Wolbachia-induced cytoplasmic incompatibility is associated with decreased Hira expression in male Drosophila

NDUFA8 potentially rescues Wolbachia-induced cytoplasmic incompatibility in Laodelphax striatellus

The endosymbiont Wolbachia manipulates host reproduction by several strategies, one of the most important of which is cytoplasmic incompatibility (CI). CI can be rescued when Wolbachia-infected males mate with females infected with the same Wolbachia strain. However, the potential rescue mechanism of CI in the small brown planthopper Laodelphax striatellus is unclear. In this study, comparative transcriptome analysis was applied to explore the effect of Wolbachia on L. striatellus eggs. A total of 1387 differentially expressed genes were identified. RNA interference of 7 Wolbachia-upregulated key planthopper genes reduced egg reproduction, suggesting that Wolbachia might improve fecundity in L. striatellus by affecting these 7 genes. Suppressing the expression of another upregulated gene, NDUFA8 (encoding NADH dehydrogenase [ubiquinone] 1 α subcomplex subunit 8-like) by RNA interference significantly increased the mortality of early embryos without affecting the number of deposited eggs. Wolbachia infection upregulated the mRNA level of NDUFA8, and dsNDUFA8 treatment of Wolbachia-infected females recreated CI-like symptoms, suggesting that NDUFA8 is associated with the rescue phenotype. Because all L. striatellus populations worldwide are infected with Wolbachia, NDUFA8 is a potential pest control target.

RpS3 Is Required for Spermatogenesis of Drosophila melanogaster

Ribosomal proteins (RPs) constitute the ribosome, thus participating in the protein biosynthesis process. Emerging studies have suggested that many RPs exhibit different expression levels across various tissues and function in a context-dependent manner for animal development. Drosophila melanogaster RpS3 encodes the ribosomal protein S3, one component of the 40S subunit of ribosomes. We found that RpS3 is highly expressed in the reproductive organs of adult flies and its depletion in male germline cells led to severe defects in sperm production and male fertility. Immunofluorescence staining showed that RpS3 knockdown had little effect on early germ cell differentiation, but strongly disrupted the spermatid elongation and individualization processes. Furthermore, we observed abnormal morphology and activity of mitochondrial derivatives in the elongating spermatids of RpS3-knockdown testes, which could cause the failure of axoneme elongation. We also found that RpS3 RNAi inhibited the formation of the individualization complex that takes charge of disassociating the spermatid bundle. In addition, excessive apoptotic cells were detected in the RpS3-knockdown testes, possibly to clean the defective spermatids. Together, our data demonstrated that RpS3 plays an important role in regulating spermatid elongation and individualization processes and, therefore, is required for normal Drosophila spermatogenesis.

Toward an accurate mechanistic understanding of Wolbachia-induced cytoplasmic incompatibility

Wolbachia are the most successful intracellular bacteria in arthropods. They can manipulate host reproduction to favour infected females, which transmit Wolbachia to their progeny and increase the presence of Wolbachia in the population. The reproductive alterations caused by Wolbachia include feminization, parthenogenesis, male killing and cytoplasmic incompatibility (CI), among which CI is the most common. CI leads to embryonic lethality when Wolbachia-infected males mate with uninfected females or those infected with an incompatible strain. This lethality can be rescued if females are infected with a compatible strain. Although CI was described in the 1960s and its connection to Wolbachia was made in the 1970s, the genes responsible for CI, called CI factors, were not identified until recently. Since then, significant progress has been made in understanding the molecular mechanism of CI using a combination of genetic, phylogenetic, biochemical and structural approaches. The detailed molecular mechanisms behind this fascinating endosymbiotic bacteria-induced phenotype have begun to emerge. Here, we summarize recent progress in understanding the molecular mechanism of CI, especially focusing on the recently solved CI factor structures and discussing what these new structures brought in terms of CI mechanism.

Association of Wolbachia with Gene Expression in Drosophila Testes

Wolbachia is a genus of intracellular symbiotic bacteria that are widely distributed in arthropods and nematodes. These maternally inherited bacteria regulate host reproductive systems in various ways to facilitate their vertical transmission. Since the identification of Wolbachia in many insects, the relationship between Wolbachia and the host has attracted great interest. Numerous studies have indicated that Wolbachia modifies a variety of biological processes in the host. Previous studies in Drosophila melanogaster (D. melanogaster) have demonstrated that Wolbachia can affect spermatid differentiation, chromosome deposition, and sperm activity in the early stages of spermatogenesis, leading to sperm dysfunction. Here, we explored the putative effect of Wolbachia in sperm maturation using transcriptomic approaches to compare gene expression in Wolbachia-infected and Wolbachia-free D. melanogaster adult testes. Our findings show that Wolbachia affects many biological processes in D. melanogaster adult testes, and most of the differentially expressed genes involved in carbohydrate metabolism, lysosomal degradation, proteolysis, lipid metabolism, and immune response were upregulated in the presence of Wolbachia. In contrast, some genes that are putatively associated with cutin and wax biosynthesis and peroxisome pathways were downregulated. We did not find any differentially expressed genes that are predicted to be related to spermatogenesis in the datasets. This work provides additional information for understanding the Wolbachia-host intracellular relationships.

