incompatibilities on reinforcement and speciation

Wolbachia is a widespread group of intracellular bacteria commonly found in arthropods. In many insect species, Wolbachia induce a cytoplasmic mating incompatibility (CI). If different Wolbachia infections occur in the same host species, bidirectional CI is often induced. Bidirectional CI acts as a postzygotic isolation mechanism if parapatric host populations are infected with different Wolbachia strains. Therefore, it has been suggested that Wolbachia could promote speciation in their hosts. In this article we investigate theoretically whether Wolbachia-induced bidirectional CI selects for premating isolation and therefore reinforces genetic divergence between parapatric host populations. To achieve this we combined models for Wolbachia dynamics with a well-studied reinforcement model. This new model allows us to compare the effect of bidirectional CI on the evolution of female mating preferences with a situation in which postzygotic isolation is caused by nuclear genetic incompatibilities (NI). We distinguish between nuclear incompatibilities caused by two loci with epistatic interactions, and a single locus with incompatibility among heterozygotes in the diploid phase. Our main findings are: (1) bidirectional CI and single locus NI select for premating isolation with a higher speed and for a wider parameter range than epistatic NI; (2) under certain parameter values, runaway sexual selection leads to the increase of an introduced female preference allele and fixation of its preferred male trait allele in both populations, whereas under others it leads to divergence in the two populations in preference and trait alleles; and (3) bidirectional CI and single locus NI can stably persist up to migration rates that are two times higher than seen for epistatic NI. The latter finding is important because the speed with which mutants at the preference locus spread increases exponentially with the migration rate. In summary, our results show that bidirectional CI selects for rapid premating isolation and so generally support the view that Wolbachia can promote speciation in their hosts.     

Detection of a novel intracellular microbiome hosted in arbuscular mycorrhizal fungi

Arbuscular mycorrhizal fungi (AMF) are important members of the plant microbiome. They are obligate biotrophs that colonize the roots of most land plants and enhance host nutrient acquisition. Many AMF themselves harbor endobacteria in their hyphae and spores. Two types of endobacteria are known in Glomeromycota: rod-shaped Gram-negative Candidatus Glomeribacter gigasporarum, CaGg, limited in distribution to members of the Gigasporaceae family, and coccoid Mollicutes-related endobacteria, Mre, widely distributed across different lineages of AMF. The goal of the present study is to investigate the patterns of distribution and coexistence of the two endosymbionts, CaGg and Mre, in spore samples of several strains of Gigaspora margarita. Based on previous observations, we hypothesized that some AMF could host populations of both endobacteria. To test this hypothesis, we performed an extensive investigation of both endosymbionts in G. margarita spores sampled from Cameroonian soils as well as in the Japanese G. margarita MAFF520054 isolate using different approaches (molecular phylotyping, electron microscopy, fluorescence in situ hybridization and quantitative real-time PCR). We found that a single AMF host can harbour both types of endobacteria, with Mre population being more abundant, variable and prone to recombination than the CaGg one. Both endosymbionts seem to retain their genetic and lifestyle peculiarities regardless of whether they colonize the host alone or together. These findings show for the first time that fungi support an intracellular bacterial microbiome, in which distinct types of endobacteria coexist in a single cell.     

Molecular identification of rickettsial endosymbionts in the non-phagotrophic volvocalean green algae

Background

The order Rickettsiales comprises Gram-negative obligate intracellular bacteria (also called rickettsias) that are mainly associated with arthropod hosts. This group is medically important because it contains human-pathogenic species that cause dangerous diseases. Until now, there has been no report of non-phagotrophic photosynthetic eukaryotes, such as green plants, harboring rickettsias.

Methodology/Principal Findings

We examined the bacterial endosymbionts of two freshwater volvocalean green algae: unicellular Carteria cerasiformis and colonial Pleodorina japonica. Epifluorescence microscopy using 4′-6-deamidino-2-phenylindole staining revealed the presence of endosymbionts in all C. cerasiformis NIES-425 cells, and demonstrated a positive correlation between host cell size and the number of endosymbionts. Strains both containing and lacking endosymbionts of C. cerasiformis (NIES-425 and NIES-424) showed a >10-fold increase in cell number and typical sigmoid growth curves over 192 h. A phylogenetic analysis of 16 S ribosomal (r)RNA gene sequences from the endosymbionts of C. cerasiformis and P. japonica demonstrated that they formed a robust clade (hydra group) with endosymbionts of various non-arthropod hosts within the family Rickettsiaceae. There were significantly fewer differences in the 16 S rRNA sequences of the rickettsiacean endosymbionts between C. cerasiformis and P. japonica than in the chloroplast 16 S rRNA or 18 S rRNA of the host volvocalean cells. Fluorescence in situ hybridization demonstrated the existence of the rickettsiacean endosymbionts in the cytoplasm of two volvocalean species.

