The tick endosymbiont Candidatus Midichloria mitochondrii and selenoproteins are essential for the growth of Rickettsia parkeri in the Gulf Coast tick vector

BACKGROUND

Pathogen colonization inside tick tissues is a significant aspect of the overall competence of a vector. Amblyomma maculatum is a competent vector of the spotted fever group rickettsiae, Rickettsia parkeri. When R. parkeri colonizes its tick host, it has the opportunity to dynamically interact with not just its host but with the endosymbionts living within it, and this enables it to modulate the tick's defenses by regulating tick gene expression. The microbiome in A. maculatum is dominated by two endosymbiont microbes: a Francisella-like endosymbiont (FLE) and Candidatus Midichloria mitochondrii (CMM). A range of selenium-containing proteins (selenoproteins) in A. maculatum ticks protects them from oxidative stress during blood feeding and pathogen infections. Here, we investigated rickettsial multiplication in the presence of tick endosymbionts and characterized the functional significance of selenoproteins during R. parkeri replication in the tick.

RESULTS

FLE and CMM were quantified throughout the tick life stages by quantitative PCR in R. parkeri-infected and uninfected ticks. R. parkeri infection was found to decrease the FLE numbers but CMM thrived across the tick life cycle. Our qRT-PCR analysis indicated that the transcripts of genes with functions related to redox (selenogenes) were upregulated in ticks infected with R. parkeri. Three differentially expressed proteins, selenoprotein M, selenoprotein O, and selenoprotein S were silenced to examine their functional significance during rickettsial replication within the tick tissues. Gene silencing of the target genes was found to impair R. parkeri colonization in the tick vector. Knockdown of the selenogenes triggered a compensatory response from other selenogenes, as observed by changes in gene expression, but oxidative stress levels and endoplasmic reticulum stress inside the ticks were also found to have heightened.

CONCLUSIONS

This study illustrates the potential of this new research model for augmenting our understanding of the pathogen interactions occurring within tick hosts and the important roles that symbionts and various tick factors play in regulating pathogen growth.

Verified and potential pathogens of predatory mites (Acari: Phytoseiidae)

Several species of phytoseiid mites (Acari: Phytoseiidae), including species of the genera Amblyseius, Galendromus, Metaseiulus, Neoseiulus, Phytoseiulus and Typhlodromus, are currently reared for biological control of various crop pests and/or as model organisms for the study of predator-prey interactions. Pathogen-free phytoseiid mites are important to obtain high efficacy in biological pest control and to get reliable data in mite research, as pathogens may affect the performance of their host or alter their reproduction and behaviour. Potential and verified pathogens have been reported for phytoseiid mites during the past 25 years. The present review provides an overview, including potential pathogens with unknown host effects (17 reports), endosymbiotic Wolbachia (seven reports), other bacteria (including Cardinium and Spiroplasma) (four reports), cases of unidentified diseases (three reports) and cases of verified pathogens (six reports). From the latter group four reports refer to Microsporidia, one to a fungus and one to a bacterium. Only five entities have been studied in detail, including Wolbachia infecting seven predatory mite species, other endosymbiotic bacteria infecting Metaseiulus (Galendromus, Typhlodromus) occidentalis (Nesbitt), the bacterium Acaricomes phytoseiuli infecting Phytoseiulus persimilis Athias-Henriot, the microsporidium Microsporidium phytoseiuli infecting P. persimilis and the microsporidium Oligosproridium occidentalis infecting M. occidentalis. In four cases (Wolbachia, A. phytoseiuli, M. phytoseiuli and O. occidentalis) an infection may be connected with fitness costs of the host. Moreover, infection is not always readily visible as no obvious gross symptoms are present. Monitoring of these entities on a routine and continuous basis should therefore get more attention, especially in commercial mass-production. Special attention should be paid to field-collected mites before introduction into the laboratory or mass rearing, and to mites that are exchanged among rearing facilities. However, at present general pathogen monitoring is not yet practical as effects of many entities are unknown. More research effort is needed concerning verified and potential pathogens of commercially reared arthropods and those used as model organisms in research.

[Bacterial symbionts (Wolbachia) of filarial nematodes: implications for the treatment and pathology of filariasis]

Filarial nematodes harbour intracellular, Gram-negative bacteria belonging to the genus Wolbachia. These bacteria have been observed in various species of filariae, including the main filariasis agents of humans and animals. It has been suggested that Wolbachia could play an important role in the biology of filarial nematodes and could be implicated in the pathogenesis of filarial diseases. Wolbachia could thus represent a target for the control of filariasis and key to the understanding of these diseases. Indeed, in various species of filariae, tetracycline treatments have been shown both to reduce/eliminate the Wolbachia population and to determine detrimental effects on the nematodes. In addition, proteins of Wolbachia have been shown to determine specific IgG responses in animals infected by filariae and some Wolbachia molecules (e.g. LPS) have been shown to stimulate innate-immunity responses (e.g. production of cytokines such as IL1, IL6, IL10, TNF-alpha and IFN-gamma by macrophages).