Wolbachia affects reproduction in the spider mite Tetranychus truncatus (Acari: Tetranychidae) by regulating chorion protein S38-like and Rop

Wolbachia-induced reproductive regulation in hosts has been used to control pest populations, but little is known about the molecular mechanism underlying Wolbachia regulation of host genes. Here, reproductive regulation by Wolbachia in the spider mite Tetranychus truncatus was studied at the molecular level. Infection with Wolbachia resulted in decreasing oviposition and cytoplasmic incompatibility in T. truncatus. Further RNA-seq revealed genes regulated by Wolbachia in T. truncatus. Real-time quantitative polymerase chain reaction (qPCR) showed that genes, including chorion protein S38-like and Rop were down-regulated by Wolbachia. RNA interference (RNAi) of chorion protein S38-like and Rop in Wolbachia-uninfected T. truncatus decreased oviposition, which was consistent with Wolbachia-induced oviposition decrease. Interestingly, suppressing Rop in Wolbachia-infected T. truncatus led to increased Wolbachia titres in eggs; however, this did not occur after RNAi of chorion protein S38-like. This is the first study to show that chorion protein S38-like and Rop facilitate Wolbachia-mediated changes in T. truncatus fertility. In addition, RNAi of Rop turned the body colour of Wolbachia-uninfected T. truncatus black, which indicates that the role of Rop is not limited to the reproductive regulation of T. truncatus.

Cardinium Localization During Its Parasitoid Wasp Host's Development Provides Insights Into Cytoplasmic Incompatibility

Arthropods harbor heritable intracellular symbionts that may manipulate host reproduction to favor symbiont transmission. In cytoplasmic incompatibility (CI), the symbiont sabotages the reproduction of infected males such that high levels of offspring mortality result when they mate with uninfected females. In crosses with infected males and infected females, however (the “rescue” cross), normal numbers of offspring are produced. A common CI-inducing symbiont, Cardinium hertigii, causes variable levels of CI mortality in the parasitoid wasp, Encarsia suzannae. Previous work correlated CI-induced mortality with male development time in this system, although the timing of Cardinium CI-induction and the relationship between development time and CI mortality was not well understood. Here, using a combination of crosses, manipulation of development time, and fluorescence microscopy, we identify the localization and the timing of the CI-induction step in the Cardinium-E. suzannae system. Antibiotic treatment of adult Cardinium-infected males did not reduce the mortality associated with the CI phenotype, suggesting that CI-alteration occurs prior to adulthood. Our results suggest that the alteration step occurs during the pupal period, and is limited by the duration of pupal development: 1) Encarsia produces most sperm prior to adulthood, 2) FISH localization of Cardinium in testes showed an association with sperm nuclei throughout spermatogenesis but not with mature sperm, and 3) two methods of prolonging the pupal period (cool temperatures and the juvenile hormone analog methoprene) both caused greater CI mortality, suggesting the degree of alteration is limited by the duration of the pupal stage. Based on these results, we compare two models for potential mechanisms of Cardinium sperm modification in the context of what is known about analogous mechanisms of Wolbachia, a more extensively studied CI-inducing symbiont.

Wolbachia-induced expression of kenny gene in testes affects male fertility in Drosophila melanogaster

Wolbachia are Gram-negative endosymbionts that are known to cause embryonic lethality when infected male insects mate with uninfected females or with females carrying a different strain of Wolbachia, a situation characterized as cytoplasmic incompatibility (CI). However, the mechanism of CI is not yet fully understood, although recent studies on Drosophila melanogaster have achieved great progress. Here, we found that Wolbachia infection caused changes in the expressions of several immunity-related genes, including significant upregulation of kenny (key), in the testes of D. melanogaster. Overexpression of key in fly testes led to a significant decrease in egg hatch rates when these flies mate with wild-type females. Wolbachia-infected females could rescue this embryonic lethality. Furthermore, in key overexpressing testes terminal deoxynucleotidyl transferase-mediated dUTP-biotin nick-end labeling signal was significantly stronger than in the control testes, and the level of reactive oxygen species was significantly increased. Overexpression of key also resulted in alterations of some other immunity-related gene expressions, including the downregulation of Zn72D. Knockdown of Zn72D in fly testes also led to a significant decrease in egg hatch rates. These results suggest that Wolbachia might induce the defect in male host fertility by immunity-related pathways and thus cause an oxidative damage and cell death in male testes.

