Efficient Vertical Transmission Research Articles

Hypovirulence-associated mycovirus epidemics cause pathogenicity degeneration of Beauveria bassiana in the field

BackgroundThe entomogenous fungus Beauveria bassiana is used as a biological insecticide worldwide, wild B. bassiana strains with high pathogenicity in the field play an important role in controlling insect pests via not only screening of highly virulent strains but also natural infection, but the pathogenicity degeneration of wild strains severely affected aforementioned effects. Previous studies have showed that multiple factors contributed to this phenomenon. It has been extensively proved that the mycovirus infection caused hypovirulence of phytopathogenic fungi, which has been used for plant disease biocontrol. However, it remains unknown whether the mycovirus epidemics is a key factor causing hypovirulence of B. bassiana naturally in the field.MethodsWild strains of B. bassiana were collected from different geographic locations in Jilin Province, China, to clarify the epidemic and diversity of the mycoviruses. A mycovirus Beauveria bassiana chrysovirus 2 (BbCV2) we have previously identified was employed to clarify its impact on the pathogenicity of host fungi B. bassiana against the larvae of insect pest Ostrinia furnacalis. The serological analysis was conducted by preparing polyclonal antibody against a BbCV2 coat protein, to determine whether it can dissociate outside the host fungal cells and subsequently infect new hosts. Transcriptome analysis was used to reveal the interactions between viruses and hosts.ResultsWe surprisingly found that the mycovirus BbCV2 was prevalent in the field as a core virus in wild B. bassiana strains, without obvious genetic differentiation, this virus possessed efficient and stable horizontal and vertical transmission capabilities. The serological results showed that the virus could not only replicate within but also dissociate outside the host cells, and the purified virions could infect B. bassiana by co-incubation. The virus infection causes B. bassiana hypovirulence. Transcriptome analysis revealed decreased expression of genes related to insect epidermis penetration, hypha growth and toxin metabolism in B. bassiana caused by mycovirus infection.ConclusionBeauveria bassiana infected by hypovirulence-associated mycovirus can spread the virus to new host strains after infecting insects, and cause the virus epidemics in the field. The findings confirmed that mycovirus infection may be an important factor affecting the pathogenicity degradation of B. bassiana in the field.

The cellular lives of Wolbachia.

Wolbachia are successful Gram-negative bacterial endosymbionts, globally infecting a large fraction of arthropod species and filarial nematodes. Efficient vertical transmission, the capacity for horizontal transmission, manipulation of host reproduction and enhancement of host fitness can promote the spread both within and between species. Wolbachia are abundant and can occupy extraordinary diverse and evolutionary distant host species, suggesting that they have evolved to engage and manipulate highly conserved core cellular processes. Here, we review recent studies identifying Wolbachia-host interactions at the molecular and cellular levels. We explore how Wolbachia interact with a wide array of host cytoplasmic and nuclear components in order to thrive in a diversity of cell types and cellular environments. This endosymbiont has also evolved the ability to precisely target and manipulate specific phases of the host cell cycle. The remarkable diversity of cellular interactions distinguishes Wolbachia from other endosymbionts and is largely responsible for facilitating its global propagation through host populations. Finally, we describe how insights into Wolbachia-host cellular interactions have led to promising applications in controlling insect-borne and filarial nematode-based diseases.

Posterior concentration of Wolbachia during the early embryogenesis of the host dynamically shapes the tissue tropism of Wolbachia in host Trichogramma wasps.