Feminization of genetic males by a symbiotic bacterium in a butterfly, Eurema hecabe (Lepidoptera: Pieridae)

Wolbachia are symbiotic bacteria found in many arthropods and filarian nematodes. They often manipulate the reproduction of host arthropods. In the present study, female-biased sex-ratio distortion in the butterfly Eurema hecabe was investigated. Breeding experiments showed that this distorted sex ratio is maternally inherited. When treated with tetracycline, adult females of the thelygenic line produced male progeny only. After PCR using Wolbachia-specific primers for the ftsZ gene a positive result was seen in the thelygenic females, but not in male progeny from tetracycline-treated females, or individuals from a Tokyo population with normal sex ratio and reproduction. Cytological observations showed that thelygenic females lack the sex chromatin body (W chromosome). The results strongly suggest that the sex-ratio distortion in E. hecabe is due to feminization of genetic males by Wolbachia.

Evolution of Wolbachia pipientis transmission dynamics in insects

Wolbachia pipientis is an intracellular bacterial parasite of arthropods that enhances its transmission by manipulating host reproduction, most commonly by inducing cytoplasmic incompatibility. The discovery of isolates with modified cytoplasmic incompatibility phenotypes and others with novel virulence properties is an indication of the potential breadth of evolutionary strategies employed by Wolbachia.

A stable triple Wolbachia infection in Drosophila with nearly additive incompatibility effects

Drosophila simulans strains infected with three different Wolbachia strains were generated by experimental injection of a third symbiont into a naturally double-infected strain. This transfer led to a substantial increase in total Wolbachia density in the host strain. Each of the three symbionts was stably transmitted in the presence of the other two. Triple-infected males were incompatible with double-infected females. No evidence was obtained for interference between modification effects of the different Wolbachia strains in males. Some incompatibility was observed between triple-infected males and females. However, this incompatibility reaction is not a specific property of triple-infected flies, because it was also observed in double-infected strains.

Horizontal transfer of Wolbachia between phylogenetically distant insect species by a naturally occurring mechanism

Wolbachia is a genus of alpha-proteobacteria found in obligate intracellular association with a wide variety of arthropods, including an estimated 10-20% of all insect species [1]. Wolbachia represents one of a number of recently identified 'reproductive parasites' [2] which manipulate the reproduction of their hosts in ways that enhance their own transmission [3] [4] [5] [6] [7] [8] [9]. The influence of Wolbachia infection on the dynamics of host populations has focused considerable interest on its possible role in speciation through reproductive isolation [3] [10] [11] and as an agent of biological control [2] [12] [13]. Although Wolbachia normally undergoes vertical transmission through the maternal line of its host population [14], there is compelling evidence from molecular phylogenies that extensive horizontal (intertaxon) transmission must have occurred [1] [9] [15] [16] [17]. Some of the best candidate vectors for the horizontal transmission of Wolbachia are insect parasitoids [15], which comprise around 25% of all insect species and attack arthropods from an enormous range of taxa [18]. In this study, we used both fluorescence microscopy and PCR amplification with Wolbachia-specific primers to show that Wolbachia can be transmitted to a parasitic wasp (Leptopilina boulardi) from its infected host (Drosophila simulans) and subsequently undergo diminishing vertical transmission in this novel host species. These results are, to our knowledge, the first to reveal a natural horizontal transfer route for Wolbachia between phylogenetically distant insect species.

Widespread occurrence of the micro-organism Wolbachia in ants

For more than 20 years, sex allocation in hymenopteran societies has been a major topic in insect sociobiology. A recurring idea was that relatedness asymmetrics arising from their haplodiploid sex determination system would lead to various parent-offspring conflicts over optimal reproduction. A possible weakness of existing theory is that only interests of nuclear genes are properly accounted for. Yet, a diversity of maternally transmitted elements manipulate the reproduction of their host in many solitary arthropod groups. The bacterium Wolbachia is a striking example of such a selfish cytoplasmic element, with effects ranging from reproductive incompatibility between host strains, induction of parthenogenesis and feminization of males. This paper reports on a first PCR-based Wolbachia screening in ants. Out of 50 Indo-Australian species, 50% screened positive for an A-group strain. One of these species also harboured a B-group strain in a double infection. Various factors that might explain the unusually high incidence of Wolbachia in ants are discussed. In general, Wolbachia may represent a widespread and previously unrecognized party active in the conflicts of interest within social insect colonies.

Horizontal transfer of parasitic sex ratio distorters between crustacean hosts

Parasitic sex distorters were artificially transferred within and between crustacean host species in order to study the effects of parasitism on host fitness and sex determination and to investigate parasite host specificity. Implantation of Nosema sp. to uninfected strains of its Gammarus duebeni host resulted in an active parasite infection in the gonad of recipient females and subsequent transovarial parasite transmission. The young of artificially infected females were feminized by the parasite, demonstrating that Nosema sp. is a cause of a sex ratio distortion in its host. In contrast, we were unable to cross-infect Armadillidium vulgare with the feminizing microsporidian from G. duebeni or to cross-infect G. duebeni with the feminizing bacterium Wolbachia sp. from A. vulgare.

