Genomic insights into Spiroplasma endosymbionts that induce male-killing and protective phenotypes in the pea aphid

The endosymbiotic bacteria Spiroplasma (Mollicutes) infect diverse plants and arthropods, and some of which induce male killing, where male hosts are killed during development. Male-killing Spiroplasma strains belong to either the phylogenetically distant Citri-Poulsonii or Ixodetis groups. In Drosophila flies, Spiroplasma poulsonii induces male killing via the Spaid toxin. While Spiroplasma ixodetis infects a wide range of insects and arachnids, little is known about the genetic basis of S. ixodetis-induced male killing. Here, we analyzed the genome of S. ixodetis strains in the pea aphid Acyrthosiphon pisum (Aphididae, Hemiptera). Genome sequencing constructed a complete genome of a male-killing strain, sAp269, consisting of a 1.5 Mb circular chromosome and an 80 Kb plasmid. sAp269 encoded putative virulence factors containing either ankyrin repeat, ovarian tumor-like deubiquitinase, or ribosome inactivating protein domains, but lacked the Spaid toxin. Further comparative genomics of Spiroplasma strains in A. pisum biotypes adapted to different host plants revealed their phylogenetic associations and the diversity of putative virulence factors. Although the mechanisms of S. ixodetis-induced male killing in pea aphids remain elusive, this study underlines the dynamic genome evolution of S. ixodetis and proposes independent acquisition events of male-killing mechanisms in insects.

Integrated analysis of mRNA and microRNA transcriptome related to immunity and autophagy in shrimp hemocytes infected with Spiroplasma eriocheiris

In recent years, the industry in charge of the cultivation of Macrobrachium nipponense (M.nipponense) has suffered significant economic losses due to an infectious pathogen called Spiroplasma eriocheiris (S.eriocheiris). There has therefore been a need to identify the key immune and autophagy genes that respond to M.nipponense's infection with S. eriocheiris to analyze its immune response mechanism and the regulation of related microRNAs (miRNAs). In this study, the mRNA and miRNA transcriptome of M.nipponense's hemocytes were analyzed at different stages of infection. This analysis employed the second and third-generation sequencing technologies. In the mRNA transcriptome, 1656 genes were expressed in healthy and susceptible M.nipponense. 892 of these were significantly up-regulated, while 764 were down-regulated. 118 genes with significant differences in autophagy, endocytosis, lysosome, Toll, IMD, and VEGF pathways were obtained from the transcriptome. In the miRNA transcriptome, 312 miRNAs (Conserved: 112, PN-type: 18, PC-type: 182) were sequenced. 74 were significantly up-regulated, and 57 were down-regulated. There were 25 miRNAs involved in regulating the Toll and IMD pathways, 41 in endocytosis, 30 in lysosome, and 12 in the VEGF pathway. An integrated analysis of immune-related miRNAs and mRNAs showed that miRNAs with significant differences (P < 0.05) such as ame-miR-29b-3p, dpu-miR-1and PC-3p-945_4074, had corresponding regulatory relationships with 118 important immune genes such as Relish, Dorsal, Caspase-3, and NF-κB. This study obtained the key immune and autophagy-related genes and corresponding regulatory miRNAs in M. nipponense's hemocytes in response to an infection by S.eriocheiris. The results can provide vital data that further reveals the defense mechanism of M.nipponense's immune system against S.eriocheiris. It can also help further comprehension and interpretation of M.nipponense's resistance mechanism to the invading S.eriocheiris, and provide molecular research information for the realization of host-directed therapies (HDT) for M.nipponense.

Eriocheir sinensis vesicle-associated membrane protein can enhance host cell phagocytosis to resist Spiroplasma eriocheiris infection

Vesicle-associated membrane protein (VAMP) belongs to the receptor protein on the membrane of the secretory transport vesicle and involves in host immune function. The intracellular pathogen Spiroplasma eriocheiris could cause Eriocheir sinensis tremor disease. In a previous study, it was found E. sinensis VAMP (EsVAMP) was differently expressed in S. eriocheiris infection by proteomics analysis. This study mainly aims at the function of EsVAMP in the process of the S. eriocheiris infection. The length of EsVAMP gene was 1681 bp, which contained a 395 bp open reading frame, 90 bp 5'-non-coding region (UTR) and 1277 bp 3'-UTR. The results of qPCR showed that EsVAMP was expressed highly in hemocytes and nerves, followed by gills, intestines and hepatopancreas, and lowly expressed in heart and muscles. EsVAMP in hemocytes was up-regulated after S. eriocheiris infection. After EsVAMP over-expression and S. eriocheiris infection, the RAW264.7 cell morphology and cell viability of the experiment group were significantly better than the control group. Meanwhile, the copy number of S. eriocheiris in the experiment group was significantly lower than that in the control group. After EsVAMP and pCMV-Cre-mCherry were ligated and transfected into RAW264.7 cells, it was found that EsVAMP and lysosome co-localized. Meanwhile, the phagocytosed inactivated S. eriocheiris number and phagocytosed efficiency in RAW264.7 cells were increased significantly. The interference experiment was carried out by synthesizing EsVAMP dsRNA to verify that the EsVAMP transcriptions were successfully suppressed. The S. eriocheiris copy number and the mortality of crab increased significantly after EsVAMP RNAi and S. eriocheiris infection. Meanwhile, the phagocytosed inactivated S. eriocheiris number and phagocytosed efficiency in hemocytes decreased significantly after EsVAMP RNAi and S. eriocheiris infection. These results showed that VAMP was involved in the cell phagocytosis to resist pathogen infection.

Ubiquitin-modified proteome analysis of Eriocheir sinensis hemocytes during Spiroplasma eriocheiris infection

Spiroplasma eriocheiris, the pathogen of Eriocheir sinensis tremor disease (TD), has bring a huge economic loss to China aquaculture. The hemocytes of crab as the first target cells of S. eriocheiris, but the interactive relationship between the E. sinensis and this pathogen not particularly clear. The present study is the first time to analysis the role of protein ubiquitination in the process of E. sinensis hemocytes response S. eriocheiris infection. By applying label-free quantitative liquid chromatography with tandem mass spectrometry proteomics, 950 lysine ubiquitination sites and 803 ubiquitination peptides on 458 proteins were identified, of which 48 ubiquitination sites on 40 proteins were quantified as significantly changed after the S. eriocheiris infection. Bioinformatics analysis of ubiquitination different proteins suggested many biological process and pathways were participated in the interaction between S. eriocheiris and host cell, such as ubiquitin system, endocytosis, prophenoloxidase system (proPO system), cell apoptosis, glycolysis. Our study can enhance our understanding of interaction between the crab and S. eriocheiris, and also provides basis to study the role of protein ubiquitination in other crustacean innate immune system.

Coexistence of Two Male-Killers and Their Impact on the Development of Oriental Tea Tortrix Homona magnanima

Male-killing, the death of male offspring induced by maternally transmitted microbes, is classified as early, or late, male-killing. The primary advantage afforded by early male-killing, which typically occurs during embryogenesis, is the reallocation of resources to females, that would have otherwise been consumed by males. Meanwhile, the key advantage of late male-killing, which typically occurs during late larval development, is the maximized potential for horizontal transmission. To date, no studies have reported on the associated developmental and physiological effects of host coinfection with early and late male-killers, which may have a significant impact on the population dynamics of the male-killers. Here we used a lepidopteran tea pest Homona magnanima as a model, which is a unique system wherein an early male-killer (a Spiroplasma bacterium) and a late male-killer (an RNA virus) can coexist in nature. An artificially established matriline, coinfected with both Spiroplasma and RNA virus, exhibited embryonic death (early male-killing) as seen in the host line singly infected with Spiroplasma. Moreover, the coinfected line also exhibited developmental retardation and low pupal weight similar to the host line singly infected with the RNA virus. A series of field surveys revealed that Spiroplasma-RNA virus coinfection occurs in nature at a low frequency. Hence, although the two male-killers are capable of coexisting within the H. magnanima population independently, high associated fitness cost appears to limit the prevalence of male-killer coinfection in the field host population.