Conclusions/Significance

The rickettsiacean endosymbionts are likely not harmful to their volvocalean hosts and may have been recently transmitted from other non-arthropod organisms. Because rickettsias are the closest relatives of mitochondria, incipient stages of mitochondrial endosymbiosis may be deduced using both strains with and without C. cerasiformis endosymbionts.     

Coexistence of cytoplasmic incompatibility and male-killing-inducing endosymbionts, and their impact on host gene flow

Male-killing (MK) and cytoplasmic incompatibility (CI) inducing bacteria are among the most common endosymbionts of arthropods. Previous theoretical research has demonstrated that these two types of endosymbionts cannot stably coexist within a single unstructured host population if no doubly infected host individuals occur. Here, we analyse a model of two host subpopulations connected by migration. We demonstrate that coexistence of MK- and CI-inducing endosymbionts is possible if migration rates are sufficiently low. In particular, our results suggest that for coexistence to be possible, migration rates into the subpopulation infected predominantly with MK-inducing endosymbionts must be considerably low, while migration rates from the MK- to the CI-infected subpopulation can be very high. We also analyse how the presence of MK- and CI-inducing endosymbionts affects host gene flow between the two subpopulations. Employing the concept of the 'effective migration rate', we demonstrate that compared with an uninfected subdivided population, gene flow is increased towards the MK-infected island, but decreased towards the CI-infected island. We discuss our results with respect to the butterfly Hypolimnas bolina, in which infection polymorphism of CI- and MK-inducing Wolbachia has been reported across South-Pacific island populations.     

Within-host Competition Does Not Select for Virulence in Malaria Parasites; Studies with Plasmodium yoelii

In endemic areas with high transmission intensities, malaria infections are very often composed of multiple genetically distinct strains of malaria parasites. It has been hypothesised that this leads to intra-host competition, in which parasite strains compete for resources such as space and nutrients. This competition may have repercussions for the host, the parasite, and the vector in terms of disease severity, vector fitness, and parasite transmission potential and fitness. It has also been argued that within-host competition could lead to selection for more virulent parasites. Here we use the rodent malaria parasite Plasmodium yoelii to assess the consequences of mixed strain infections on disease severity and parasite fitness. Three isogenic strains with dramatically different growth rates (and hence virulence) were maintained in mice in single infections or in mixed strain infections with a genetically distinct strain. We compared the virulence (defined as harm to the mammalian host) of mixed strain infections with that of single infections, and assessed whether competition impacted on parasite fitness, assessed by transmission potential. We found that mixed infections were associated with a higher degree of disease severity and a prolonged infection time. In the mixed infections, the strain with the slower growth rate was often responsible for the competitive exclusion of the faster growing strain, presumably through host immune-mediated mechanisms. Importantly, and in contrast to previous work conducted with Plasmodium chabaudi, we found no correlation between parasite virulence and transmission potential to mosquitoes, suggesting that within-host competition would not drive the evolution of parasite virulence in P. yoelii.     

Fitness effects of Wolbachia and Spiroplasma in Drosophila melanogaster

Maternally inherited endosymbionts that manipulate the reproduction of their insect host are very common. Aside from the reproductive manipulation they produce, the fitness of these symbionts depends in part on the direct impact they have on the female host. Although this parameter has commonly been investigated for single infections, it has much more rarely been established in dual infections. We here establish the direct effect of infection with two different symbionts exhibiting different reproductive manipulation phenotypes, both alone and in combination, in the fruit fly Drosophila melanogaster. This species carries a cytoplasmic incompatibility inducing Wolbachia and a male-killing Spiroplasma, occurring as single or double (co-) infections in natural populations. We assessed direct fitness effects of these bacteria on their host, by comparing larval competitiveness and adult fecundity of uninfected, Wolbachia, Spiroplasma and Wolbachia–Spiroplasma co-infected females. We found no effect of infection status on the fitness of females for both estimates, that is, no evidence of any benefits or costs to either single or co-infection. This leads to the conclusion that both bacteria probably have other sources of benefits to persist in D. melanogaster populations, either by means of their reproductive manipulations (fitness compensation from male death in Spiroplasma infection and cytoplasmic incompatibility in Wolbachia infection) or by positive fitness interactions on other fitness components.     