Effects of A and B Wolbachia and host genotype on interspecies cytoplasmic incompatibility in Nasonia

Wolbachia endosymbionts cause postmating reproductive isolation between the sibling species Nasonia vitripennis and N. giraulti. Most Nasonia are doubly infected with a representative from each of the two major Wolbachia groups (A and B). This study investigates the role of single (A or B) and double (A and B) Wolbachia infections in interspecies cytoplasmic incompatibility (CI) and host genomic influences on the incompatibility phenotype. Results show that the single A Wolbachia harbored in N. vitripennis (wAv) is bidirectionally incompatible with the single A Wolbachia harbored in N. giraulti (wAg). Results also indirectly show that the N. vitripennis wBv is bidirectionally incompatible with the N. giraulti wBg. The findings support current phylogenetic evidence that suggests these single infections have independent origins and were acquired via horizontal transfer. The wAv Wolbachia expresses partial CI in the N. vitripennis nuclear background. However, following genomic replacement by introgression, wAv expresses complete CI in the N. giraulti background and remains bidirectionally incompatible with wAg. Results show that double infections can reinforce interspecies reproductive isolation through the addition of incompatibility types and indicate that the host genome can influence incompatibility levels. This study has implications for host-symbiont coevolution and the role of Wolbachia in speciation.

Overcoming cytoplasmic incompatibility in Drosophila

The endocellular microbe Wolbachia pipientis infects a wide variety of invertebrate species, in which its presence is closely linked to a form of reproductive failure termed cytoplasmic incompatibility (CI). CI renders infected males unable to father offspring when mated to uninfected females. Because CI can dramatically affect fitness in natural populations, mechanisms that abate CI can have equally large impacts on fitness. We have discovered that repeated copulation by Wolbachia-infected male Drosophila simulans significantly diminishes CI. Repeated copulation does not prevent Wolbachia from populating developing spermatids, but may reduce the time during spermatogenesis when Wolbachia can express CI. This restoration of fertility in premated infected males could have important implications for Wolbachia transmission and persistence in nature and for its exploitation as an agent of biological pest control.

The diverse habitats of obligate intracellular parasites

Bacterial obligate intracellular parasites have evolved diverse mechanisms for evasion of host cellular defenses. These mechanisms involve adaptations for survival in distinct intracellular compartments. Intracellular niches inhabited by obligate intracellular parasites include the cytoplasm, arrested early endosomes, lysosomes, and vesicles that do not fuse with the endosomal compartment but intersect with an exocytic pathway.

Rhizobium gallicum sp. nov. and Rhizobium giardinii sp. nov., from Phaseolus vulgaris nodules

Thirty-one strains of two new genomic species (genomic species 1 and 2) of rhizobia isolated from root nodules of Phaseolus vulgaris and originating from various locations in France were compared with reference strains of rhizobia by performing a numerical analysis of 64 phenotypic features. Each genomic species formed a distinct phenon and was separated from the other rhizobial species. A comparison of the complete 16S rRNA gene sequences of a representative of genomic species 1 (strain R602spT) and a representative of genomic species 2 (strain H152T) with the sequences of other rhizobia and related bacteria revealed that each genomic species formed a lineage independent of the lineages formed by the previously recognized species of rhizobia. Genomic species 1 clustered with the species that include the bean-nodulating rhizobia, Rhizobium leguminosarum, Rhizobium etli, and Rhizobium tropici, and branched with unclassified rhizobial strain OK50, which was isolated from root nodules of Pterocarpus klemmei in Japan. Genomic species 2 was distantly related to all other Rhizobium species and related taxa, and the most closely related organisms were Rhizobium galegae and several Agrobacterium species. On the basis of the results of phenotypic and phylogenetic analyses and genotypic data previously published and reviewed in this paper, two new species of the genus Rhizobium, Rhizobium gallicum and Rhizobium giardinii, are proposed for genomic species 1 and 2, respectively. Each species could be divided in two subgroups on the basis of symbiotic characteristics, as shown by phenotypic (host range and nitrogen fixation effectiveness) and genotypic data. For each species, one subgroup had the same symbiotic characteristics as R. leguminosarum biovar phaseoli and R. etli biovar phaseoli. The other subgroup had a species-specific symbiotic phenotype and genotype. Therefore, we propose that each species should be subdivided into two biovars, as follows: R. gallicum biovar gallicum and R. gallicum biovar phaseoli; and R. giardinii biovar giardinii and R. giardinii biovar phaseoli.

Classification of Austrian rhizobia and the Mexican isolate FL27 obtained from Phaseolus vulgaris L. as Rhizobium gallicum

The phylogenetic positions of four rhizobial strains obtained from nodules of common bean plants (Phaseolus vulgaris L.) grown in an Austrian soil and of the Mexican bean isolate FL27 are described. Analysis of the 16S rRNA genes revealed sequences almost identical to that of the Rhizobium gallicum type strain, R602sp, with a maximum of two nucleotide substitutions. Comparison of the 16S rRNA gene sequences with those from other bacteria indicated highest similarity to Rhizobium sp. strain OK-50, Rhizobium leguminosarum IAM 12609, and Rhizobium etli. DNA homology determined by DNA-DNA hybridization was high among the Austrian isolates and R602spT (45 to 90%) and ranged from 21 to 65% with FL27, but hybridization analysis revealed very low homology to the recognized common bean-nodulating species, R. leguminosarum bv. phaseoli, R. etli, and Rhizobium tropici. Ribosomal gene organization was studied by Southern hybridization with the 16S rRNA gene and temperature gradient gel electrophoresis, indicating identical organizations and the presence of three identical 16S rRNA copies in the genome of this species. The six strains investigated showed different plasmid profiles based on their geographical origins. We propose that the Austrian isolates and the Mexican strain FL27 are members of the species R. gallicum.