The effect of the endosymbiont Wolbachia on the behavior of insect hosts

As one of the most successful intracellular symbiotic bacteria, Wolbachia can infect many arthropods and nematodes. Wolbachia infection usually affects the reproduction of their hosts to promote their own proliferation and transmission. Currently, most of the studies focus on the mechanisms of Wolbachia interactions with host reproduction. However, in addition to distribution in the reproductive tissues, Wolbachia also infect various somatic tissues of their hosts, including the brain. This raises the potential that Wolbachia may influence some somatic processes, such as behaviors in their hosts. So far, information about the effects of Wolbachia infection on host behavior is still very limited. The present review presents the current literature on different aspects of the influence of Wolbachia on various behaviors, including sleep, learning and memory, mating, feeding and aggression in their insect hosts. We then highlight ongoing scientific efforts in the field that need addressing to advance this field, which can have significant implications for further developing Wolbachia as environmentally friendly biocontrol agents to control insect-borne diseases and agricultural pests.

Symbiont-mediated cytoplasmic incompatibility: what have we learned in 50 years?

Cytoplasmic incompatibility (CI) is the most common symbiont-induced reproductive manipulation. Specifically, symbiont-induced sperm modifications cause catastrophic mitotic defects in the fertilized embryo and ensuing lethality in crosses between symbiotic males and either aposymbiotic females or females harboring a different symbiont strain. However, if the female carries the same symbiont strain, then embryos develop properly, thereby imparting a relative fitness benefit to symbiont-transmitting mothers. Thus, CI drives maternally-transmitted bacteria to high frequencies in arthropods worldwide. In the past two decades, CI experienced a boom in interest due to its (i) deployment in worldwide efforts to curb mosquito-borne diseases, (ii) causation by bacteriophage genes, cifA and cifB, that modify sexual reproduction, and (iii) important impacts on arthropod speciation. This review serves as a gateway to experimental, conceptual, and quantitative themes of CI and outlines significant gaps in understanding CI’s mechanism that are ripe for investigation from diverse subdisciplines in the life sciences.

How do Wolbachia modify the Drosophila ovary? New evidences support the "titration-restitution" model for the mechanisms of Wolbachia-induced CI

Background

Cytoplasmic incompatibility (CI) is the most common phenotype induced by endosymbiont Wolbachia and results in embryonic lethality when Wolbachia-modified sperm fertilize eggs without Wolbachia. However, eggs carrying the same strain of Wolbachia can rescue this embryonic death, thus producing viable Wolbachia-infected offspring. Hence Wolbachia can be transmitted mainly by hosts’ eggs. One of the models explaining CI is “titration-restitution”, which hypothesized that Wolbachia titrated-out some factors from the sperm and the Wolbachia in the egg would restitute the factors after fertilization. However, how infected eggs rescue CI and how hosts’ eggs ensure the proliferation and transmission of Wolbachia are not well understood.

Results

By RNA-seq analyses, we first compared the transcription profiles of Drosophila melanogaster adult ovaries with and without the wMel Wolbachia and identified 149 differentially expressed genes (DEGs), of which 116 genes were upregulated and 33 were downregulated by Wolbachia infection. To confirm the results obtained from RNA-seq and to screen genes potentially associated with reproduction, 15 DEGs were selected for quantitative RT-PCR (qRT-PCR). Thirteen genes showed the same changing trend as RNA-seq analyses. To test whether these genes are associated with CI, we also detected their expression levels in testes. Nine of them exhibited different changing trends in testes from those in ovaries. To investigate how these DEGs were regulated, sRNA sequencing was performed and identified seven microRNAs (miRNAs) that were all upregulated in fly ovaries by Wolbachia infection. Matching of miRNA and mRNA data showed that these seven miRNAs regulated 15 DEGs. Wolbachia-responsive genes in fly ovaries were involved in biological processes including metabolism, transportation, oxidation-reduction, immunity, and development.