Wolbachia in the spittlebug Prosapia ignipectus: Variable infection frequencies,but no apparent effect on host reproductive isolation

Animals serve as hosts for complex communities of microorganisms, including endosymbionts that live inside their cells. Wolbachia bacteria are perhaps the most common endosymbionts, manipulating host reproduction to propagate. Many Wolbachia cause cytoplasmic incompatibility (CI), which results in reduced egg hatch when uninfected females mate with infected males. Wolbachia that cause intense CI spread to high and relatively stable frequencies, while strains that cause weak or no CI tend to persist at intermediate, often variable, frequencies. Wolbachia could also contribute to host reproductive isolation (RI), although current support for such contributions is limited to a few systems. To test for Wolbachia frequency variation and effects on host RI, we sampled several local Prosapia ignipectus (Fitch) (Hemiptera: Cercopidae) spittlebug populations in the northeastern United States over two years, including closely juxtaposed Maine populations with different monomorphic color forms, “black” and “lined.” We discovered a group‐B Wolbachia (wPig) infecting P. ignipectus that diverged from group‐A Wolbachia—like model wMel and wRi strains in Drosophila—6 to 46 MYA. Populations of the sister species Prosapia bicincta (Say) from Hawaii and Florida are uninfected, suggesting that P. ignipectus acquired wPig after their initial divergence. wPig frequencies were generally high and variable among sites and between years. While phenotyping wPig effects on host reproduction is not currently feasible, the wPig genome contains three divergent sets of CI loci, consistent with high wPig frequencies. Finally, Maine monomorphic black and monomorphic lined populations of P. ignipectus share both wPig and mtDNA haplotypes, implying no apparent effect of wPig on the maintenance of this morphological contact zone. We hypothesize P. ignipectus acquired wPig horizontally as observed for many Drosophila species, and that significant CI and variable transmission produce high but variable wPig frequencies.     

共生菌Wolbachia引起宿主细胞质不亲和的研究进展

Wolbachia 是一类广泛存在于节肢动物以及线虫体内细胞质中呈母系遗传的共生细菌,能够在宿主中产生细胞质不亲和、孤雌生殖、雌性化及杀雄等多种生殖调控作用,其中细胞质不亲和是指被 Wolbachia 感染的雄性个体与未感染的雌性个体（单向不亲和）,或者感染不同株系 Wolbachia 的雌性个体（双向不亲和）交配后不能或很少产生后代,或者后代偏雄性的现象。细胞质不亲和作用使感染的雌性个体在种群中具有很大的生殖优势,凭借这种生殖优势,Wolbachia 能够迅速在宿主种群中扩张。细胞质不亲和的机理探索主要集中在细胞学水平上,其中广为接受的精子“修饰”和“拯救”理论认为,精巢中的 Wolbachia 能够修饰宿主的精细胞,使其不能和卵细胞正常融合,但是当母本感染相同的 Wolbachia 时,就能够将“修饰”过的精子细胞“拯救”过来,使其恢复与卵细胞的正常融合。而分子机理上的探索也开始在转录组、基因组和miRNA水平上对部分昆虫展开了研究。影响细胞质不亲和的因素有很多,包括宿主遗传背景、 Wolbachia 株系、Wolbachia 基因型、共生菌密度（浓度、滴度）、雄虫年龄、环境因素以及共生菌在宿主生殖组织的分布等。近年来,人类也应用细胞质不亲和控制害虫（主要是蚊虫）和人类疾病,取得了较好的进展。     

Palaeosymbiosis Revealed by Genomic Fossils of Wolbachia in a Strongyloidean Nematode

Wolbachia are common endosymbionts of terrestrial arthropods, and are also found in nematodes: the animal-parasitic filaria, and the plant-parasite Radopholus similis. Lateral transfer of Wolbachia DNA to the host genome is common. We generated a draft genome sequence for the strongyloidean nematode parasite Dictyocaulus viviparus, the cattle lungworm. In the assembly, we identified nearly 1 Mb of sequence with similarity to Wolbachia. The fragments were unlikely to derive from a live Wolbachia infection: most were short, and the genes were disabled through inactivating mutations. Many fragments were co-assembled with definitively nematode-derived sequence. We found limited evidence of expression of the Wolbachia-derived genes. The D. viviparus Wolbachia genes were most similar to filarial strains and strains from the host-promiscuous clade F. We conclude that D. viviparus was infected by Wolbachia in the past, and that clade F-like symbionts may have been the source of filarial Wolbachia infections.     