Biodiversity of bradyrhizobia nodulating Lupinus spp

The genetic structure of Bradyrhizobium isolates recovered from three Lupinus species (Lupinus campestris, Lupinus montanus, and Lupinus exaltatus) grown in Mexico was examined. Among 41 Bradyrhizobium isolates, 18 electrophoretic types (ETs) were distinguished by multilocus enzyme electrophoresis of five metabolic enzymes. The mean genetic diversity, 0.64, indicated that there was great genetic diversity in the population sampled. Most isolates (63%) fell into two closely related clusters (clusters I and II) and were the types most frequently isolated from the root nodules of L. montanus and L. campestris. ET cluster III isolates were frequent nodule occupants of L. exaltatus. The isolates also were assigned to three main groups by using Curie point pyrolysis mass spectrometry. In general, the multilocus enzyme electrophoretic data and pyrolysis mass spectrometric data agreed. We determined the 16S rRNA sequences of representative Lupinus isolates and of Bradyrhizobium japonicum USDA 6T and found that the lupine isolates were highly related to the B. japonicum type strain, although not all B. japonicum type strains (subcultures maintained in different bacterial collections) had identical small-subunit rRNA.

Thanks to a parasite, asexual reproduction catches on

Can you catch a new kind of sex life? Wasps can, according to new research on the speciation patterns of one family of asexually reproducing wasps and its bacterial parasite, Wolbachia. Wolbachia live in their hosts' egg cells and alter reproduction. And if a wasp picks up a Wolbachia infection from another wasp, it may be forever doomed to an asexual state.

In vitro infection of a cell line from Ictalurus nebulosus with Piscirickettsia salmonis

Piscirickettsia salmonis, the etiologic agent of salmonid rickettsial septicemia (SRS), affects several species of salmonids. Previous reports using the appearance of cytopathic effect (CPE) as the criterion for susceptibility, showed that Piscirickettsia salmonis (ATCC strain) can be grown in vitro in some cells lines derived from salmonid fish, but not in BB cells from brown bullhead (Ictalurus nebulosus) and BF-2 cells from bluegill (Lepomis macrochirus). In this study we describe growth of P. salmonis (ATCC strain VR 1361) in a cell line previously believed to be nonpermissive for this organism. CPE was first detected in chinook salmon embryo (CHSE-214) and epithelioma papulosum ciprini (EPC) cell lines at 6 d postinfection (dpi). In contrast, using BB cell line, CPE was first detected 45 dpi and the monolayer completed CPE by 78 dpi. Electron microscopic examination of BB cells 78 dpi revealed free, intracytoplasmic and extracellular localization of the agent. P. salmonis was also observed within membrane-bounded vacuoles in BB cells, similar to that described in CHSE 214 cells. Contrary to earlier reports, results from the present study show that the BB cell line, is susceptible to Piscirickettsia salmonis infection. The delayed onset of CPE in BB cells in comparison to other permissive cell lines suggests that BB cells are not ideal hosts for P. salmonis. Interestingly, however, these results demonstrate that P. salmonis can infect non-salmonid cell lines, and raises the possibility that non-salmonid fish may play a role in the persistence and transmission of SRS in the natural environment.

An improved microinjection protocol for the transfer of Wolbachia pipientis between infected and uninfected strains of the flour beetle Tribolium confusum

The lethality of halocarbon and other oils to hatching larvae of the flour beetle Tribolium confusum limits existing microinjection protocols, because postinjection survivorship is only 5.6% of the eggs injected. We report the development of an oil-free protocol that improves survivorship fivefold. We used this protocol to transfect the cytoplasmic endosymbiont Wolbachia pipientis from infected donor eggs to uninfected host eggs and observed reproductive incompatibility in 40% of the surviving, injected eggs. Compared with mock injected controls (35.9% survival) or microinjection of uninfected cytoplasm (32.3% survival), injection of Wolbachia-infected cytoplasm into host eggs significantly reduced egg-to-adult survival (25.1%).

Naturally-occurring Wolbachia infection in Drosophila simulans that does not cause cytoplasmic incompatibility

Microbes of the genus Wolbachia are transmitted by their hosts via the maternal parent and are responsible for cytoplasmic incompatibility among insect populations. This phenomenon can result in Wolbachia spreading through natural populations as previously demonstrated in Drosophila simulans. Here we describe another Wolbachia infection in D. simulans that does not cause cytoplasmic incompatibility. This is a property of the Wolbachia rather than the nuclear background. The infection occurs at a low frequency in natural populations from eastern Australia. The infection shows perfect maternal transmission in the field and does not cause any detectable deleterious effects on its host. These findings suggest that the Wolbachia infection behaves like a neutral variant in populations. The infection may represent an evolutionary outcome of interactions between Wolbachia infections and their hosts.