Co-infection of Wolbachia and Spiroplasma in spider mite Tetranychus truncatus increases male fitness

Wolbachia and Spiroplasma are intracellular bacteria that are of great interest to entomologists, because of their ability to alter insect host biology in multiple ways. In the spider mite Tetranychus truncatus, co-infection of Wolbachia and Spiroplasma can induce cytoplasmic incompatibility (CI) and fitness costs; however, little is known about the effect of co-infection at the genetic level and the molecular mechanisms underlying CI. In this study, we explored the influence of the two symbionts on male mite host fitness and used RNA sequencing to generate the transcriptomes of T. truncatus with four different types of infection. In total, we found symbiont-infected lines had a higher hatch proportion than the uninfected line, and the development time of the uninfected line was longer than that of the other lines. Co-infection changed the expression of many genes related to digestion detoxification, reproduction, immunity and oxidation reduction. Our results indicate that co-infection of Wolbachia and Spiroplasma confers multiple effects on their hosts, and helps illuminate the complex interactions between endosymbionts and arthropods.

An ShK-domain serine protease of Eriocheir sinensis regulates the PO activity to resist Spiroplasma eriocheiris infection

A novel serine protease contains two ShK-domain was found from the Chinese mitten crab Eriocheir sinensis (EsShK-SP). The full-length EsShK-SP cDNA is 1927 bp and contains a 1260-bp open reading frame encoding a protein of 420 amino acids, including a signal peptide, two ShK domain, and Tryp-SPC domain. Quantitative real-time PCR showed that EsShK-SP was expressed mainly in the hemocytes, gills, intestine, and nerve, but weakly in heart, muscle, and hepatopancreas. After infected with Spiroplasma eriocheiris, the expression of EsShK-SP was significantly up-regulated from 1 d to 9 d. The Tryp-SPC domain was ligated with pGEX-4T-1 vector and prokaryotic expressed to obtain recombinant protein rSPC. When rSPC and S. eriocheiris stimulated the hemocytes of E. sinensis, the PO activity was significantly up-regulated. The subcellular localization revealed that recombinant EsShK-SP was mainly located in the cytoplasm of Drosophila S2 cells. Both absolute real-time PCR and confocal laser scanning microscope results showed that over-expression of EsShK-SP in S2 cells could decrease the copy number of S. eriocheiris. Meanwhile, the over-expression of EsShK-SP also increased the PO activity and cell viability of S2 cells. After EsShK-SP RNA interference using dsRNA, the expression levels of proPO and activity of PO decreased significantly from 48 h to 96 h. The knockdown of EsShK-SP by RNAi resulted in the copy number of S. eriocheiris in the EsShK-SP silenced group was significantly increased compared to the control groups during S. eriocheiris infection. Meanwhile, the survival rate of crabs decreased in the EsShK-SP-dsRNA group. The above results indicated that EsShK-SP plays an important immune role during E. sinensis against S. eriocheiris through regulation of the proPO system.

Wolbachia dominate Spiroplasma in the co-infected spider mite Tetranychus truncatus

Wolbachia and Spiroplasma are both maternally inherited endosymbionts in arthropods, and they can co-infect the same species. However, how they interact with each other in the same host is not clear. Here we investigate a co-infected Tetranychus truncatus spider mite strain that shares the same genetic background with singly infected and uninfected strains to detect the impacts of the two symbionts on their host. We found that Wolbachia-infected and Spiroplasma-infected mites can suffer significant fitness costs involving decreased fecundity, although with no effect on lifespan or development. Wolbachia induced incomplete cytoplasmic incompatibility in T. truncatus both in singly infected and doubly infected strains, resulting in female killing. In both females and males of the co-infected spider mite strain, Wolbachia density was higher than Spiroplasma density. Transcriptome analysis of female adults showed that the most differentially expressed genes were found between the co-infected strain and both the singly infected Spiroplasma strain and uninfected strain. The Wolbachia strain had the fewest differentially expressed genes compared with the co-infected strain, consistent with the higher density of Wolbachia in the co-infected strain. Wolbachia, therefore, appears to have a competitive advantage in host mites over Spiroplasma and is likely maintained in populations by cytoplasmic incompatibility despite having deleterious fitness effects.

TMT-based quantitative proteomic analysis of Eriocheir sinensis hemocytes and thoracic ganglion during Spiroplasma eriocheiris infection

Spiroplasma eriocheiris, a novel pathogen of Chinese mitten crab Eriocheir sinensis tremor disease, has led into catastrophic economic losses in aquaculture. S. eriocheiris invaded the hemocytes in the early stage, then invaded nerve tissue and caused typically paroxysmal tremors of pereiopod in the late stage of infection. The purpose of this study was to detect the infection mechanism of hemocytes in the early stage and thoracic ganglion in the late stage of S. eriocheiris infection at the protein level. Hemocytes and thoracic ganglion were collected at 24 h and 10 d after injection (the crabs with typical paroxysmal tremors of the pereiopod), respectively. TMT was performed with isobaric markers, followed by liquid chromatography tandem mass spectrometry (LC-MS/MS). In hemocytes, 127 proteins were up-regulated and 85 proteins were down-regulated in 2747 quantified proteins. Many proteins and process including proPO system proteins, hemolymph coagulation system proteins and lectins were differently expressed in hemocytes and involved in the early immune process of E. sinensis against S. eriocheiris infection. Meanwhile, 545 significantly different expression proteins (292 down-regulated and 253 up-regulated protein including a number of immune-associated, nervous system development and signal transmission related proteins) were identified in thoracic ganglion in the late stage of S. eriocheiris infection. The qRT-PCR analysis results shown that the selected significantly changed proteins in hemocytes and thoracic ganglion were consistent with the TMT proteomics. This paper reported for the first time to study the responses of crab hemocyte and thoracic ganglion against the S. eriocheiris infection at different stages. These findings help us understand the infection mechanism of S. eriocheiris at different stage with the different tissue.

Spatio-temporal distribution of Spiroplasma infections in the tsetse fly (Glossina fuscipes fuscipes) in northern Uganda

Tsetse flies (Glossina spp.) are vectors of parasitic trypanosomes, which cause human (HAT) and animal African trypanosomiasis (AAT) in sub-Saharan Africa. In Uganda, Glossina fuscipes fuscipes (Gff) is the main vector of HAT, where it transmits Gambiense disease in the northwest and Rhodesiense disease in central, southeast and western regions. Endosymbionts can influence transmission efficiency of parasites through their insect vectors via conferring a protective effect against the parasite. It is known that the bacterium Spiroplasma is capable of protecting its Drosophila host from infection with a parasitic nematode. This endosymbiont can also impact its host's population structure via altering host reproductive traits. Here, we used field collections across 26 different Gff sampling sites in northern and western Uganda to investigate the association of Spiroplasma with geographic origin, seasonal conditions, Gff genetic background and sex, and trypanosome infection status. We also investigated the influence of Spiroplasma on Gff vector competence to trypanosome infections under laboratory conditions. Generalized linear models (GLM) showed that Spiroplasma probability was correlated with the geographic origin of Gff host and with the season of collection, with higher prevalence found in flies within the Albert Nile (0.42 vs 0.16) and Achwa River (0.36 vs 0.08) watersheds and with higher prevalence detected in flies collected in the intermediate than wet season. In contrast, there was no significant correlation of Spiroplasma prevalence with Gff host genetic background or sex once geographic origin was accounted for in generalized linear models. Additionally, we found a potential negative correlation of Spiroplasma with trypanosome infection, with only 2% of Spiroplasma infected flies harboring trypanosome co-infections. We also found that in a laboratory line of Gff, parasitic trypanosomes are less likely to colonize the midgut in individuals that harbor Spiroplasma infection. These results indicate that Spiroplasma infections in tsetse may be maintained by not only maternal but also via horizontal transmission routes, and Spiroplasma infections may also have important effects on trypanosome transmission efficiency of the host tsetse. Potential functional effects of Spiroplasma infection in Gff could have impacts on vector control approaches to reduce trypanosome infections.

The defensive Spiroplasma

Defensive microbes are of great interest for their roles in arthropod health, disease transmission, and biocontrol efforts. Obligate bacterial passengers of arthropods, such as Spiroplasma, confer protection against the natural enemies of their hosts to improve their own fitness. Although known for less than a decade, Spiroplasma's defensive reach extends to diverse parasites, both microbial and multicellular. We provide an overview of known defensive phenotypes against nematodes, parasitoid wasps, and fungi, and highlight recent studies supporting the role of Spiroplasma-encoded ribosome-inactivating proteins in protection. With cellular features well-suited for life in the hemolymph, broad distribution among invertebrate hosts, and the capacity to repeatedly evolve vertical transmission, Spiroplasma may be uniquely equipped to form intimate, defensive associations to combat extracellular parasites. Along with insights into defensive mechanisms, recent significant advances have been made in male-killing - a phenotype with interesting evolutionary ties to defense. Finally, we look forward to an exciting decade using the genetic tools of Drosophila, and the rapidly-advancing tractability of Spiroplasma itself, to better understand mechanisms and evolution in defensive symbiosis.