Conclusions

Comparisons of mRNA and miRNA data from fly ovaries revealed 149 mRNAs and seven miRNAs that exhibit significant changes in expression due to Wolbachia infection. Notably, most of the DEGs showed variation in opposite directions in ovaries versus testes in the presence of Wolbachia, which generally supports the “titration-restitution” model for CI. Furthermore, genes related to metabolism were upregulated, which may benefit maximum proliferation and transmission of Wolbachia. This provides new insights into the molecular mechanisms of Wolbachia-induced CI and Wolbachia dependence on host ovaries.

Electronic supplementary material

The online version of this article (10.1186/s12864-019-5977-6) contains supplementary material, which is available to authorized users.

Ocnus is essential for male germ cell development in Drosophila melanogaster

The ocnus (ocn) gene encodes a protein abundant in the testes, implying its role in testis development. When Drosophila melanogaster is infected with the endosymbiont wMel Wolbachia, which affects the spermatogenesis of its hosts, ocn is downregulated in the third-instar larval testes, suggesting a role of ocn in spermatogenesis. In this study, we knocked down ocn in the testes and found that the hatch rates of embryos derived from ocn-knockdown males were significantly decreased, and 84.38% of the testes were much smaller in comparison to controls. Analysis of the smaller testes showed no germ cells but they had an extended hub. Using RNA-sequencing (RNA-Seq), we identified 69 genes with at least a twofold change (q-value < 5%) in their expression after ocn knockdown; of these, eight testes-specific and three reproduction-related genes were verified to be significantly downregulated using quantitative reverse transcription-PCR. Three genes (orientation disruptor, p24-2 and CG13541) were also significantly downregulated in the presence of Wolbachia. Furthermore, 98 genes were not expressed when ocn was knocked down in testes. These results suggest that ocn plays a crucial role in male germ cell development in Drosophila, possibly by regulating the expression of multiple spermatogenesis-related genes. Our data provide important information to help understand the molecular regulatory mechanisms underlying spermatogenesis.

High Ozone (O3) Affects the Fitness Associated with the Microbial Composition and Abundance of Q Biotype Bemisia tabaci

Ozone (O₃) affects the fitness of an insect, such as its development, reproduction and protection against fungal pathogens, but the mechanism by which it does so remains unclear. Here, we compared the fitness (i.e., the growth and development time, reproduction and protection against Beauveria bassiana (B. bassiana) of Q biotype whiteflies fumigated under hO₃ (280 ± 20 ppb) and control O₃ (50 ± 10 ppb) concentrations. Moreover, we determined that gene expression was related to development, reproduction and immunity to B. bassiana and examined the abundance and composition of bacteria and fungi inside of the body and on the surface of the Q biotype whitefly. We observed a significantly enhanced number of eggs that were laid by a female, shortened developmental time, prolonged adult lifespan, decreased weight of one eclosion, and reduced immunity to B. bassiana in whiteflies under hO₃, but hO₃ did not significantly affect the expression of genes related to development, reproduction and immunity. However, hO₃ obviously changed the composition of the bacterial communities inside of the body and on the surface of the whiteflies, significantly reducing Rickettsia and enhancing Candidatus_Cardinium. Similarly, hO₃ significantly enhanced Thysanophora penicillioides from the Trichocomaceae family and reduced Dothideomycetes (at the class level) inside of the body. Furthermore, positive correlations were found between the abundance of Candidatus_Cardinium and the female whitefly ratio and the fecundity of a single female, and positive correlations were found between the abundance of Rickettsia and the weight of adult whiteflies just after eclosion and immunity to B. bassiana. We conclude that hO₃ enhances whitefly development and reproduction but impairs immunity to B. bassiana, and our results also suggest that the changes to the microbial environments inside of the body and on the surface could be crucial factors that alter whitefly fitness under hO₃.

A draft genome sequence of an invasive mosquito: an Italian Aedes albopictus

The draft genome sequence of Italian specimens of the Asian tiger mosquito Aedes (Stegomyia) albopictus (Diptera: Culicidae) was determined using a standard NGS (next generation sequencing) approach. The size of the assembled genome is comparable to that of Aedes aegypti; the two mosquitoes are also similar as far as the high content of repetitive DNA is concerned, most of which is made up of transposable elements. Although, based on BUSCO (Benchmarking Universal Single-Copy Orthologues) analysis, the genome assembly reported here contains more than 99% of protein-coding genes, several of those are expected to be represented in the assembly in a fragmented state. We also present here the annotation of several families of genes (tRNA genes, miRNA genes, the sialome, genes involved in chromatin condensation, sex determination genes, odorant binding proteins and odorant receptors). These analyses confirm that the assembly can be used for the study of the biology of this invasive vector of disease.