Wolbachia superinfections and the expression of cytoplasmic incompatibility

Strains of Drosophila simulans from Riverside, California (DSR) and Hawaii (DSH) harbour distinct strains of the cytoplasmic incompatibility microorganism Wolbachia, resulting in the expression of bidirectional incompatibility when crossed. D. simulans lines carrying both of these (superinfected) Wolbachia strains were generated by the transfer of infected DSH cytoplasm into DSR embryos by microinjection. The superinfected flies were unidirectionally incompatible with both DSR and DSH individuals. As a result of this pattern, the superinfected state was observed to replace single infections in laboratory populations. The ability of the superinfection to spread was modulated by the production of singly infected offspring from superinfected mothers: strain segregation was observed under crowded larval rearing conditions. An inverse correlation between the penetrance of the cytoplasmic incompatibility phenotype and the degree of larval crowding was also observed. The findings have implications for the evolution of bidirectionally incompatible strains, and lead to the prediction that superinfections should be relatively common in field populations. Evidence for a natural superinfection in the mosquito Aedes albopictus is discussed. The results also have applied significance for the generation of insect lines capable of driving desirable genes into populations already infected with Wolbachia, thus allowing repeated opportunities for population replacement.

Wolbachia and cytoplasmic incompatibility in mycophagous Drosophila and their relatives

Bacterial symbionts belonging to the genus Wolbachia are associated with postzygotic reproductive incompatibility in a number of insect species. Using polymerase chain reaction (PCR) amplification of bacterial gene sequences, strains from 10 species belonging to the closely related quinaria, testacea and tripunctata groups of Drosophila were screened for the presence of Wolbachia in their reproductive tissues. Those screened included the mycophagous species D. falleni, D. recens, D. phalerata, D. testacea, D. neotestacea, D. orientacea, D. putrida and D. tripunctata, and the nonmycophagous species D. palustris and D. quinaria. Two species, D. recens and D. orientacea, were found to be infected with Wolbachia. Subsequent tests of four additional strains of D. recens found all to be infected with the bacteria. It was established that these bacteria cause partial cytoplasmic incompatibility in D. recens by antibiotic curing followed by crosses between cured and uncured strains. Curing was confirmed by a PCR assay. Although most species of insects shown to be infected with Wolbachia are cosmopolitan and/or have undergone recent range expansion in association with human activity, D. recens and D. orientacea are endemic species with specialized ecological habits. Preliminary molecular phylogenetic analysis indicates that, among the species we examined, D. quinaria is most closely related to D. recens. To determine whether the bacteria are involved in reproductive isolation between these two species, reciprocal crosses were carried out between D. quinaria and both infected and uninfected (cured) strains of D. recens. Although these species did mate with each other, all interspecific crosses failed to yield hybrid progeny, indicating that the bacteria are not responsible for reproductive incompatibility between these species.

Parthenogenesis-inducing microorganisms in Aphytis (Hymenoptera: Aphelinidae)

Production of males in uniparental lines of two species in the parasitic wasp genus Aphytis was induced by rifampicin, and male sexual functioning was determined. Wolbachia-specific 16S rDNA primers were used in a PCR in order to: (1) assess correlation between thelytokous reproduction and the presence of Wolbachia; (2) detect the loss of Wolbachia DNA in uniparental A. lingnanensis following antibiotic treatments, with or without the presence of a host; and (3) clone and sequence part of the Wolbachia 16S rDNA from the uniparental Aphytis species for phylogenetic studies. Males produced viable sperm that was transferred to the female spermatheca following mating. However, sperm failure to effect egg fertilization resulted in all-male progeny. Wolbachia were found in the two uniparental (A. lingnanensis and A. diaspidis) but not in the two biparental (A. lingnanensis and A. melinus) Aphytis lines tested. They can be detected in wasps up to 7 days following antibiotic treatments, regardless of the presence of host. The 16S rDNA for the symbionts in the two Aphytis species is virtually identical, and is most closely related to the Wolbachia found in Muscidifurax uniraptor (Pteromalidae).

Wolbachia infections and the expression of cytoplasmic incompatibility in Drosophila sechellia and D. mauritiana

Various stocks of Drosophila mauritiana and D. sechellia were found to be infected with Wolbachia, a Rickettsia-like bacterium that is known to cause cytoplasmic incompatibility and other reproductive abnormalities in arthropods. Testing for the expression of cytoplasmic incompatibility in these two species showed partial incompatibility in D. sechellia but no expression of incompatibility in D. mauritiana. To determine whether absence of cytoplasmic incompatibility in D. mauritiana was due to either the bacterial or host genome, we transferred bacteria from D. mauritiana into an uninfected strain of D. simulans, a host species known to express high levels of incompatibility with endogenous Wolbachia. We also performed the reciprocal transfer of the natural D. simulans Riverside infection into a tetracycline-treated stock of D. mauritiana. In each case, the ability to express incompatibility was unaltered by the different host genetic background. These experiments indicate that in D. simulans and D. mauritiana expression of the cytoplasmic incompatibility phenotype is determined by the bacterial strain and that D. mauritiana harbors a neutral strain of Wolbachia.

Cytoplasmic incompatibility in Drosophila simulans: dynamics and parameter estimates from natural populations

In Drosophila simulans, cytoplasmically transmitted Wolbachia microbes cause reduced egg hatch when infected males mate with uninfected females. A Wolbachia infection and an associated mtDNA variant have spread northward through California since 1986. PCR assays show that Wolbachia infection is prevalent throughout the continental US and Central and South America, but some lines from Florida and Ecuador that are PCR-positive for Wolbachia do not cause incompatibility. We estimate from natural populations infection frequencies and the transmission and incompatibility parameter values that affect the spread of the infection. On average, infected females from nature produce 3-4% uninfected ova. Infected females with relatively low fidelity of maternal transmission show partial incompatibility with very young infected laboratory males. Nevertheless, crosses between infected flies in nature produce egg-hatch rates indistinguishable from those produced by crosses between uninfected individuals. Incompatible crosses in nature produce hatch rates 30-70% as high as those from compatible crosses. Wild-caught infected and uninfected females are equally fecund in the laboratory. Incompatibility decreases with male age, and age-specific incompatibility levels suggest that males mating in nature may often be 2 or 3 weeks old. Our parameter estimates accurately predict the frequency of Wolbachia infection in California populations.