The bacterial endosymbiont, Wolbachia spp. induce thelytokous parthenogenesis in certain parasitoid wasps, such as the egg parasitoid wasps Trichogramma spp. To complete the cycle of vertical transmission, Wolbachia displays efficient transovarial transmission by targeting the reproductive tissues and often exhibits strong tissue-specific tropism in their host. The present study aimed to describe the basic Wolbachia distribution patterns that occur during the development of Wolbachia-infected, thelytokous Trichogramma dendrolimi, and T. pretiosum. We used fluorescence in situ hybridization (FISH) to investigate Wolbachia signal dynamics during early embryogenesis (from 30 to 120 min). Wolbachia titers and distributions from the embryo to adult stages of Trichogramma after early embryogenesis were detected by absolute quantitative polymerase chain reaction (AQ-PCR) and FISH. The symmetry ratios (SR) of the Wolbachia signals were calculated using the SR odds ratios in the anterior and posterior parts of the host. The SR was determined to describe Wolbachia tropism during early embryogenesis and various developmental stages of Trichogramma. Wolbachia was concentrated in the posterior part of the embryo during early embryogenesis and the various developmental stages of both T. dendrolimi and T. pretiosum. Wolbachia density increased with the number of nuclei and the initial mitotic division frequency during early embryogenesis. The total Wolbachia titer increased with postembryogenesis development in both T. dendrolimi and T. pretiosum. However, the Wolbachia densities relative to body size were significantly lower at the adult and pupal stages than they were at the embryonic stage. The present work revealed that posterior Wolbachia concentration during early host embryogenesis determined Wolbachia localization in adult wasps. By this mechanism, Wolbachia exhibits efficient vertical transmission across generations by depositing only female Wolbachia-infected offspring. The results of this study describe the dynamics of Wolbachia during the development of their Trichogramma host. The findings of this investigation helped clarify Wolbachia tropism in Trichogramma wasps.

Transovarial transmission of pathogenic protozoa and rickettsial organisms in ticks.

Transovarial transmission (TOT) is an efficient vertical transmission of pathogens that is observed in many arthropod vectors. This method seems to be an evolutionarily unique development observed only in Babesia sensu stricto (clade VI) and Rickettsia spp., whereas transstadial transmission is the common/default way of transmission. Transovarial transmission does not necessarily contribute to the amplification of tick-borne pathogens but does contribute to the maintenance of disease in the environment. This review aims to provide an updated summary of previous reports on TOT of tick-borne pathogens.

Wolbachia infection dynamics in a natural population of the pear psyllid Cacopsylla pyri (Hemiptera: Psylloidea) across its seasonal generations

Wolbachia is one of the most abundant intracellular symbionts of arthropods and has profound effects on host biology. Wolbachia transmission and host phenotypes often depend on its density within the host, which can be affected by multiple biotic and abiotic factors. However, very few studies measured Wolbachia density in natural host populations. Here, we describe Wolbachia in the pear psyllid Cacopsylla pyri from three populations in the Czech Republic. Using phylogenetic analyses based on wsp and multilocus sequence typing genes, we demonstrate that C. pyri harbours three new Wolbachia strains from supergroup B. A fourth Wolbachia strain from supergroup A was also detected in parasitised immatures of C. pyri, but likely came from a hymenopteran parasitoid. To obtain insights into natural Wolbachia infection dynamics, we quantified Wolbachia in psyllid individuals from the locality with the highest prevalence across an entire year, spanning several seasonal generations of the host. All tested females were infected and Wolbachia density remained stable across the entire period, suggesting a highly efficient vertical transmission and little influence from the environment and different host generations. In contrast, we observed a tendency towards reduced Wolbachia density in males which may suggest sex-related differences in Wolbachia-psyllid interactions.

Microsporidia MB is found predominantly associated with Anopheles gambiae s.s and Anopheles coluzzii in Ghana

A vertically transmitted microsporidian, Microsporidia MB, with the ability to disrupt Plasmodium development was reported in Anopheles arabiensis from Kenya, East Africa. To demonstrate its range of incidence, archived DNA samples from 7575 Anopheles mosquitoes collected from Ghana were screened. MB prevalence was observed at 1.8%. An. gambiae s.s constituted 87% of positive mosquitoes while the remaining were from An. coluzzii. Both sibling species had similar positivity rates (24% and 19%; p = 0.42) despite the significantly higher number of An. gambiae s.s analysed (An. gambiae s.s = 487; An. coluzzii = 94; p = 0.0005). The microsporidian was also more prevalent in emerged adults from field-collected larvae than field-caught adults (p < 0.0001) suggestive of an efficient vertical transmission and/or horizontal transfer among larvae. This is the first report of Microsporidia MB in Anopheles mosquitoes in West Africa. It indicates possible widespread among malaria vector species and warrants investigations into the symbiont’s diversity across sub-Saharan Africa.