The function of Eriocheir sinensis transferrin and iron in Spiroplasma eriocheiris infection

Transferrin, a member of the iron binding superfamily protein, plays an extremely important role in the transport of iron in the biological process of cells. The result of preliminary proteomic study on E. sinensis hemocytes infected Spiroplasma eriocheiris showed the expression of transferrin (EsTF) and ferrin (EsFe) significantly changed. In addition, other reports have confirmed that transferrin, ferritin and iron are involved in the immune response of hosts. In order to validate the immune function of EsTF, the whole length of EsTF was successfully amplified by the gene cloning and RACE technique. The results showed that the full-length cDNA of the EsTF gene was 2748 bp, including a 2193 bp open reading frame which encodes 730 amino acids. The result of bioinformatics analysis showed EsTF contains two highly conserved TR_FER domains. Evolutionary analysis showed that EsTF has a close genetic relationship with other TFs of invertebrates. In addition, EsTF mRNA was highly transcripted in nerve and intestine tissues, followed by hemocytes. The expression of EsTF, EsFe1 and EsFe2 increased after exogenous supplemental of iron under the concentration of 100 nmol/L in water. After exogenous supplement of iron and injection with S. eriocheiris, these three gene transcription of mRNA levels were higher than that of PBS group, while lower than the S. eriocheiris group and the iron group. Besides, the copy number of S. eriocheiris in the experimental group was significantly reduced, and the death rate decreased. As can be seen, iron made transferrin and ferritin return to normal levels during the infection of S. eriocheiris and help the host maintain normal immunity levels to resist S. eriocheiris. These results further demonstrated that EsTF, EsFe1, EsFe2 and iron play a role in the immune defense mechanism of the crabs to resist S. eriocheiris infection.

iTRAQ-based quantitative proteomic analysis of Procambarus clakii hemocytes during Spiroplasma eriocheiris infection

As a new-found aquaculture pathogen, Spiroplasma eriocheiris, has resulted in inconceivable economic losses in aquaculture. In the infection of S. eriocheiris, the Procambarus clakii hemocytes have indicated to be major target cells. What was designed to examine in our study is the hemocytes' immune response at the protein levels. Before the pathogen was injected and after 192 h of post-injection, the differential proteomes of the crayfish hemocytes were analyzed immediately by isobaric tags for relative and absolute quantization (iTRAQ) labeling, followed by liquid chromatogramphytandem mass spectrometry (LC-MS/MS). This research had identified a total of 285 differentially expressed proteins. Eighty-three and 202 proteins were up-regulated and down-regulated, respectively, caused by the S. eriocheiris infection. Up-regulated proteins included alpha-2-macroglobulin (α2M), vitellogenin, ferritin, etc. Down-regulated proteins, involved with serine protease, peroxiredoxin 6, 14-3-3-like protein, C-type lectin, cdc42 homolog precursor, etc. The prophenoloxidase-activating system, antimicrobial action involved in the immune responses of P. clarkii is considered to be damaged due to S. eriocheiris infection. The present work could lay the foundation for future research on the proteins related to the susceptibility/resistance of P. clarkii to S. eriocheiris. In addition, it is helpful for our understanding molecular mechanism of disease processes in crayfishes.

Viscotaxis: Microswimmer Navigation in Viscosity Gradients

The survival of many microorganisms, like Leptospira or Spiroplasma bacteria, can depend on their ability to navigate towards regions of favorable viscosity. While this ability, called viscotaxis, has been observed in several bacterial experiments, the underlying mechanism remains unclear. We provide a framework to study viscotaxis of biological or synthetic self-propelled swimmers in slowly varying viscosity fields and show that suitable body shapes create viscotaxis based on a systematic asymmetry of viscous forces acting on a microswimmer. Our results shed new light on viscotaxis in Spiroplasma and Leptospira and suggest that dynamic body shape changes exhibited by both types of microorganisms may have an unrecognized functionality: to prevent them from drifting to low viscosity regions where they swim poorly. The present theory classifies microswimmers regarding their ability to show viscotaxis and can be used to design synthetic viscotactic swimmers, e.g., for delivering drugs to a target region distinguished by viscosity.

The function of serpin-2 from Eriocheir sinensis in Spiroplasma eriocheiris infection

Serpin families classified serine protease inhibitors regulate various physiological processes. However, there is not study on the role of serpin in immune responses against Spiroplasma eriocheiris as a novel causative pathogen in the Chinese mitten crab, Eriocheir sinensis. In our study, quantitative real-time PCR (qRT-PCR) revealed that the mRNA transcripts of Esserpin-2 were ubiquitous in every tissue, relative higher expression in hepatopancreas, gill and hemocytes, while the intestine, muscle, heart and nerve showed relative lower expression. Followed by infection with S. eriocheiris, the transcripts of Esserpin-2 were significantly down-regulated from 1 d to 7 d. After double-stranded RNA injection, the transcripts of Esserpin-2 dramatically declined from 48 h to 96 h. The transcripts of proPO were found to be obviously increased after Esserpin-2 silenced, meanwhile, LGBP with no significant difference. The copy number of S. eriocheiris and subsequently the mortality of crabs in a silencing Esserpin-2 group were significantly less than control groups during infection. The subcellular localization experiment suggested that recombinant Esserpin-2 was mainly located in the cytoplasm. Finally, over-expression assay in Drosophila S2 cells indicated that Esserpin-2 could increase copies of S. eriocheiris and result in cell death. These findings demonstrated that Esserpin-2 involved in the innate immune mechanism of E. sinensis in response to S. eriocheiris infection.

Introduction of a putative biocontrol agent into a range of phytoplasma- and liberibacter-susceptible crop plants

BACKGROUND

Phytoplasma, the causative agent of Bois Noir disease of grapevines, are vectored by the planthopper Hyalesthes obsoletus (Hemiptera: Cixiidae). A Dyella-like bacterium (DLB) isolated from H. obsoletus inhibits the growth of Spiroplasma melliferum, a cultivable relative of phytoplasma. Additional evidence suggests that DLB can reduce the symptoms of yellows disease in grapevine plantlets. The aim of this study was to test whether DLB could colonize a range of phytoplasma- and liberibacter-sensitive crop plants, and thus assess its potential agricultural use.

RESULTS

Vitex agnus-castus, the preferred host plant of H. obsoletus was found to be a natural host of DLB, which was successfully introduced into a range of crop plants belonging to seven families. The most effective DLB application method was foliar spraying. Microscopy observation revealed that DLB aggregated on the leaf surface and around the stomata, suggesting that this is its route of entry. DLB was also present in the vascular tissues of plants, indicating that it moved systemically through the plant.

CONCLUSIONS

DLB is a potential biocontrol agent and its broad spectrum of host plants indicates the possibility of its future use against a range of diseases caused by phloem-limited bacteria. © 2017 Society of Chemical Industry.

Incidence of Facultative Bacterial Endosymbionts in Spider Mites Associated with Local Environments and Host Plants

Spider mites are frequently associated with multiple endosymbionts whose infection patterns often exhibit spatial and temporal variation. However, the association between endosymbiont prevalence and environmental factors remains unclear. Here, we surveyed endosymbionts in natural populations of the spider mite, Tetranychus truncatus, in China, screening 935 spider mites from 21 localities and 12 host plant species. Three facultative endosymbiont lineages, Wolbachia, Cardinium, and Spiroplasma, were detected at different infection frequencies (52.5%, 26.3%, and 8.6%, respectively). Multiple endosymbiont infections were observed in most local populations, and the incidence of individuals with the Wolbachia-Spiroplasma coinfection was higher than expected from the frequency of each infection within a population. Endosymbiont infection frequencies exhibited associations with environmental factors: Wolbachia infection rates increased at localities with higher annual mean temperatures, while Cardinium and Spiroplasma infection rates increased at localities from higher altitudes. Wolbachia was more common in mites from Lycopersicon esculentum and Glycine max compared to those from Zea mays This study highlights that host-endosymbiont interactions may be associated with environmental factors, including climate and other geographically linked factors, as well as the host's food plant.IMPORTANCE The aim of this study was to examine the incidence of endosymbiont distribution and the infection patterns in spider mites. The main findings are that multiple endosymbiont infections were more common than expected and that endosymbiont infection frequencies were associated with environmental factors. This work highlights that host-endosymbiont interactions need to be studied within an environmental and geographic context.