Epigenetic modifications underlying symbiont-host interactions

The development, existence, and functioning of numerous animals and plants depend on their symbiotic interactions with other organisms, mainly microorganisms. In return, the symbionts benefit from safe habitats and nutrient-rich environments provided by their hosts. In these interactions, genetic changes in either of the partners may provide fitness advantages and become subjects to natural selection. Recent findings suggest that epigenetic changes, heritable or within the organism's life time, in either of the partners play significant roles in the establishment of symbiotic relationships. In this review, a variety of epigenetic effects underlying the most common host-symbiont interactions will be examined to determine to what extent these effects are shared in various interactions and how the epigenetic pathways could possibly be manipulated to benefit the interacting symbionts.

Knockdown of ATPsyn-b caused larval growth defect and male infertility in Drosophila

The ATPsyn-b encoding for subunit b of ATP synthase in Drosophila melanogaster is proposed to act in ATP synthesis and phagocytosis, and has been identified as one of the sperm proteins in both Drosophila and mammals. At present, its details of functions in animal growth and spermatogenesis have not been reported. In this study, we knocked down ATPsyn-b using Drosophila lines expressing inducible hairpin RNAi constructs and Gal4 drivers. Ubiquitous knockdown of ATPsyn-b resulted in growth defects in larval stage as the larvae did not grow bigger than the size of normal second-instar larvae. Knockdown in testes did not interrupt the developmental excursion to viable adult flies, however, these male adults were sterile. Analyses of testes revealed disrupted nuclear bundles during spermatogenesis and abnormal shaping in spermatid elongation. There were no mature sperm in the seminal vesicle of ATPsyn-b knockdown male testes. These findings suggest us that ATPsyn-b acts in growth and male fertility of Drosophila.

Sexually antagonistic zygotic drive: a new form of genetic conflict between the sex chromosomes

Sisters and brothers are completely unrelated with respect to the sex chromosomes they inherit from their heterogametic parent. This has the potential to result in a previously unappreciated form of genetic conflict between the sex chromosomes, called sexually antagonistic zygotic drive (SA-ZD). SA-ZD can arise whenever brothers and sisters compete over limited resources or there is brother-sister mating coupled with inbreeding depression. Although theory predicts that SA-ZD should be common and influence important evolutionary processes, there is little empirical evidence for its existence. Here we discuss the current understanding of SA-ZD, why it would be expected to elude empirical detection when present, and how it relates to other forms of genetic conflict.

The relative importance of DNA methylation and Dnmt2-mediated epigenetic regulation on Wolbachia densities and cytoplasmic incompatibility

Wolbachia pipientis is a worldwide bacterial parasite of arthropods that infects germline cells and manipulates host reproduction to increase the ratio of infected females, the transmitting sex of the bacteria. The most common reproductive manipulation, cytoplasmic incompatibility (CI), is expressed as embryonic death in crosses between infected males and uninfected females. Specifically, Wolbachia modify developing sperm in the testes by unknown means to cause a post-fertilization disruption of the sperm chromatin that incapacitates the first mitosis of the embryo. As these Wolbachia-induced changes are stable, reversible, and affect the host cell cycle machinery including DNA replication and chromosome segregation, we hypothesized that the host methylation pathway is targeted for modulation during cytoplasmic incompatibility because it accounts for all of these traits. Here we show that infection of the testes is associated with a 55% increase of host DNA methylation in Drosophila melanogaster, but methylation of the paternal genome does not correlate with penetrance of CI. Overexpression and knock out of the Drosophila DNA methyltransferase Dnmt2 neither induces nor increases CI. Instead, overexpression decreases Wolbachia titers in host testes by approximately 17%, leading to a similar reduction in CI levels. Finally, strength of CI induced by several different strains of Wolbachia does not correlate with levels of DNA methylation in the host testes. We conclude that DNA methylation mediated by Drosophila’s only known methyltransferase is not required for the transgenerational sperm modification that causes CI.

Genome diversity and divergence in Drosophila mauritiana: multiple signatures of faster X evolution