Insect-to-insect horizontal transmission of a phytoreovirus in the absence of an infected plant host

Vertical transmission of Homalodisca vitripennis reovirus (HoVRV) from glassy-winged sharpshooter (GWSS, Homalodisca vitripennis (Germar)) females to progeny occurred in laboratory assays at frequencies too low (2.6%–15.4%) to account for HoVRV incidence (90–100%) in field populations resident in citrus. Because citrus is immune to HoVRV and no plant host is known, horizontal transmission of HoVRV from insect-to-insect was evaluated. Exposure of colony-reared, virus-free test nymphs to HoVRV-infected source adults held in the same cage for 10 days on virus-immune cowpea resulted in HoVRV transmission (13.3%–30.7%) to test nymphs. HoVRV was not transmitted when exposure was indirect and required passive movement of virions through the xylem of immune citrus seedlings. Collectively, these results demonstrate direct insect-to-insect horizontal transmission of HoVRV, providing a plausible explanation for high incidence of HoVRV in GWSS field populations in the absence of efficient vertical transmission or a plant host.

Highly Efficient Vertical Transmission for Zika Virus in Aedes aegypti after Long Extrinsic Incubation Time

While the Zika virus (ZIKV) 2014–2017 pandemic has subsided, there remains active transmission. Apart from horizontal transmission to humans, the main vector Aedes aegypti can transmit the virus vertically from mother to offspring. Large variation in vertical transmission (VT) efficiency between studies indicates the influence of parameters, which remain to be characterized. To determine the roles of extrinsic incubation time and gonotrophic cycle, we deployed an experimental design that quantifies ZIKV in individual progeny and larvae. We observed an early infection of ovaries that exponentially progressed. We quantified VT rate, filial infection rate, and viral load per infected larvae at 10 days post oral infection (d.p.i.) on the second gonotrophic cycle and at 17 d.p.i. on the second and third gonotrophic cycle. As compared to previous reports that studied pooled samples, we detected a relatively high VT efficiency from 1.79% at 10 d.p.i. and second gonotrophic cycle to 66% at 17 d.p.i. and second gonotrophic cycle. At 17 d.p.i., viral load largely varied and averaged around 800 genomic RNA (gRNA) copies. Longer incubation time and fewer gonotrophic cycles promoted VT. These results shed light on the mechanism of VT, how environmental conditions favor VT, and whether VT can maintain ZIKV circulation.

Sex chromosomes control vertical transmission of feminizing Wolbachiasymbionts in an isopod.

Microbial endosymbiosis is widespread in animals, with major ecological and evolutionary implications. Successful symbiosis relies on efficient vertical transmission through host generations. However, when symbionts negatively affect host fitness, hosts are expected to evolve suppression of symbiont effects or transmission. Here, we show that sex chromosomes control vertical transmission of feminizing Wolbachia endosymbionts in the isopod Armadillidium nasatum. Theory predicts that the invasion of an XY/XX species by cytoplasmic sex ratio distorters is unlikely because it leads to fixation of the unusual (and often lethal or infertile) YY genotype. We demonstrate that A. nasatum X and Y sex chromosomes are genetically highly similar and that YY individuals are viable and fertile, thereby enabling Wolbachia spread in this XY-XX species. Nevertheless, we show that Wolbachia cannot drive fixation of YY individuals, because infected YY females do not transmit Wolbachia to their offspring, unlike XX and XY females. The genetic basis fits the model of a Y-linked recessive allele (associated with an X-linked dominant allele), in which the homozygous state suppresses Wolbachia transmission. Moreover, production of all-male progenies by infected YY females restores a balanced sex ratio at the host population level. This suggests that blocking of Wolbachia transmission by YY females may have evolved to suppress feminization, thereby offering a whole new perspective on the evolutionary interplay between microbial symbionts and host sex chromosomes.

A Nonstructural Protein Responsible for Viral Spread of a Novel Insect Reovirus Provides a Safe Channel for Biparental Virus Transmission to Progeny.