The Enigmatic Genome of an Obligate Ancient Spiroplasma Symbiont in a Hadal Holothurian

Protective symbiosis has been reported in many organisms, but the molecular mechanisms of the mutualistic interactions between the symbionts and their hosts are unclear. Here, we sequenced the 424-kbp genome of "Candidatus Spiroplasma holothuricola," which dominated the hindgut microbiome of a sea cucumber, a major scavenger captured in the Mariana Trench (6,140 m depth). Phylogenetic relationships indicated that the dominant bacterium in the hindgut was derived from a basal group of Spiroplasma species. In this organism, the genes responsible for the biosynthesis of amino acids, glycolysis, and sugar transporters were lost, strongly suggesting endosymbiosis. The highly decayed genome consists of two chromosomes and harbors genes coding for proteolysis, microbial toxin, restriction-methylation systems, and clustered regularly interspaced short palindromic repeats (CRISPRs), composed of three cas genes and 76 CRISPR spacers. The holothurian host is probably protected against invading viruses from sediments by the CRISPRs/Cas and restriction systems of the endosymbiotic spiroplasma. The protective endosymbiosis indicates the important ecological role of the ancient Spiroplasma symbiont in the maintenance of hadal ecosystems.IMPORTANCE Sea cucumbers are major inhabitants in hadal trenches. They collect microbes in surface sediment and remain tolerant against potential pathogenic bacteria and viruses. This study presents the genome of endosymbiotic spiroplasmas in the gut of a sea cucumber captured in the Mariana Trench. The extreme reduction of the genome and loss of essential metabolic pathways strongly support its endosymbiotic lifestyle. Moreover, a considerable part of the genome was occupied by a CRISPR/Cas system to provide immunity against viruses and antimicrobial toxin-encoding genes for the degradation of microbes. This novel species of Spiroplasma is probably an important protective symbiont for the sea cucumbers in the hadal zone.

Identification and function analysis of ras-related nuclear protein from Macrobrachium rosenbergii involved in Spiroplasma eriocheiris infection

A ras-related nuclear protein (Ran) protein was obtained from Macrobrachium rosenbergii, named MrRan. Phylogenetic analysis results showed that MrRan was clustered in one group together with other crustaceans. Tissue distribution analysis revealed that MrRan was expressed mainly in gill, intestine and stomach, and expressed weakly in muscle. The MrRan expression levels in gill and hemocyte of prawns were significantly up-regulated after challenged by Spiroplasma eriocheiris. The copy number of S. eriocheiris in MrRan dsRNA injection group was significantly less than control groups during infection. Meanwhile, silencing MrRan obviously increased the survival rate of prawns. The subcellular localization experiment suggested that recombinant MrRan was mainly located in the nucleus, and relatively weak in the cytoplasm. Finally, over-expression in Drosophila S2 cell indicated that MrRan could increase copies of S. eriocheiris and decrease of cell viability. The present study suggested that MrRan participated in regulating the phagocytosis of S. eriocheiris in M. rosenbergii.

Integrated analysis of mRNA-seq in the haemocytes of Eriocheir sinensis in response to Spiroplasma eriocheiris infection

The Chinese mitten crab Eriocheir sinensis is an important economic crustacean that has been exposed to various diseases. Spiroplasma eriocheiris, isolated from tremor-diseased E. sinensis, was first identified as a lethal pathogen of freshwater crustaceans. To understand the pathogenesis of S. eriocheiris to E. sinensis, the transcriptomic profiles of haemocytes in the experimental and control groups at 1 d and 7 d post-injection were obtained using Illumina HiSeq 2500. These results showed that 40,358,724, 44,462,112, 45,516,576 and 37,713,728 paired-end clean reads were obtained from the cDNA libraries of DZ1 (the control group at 1 d), DZ7 (the control group at 7 d), SY1 (the experimental group at 1 d) and SY7 (the experimental group at 7 d), respectively. In total, 106,641 unique transcript fragments (unigenes) were assembled, with an average length of 710 bp. On the first day of stimulation, 33,084 up-regulated transcripts and 19,208 down-regulated transcripts were found in the experimental group compared with those in the control group. On the seventh day of stimulation, 40,198 up-regulated transcripts and 12,032 down-regulated transcripts were found in the experimental group compared with those in the control group. Some canonical immune-related pathways were identified via KEGG pathway analysis, including complement and coagulation cascades, the VEGF signalling pathway, the Wnt signalling pathway, natural killer cell-mediated cytotoxicity, the MAPK signalling pathway, neuroactive ligand-receptor interactions, and the Lysosome pathway. We found important immune-related genes (GNPTAB, MASP2, F7, F5, NFATC, TRAF6, MAP3K5, and TRa) in the KEGG pathway, and those genes were confirmed by qRT-PCR analysis. In addition, the significantly enriched neuroactive ligand-receptor interaction pathway was associated with intense paroxysmal tremors of infected crabs. Our results provide valuable information for the further analysis of the mechanisms of E. sinensis defence against S. eriocheiris invasion.

Interaction of heat shock protein 60 (HSP60) with microRNA in Chinese mitten crab during Spiroplasma eriocheiris infection

Heat shock protein 60 from the Chinese mitten crab Eriocheir sinensis (EsHSP60) was previously identified in relation to Spiroplasma eriocheiris infection by isobaric tags for relative and absolute quantitation labelling followed by liquid chromatography-tandem mass spectrometry. In the present study, to validate the immune function of this protein, the cDNA of the EsHSP60 gene was cloned. Various crab tissues were assessed using real-time PCR, which showed that EsHSP60 transcription occurred in all tissues examined. The expression profiles of EsHSP60 in haemolymph at transcription and protein levels when infected with S. eriocheiris were investigated by real-time PCR and Western blot analysis, respectively. A significant increase of EsHSP60 transcription and protein expression appeared post-injection in response to S. eriocheiris infection when compared to the control group. The double-luciferase reporter gene assay showed that the microRNA PC-533-3p interacted with the 3'-untranslated region of EsHSP60 and inhibited the translation of EsHSP60. The expression profiles of PC-533-3p during S. eriocheiris infection were also investigated by real-time PCR. However, the change tendency of PC-533-3p was opposite to that of the EsHSP60 after S. eriocheiris challenge. These data indicate that the EsHSP60 proteins may play an important role in mediating the immune responses of E. sinensis to an S. eriocheiris challenge.

Are sex ratio distorting endosymbionts responsible for mating system variation among dance flies (Diptera: Empidinae)?

Maternally inherited bacterial endosymbionts are common in many arthropod species. Some endosymbionts cause female-biased sex ratio distortion in their hosts that can result in profound changes to a host’s mating behaviour and reproductive biology. Dance flies (Diptera: Empidinae) are well known for their unusual reproductive biology, including species with female-specific ornamentation and female-biased lek-like swarming behaviour. The cause of the repeated evolution of female ornaments in these flies remains unknown, but is probably associated with female-biased sex ratios in individual species. In this study we assessed whether dance flies harbour sex ratio distorting endosymbionts that might have driven these mating system evolutionary changes. We measured the incidence and prevalence of infection by three endosymbionts that are known to cause female-biased sex ratios in other insect hosts (Wolbachia, Rickettsia and Spiroplasma) across 20 species of dance flies. We found evidence of widespread infection by all three symbionts and variation in sex-specific prevalence across the taxa sampled. However, there was no relationship between infection prevalence and adult sex ratio measures and no evidence that female ornaments are associated with high prevalences of sex-biased symbiont infections. We conclude that the current distribution of endosymbiont infections is unlikely to explain the diversity in mating systems among dance fly species.