Drosophila mauritiana is an Indian Ocean island endemic species that diverged from its two sister species, Drosophila simulans and Drosophila sechellia, approximately 240,000 years ago. Multiple forms of incomplete reproductive isolation have evolved among these species, including sexual, gametic, ecological, and intrinsic postzygotic barriers, with crosses among all three species conforming to Haldane’s rule: F₁ hybrid males are sterile and F₁ hybrid females are fertile. Extensive genetic resources and the fertility of hybrid females have made D. mauritiana, in particular, an important model for speciation genetics. Analyses between D. mauritiana and both of its siblings have shown that the X chromosome makes a disproportionate contribution to hybrid male sterility. But why the X plays a special role in the evolution of hybrid sterility in these, and other, species remains an unsolved problem. To complement functional genetic analyses, we have investigated the population genomics of D. mauritiana, giving special attention to differences between the X and the autosomes. We present a de novo genome assembly of D. mauritiana annotated with RNAseq data and a whole-genome analysis of polymorphism and divergence from ten individuals. Our analyses show that, relative to the autosomes, the X chromosome has reduced nucleotide diversity but elevated nucleotide divergence; an excess of recurrent adaptive evolution at its protein-coding genes; an excess of recent, strong selective sweeps; and a large excess of satellite DNA. Interestingly, one of two centimorgan-scale selective sweeps on the D. mauritiana X chromosome spans a region containing two sex-ratio meiotic drive elements and a high concentration of satellite DNA. Furthermore, genes with roles in reproduction and chromosome biology are enriched among genes that have histories of recurrent adaptive protein evolution. Together, these genome-wide analyses suggest that genetic conflict and frequent positive natural selection on the X chromosome have shaped the molecular evolutionary history of D. mauritiana, refining our understanding of the possible causes of the large X-effect in speciation.

Wolbachia-induced paternal defect in Drosophila is likely by interaction with the juvenile hormone pathway

Wolbachia are endosymbionts that infect many insect species. They can manipulate the host's reproduction to increase their own maternal transmission. Cytoplasmic incompatibility (CI) is one such manipulation, which is expressed as embryonic lethality when Wolbachia-infected males mate with uninfected females. However, matings between males and females carrying the same Wolbachia strain result in viable progeny. The molecular mechanisms of CI are currently not clear. We have previously reported that the gene Juvenile hormone-inducible protein 26 (JhI-26) exhibited the highest upregulation in the 3rd instar larval testes of Drosophila melanogaster when infected by Wolbachia. This is reminiscent of an interaction between Wolbachia and juvenile hormone (JH) pathway in flies. Considering that Jhamt gene encodes JH acid methyltransferase, a key regulatory enzyme of JH biosynthesis, and that methoprene-tolerant (Met) has been regarded as the best JH receptor candidate, we first compared the expression of Jhamt and Met between Wolbachia-infected and uninfected fly testes to investigate whether Wolbachia infection influence the JH signaling pathway. We found that the expressions of Jhamt and Met were significantly increased in the presence of Wolbachia, suggesting an interaction of Wolbachia with the JH signaling pathway. Then, we found that overexpression of JhI-26 in Wolbachia-free transgenic male flies caused paternal-effect lethality that mimics the defects associated with CI. JhI-26 overexpressing males resulted in significantly decrease in hatch rate. Surprisingly, Wolbachia-infected females could rescue the egg hatch. In addition, we showed that overexpression of JhI-26 caused upregulation of the male accessory gland protein (Acp) gene CG10433, but not vice versa. This result suggests that JhI-26 may function at the upstream of CG10433. Likewise, overexpression of CG10433 also resulted in paternal-effect lethality. Both JhI-26 and CG10433 overexpressing males resulted in nuclear division defects in the early embryos. Finally, we found that Wolbachia-infected males decreased the propensity of the mated females to remating, a phenotype also caused by both JhI-26 and CG10433 overexpressing males. Taken together, our results provide a working hypothesis whereby Wolbachia induce paternal defects in Drosophila probably by interaction with the JH pathway via JH response genes JhI-26 and CG10433.

HIRA is essential for the development of gibel carp

HIRA is one of the chaperones of histone H3.3. Mutation of Hira results in embryonic lethality in mice, suggesting a critical role in embryogenesis. However, Hira-mutated Drosophila may survive to adults, indicating that it is dispensable in Drosophila development. The role of Hira in fish development is unknown. In this study we first investigated the expression of Hira during embryogenesis of gibel carp (Carassius auratus gibelio) by whole-mount in situ hybridization. We found that Hira signal appeared ubiquitously in the early embryos. After gastrulation, it appeared mainly along the anterior-posterior axis, including the tail bud. In hatching period, the signal was detected in head, heart, and the endoderm region on the back of yolk. Then by microinjection with morpholino-HIRA at the beginning of development, we observed delayed gastrulation and abnormal somitogenesis in gibel carp embryos. The HIRA morphants exhibited short trunk, limited yolk extension, and twisted tail. Most of the mutants died during embryogenesis or shortly after hatching. The rest of the HIRA morphants could survive to larvae but with severe defects in organogenesis. These data suggest that HIRA may be essential for the development of gibel carp, and this function is conserved in vertebrates.