Diaphorina citri reovirus (DcRV) was previously identified based on metagenomics surveys for virus discovery. Here, we demonstrated that DcRV induces persistent infection in its psyllid host, Diaphorina citri DcRV was efficiently vertically passed to offspring in a biparental manner. Transmission electron microscopic and immunological analyses showed that the DcRV-encoded nonstructural protein P10 assembled into a virion-packaging tubular structure which is associated with the spread of DcRV throughout the bodies of D. citri insects. P10 tubules containing virions were associated with oocytes of female and sperm of male D. citri insects, suggesting a role in the highly efficient biparental transmission of DcRV. Knocking down P10 by RNA interference for males reduced the percentage of DcRV-infected progeny and for females reduced the viral accumulation in progeny. These results, for the first time, show that a nonstructural protein of a novel insect reovirus provides a safe and pivotal channel for virus spread and biparental transmission to progeny.IMPORTANCE The Asian citrus psyllid, Diaphorina citri Kuwayama, is an important pest in the worldwide citrus industry. It is the vector of "Candidatus Liberibacter asiaticus," the bacterial pathogen of Huanglongbing, which is currently considered the most destructive disease of citrus worldwide. DcRV was previously identified based on metagenomics surveys for virus discovery. Here, we found that this novel and persistent insect reovirus took advantage of a virus-encoded nonstructural protein, P10, for efficient vertical transmission from parents to progeny. P10 assembled into a virion-packaging tubular structure and was associated with oocytes of female D. citri and sperm of males. Consistent with this, knockdown of P10 for either male or female D. citri insects inhibited DcRV transmission to offspring. This tubular strategy for viral spread and biparental transmission might serve as a target for controlling viral vertical transmission and population expansion.

Wolbachia localization during Laodelphax striatellus embryogenesis

Wolbachia are intracellular bacteria carried by thousands of arthropod species. The success of Wolbachia is due to efficient vertical transmission by the host maternal germline. Wolbachia’s behavior during host oogenesis is well characterized, although their behavior during embryogenesis is unclear. Vertical transmission of Wolbachia wStri in the small brown planthopper, Laodelphax striatellus is extraordinarily efficient. To understand why, we investigated its localization and dynamics in L. striatellus embryos. Microscopic observations indicated that the Wolbachia were mainly localized at the anterior region of the embryo during early embryogenesis. The distribution of Wolbachia within the anterior region was established during oogenesis, and according to a phylogenetic analysis, may be due to intrinsic factors in Wolbachia. We observed that wStri migrated to the posterior part cells during late embryogenesis, in the region where gonads were formed. An expression profile of Wolbachia-infected host embryonic development genes revealed Ddx1 mRNAs, which is required for host viability and in the germ line, accumulated in the posterior region of 3-day-old embryos, while other development genes mRNAs were significantly more abundant in the posterior region of 6-day-old embryos. These genes thus appear to be associated with the localization of Wolbachia wStri in the anterior region, although their functions remain unclear. These results can explain Wolbachia wStri high prevalence in L. striatellus.

Correction: Wolbachia and host germline components compete for kinesin-mediated transport to the posterior pole of the Drosophila oocyte.

[This corrects the article DOI: 10.1371/journal.ppat.1007216.].

Endornaviruses: persistent dsRNA viruses with symbiotic properties in diverse eukaryotes.

Endornaviruses are unique, persistent, double-stranded RNA (dsRNA) viruses with symbiotic properties that infect diverse eukaryotes, such as plants, fungi, and oomycetes. Endornaviruses contain a linear dsRNA genome of approximately 10 to 17 kbp in length and are classified in the family Endornaviridae, which consists of two genera, Alphaendornavirus and Betaendornavirus. The endornaviruses encode a single long open reading frame encoding approximately 3200 to 5800 amino acid residues of conserved viral RNA helicase and RNA-dependent RNA polymerase domains, and some endornaviruses contain a site-specific nick in the coding strand of their dsRNA genome. Acute plant viruses propagate rapidly and systemically, eventually killing the host plant, and are then transmitted horizontally. In contrast, plant endornaviruses have several common persistent (symbiotic) properties: a stable low copy number in the host plant, no obvious effect on the host plant, and efficient vertical transmission via gametes. Plant endornaviruses are likely maintained within host plants for hundreds of generations, so the host must stringently regulate their propagation. Whereas RNA silencing functions as a defense system against acute viruses in plants, it may be necessary for the persistent infection (symbiotic life cycle) of endornaviruses. This process includes the stringent regulation of low virus copy number (steady replication before every host cell division) and efficient vertical transmission of the virus to the next generation.

Demonstration of efficient vertical and venereal transmission of dengue virus type-2 in a genetically diverse laboratory strain of Aedes aegypti.