Evolution of single and double Wolbachia symbioses during speciation in the Drosophila simulans complex

Maternally inherited bacteria of the genus Wolbachia are responsible for the early death of embryos in crosses between uninfected females and infected males in several insect species. This phenomenon, known as cytoplasmic incompatibility, also occurs between strains infected by different symbionts in some species, including Drosophila simulans. Wolbachia was found in two species closely related to D. simulans, Drosophila mauritiana, and Drosophila sechellia, and shown to cause incompatibility in the latter species but not in D. mauritiana. Comparison of bacterial and mtDNA history clarifies the origins of bacterial and incompatibility polymorphisms in D. simulans. Infection in D. mauritiana is probably the result of introgression of an infected D. simulans cytoplasm. Some D. simulans and D. sechellia cytoplasmic lineages harbor two bacteria as a consequence of a double infection which probably occurred in a common ancestor. The descendant symbionts in each species are associated with similar incompatibility relationships, which suggests that little variation of incompatibility types has occurred within maternal lineages beyond that related to the density of symbionts in their hosts.

Increased male fertility in Tribolium confusum beetles after infection with the intracellular parasite Wolbachia

The cytoplasmically inherited microorganism Wolbachia pipientis behaves like a sexually selected trait in its host, the flour beetle Tribolium confusum, enhancing male fertility at the expense of female fecundity. Here we show that infected females have fewer offspring than uninfected females but infected males have a large fertility advantage over uninfected males within multiply-inseminated infected or uninfected females. The male fertility effect accelerates the spread of the Wolbachia through the host population and expands the initial opportunity for hitch-hiking of host nuclear genes. Sperm competition in a host, mediated by endosymbionts, has not been previously described.

Cytoplasmic incompatibility. Giant steps sideways

The horizontal transfer of a bacterial endosymbiont that is intimately associated with reproductive isolation in insects is now feasible and may, in principle, lead to new strategies for biological pest control.

Properties of Drosophila simulans strains experimentally infected by different clones of the bacterium Wolbachia

Maternally inherited bacteria of the genus Wolbachia are responsible for reproductive incompatibilities between strains of Drosophila simulans. Such incompatibilities are known in several types of crosses, including both directions of crossing between two types of infected strains, 'R' and 'S'. To determine whether the bidirectional incompatibility between R and S flies is due only to differences between their bacteria, flies from an uninfected strain have been experimentally infected with bacteria associated with each type. The incompatibility properties of experimental strains are close to those of original strains harbouring the same bacteria and therefore independent of nuclear background. Backcross experiments, however, show that the infection level of a strain depends on the nature of paternal ancestors. This is not explained by nuclear effects but is possibly the result of an interaction between the infection levels of both parents, in which the infection level of S strains is an equilibrium between a tendency for females to produce weakly infected offspring and selection of more infected eggs by sperm from infected males.

Proposed tests for the routine identification of Rochalimaea species

A study was conducted to establish tests for the routine identification of Rochalimaea species. Strains used were reference strains of Rochalimaea vinsonii and Rochalimaea quintana, and a type strain and six human isolates of Rochalimaea henselae. Rochalimaea species were confirmed to be gram-negative, oxidase-negative, non-motile, urease-negative, indole-negative, catalase-negative, glucose-nonfermenting organisms which failed to grow on MacConkey agar. Further testing of the organisms in a commercial identification system with the addition of hemin (100 micrograms/ml) to the medium revealed biochemical reactivity of the organisms not previously observed. The Voges-Proskauer reaction, tests for hydrolysis of hippurate and esculin, leucine arylamidase activity and the lactose test allowed identification and differentiation of the three species. Rochalimaea henselae was the only species with a positive lactose test and Rochalimaea quintana was the only species with a positive Voges-Proskauer reaction. Further studies are needed to confirm the validity of these tests for identification of Rochalimaea species.

Interspecific and intraspecific horizontal transfer of Wolbachia in Drosophila

Cytoplasmic incompatibility (CI) in Drosophila simulans is related to infection of the germ line by a rickettsial endosymbiont (genus Wolbachia). Wolbachia were transferred by microinjection of egg cytoplasm into uninfected eggs of both D. simulans and D. melanogaster to generate infected populations. Transinfected strains of D. melanogaster with lower densities of Wolbachia than the naturally infected D. simulans strain did not express high levels of CI. However, transinfected D. melanogaster egg cytoplasm, transferred back into D. simulans, generated infected populations that expressed CI at levels near those of the naturally infected strain. A transinfected D. melanogaster line selected for increased levels of CI expression also displayed increased symbiont densities. These data suggest that a threshold level of infection is required for normal expression of CI and that host factors help determine the density of the symbiont in the host.