Within-host competition between Borrelia afzelii ospC strains in wild hosts as revealed by massively parallel amplicon sequencing

Infections frequently consist of more than one strain of a given pathogen. Experiments have shown that co-infecting strains often compete, so that the infection intensity of each strain in mixed infections is lower than in single strain infections. Such within-host competition can have important epidemiological and evolutionary consequences. However, the extent of competition has rarely been investigated in wild, naturally infected hosts, where there is noise in the form of varying inoculation doses, asynchronous infections and host heterogeneity, which can potentially alleviate or eliminate competition. Here, we investigated the extent of competition between Borrelia afzelii strains (as determined by ospC genotype) in three host species sampled in the wild. For this purpose, we developed a protocol for 454 amplicon sequencing of ospC, which allows both detection and quantification of each individual strain in an infection. Each host individual was infected with one to six ospC strains. The infection intensity of each strain was lower in mixed infections than in single ones, showing that there was competition. Rank-abundance plots revealed that there was typically one dominant strain, but that the evenness of the relative infection intensity of the different strains in an infection increased with the multiplicity of infection. We conclude that within-host competition can play an important role under natural conditions despite many potential sources of noise, and that quantification by next-generation amplicon sequencing offers new possibilities to dissect within-host interactions in naturally infected hosts.     

Wolbachia Strengthens Cardinium-Induced Cytoplasmic Incompatibility in the Spider Mite Tetranychus piercei McGregor

Wolbachia and Cardinium are maternally inherited intracellular bacteria that can manipulate the reproduction of their arthropod hosts, such as by inducing cytoplasmic incompatibility (CI). Although the reproductive alteration induced by Wolbachia or Cardinium have been well investigated, the effects of these two endosymbionts co-infecting the same host are poorly understood. We found that Tetranychus piercei McGregor is naturally infected with Wolbachia and Cardinium. We performed all possible crossing combinations using naturally infected and cured strains, and the results show that Wolbachia induced a weak level of CI, while Cardinium-infected and doubly infected males caused severe CI. Wolbachia and Cardinium could not rescue CI each other; however, Wolbachia boosted the expression of Cardinium-induced CI. Quantitative PCR results demonstrated that CI was associated with the infection density of Wolbachia and Cardinium.     

Host genetic architecture in single and multiple infections

Hosts are often target to multiple simultaneous infections by genetically diverse parasite strains. The interaction among these strains and the interaction of each strain with the host was shown to have profound effects on the evolution of parasite traits. Host factors like genetic architecture of resistance have so far been largely neglected. To see whether genetic architecture differs between different kinds of infections we used joint scaling analysis to compare the genetic components of resistance in the red flour beetle Tribolium castaneum exposed to single and multiple strains of the microsporidian Nosema whitei. Our results indicate that additive, dominance and epistatic components were more important in single infections whereas maternal components play a decisive role in multiple infections. In detail, parameter estimates of additive, dominance and epistatic components correlated positively between single and multiple infections, whereas maternal components correlated negatively. These findings may suggest that specificity of host–parasite interactions are mediated by genetic and especially epistatic components whereas maternal effects constitute a more general form of resistance.     

Host genotype changes bidirectional to unidirectional cytoplasmic incompatibility in Nasonia longicornis

Wolbachia are the most abundant maternally inherited endosymbionts of insects and cause various reproductive alterations in their hosts. One such manipulation is cytoplasmic incompatibility (CI), which is a sperm-egg incompatibility typically resulting in zygotic death. Nasonia longicornis (Hymenoptera: Pteromalidae) has an A supergroup and two closely related B supergroup Wolbachia infections. The B supergroup bacteria co-diverged in this host genus. Both triple (wNlonAwNlonB1wNlonB2) and double infections (wNlonAwNlonB1, wNlonAwNlonB2) have been obtained from the field. In the present study, CI was determined among the three Wolbachia types in different host genetic backgrounds. Results show that host genetic background determines whether bidirectional CI or unidirectional CI occurs between the two closely related B group Wolbachia. Results show that the wNlonB1-infected males are bidirectionally incompatible with wNlonB2 in their 'native' nuclear genetic background, whereas wNlonB1 males are compatible with wNlonB2 in two other N. longicornis genetic backgrounds, resulting in unidirectional CI. In contrast, wNlonB2-infected males are incompatible with wNlonB1 females in all three host genetic backgrounds. These changes in incompatibility are not due to the loss of the bacteria. We hypothesize that a repressor gene for sperm modification by wNlonB1 is segregating in N. longicornis populations. The relevance of these findings to the potential role of Wolbachia in host-reproductive divergence and speciation is discussed.