Insights from natural host-parasite interactions: the Drosophila model

Immune responses against opportunistic pathogens have been extensively studied in Drosophila, leading to a detailed map of the genetics behind innate immunity networks including the Toll, Imd, Jak-Stat, and JNK pathways. However, immune mechanisms of other organisms, such as plants, have primarily been investigated using natural pathogens. It was the use of natural pathogens in plant research that revealed the plant R-Avr system, a specialized immune response derived from antagonistic coevolution between plant immune proteins and their natural pathogens' virulence proteins. Thus, we recommend that researchers begin to use natural Drosophila pathogens to identify novel immune strategies that may have arisen through antagonistic coevolution with common natural pathogens. In this review, we address the benefits of using natural pathogens in research, describe the known natural pathogens of Drosophila, and discuss the future prospects for research on natural pathogens of Drosophila.

Transcriptional regulation of Culex pipiens mosquitoes by Wolbachia influences cytoplasmic incompatibility

Cytoplasmic incompatibility (CI) induced by the endosymbiont Wolbachia pipientis causes complex patterns of crossing sterility between populations of the Culex pipiens group of mosquitoes. The molecular basis of the phenotype is yet to be defined. In order to investigate what host changes may underlie CI at the molecular level, we examined the transcription of a homolog of the Drosophila melanogaster gene grauzone that encodes a zinc finger protein and acts as a regulator of female meiosis, in which mutations can cause sterility. Upregulation was observed in Wolbachia-infected C. pipiens group individuals relative to Wolbachia-cured lines and the level of upregulation differed between lines that were reproductively incompatible. Knockdown analysis of this gene using RNAi showed an effect on hatch rates in a Wolbachia infected Culex molestus line. Furthermore, in later stages of development an effect on developmental progression in CI embryos occurs in bidirectionally incompatible crosses. The genome of a wPip Wolbachia strain variant from Culex molestus was sequenced and compared with the genome of a wPip variant with which it was incompatible. Three genes in inserted or deleted regions were newly identified in the C. molestus wPip genome, one of which is a transcriptional regulator labelled wtrM. When this gene was transfected into adult Culex mosquitoes, upregulation of the grauzone homolog was observed. These data suggest that Wolbachia-mediated regulation of host gene expression is a component of the mechanism of cytoplasmic incompatibility.

Wolbachia are maternally inherited bacteria that manipulate invertebrate reproduction. Cytoplasmic incompatibility is embryo death that occurs when males carrying Wolbachia mate with females that do not, or that carry a different Wolbachia variant; its mechanism is poorly understood. In Culex mosquitoes, in the presence of Wolbachia a gene related to a Drosophila melanogaster gene, grauzone, which has been shown to act as a regulator of the meiotic cell cycle, showed an elevated level of expression. When lower levels of expression were achieved through RNA interference, embryo hatch rates were affected and the stage of development at which embryo death occurs was altered. To find Wolbachia genes that influence cytoplasmic incompatibility, we compared the genomes of two variants of Wolbachia from Culex that produce cytoplasmic incompatibility with one another. Although most segments of these genomes were very similar, one newly identified gene is predicted to be a regulator of gene transcription. We cloned this gene into a plasmid, expressed it in adult mosquitoes and found higher levels of expression of the Culex grauzone homolog. This suggests that the Wolbachia transcriptional regulator may play an important role in manipulating the host in order to induce cytoplasmic incompatibility.

Wolbachia: Can we save lives with a great pandemic?

Wolbachia pipientis is the most common bacterial infection in the animal world and wields a vast influence on invertebrate reproduction, sex determination, speciation, and behavior worldwide. These avenues of research have made seminal gains, including the latest use of Wolbachia to alter mosquito populations and a strengthened focus on using anti-Wolbachia therapies against filarial nematode infections. This work is further bolstered by a more refined knowledge of Wolbachia biology spanning mechanisms to relevance. Here we tally the most up-to-date knowledge in the field and review the immense implications that this global infection has for the basic and applied life sciences.

Infection with a Virulent Strain of Wolbachia Disrupts Genome Wide-Patterns of Cytosine Methylation in the Mosquito Aedes aegypti

Background

Cytosine methylation is one of several reversible epigenetic modifications of DNA that allow a greater flexibility in the relationship between genotype and phenotype. Methylation in the simplest models dampens gene expression by modifying regions of DNA critical for transcription factor binding. The capacity to methylate DNA is variable in the insects due to diverse histories of gene loss and duplication of DNA methylases. Mosquitoes like Drosophila melanogaster possess only a single methylase, DNMT2.