Wolbachia endosymbionts responsible for various alterations of sexuality in arthropods

Rickettsia-like maternally inherited bacteria have been shown to be involved in a variety of alterations of arthropod sexuality, such as female-biased sex ratios, parthenogenesis, and sterility of crosses either between infected males and uninfected females or between infected individuals (cytoplasmic incompatibility). We have characterized several of these microorganisms through partial sequences of the small (16S) and large (23S) subunit ribosomal DNA. All the symbionts identified, which include several cytoplasmic incompatibility microorganisms, several endosymbionts of terrestrial isopods, and symbionts of two thelytokous Trichogramma wasp species, belong to a monophyletic group of related symbionts, some of which have previously been detected in several insects exhibiting cytoplasmic incompatibility. Three molecular lineages can be identified on the basis of 16S as well as 23S sequences. Although they are only known as endocellular symbionts, Wolbachia spread by horizontal transfer across host lineages as evidenced by their diversification which occurred long after that of their hosts, and by the non-congruence of the phylogenetic relationships of symbionts and their hosts. Indeed, symbionts of two different lineages have been found in the same host species, whereas closely related endosymbionts are found in distinct insect orders. Isopod endosymbionts form a separate lineage, and they can determine feminization as well as cytoplasmic incompatibility. The ability to determine cytoplasmic incompatibility, found in all lineages, is probably ancestral to this group.

Molecular identification of microorganisms associated with parthenogenesis

Cytoplasmically interited microorganisms are widespread in insects and have been implicated as causes of female parthenogenesis (females developing from unfertilized eggs) and cytoplasmic incompatibility. Normal sexual reproduction can be restored by treatment with antibiotics. Sequence analysis of the DNA encoding 16S ribosomal RNA has shown that cytoplasmic incompatibility bacteria from diverse insect taxa are closely related (they share >95% sequence sililarity) and belong to the alpha subdivision of Proteobacteria. Here we show that parthenogenesis-associated bacteria from parasitoid Hymenoptera also fall into this bacterial group, having up to 99% sequence similarity to some incompatibility microorganisms. Both incompatibility and parthenogenesis microorganisms alter host chromosome behaviour during early mitotic divisions of the egg. Incompatibility bacteria act by interfering with paternal chromosome incorporation in fertilized eggs, whereas parthenogenesis bacteria prevent segregation of chromosomes in unfertilized eggs. These traits are adaptive for the microorganisms. On the basis of their sequence similarities, we conclude that parthenogenesis bacteria and cytoplasmic incompatibility bacteria form a monophyletic group of microorganisms that 'specialize' in manipulating chromosome behaviour and reproduction of insects.

Mitochondrial DNA polymorphism and feminizing sex factors dynamics in a natural population of Armadillidium vulgare (Crustacea, Isopoda)

Sex determination in Armadillidium vulgare may be under the control of two parasitic sex factors that reverse genetic males into functional neo-females. The first feminizing factor (F) is a Wolbachia and the other (f) is probably a sequence of the F bacterial DNA unstably integrated into the host genome. Both of these feminizing factors are mainly maternally transmitted. Here we investigate the mitochondrial DNA polymorphism of wild iso-female lineages harbouring either F or f. Among the four haplotypes present in the population, two were the f-harbouring lineages, while two were common to the F- and f-harbouring lineages. This result suggests that there has been an introgression of the f factor into lineages infected by F Wolbachia. Based on previous data, we propose two different ways to account for such introgression. Given the particular dynamics of feminizing factors (f-harbouring lineages increase in populations at the expense of F-harbouring lineages), such an introgression should prevent the replacement of F-linked mitochondrial types by f-linked mitochondrial types in wild populations.

Cytoplasmic incompatibility in Drosophila populations: influence of assortative mating on symbiont distribution

Cytoplasmic incompatibility is known to occur between strains of both Drosophila simulans and D. melanogaster. Incompatibility is associated with the infection of Drosophila with microorganismal endosymbionts. This paper reports survey work conducted on strains of D. simulans and D. melanogaster from diverse geographical locations finding that infected populations are relatively rare and scattered in their distribution. The distribution of infected populations of D. simulans appears to be at odds with deterministic models predicting the rapid spread of the infection through uninfected populations. Examination of isofemale lines from four localities in California where populations appear to be polymorphic for the infection failed to find evidence for consistent assortative mating preferences between infected and uninfected populations that may explain the basis for the observed polymorphism.

Lack of lysosomal fusion with phagosomes containing Ehrlichia risticii in P388D1 cells: abrogation of inhibition with oxytetracycline

Fusion of lysosomes with phagosomes containing Ehrlichia risticii, an obligate intracellular parasite, was evaluated in P388D1 murine macrophagelike cells. Lysosomes in cells ranging in infectivity from 30 to 70% were labeled cytochemically with acid phosphatase or via endocytosis of thorium dioxide or cationized ferritin to document phagosome-lysosome (P-L) fusion in untreated cells and cells treated with oxytetracycline. Regardless of the marker used, P-L fusion was generally not observed in E. risticii-containing vacuoles in untreated cells, while significantly greater P-L fusion with ehrlichia-containing vacuoles was observed after oxytetracycline treatment. When latex beads were introduced into uninfected cell cultures, P-L fusion was observed with vacuoles containing latex. Fusion of lysosomes with latex-containing vacuoles in cells was significantly greater than fusion of lysosomes with ehrlichia-containing vacuoles in the same infected cells. These findings indicate that E. risticii is able to inhibit P-L fusion, whereas oxytetracycline deprives organisms of this ability.