Heat Stress Affects Facultative Symbiont-Mediated Protection from a Parasitoid Wasp

Many insects carry facultative bacterial symbionts, which provide benefits including resistance to natural enemies and abiotic stresses. Little is known about how these beneficial phenotypes are affected when biotic or abiotic threats occur simultaneously. The pea aphid (Acyrthosiphon pisum) can host several well-characterized symbiont species. The symbiont known as X-type can protect against both parasitoid wasps and heat stress. Here, we used three pea aphid genotypes that were naturally infected with X-type and the symbiont Spiroplasma sp. We compared aphids coinfected with these two symbionts with those cured from X-type and infected with only Spiroplasma to investigate the ability of X-type to confer benefits to the host when two threats are experienced simultaneously. Our aim is to explore how robust symbiont protection may be outside a benign laboratory environment. Aphids were subjected to heat shock either before or after attack by parasitoid wasps. Under a benign temperature regime, the aphids carrying X-type tended to be better protected from the parasitoid than those cured. When the aphids experienced a heat shock before being parasitized aphids carrying X-type were more susceptible than those cured. Regardless of infection with the symbiont, the aphids benefitted from being heat shocked after parasitization. The results demonstrate how resistance to parasitoid wasps can be strongly environment-dependent and that a beneficial phenotype conferred by a symbiont under controlled conditions in the laboratory does not necessarily equate to a consistently useful effect in natural populations.

A Nightmare for Males? A Maternally Transmitted Male-Killing Bacterium and Strong Female Bias in a Green Lacewing Population

For maternally transmitted microbes, a female-biased host sex ratio is of reproductive advantage. Here we found a strong female bias in a field population of the green lacewing, Mallada desjardinsi (Insecta; Neuroptera). This bias was attributed to the predominance of individuals harboring a maternally inherited male-killing bacterium that was phylogenetically closely related to the plant-pathogenic Spiroplasma phoeniceum and Spiroplasma kunkelii. Among 35 laboratory-reared broods produced by wild-caught females, 21 broods (60%)—all infected with Spiroplasma—consisted of only females (940 individuals). Among 14 broods consisting of both males and females (516 and 635 individuals, respectively), 4 broods were doubly infected with Spiroplasma and Rickettsia, 6 broods were singly infected with Rickettsia, and 3 broods were uninfected (remaining one brood was unknown). Mortality during embryonic and larval development was prominent in all-female broods but not in normal sex ratio broods. Following antibiotic treatment on all-female broods, mortality was significantly reduced and the sex ratio was restored to 1:1. Strong expression and high prevalence of this male-killer is remarkable considering its low density (~10⁻⁵–10⁻⁴ cells per host mitochondrial gene copy based on quantitative PCR). In addition, a bacterium closely related to Rickettsia bellii was present in 25 of 34 broods (73.5%), irrespective of the sex ratio, with the infection density comparable to other cases of endosymbiosis (~10⁻²–10⁻¹ cells per mitochondrial gene copy). Higher density of Rickettsia than Spiroplasma was also demonstrated by electron microscopy which visualized both Spiroplasma-like cells and Rickettsia-like cells inside and outside the ovarian cells.

Spiroplasma - an emerging arthropod-borne pathogen?

Spiroplasma is a genus of wall-less, low-GC, small Gram-positive bacteria of the internal contractile cytoskeleton, with helical morphology and motility. The genus is classified within the class Mollicutes. Spiroplasma / host interactions can be classified as commensal, pathogenic or mutualist. The majority of spiroplasmas are found to be commensals of insects, arachnids, crustaceans or plants, whereas a small number of species are pathogens of plants, insects, and crustaceans. Insects are particularly rich sources of spiroplasmas. The bacteria are common in haematophagous arthropods: deerflies, horseflies, mosquitoes, and in ticks, where they may occur abundantly in salivary glands. The ability of spiroplasmas to propagate in rodents was experimentally proven, and Spiroplasma infections have been reported recently in humans. Some authors have purported an etiological role of Spiroplasma in causing transmissible spongiform encephalopathies (TSEs), but convincing proof is lacking. The possibility for humans and other vertebrates to be infected with Spiroplasma spp. in natural conditions is largely unknown, as well as the possibility of the transmission of these bacteria by ticks and haematophagous insects. Nevertheless, in the light of new data, such possibilities cannot be excluded.

iTRAQ-based proteomic study of the effects of Spiroplasma eriocheiris on Chinese mitten crab Eriocheir sinensis hemocytes

Spiroplasma eriocheiris is as a novel pathogen of Chinese mitten crab Eriocheir sinensis tremor disease. The hemocytes have been shown to be major target cells in S. eriocheiris infection. The aim of this study was to examine the hemocytes' immune response at the protein levels. The differential proteomes of the crab hemocytes were analyzed immediately prior to injection with the pathogen, and at 10 d post-injection by isobaric tags for relative and absolute quantization (iTRAQ) labeling, followed by liquid chromatography-tandem mass spectrometry (LC-MS/MS). A total of 1075 proteins were identified by LC-MS/MS and de novo sequencing data. Using a 1.2-fold change in expression as a physiologically significant benchmark, 76 differentially expressed proteins (7.07%) were reliably quantified by iTRAQ analysis. Thirty-five (3.26%) proteins were up-regulated and 41 (3.81%) proteins were down-regulated resulting from a S. eriocheiris infection. Approximately 20 differential proteins in hemocytes were involved in the stress and immune responses. Up-regulated proteins included alpha-2-macroglobulin (α2M), prostaglandin D synthase (GST), ferritin, and heat shock protein 60. Down-regulated proteins included two lectins (mannose-binding protein and hemocytin), three kinds of serine proteinase inhibitors (two serpins and pacifastin), three different kinds of serine proteases, mitogen-activated protein kinase kinase (MAPKK), and two thioredoxins (Trx), crustin, etc. Selected bioactive factors (α2M, GST, ferritin, tubulin, crustin, thioredoxin, clip domain serine protease and serpin) are verified by their immune roles in the S. eriocheiris infection using Real-time PCR. The variation trend of immune gene's mRNA expression is similar with the result of iTRAQ, except the tubulin. The prophenoloxidase-activating system, antimicrobial action and antioxidant system involved in the immune responses of E. sinensis is believed to be a resistance to S. eriocheiris infection. This is the first report of the proteome response of crab hemocytes against S. eriocheiris infection. These findings contribute to our understanding of tremor disease processes in crabs, and provide the first evidence to promote a search for potential biomarkers of the disease.

A structural framework for a near-minimal form of life: mass and compositional analysis of the helical mollicute Spiroplasma melliferum BC3

Spiroplasma melliferum is a wall-less bacterium with dynamic helical geometry. This organism is geometrically well defined and internally well ordered, and has an exceedingly small genome. Individual cells are chemotactic, polar, and swim actively. Their dynamic helicity can be traced at the molecular level to a highly ordered linear motor (composed essentially of the proteins fib and MreB) that is positioned on a defined helical line along the internal face of the cell's membrane. Using an array of complementary, informationally overlapping approaches, we have taken advantage of this uniquely simple, near-minimal life-form and its helical geometry to analyze the copy numbers of Spiroplasma's essential parts, as well as to elucidate how these components are spatially organized to subserve the whole living cell. Scanning transmission electron microscopy (STEM) was used to measure the mass-per-length and mass-per-area of whole cells, membrane fractions, intact cytoskeletons and cytoskeletal components. These local data were fit into whole-cell geometric parameters determined by a variety of light microscopy modalities. Hydrodynamic data obtained by analytical ultracentrifugation allowed computation of the hydration state of whole living cells, for which the relative amounts of protein, lipid, carbohydrate, DNA, and RNA were also estimated analytically. Finally, ribosome and RNA content, genome size and gene expression were also estimated (using stereology, spectroscopy and 2D-gel analysis, respectively). Taken together, the results provide a general framework for a minimal inventory and arrangement of the major cellular components needed to support life.

Male killing Spiroplasma preferentially disrupts neural development in the Drosophila melanogaster embryo

Male killing bacteria such as Spiroplasma are widespread pathogens of numerous arthropods including Drosophila melanogaster. These maternally transmitted bacteria can bias host sex ratios toward the female sex in order to ‘selfishly’ enhance bacterial transmission. However, little is known about the specific means by which these pathogens disrupt host development in order to kill males. Here we show that a male-killing Spiroplasma strain severely disrupts nervous tissue development in male but not female D. melanogaster embryos. The neuroblasts, or neuron progenitors, form properly and their daughter cells differentiate into neurons of the ventral nerve chord. However, the neurons fail to pack together properly and they produce highly abnormal axons. In contrast, non-neural tissue, such as mesoderm, and body segmentation appear normal during this time, although the entire male embryo becomes highly abnormal during later stages. Finally, we found that Spiroplasma is altogether absent from the neural tissue but localizes within the gut and the epithelium immediately surrounding the neural tissue, suggesting that the bacterium secretes a toxin that affects neural tissue development across tissue boundaries. Together these findings demonstrate the unique ability of this insect pathogen to preferentially affect development of a specific embryonic tissue to induce male killing.