Aedes aegypti is the primary mosquito vector of dengue viruses (DENV; serotypes 1–4). Human-mosquito transmission cycles maintain DENV during epidemics but questions remain regarding how these viruses survive when human infections and vector abundance are minimal. Aedes mosquitoes can transmit DENV within the vector population through two alternate routes: vertical and venereal transmission (VT and VNT, respectively). We tested the efficiency of VT and VNT in a genetically diverse laboratory (GDLS) strain of Ae. aegypti orally infected with DENV2 (Jamaica 1409). We examined F1 larvae from infected females generated during the first and second gonotrophic cycles (E1 and E2) for viral envelope (E) antigen by amplifying virus in C6/36 cells and then performing an indirect immunofluorescence assay (IFA). RT-PCR/nested PCR analyses confirmed DENV2 RNA in samples positive by IFA. We observed VT of virus to larvae and adult male progeny and VNT of virus to uninfected virgin females after mating with males that had acquired virus by the VT route. We detected no DENV2 in 30 pools (20 larvae/pool) of F1 larvae following the first gonotrophic cycle, suggesting limited virus dissemination at 7 days post-infection. DENV2 was detected by IFA in 27 of 49 (55%) and 35 of 51 (68.6%) F1 larval pools (20 larvae/pool) from infected E2 females that received a second blood meal without virus at 10 or 21 days post-infection (E2-10d-F1 and E2-21-F1), respectively. The minimum filial infection rates by IFA for E2-10d-F1 and E2-21d-F1 mosquitoes were 1:36 and 1:29, respectively. The VNT rate from E2-10d-F1 males to virgin (uninfected) GDLS females was 31.6% (118 of 374) at 8 days post mating. Twenty one percent of VNT-infected females receiving a blood meal prior to mating had disseminated virus in their heads, suggesting a potential pathway for virus to re-enter the human-mosquito transmission cycle. This is the first report of VNT of DENV by male Ae. aegypti and the first demonstration of sexual transmission in Aedes by naturally infected males. Our results demonstrate the potential for VT and VNT of DENV in nature as mechanisms for virus maintenance during inter-epidemic periods.

Wolbachia and host germline components compete for kinesin-mediated transport to the posterior pole of the Drosophila oocyte.

Widespread success of the intracellular bacterium Wolbachia across insects and nematodes is due to efficient vertical transmission and reproductive manipulations. Many strains, including wMel from Drosophila melanogaster, exhibit a specific concentration to the germplasm at the posterior pole of the mature oocyte, thereby ensuring high fidelity of parent-offspring transmission. Transport of Wolbachia to the pole relies on microtubules and the plus-end directed motor kinesin heavy chain (KHC). However, the mechanisms mediating Wolbachia’s association with KHC remain unknown. Here we show that reduced levels of the host canonical linker protein KLC results in dramatically increased levels of Wolbachia at the oocyte’s posterior, suggesting that KLC and some key associated host cargos outcompete Wolbachia for association with a limited amount of KHC motor proteins. Consistent with this interpretation, over-expression of KHC causes similarly increased levels of posteriorly localized Wolbachia. However, excess KHC has no effect on levels of Vasa, a germplasm component that also requires KHC for posterior localization. Thus, Wolbachia transport is uniquely KHC-limited because these bacteria are likely outcompeted for binding to KHC by some host cargo/linker complexes. These results reveal a novel host-symbiont interaction that underscores the precise regulation required for an intracellular bacterium to co-opt, but not disrupt, vital host processes.

The alkalophilic fungus Sodiomyces alkalinus hosts beta- and gammapartitiviruses together with a new fusarivirus.

Mixed infection by three dsRNA viruses, a novel betapartitivirus, a gammapartitivirus, and a novel fusarivirus, has been identified in four isolates of the obligate alkalophilic fungus Sodiomyces alkalinus. The first, Sodiomyces alkalinus partitivirus 1 (SaPV1), is placed within the genus Betapartitivirus and is related to Ustilaginoidea virens partitivirus 2. The taxonomic position of the second virus is less clear as it shares high (85%) amino acid sequence identity but significantly low (77%) nucleotide sequence identity of the capsid protein with Colletotrichum truncatum partitivirus 1. The third, the novel Sodiomyces alkalinus fusarivirus 1 (SaFV1), is related to Fusarium poae fusarivirus 1. All the viruses show efficient vertical transmission through asexual and sexual spores. These novel coexisting viruses do not evoke apparent phenotypic alteration to their fungal host. This is the first description of a viral infection in an alkalophilic fungus.