Description

Here we characterise the methylome of the mosquito Aedes aegypti and examine its relationship to transcription and test the effects of infection with a virulent strain of the endosymbiont Wolbachia on the stability of methylation patterns.

Conclusion

We see that methylation in the A. aegypti genome is associated with reduced transcription and is most common in the promoters of genes relating to regulation of transcription and metabolism. Similar gene classes are also methylated in aphids and honeybees, suggesting either conservation or convergence of methylation patterns. In addition to this evidence of evolutionary stability, we also show that infection with the virulent wMelPop Wolbachia strain induces additional methylation and demethylation events in the genome. While most of these changes seem random with respect to gene function and have no detected effect on transcription, there does appear to be enrichment of genes associated with membrane function. Given that Wolbachia lives within a membrane-bound vacuole of host origin and retains a large number of genes for transporting host amino acids, inorganic ions and ATP despite a severely reduced genome, these changes might represent an evolved strategy for manipulating the host environments for its own gain. Testing for a direct link between these methylation changes and expression, however, will require study across a broader range of developmental stages and tissues with methods that detect splice variants.

Mapping Wolbachia distributions in the adult Drosophila brain

The maternally inherited bacterium Wolbachia infects the germline of most arthropod species. Using Drosophila simulans and D. melanogaster, we demonstrate that localization of Wolbachia to the fat bodies and adult brain is likely also a conserved feature of Wolbachia infection. Examination of three Wolbachia strains (WMel , WRiv , WPop ) revealed that the bacteria preferentially concentrate in the central brain with low titres in the optic lobes. Distribution within regions of the central brain is largely determined by the Wolbachia strain, while the titre is influenced by both, the host species and the bacteria strain. In neurons of the central brain and ventral nerve cord, Wolbachia preferentially localizes to the neuronal cell bodies but not to axons. All examined Wolbachia strains are present intracellularly or in extracellular clusters, with the pathogenic WPop strain exhibiting the largest and most abundant clusters. We also discovered that 16 of 40 lines from the Drosophila Genetic Reference Panel are Wolbachia infected. Direct comparison of Wolbachia infected and cured lines from this panel reveals that differences in physiological traits (chill coma recovery, starvation, longevity) are partially due to host line influences. In addition, a tetracycline-induced increase in Drosophila longevity was detected many generations after treatment.

Differentially expressed profiles in the larval testes of Wolbachia infected and uninfected Drosophila

Background

Wolbachia are endosymbiotic bacteria that are frequently found in arthropods and nematodes. These maternally inherited bacteria manipulate host reproduction by several mechanisms including cytoplasmic incompatibility (CI). CI is the most common phenotype induced by Wolbachia and results in the developmental arrest of embryos derived from crosses between Wolbachia-infected males and uninfected females. Although the molecular mechanisms of CI are currently unknown, several studies suggest that host sperm is modified by Wolbachia during spermatogenesis.

Results

We compared the gene expression of Drosophila melanogaster larval testes with and without the wMel strain of Wolbachia to identify candidate genes that could be involved in the interaction between Wolbachia and the insect host. Microarray, quantitative RT-PCR and in situ hybridization analyses were carried out on D. melanogaster larval testes to determine the effect of Wolbachia infection on host gene expression. A total of 296 genes were identified by microarray analysis to have at least a 1.5 fold change [q-value < 5%] in expression. When comparing Wolbachia-infected flies to uninfected flies, 167 genes were up-regulated and 129 genes down-regulated. Differential expression of genes related to metabolism, immunity, reproduction and other functions were observed. Quantitative RT-PCR (qRT-PCR) confirmed 12 genes are differentially expressed in the testes of the 3^rd instar larvae of Wolbachia-infected and uninfected flies. In situ hybridization demonstrated that Wolbachia infection changes the expression of several genes putatively associated with spermatogenesis including JH induced protein-26 and Mst84Db, or involved in immune (kenny) or metabolism (CG4988-RA).

Conclusions

Wolbachia change the gene expression of 296 genes in the larval testes of D. melanogaster including genes related to metabolism, immunity and reproduction. Interestingly, most of the genes putatively involved in immunity were up-regulated in the presence of Wolbachia. In contrast, most of the genes putatively associated with reproduction (especially spermatogenesis) were down-regulated in the presence of Wolbachia. These results suggest Wolbachia may activate the immune pathway but inhibit spermatogenesis. Our data provide a significant panel of candidate genes that may be involved in the interaction between Wolbachia and their insect hosts. This forms a basis to help elucidate the underlying mechanisms of Wolbachia-induced CI in Drosophila and the influence of Wolbachia on spermatogenesis.