RNA-Seq analysis reveals candidate genes for ontogenic resistance in Malus-Venturia pathosystem

Ontogenic scab resistance in apple leaves and fruits is a horizontal resistance against the plant pathogen Venturia inaequalis and is expressed as a decrease in disease symptoms and incidence with the ageing of the leaves. Several studies at the biochemical level tried to unveil the nature of this resistance; however, no conclusive results were reported. We decided therefore to investigate the genetic origin of this phenomenon by performing a full quantitative transcriptome sequencing and comparison of young (susceptible) and old (ontogenic resistant) leaves, infected or not with the pathogen. Two time points at 72 and 96 hours post-inoculation were chosen for RNA sampling and sequencing. Comparison between the different conditions (young and old leaves, inoculated or not) should allow the identification of differentially expressed genes which may represent different induced plant defence reactions leading to ontogenic resistance or may be the cause of a constitutive (uninoculated with the pathogen) shift toward resistance in old leaves. Differentially expressed genes were then characterised for their function by homology to A. thaliana and other plant genes, particularly looking for genes involved in pathways already suspected of appertaining to ontogenic resistance in apple or other hosts, or to plant defence mechanisms in general. IN THIS WORK, FIVE CANDIDATE GENES PUTATIVELY INVOLVED IN THE ONTOGENIC RESISTANCE OF APPLE WERE IDENTIFIED: a gene encoding an "enhanced disease susceptibility 1 protein" was found to be down-regulated in both uninoculated and inoculated old leaves at 96 hpi, while the other four genes encoding proteins (metallothionein3-like protein, lipoxygenase, lipid transfer protein, and a peroxidase 3) were found to be constitutively up-regulated in inoculated and uninoculated old leaves. The modulation of the five candidate genes has been validated using the real-time quantitative PCR. Thus, ontogenic resistance may be the result of the corresponding up- and down-regulation of these genes.

Diversity of lectins in Macrobrachium rosenbergii and their expression patterns under spiroplasma MR-1008 stimulation

Lectins play important roles in crustacean innate immunity through recognition of foreign pathogens. In this study, 20 lectins including C-type lectins [dual-carbohydrate recognition domain (CRD) type and single-CRD type], L-type lectin, and lectin with low-density lipoprotein class A (LDLa) domain were identified from the freshwater prawn Macrobrachium rosenbergii. The tissue distribution and expression patterns of these lectins under spiroplasma strain MR-1008 challenge were investigated. Most of the lectins were found to be mainly distributed in the hepatopancreas. Lectin5, Lectin14, Lectin17, and Lectin18 exhibited the highest expression level in the hemocytes, nerve, intestine, and heart, respectively. MrLec1 to MrLec6 (dual-CRD lectins) in the hepatopancreas were up-regulated by spiroplasma challenge. Single-CRD lectins reached the highest level at 72 h after spiroplasma challenge. Lectin9 and Lectin15 both belong to L-type lectins. At post-spiroplasma challenge, Lectin9 expression was up-regulated, whereas Lectin15 expression was down-regulated. Lectin11 with LDLa domain showed the highest level after 12 h Lectin18 and Lectin20, namely, CD209, were also up-regulated by spiroplasma challenge. Lectin14, a C-type lectin, quickly reached the highest level after 2 h Lectin16 showed the highest level after 72 h Lectin5 reached the highest level in cultured hemocytes after 6 h Lectin17 in the intestine and Lectin14 in the nerve were slightly up-regulated after 6 and 2 h, respectively. Our research results indicate that lectins may play important roles in early or late immune responses against spiroplasma challenge.

Transcriptome-wide identification and characterization of the Procambarus clarkii microRNAs potentially related to immunity against Spiroplasma eriocheiris infection

We used the Illumina/Solexa deep sequencing technology to sequence two small RNA libraries prepared from hemocytes of Procambarus clarkii under normal and infection conditions. The high-throughput sequencing approach resulted in approximately 12,801,827 and 8,410,455 raw reads corresponding to 10,949,754 and 6,648,161 high-quality mappable reads for the normal and infected hemocyte samples, respectively. Bioinformatic analyses identified 195 unique miRNAs, including 30 that are conserved in crustaceans, 48 that are novel to crayfish but are present in other arthropods (PN-type), and 117 that are completely new (PC-type). Thirty-three miRNAs displayed significant differential expressions between the two hemocyte samples (p < 0.0001). Of these, 15 (45.5%) were significantly up-regulated and 18 (54.5%) were significantly down-regulated upon challenge with Spiroplasma eriocheiris. Integrating comparative genomic and bibliomic analysis, of the 33 significant miRNAs identified, 19 were conserved and immune-related in P. clarkii and Eriocheir sinensis infected with S. eriocheiris infection; 24 were conserved and immune-related in P. clarkii and Marsupenaeus japonicus immune response to S. eriocheiris or white spot syndrome virus (WSSV) infection. Function annotation of target genes revealed a broad range of biological processes and signal transduction pathways that regulated by crayfish miRNAs. Thereinto, pcl-miR-34, pcl-miR-7, PN-pcl-let-7, pcl-miR-1, and pcl-miR-2b are highly conserved in vertebrates and invertebrates and function in the similar pathways. To our knowledge, this is the first report of comprehensive identification of P. clarkii miRNAs and of expression analysis of P. clarkii miRNAs after exposure to S. eriocheiris in crayfish, and many miRNAs were differentially regulated under normal and infection conditions. Our results should help develop new control strategies for efficient immune protection against S. eriocheiris infections in crustaceans.

Presence of Dalbulus maidis (Hemiptera: Cicadellidae) and of Spiroplasma kunkelii in the temperate region of Argentina

"Corn stunt" is one of the main corn (Zea mays L.) diseases in the Americas and Dalbulus maidis (DeLong & Wolcott) is the key vector of the pathogen Spiroplasma kunkelii Whitcomb. In Argentina, the corn-producing area is in the temperate region, where vector and pathogen prevalence levels are unknown. In this study, the prevalence and distribution of D. maidis and S. kunkelii in the temperate region of Argentina and D. maidis overwintering ability in this region were determined. Surveys were conducted in 2005-2006 and 2006-2007 seasons to determine D. maidis and S. kunkelii presence, and in winter 2006 to determine the vector overwintering ability. The highest S. kunkelii prevalence and incidence levels were found in the transition area from the temperate to the subtropical region, related to the highest D. maidis prevalence and insects sampled per location. D. maidis adults were found in volunteer corn plants and spontaneous vegetation in autumn and winter months, which were inoculative for the pathogen S. kunkelii. This overwintering ability was related to detection of D. maidis insects in corn crops at early growth stages in the following growing season. This work emphasizes that corn stunt disease is present in the temperate region of Argentina, and this highlights the need to develop proper agronomic practices like monitoring insect vector populations and controlling voluntary plants. This study also indicates that further research is needed to understand the potential yield reduction caused by this pathogen on symptomless plants and population dynamics of the insect vector.

Spiroplasma in Drosophila melanogaster populations: prevalence, male-killing, molecular identification, and no association with Wolbachia

Spiroplasma endosymbionts are maternally transmitted bacteria that may kill infected sons resulting in the production of female-biased broods. The prevalence of male killers varies considerably both between and within species. Here, we evaluate the spatial and temporal status of male-killing and non-male-killing Spiroplasma infection in three Brazilian populations of Drosophila melanogaster, nearly a decade after the first occurrence report for this species. The incidence of the male-killing Spiroplasma ranged from close to 0 to 17.7 % (so far the highest estimate for a Drosophila species) with a suggestion of temporal decline in a population. We also found non-male-killing Spiroplasma coexisting in one population at lower prevalence (3-5 %), and we did not detect it in the other two. This may be taken as a suggestion of a spreading advantage conferred by the male-killing strategy. Sequencing two loci, we identified the phylogenetic position of Spiroplasma strains from the three localities, showing that all strains group closely in the poulsonii clade. Due to intensive sampling effort, we were able to test the association between Spiroplasma infections and another widespread endosymbiont, Wolbachia, whose prevalence ranged from 81.8 to 100 %. The prevalence of Wolbachia did not differ between Spiroplasma-infected and uninfected strains in our largest sample nor were the prevalences of the two endosymbionts associated across localities.