Cowdria ruminantium: stability and preservation of the organism

Blood collected in either sodium heparin or disodium edetate vacutainers from febrile goats infected with 4 isolates of Cowdria ruminantium and cryopreserved with 10% dimethyl sulphoxide at -70 degrees C and -196 degrees C was an effective stabilate to initiate heartwater infections in goats. A homogenized pool of whole Amblyomma variegatum ticks in Snyder's buffer, maintained at -196 degrees C, was used to infect a goat with C. ruminantium. Liver and spleen collected from Swiss mice infected with the Kwanyanga isolate of C. ruminantium were homogenized in Snyder's buffer, maintained at -196 degrees C and were used to initiate infections in mice. Fresh blood collected from febrile goats and maintained at 4 degrees C for as long as 72 h was infectious to mice. Neutrophils separated from blood of C. ruminantium infected goats and maintained in modified RPMI medium at 37 degrees C for 68 h were infectious for a goat. Similarly neutrophils from a 2nd infected goat maintained for 96 h at 37 degrees C were infectious for mice.

Biochemistry of rickettsiae: recent advances

The application of new biotechnology to the study of the biochemistry of rickettsiae was a prominent feature of the presentations at the 3rd International Symposium on Rickettsiae and Rickettsial Diseases, held in Smolenice near Bratislava in September 1984. This review is an attempt to summarize recent advances leading up to these presentations as well as the studies that have been reported in the two years since the meeting. Since rickettsiae are intracellular parasites, most reviews deal with the interaction of rickettsiae with host cells. It is useful, however, to focus also--as we have done--on the properties of rickettsiae that can be demonstrated in the absence of their hosts, although, undoubtedly, many of these properties reflect adaptation to an intracellular microenvironment.

Heme synthesis in Crithidia deanei: influence of the endosymbiote

The activity of the following enzymes involved in the biosynthesis of porphyrins was determined in endosymbiote-free and endosymbiote-containing Crithidia deanei grown in a chemically defined medium: succinyl Coenzyme A synthetase (Suc.CoA-S), 5-aminolevulinate synthetase (ALA-S), 4,5-dioxovaleric acid transaminase (DOVA-T), 5-aminolevulinate dehydratase (ALA-D), porphobilinogenase (PBGase), deaminase and heme synthetase (Heme-S). The amount of 5-aminolevulinic acid (ALA) and porphobilinogen, porphyrins and heme was also determined. ALA and PBG were detected in C. deanei. The levels of free porphyrins was low. Heme concentration was nil. The activity of ALA-D, deaminase and PBGase was not detected in C. deanei. The activity of Suc.CoA-S and ALA-S were twice higher in symbiote-containing than in aposymbiotic C. deanei. Aposymbiotic cells had a higher activity of DOVA-T than symbiote-containing cells. The level of Heme-S, measured using protoporphyrin as substrate, was twice as high in symbiote-containing than in symbiote-free cells.

Induction by streptococcal L-forms of two reproductive isolating mechanisms in Drosophila paulistorum

The six semispecies of the Drosophila paulistorum complex do not normally interbreed, but when hybrids are produced in the laboratory, the males are sterile. In earlier work, cytoplasmic components responsible for this sterility had been transferred by injection of streptococcal L-forms isolated from the Mesitas and Santa Marta flies. To extend these findings, and L-form from the Centro-American flies was injected into three semispecies. Transfer of L-form from one semispecies to another resulted in sterile male progeny. The L-form isolated from Mesitas was injected into all six semispecies. Results simulated "hybrid inviability," i.e., only the Mesitas semispecies produced the expected number of offspring, and the total number of progeny from the other five semispecies was considerably reduced. Despite the small numbers of males, reflecting induced inviability, there was evidence that the transfer of L-forms from one semispecies to another simultaneously caused sterility in male progeny.

Observation of symbiote migration in human body lice with scanning and transmission electron microscopy

Bacterial symbiotes in the human body louse Pediculus humanus migrate from the mycetome to the lateral oviducts during the adult molt. Their migration was first described by Ries (E. Ries. 1931. Z. Morphol. Oekol. Tiere, 20:233-367.), who examined sectioned specimens with light microscopy. The present study is a more detailed investigation which involves the use of scanning and transmission electron micrographs. The results of our studies confirm Ries' observations. Micrographs are presented of symbiotes emerging from the mycetome, migrating to the reproductive tract, and invading the lateral oviducts.

A rickettsia-like organism showing positive immunofluorescence with antisera to Coxiella burnetii in Haemaphysalis inermis ticks

A rickettsia-like organism (RLO) was detected in the oocytes of Haemaphysalis inermis ticks. The RLOs were about 5 microns long bent rods with a definite inner structure. They were Gram-negative and could be visualized by Giemsa but not by Gimenez staining. Attempts to cultivate the RLO in chick embryo yolk sacs, various types of cell culture and tick body cavities were unsuccessful. The RLO displayed a bright immunofluorescence with antisera to Coxiella burnetii, but no immunofluorescence was obtained with antisera to representatives of typhus and spotted fever group rickettsiae, with the exception of very weak fluorescence with serum from a rabbit immunized with Rickettsia akari and one serum from Apodemus flavicollis immunized with Rickettsia conorii. These findings should be taken into consideration when studying the infestation of ticks with rickettsiae.