Congenital Zika virus infection: More than just microcephaly.

A nonhuman primate model demonstrates efficient vertical transmission of Zika virus.

How Long Does Wolbachia Remain on Board?

Wolbachia bacteria infect about half of all arthropods, with diverse and extreme consequences ranging from sex-ratio distortion and mating incompatibilities to protection against viruses. These phenotypic effects, combined with efficient vertical transmission from mothers to offspring, satisfactorily explain the invasion dynamics of Wolbachia within species. However, beyond the species level, the lack of congruence between the host and symbiont phylogenetic trees indicates that Wolbachia horizontal transfers and extinctions do happen and underlie its global distribution. But how often do they occur? And has the Wolbachia pandemic reached its equilibrium? Here, we address these questions by inferring recent acquisition/loss events from the distribution of Wolbachia lineages across the mitochondrial DNA tree of 3,600 arthropod specimens, spanning 1,100 species from Tahiti and surrounding islands. We show that most events occurred within the last million years, but are likely attributable to individual level variation (e.g., imperfect maternal transmission) rather than population level variation (e.g., Wolbachia extinction). At the population level, we estimate that mitochondria typically accumulate 4.7% substitutions per site during an infected episode, and 7.1% substitutions per site during the uninfected phase. Using a Bayesian time calibration of the mitochondrial tree, these numbers translate into infected and uninfected phases of approximately 7 and 9 million years. Infected species thus lose Wolbachia slightly more often than uninfected species acquire it, supporting the view that its present incidence, estimated here slightly below 0.5, represents an epidemiological equilibrium.

Cell Division by Longitudinal Scission in the Insect Endosymbiont Spiroplasma poulsonii.

ABSTRACTSpiroplasma bacteria are highly motile bacteria with no cell wall and a helical morphology. This clade includes many vertically transmitted insect endosymbionts, including Spiroplasma poulsonii, a natural endosymbiont of Drosophila melanogaster. S. poulsonii bacteria are mainly found in the hemolymph of infected female flies and exhibit efficient vertical transmission from mother to offspring. As is the case for many facultative endosymbionts, S. poulsonii can manipulate the reproduction of its host; in particular, S. poulsonii induces male killing in Drosophila melanogaster. Here, we analyze the morphology of S. poulsonii obtained from the hemolymph of infected Drosophila. This endosymbiont was not only found as long helical filaments, as previously described, but was also found in a Y-shaped form. The use of electron microscopy, immunogold staining of the FtsZ protein, and antibiotic treatment unambiguously linked the Y shape of S. poulsonii to cell division. Observation of the Y shape in another Spiroplasma, S. citri, and anecdotic observations from the literature suggest that cell division by longitudinal scission might be prevalent in the Spiroplasma clade. Our study is the first to report the Y-shape mode of cell division in an endosymbiotic bacterium and adds Spiroplasma to the so far limited group of bacteria known to utilize this cell division mode.

Vertical Transmission of a Drosophila Endosymbiont Via Cooption of the Yolk Transport and Internalization Machinery

ABSTRACTSpiroplasma is a diverse bacterial clade that includes many vertically transmitted insect endosymbionts, including Spiroplasma poulsonii, a natural endosymbiont of Drosophila melanogaster. These bacteria persist in the hemolymph of their adult host and exhibit efficient vertical transmission from mother to offspring. In this study, we analyzed the mechanism that underlies their vertical transmission, and here we provide strong evidence that these bacteria use the yolk uptake machinery to colonize the germ line. We show that Spiroplasma reaches the oocyte by passing through the intercellular space surrounding the ovarian follicle cells and is then endocytosed into oocytes within yolk granules during the vitellogenic stages of oogenesis. Mutations that disrupt yolk uptake by oocytes inhibit vertical Spiroplasma transmission and lead to an accumulation of these bacteria outside the oocyte. Impairment of yolk secretion by the fat body results in Spiroplasma not reaching the oocyte and a severe reduction of vertical transmission. We propose a model in which Spiroplasma first interacts with yolk in the hemolymph to gain access to the oocyte and then uses the yolk receptor, Yolkless, to be endocytosed into the oocyte. Cooption of the yolk uptake machinery is a powerful strategy for endosymbionts to target the germ line and achieve vertical transmission. This mechanism may apply to other endosymbionts and provides a possible explanation for endosymbiont host specificity.