Identification and comparative analysis of the Eriocheir sinensis microRNA transcriptome response to Spiroplasma eriocheiris infection using a deep sequencing approach

The Chinese mitten crab Eriocheir sinensis is one of the most important freshwater aquaculture crustacean species in China. MicroRNAs (miRNAs) are small non-coding RNAs that are important effectors in the intricate host-pathogen interaction network. To increase the repertoire of miRNAs characterized in crustaceans and to examine the relationship between host miRNA expression and pathogen infection, we used the Illumina/Solexa deep sequencing technology to sequence two small RNA libraries prepared from haemocytes of E. sinensis under normal conditions and during infection with Spiroplasma eriocheiris. The high-throughput sequencing resulted in approximately 30,975,151 and 30,826,277 raw reads corresponding to 12,077,088 and 16,271,545 high-quality mappable reads for the normal and infected haemocyte samples, respectively. Bioinformatic analyses identified 735 unique miRNAs, including 36 that are conserved in crustaceans, 134 that are novel to crabs but are present in other arthropods (PN-type), and 565 that are completely new (PC-type). Two hundred twenty-eight unique miRNAs displayed significant differential expression between the normal and infected haemocyte samples (p < 0.0001). Of these, 133 (58%) were significantly up-regulated and 95 (42%) were significantly down-regulated upon challenge with S. eriocheiris. Real-time quantitative PCR (RT-qPCR) experiments were preformed for 10 miRNAs of the two samples, and agreement was found between the sequencing and RT-qPCR data. To our knowledge, this is the first report of comprehensive identification of E. sinensis miRNAs and of expression analysis of E. sinensis miRNAs after exposure to S. eriocheiris. Many miRNAs were differentially regulated when exposed to the pathogen, and these findings support the hypothesis that certain miRNAs might be essential in host-pathogen interactions. Our results suggest that elucidation of the molecular mechanisms responsible for miRNA regulation of the host's innate immune system should help with the development of new control strategies to prevent or treat S. eriocheiris infections in crustaceans.

Exitianus obscurinervis (Hemiptera: Cicadellidae), a new experimental vector of Spiroplasma kunkelii

"Corn stunt" caused by the mollicute Spiroplasma kunkelii (Whitcomb) is potentially one of the most severe diseases affecting the corn (Zea mays L.) crop in the Americas, and the leafhopper Dalbulus maidis (DeLong & Wolcott) is considered its most important vector. However, other insects seen quite frequently in corn crops might well be its vectors in Argentina To identify any leafhoppers species other than D. maidis that can transmit S. kunkelii, transmission assays were conducted, using individuals of Exitianus obscurinervis (Stål) collected in field and reared under controlled conditions. S. kunkelii was transmitted to corn plants by E. obscurinervis. The pathogen was transmitted to seven of the 11 plants, which showed characteristic corn stunt symptoms, and the presence of the pathogen was confirmed by DAS-ELISA. The presence of S. kunkelii in the E. obscurinervis individuals used in transmission experiments was confirmed by polymerase chain reaction and electron microscopy. The current study shows the existence of a new experimental vector of S. kunkelii, the leafhopper E. obscurinervis, which acquired spiroplasmas from infected plants and inoculated it to healthy plants.

Facultative symbiont infections affect aphid reproduction

Some bacterial symbionts alter their hosts reproduction through various mechanisms that enhance their transmission in the host population. In addition to its obligatory symbiont Buchnera aphidicola, the pea aphid Acyrthosiphon pisum harbors several facultative symbionts influencing several aspects of host ecology. Aphids reproduce by cyclical parthenogenesis whereby clonal and sexual reproduction alternate within the annual life cycle. Many species, including the pea aphid, also show variation in their reproductive mode at the population level, with some lineages reproducing by cyclical parthenogenesis and others by permanent parthenogenesis. While the role of facultative symbionts has been well studied during the parthenogenetic phase of their aphid hosts, very little is known on their possible influence during the sexual phase. Here we investigated whether facultative symbionts modulate the capacity to produce sexual forms in various genetic backgrounds of the pea aphid with controlled symbiont composition and also in different aphid genotypes from natural populations with previously characterized infection status and reproductive mode. We found that most facultative symbionts exhibited detrimental effects on their hosts fitness under sex-inducing conditions in comparison with the reference lines. We also showed that the loss of sexual phase in permanently parthenogenetic lineages of A. pisum was not explained by facultative symbionts. Finally, we demonstrated that Spiroplasma infection annihilated the production of males in the host progeny by inducing a male-killing phenotype, an unexpected result for organisms such as aphids that reproduce primarily through clonal reproduction.

Spiroplasma infection in Drosophila melanogaster: what is the advantage of killing males?

Male-killing bacteria are maternally inherited agents that cause death of sons of infected females. Their transmission rate is commonly high but imperfect and also sensitive to different environmental factors. Therefore, the proportion of infected females should be reduced in each generation. In order to explain male-killers spread and persistence in host population, a mechanism resulting in the relative increase of infected females must outweigh the losses caused by the imperfect transmission. The resource release hypothesis states that the males' death results in increased resources available to sibling females which would otherwise be used by their male siblings. Infected females are then expected: to be larger than uninfected females in natural populations; or to have higher viability; or to have shorter development times; or any combination of these outcomes. Here, we tested the resource release hypothesis by measuring body size of infected and uninfected wild-caught Drosophila melanogaster females and carried out other fitness related measures in the laboratory. Wild-caught infected females produced more daughters than uninfected females in their first days in the laboratory. However, although no significant difference in viability was found in a controlled experiment with infected and uninfected flies from a standard laboratory strain, there was a decrease in development time probably mediated by reduced competition. Fitness effects conditioned by the host genetic background are pointed out as a possible explanation for this difference between wild and laboratory flies. Our findings are discussed in the context of the resource advantage hypothesis.

Hydrodynamics of helical-shaped bacterial motility

To reveal the underlying hydrodynamic mechanism for the directed propulsion of the bacterium Spiroplasma, we formulate a coarse-grained elastic polymer model with domains of alternating helicities along the contour. Using hydrodynamic simulations and analytic arguments, we show that the propagation of helical domain walls leads to the directed propulsion of the cell body opposite to the domain-wall traveling direction. Several key features of Spiroplasma motility are reproduced by our model. We in particular show that the helical pitch angle observed for Spiroplasma meliferum, psi=35 degrees , is optimized for maximal swimming speed and energy-conversion efficiency. Our analytic theory based on the slender-body hydrodynamic approximation agrees very well with our numerical data demonstrating how the chirality switch propagating along the helical cell body is converted to a translational thrust for the cell body itself. We in detail consider thermal effects on the propulsion efficiency in the form of orientational fluctuations and conformational fluctuations of the helix shape. The body length dependence of the cell motility is studied numerically and compared to our approximate analytic theory. For fixed pitch angle psi=35 degrees , the swimming speed is maximized at a ratio of cell-body length to domain length of about 2-3, which are typical values for real cells. We also propose simple analytic arguments for an enhancement of the swimming velocity with increasing solution viscosity by taking into account the effects of transient confinement of a helical cell body in a polymeric meshwork. Comparison with a generalized theory for the swimming speed of flagellated bacteria in polymeric meshworks shows that the presence of a finite-sized bacterial head gives rise to a maximal swimming speed at a finite solution viscosity, whereas in the absence of a head the swimming speed monotonically increases with increasing viscosity.

Differences in intracellular localization of corn stunt spiroplasmas in magnesium treated maize

Maize plants infected with Spiroplasma kunkelii show symptoms similar to that of plants in a magnesium-deficient soil, and it has been shown that the spiroplasma alters the plants' magnesium absorption. In the current study we compared changes associated to either spiroplasma infection, two soil magnesium levels and their combinations. Plant symptoms were recorded and correlated with transmission electron microscopy observations. Plants grown on a high magnesium treatment showed no macroscopical alterations nor organelle ultrastructural alterations, while plants on a low magnesium treatment showed macroscopical vein yellowing and, ultrastructurally, they had most chloroplasts and mitochondrial membranes altered. Infected plants on a low magnesium treatment had an ageing aspect, ultrastructurally showed chloroplasts and mitochondrial alterations similar to those non-infected and grown on a low magnesium treatment, and spiroplasma cells were found in phloem cells, but outside their cytoplasm. Infected plants on a high magnesium treatment showed similar symptoms and ultrastructural alterations as either non-infected plants on the low magnesium treatment or in infected plants on the low magnesium treatment, but differ from them in that the spiroplasma cells were located inside the cytoplasm. Results suggest that magnesium is involved in the plant-pathogen interaction.

Kinematics of the swimming of Spiroplasma

Spiroplasma swimming is studied with a simple model based on resistive-force theory. Specifically, we consider a bacterium shaped in the form of a helix that propagates traveling-wave distortions which flip the handedness of the helical cell body. We treat cell length, pitch angle, kink velocity, and distance between kinks as parameters and calculate the swimming velocity that arises due to the distortions. We find that, for a fixed pitch angle, scaling collapses the swimming velocity (and the swimming efficiency) to a universal curve that depends only on the ratio of the distance between kinks to the cell length. Simultaneously optimizing the swimming efficiency with respect to interkink length and pitch angle, we find that the optimal pitch angle is 35.5 degrees and the optimal interkink length ratio is 0.338, values in good agreement with experimental observations.

Variable incidence of Spiroplasma infections in natural populations of Drosophila species

Spiroplasma is widespread as a heritable bacterial symbiont in insects and some other invertebrates, in which it sometimes acts as a male-killer and causes female-biased sex ratios in hosts. Besides Wolbachia, it is the only heritable bacterium known from Drosophila, having been found in 16 of over 200 Drosophila species screened, based on samples of one or few individuals per species. To assess the extent to which Spiroplasma infection varies within and among species of Drosophila, intensive sampling consisting of 50–281 individuals per species was conducted for natural populations of 19 Drosophila species. Infection rates varied among species and among populations of the same species, and 12 of 19 species tested negative for all individuals. Spiroplasma infection never was fixed, and the highest infection rates were 60% in certain populations of D. hydei and 85% in certain populations of D. mojavensis. In infected species, infection rates were similar for males and females, indicating that these Spiroplasma infections do not confer a strong male-killing effect. These findings suggest that Spiroplasma has other effects on hosts that allow it to persist, and that environmental or host variation affects transmission or persistence leading to differences among populations in infection frequencies.

High and low temperatures differently affect infection density and vertical transmission of male-killing Spiroplasma symbionts in Drosophila hosts

We investigated the vertical transmission, reproductive phenotype, and infection density of a male-killing Spiroplasma symbiont in two Drosophila species under physiological high and low temperatures through successive host generations. In both the native host Drosophila nebulosa and the nonnative host Drosophila melanogaster, the symbiont infection and the male-killing phenotype were stably maintained at 25 degrees C, rapidly lost at 18 degrees C, and gradually lost at 28 degrees C. In the nonnative host, both the high and low temperatures significantly suppressed the infection density of the spiroplasma. In the native host, by contrast, the low temperature suppressed the infection density of the spiroplasma whereas the high temperature had little effect on the infection density. These results suggested that the low temperature suppresses both the infection density and the vertical transmission of the spiroplasma whereas the high temperature suppresses the vertical transmission preferentially. The spiroplasma density was consistently higher in the native host than in the nonnative host, suggesting that the host genotype may affect the infection density of the symbiont. The temperature- and genotype-dependent instability of the symbiont infection highlights a complex genotype-by-genotype-by-environment interaction and may be relevant to the low infection frequencies of the male-killing spiroplasmas in natural Drosophila populations.

Interspecific transmission of endosymbiotic Spiroplasma by mites

The occurrence of closely related strains of maternally transmitted endosymbionts in distantly related insect species indicates that these infections can colonize new host species by lateral transfer, although the mechanisms by which this occurs are unknown. We investigated whether ectoparasitic mites, which feed on insect haemolymph, can serve as interspecific vectors of Spiroplasma poulsonii, a male-killing endosymbiont of Drosophila. Using Spiroplasma-specific primers for PCR, we found that mites can pick up Spiroplasma from infected Drosophila nebulosa females and subsequently transfer the infection to Drosophila willistoni. Some of the progeny of the recipient D. willistoni were infected, indicating successful maternal transmission of the Spiroplasma within the new host species. However, the transmission rate of the infection from recipient flies to their offspring was low, perhaps due to low Spiroplasma density in the recipient flies.

Prevalence of a non-male-killing spiroplasma in natural populations of Drosophila hydei

Male-killing phenotypes are found in a variety of insects and are often associated with maternally inherited endosymbiotic bacteria. In several species of Drosophila, male-killing endosymbionts of the genus Spiroplasma have been found at low frequencies (0.1 to 3%). In this study, spiroplasma infection without causing male-killing was shown to be prevalent (23 to 66%) in Japanese populations of Drosophila hydei. Molecular phylogenetic analyses showed that D. hydei was infected with a single strain of spiroplasma, which was closely related to male-killing spiroplasmas from other Drosophila species. Artificial-transfer experiments suggested that the spiroplasma genotype rather than the host genotype was responsible for the absence of the male-killing phenotype. Infection densities of the spiroplasma in the natural host, D. hydei, and in the artificial host, Drosophila melanogaster, were significantly lower than those of the male-killing spiroplasma NSRO, which was in accordance with the hypothesis that a threshold infection density is needed for the spiroplasma-induced male-killing expression.

Heritable endosymbionts of Drosophila

Although heritable microorganisms are increasingly recognized as widespread in insects, no systematic screens for such symbionts have been conducted in Drosophila species (the primary insect genetic models for studies of evolution, development, and innate immunity). Previous efforts screened relatively few Drosophila lineages, mainly for Wolbachia. We conducted an extensive survey of potentially heritable endosymbionts from any bacterial lineage via PCR screens of mature ovaries in 181 recently collected fly strains representing 35 species from 11 species groups. Due to our fly sampling methods, however, we are likely to have missed fly strains infected with sex ratio-distorting endosymbionts. Only Wolbachia and Spiroplasma, both widespread in insects, were confirmed as symbionts. These findings indicate that in contrast to some other insect groups, other heritable symbionts are uncommon in Drosophila species, possibly reflecting a robust innate immune response that eliminates many bacteria. A more extensive survey targeted these two symbiont types through diagnostic PCR in 1225 strains representing 225 species from 32 species groups. Of these, 19 species were infected by Wolbachia while only 3 species had Spiroplasma. Several new strains of Wolbachia and Spiroplasma were discovered, including ones divergent from any reported to date. The phylogenetic distribution of Wolbachia and Spiroplasma in Drosophila is discussed.

Finding of male-killing Spiroplasma infecting Drosophila melanogaster in Africa implies transatlantic migration of this endosymbiont

We report the identification of male-killing Spiroplasma in a wild-caught female Drosophila melanogaster from Uganda, the first such infection to be found in this species outside of South America. Among 38 female flies collected from Namulonge, Uganda in April, 2005, one produced a total of 41 female offspring but no males. PCR testing of subsequent generations revealed that females retaining Spiroplasma infection continued to produce a large excess of female progeny, while females that had lost Spiroplasma produced offspring with normal sex ratios. Consistent with earlier work, we find that male-killing and transmission efficiency appear to increase with female age, and we note that males born in sex ratio broods display much lower survivorship than their female siblings. DNA sequence comparisons at three loci suggest that this Spiroplasma strain is closely related to the male-killing strain previously found to infect D. melanogaster in Brazil, although part of one locus appears to show a recombinant history. Implications for the origin and history of male-killing Spiroplasma in D. melanogaster are discussed.

The kinky propulsion of Spiroplasma

Bacteria have evolved many different means of generating movement. In this issue of Cell, Shaevitz et al. (2005) describe the swimming movement of a helical bacterium called Spiroplasma. They discover that Spiroplasma propels itself by generating two temporally distinct kinks that travel the length of the bacterium. These results point to the existence of a contractile apparatus that drives cell movement.

Spiroplasma swim by a processive change in body helicity

Microscopic organisms must rely on very different strategies than their macroscopic counterparts to swim through liquid. To date, the best understood method for prokaryotic swimming employs the rotation of flagella. Here, we show that Spiroplasma, tiny helical bacteria that infect plants and insects, use a very different approach. By measuring cell kinematics during free swimming, we find that propulsion is generated by the propagation of kink pairs down the length of the cell body. A processive change in the helicity of the body creates these waves and enables directional movement.