Global Wolbachia prevalence, titer fluctuations and their potential of causing cytoplasmic incompatibilities in tsetse flies and hybrids of Glossina morsitans subgroup species

The cellular lives of Wolbachia

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

Infection density pattern of Cardinium affects the responses of bacterial communities in an invasive whitefly under heat conditions

Communities of bacteria, especially symbionts, are vital for the growth and development of insects and other arthropods, including Bemisia tabaci Mediterranean (MED), a destructive and invasive insect pest. However, the infection density patterns and influence factors of bacteria in whiteflies, which mainly include symbionts, remain largely unclear. To reveal the different density patterns of Cardinium in B. tabaci MED populations and the impacts of high temperatures on whiteflies with different Cardinium density infection patterns, 2 isofemale lines isolated from B. tabaci MED from the same geographical population of China and from B. tabaci MED collected from other countries and locations were examined using several techniques and methods, including fluorescence in situ hybridization (FISH), quantitative real-time polymerase chain reaction (qPCR), 16S rRNA gene sequencing, and 2b-RAD sequencing. The results showed that there were 2 different infection density patterns of Cardinium in B. tabaci MED (including 1 high-density pattern and 1 low-density pattern). For whiteflies with low-density Cardinium, conventional PCR could not detect Cardinium, but the other techniques confirmed that there was a low level of Cardinium within hosts. High temperature significantly decreased the diversity of bacterial communities: the relative titer of Cardinium increased but the density of Rickettsia decreased in the isofemale line with high-density Cardinium. However, high temperature did not influence the diversity and symbiont density in the line with low-density Cardinium. Moreover, high temperature influenced the functions of bacterial communities in whiteflies with high-density Cardinium but did not affect the bacterial functions in whiteflies with low-density Cardinium. Our results provide novel insights into the complex associations between symbionts and host insects.

The Insect Pest Control Laboratory of the Joint FAO/IAEA Programme: Ten Years (2010-2020) of Research and Development, Achievements and Challenges in Support of the Sterile Insect Technique

The Joint FAO/IAEA Centre (formerly called Division) of Nuclear Techniques in Food and Agriculture was established in 1964 and its accompanying laboratories in 1961. One of its subprograms deals with insect pest control, and has the mandate to develop and implement the sterile insect technique (SIT) for selected key insect pests, with the goal of reducing the use of insecticides, reducing animal and crop losses, protecting the environment, facilitating international trade in agricultural commodities and improving human health. Since its inception, the Insect Pest Control Laboratory (IPCL) (formerly named Entomology Unit) has been implementing research in relation to the development of the SIT package for insect pests of crops, livestock and human health. This paper provides a review of research carried out between 2010 and 2020 at the IPCL. Research on plant pests has focused on the development of genetic sexing strains, characterizing and assessing the performance of these strains (e.g., Ceratitis capitata), elucidation of the taxonomic status of several members of the Bactrocera dorsalis and Anastrepha fraterculus complexes, the use of microbiota as probiotics, genomics, supplements to improve the performance of the reared insects, and the development of the SIT package for fruit fly species such as Bactrocera oleae and Drosophila suzukii. Research on livestock pests has focused on colony maintenance and establishment, tsetse symbionts and pathogens, sex separation, morphology, sterile male quality, radiation biology, mating behavior and transportation and release systems. Research with human disease vectors has focused on the development of genetic sexing strains (Anopheles arabiensis, Aedes aegypti and Aedes albopictus), the development of a more cost-effective larvae and adult rearing system, assessing various aspects of radiation biology, characterizing symbionts and pathogens, studying mating behavior and the development of quality control procedures, and handling and release methods. During the review period, 13 coordinated research projects (CRPs) were completed and six are still being implemented. At the end of each CRP, the results were published in a special issue of a peer-reviewed journal. The review concludes with an overview of future challenges, such as the need to adhere to a phased conditional approach for the implementation of operational SIT programs, the need to make the SIT more cost effective, to respond with demand driven research to solve the problems faced by the operational SIT programs and the use of the SIT to address a multitude of exotic species that are being introduced, due to globalization, and established in areas where they could not survive before, due to climate change.

Bacterial Symbionts of Tsetse Flies: Relationships and Functional Interactions Between Tsetse Flies and Their Symbionts

Tsetse flies (Glossina spp.) act as the sole vectors of the African trypanosome species that cause Human African Trypanosomiasis (HAT or African Sleeping Sickness) and Nagana in animals. These flies have undergone a variety of specializations during their evolution including an exclusive diet consisting solely of vertebrate blood for both sexes as well as an obligate viviparous reproductive biology. Alongside these adaptations, Glossina species have developed intricate relationships with specific microbes ranging from mutualistic to parasitic. These relationships provide fundamental support required to sustain the specializations associated with tsetse's biology. This chapter provides an overview on the knowledge to date regarding the biology behind these relationships and focuses primarily on four bacterial species that are consistently associated with Glossina species. Here their interactions with the host are reviewed at the morphological, biochemical and genetic levels. This includes: the obligate symbiont Wigglesworthia, which is found in all tsetse species and is essential for nutritional supplementation to the blood-specific diet, immune system maturation and facilitation of viviparous reproduction; the commensal symbiont Sodalis, which is a frequently associated symbiont optimized for survival within the fly via nutritional adaptation, vertical transmission through mating and may alter vectorial capacity of Glossina for trypanosomes; the parasitic symbiont Wolbachia, which can manipulate Glossina via cytoplasmic incompatibility and shows unique interactions at the genetic level via horizontal transmission of its genetic material into the genome in two Glossina species; finally, knowledge on recently observed relations between Spiroplasma and Glossina is explored and potential interactions are discussed based on knowledge of interactions between this bacterial Genera and other insect species. These flies have a simple microbiome relative to that of other insects. However, these relationships are deep, well-studied and provide a window into the complexity and function of host/symbiont interactions in an important disease vector.

Cryptic Wolbachia (Rickettsiales: Rickettsiaceae) Detection and Prevalence in Culicoides (Diptera: Ceratopogonidae) Midge Populations in the United States

Culicoides midges vector numerous veterinary and human pathogens. Many of these diseases lack effective therapeutic treatments or vaccines to limit transmission. The only effective approach to limit disease transmission is vector control. However, current vector control for Culicoides midges is complicated by the biology of many Culicoides species and is not always effective at reducing midge populations and impacting disease transmission. The endosymbiont Wolbachia pipientis Hertig may offer an alternative control approach to limit disease transmission and affect Culicoides populations. Here the detection of Wolbachia infections in nine species of Culicoides midges is reported. Infections were detected at low densities using qPCR. Wolbachia infections were confirmed with the sequencing of a partial region of the 16S gene. Fluorescence in situ hybridization of Culicoides sonorensis Wirth and Jones adults and dissected ovaries confirm the presence of Wolbachia infections in an important vector of Bluetongue and Epizootic hemorrhagic disease viruses. The presence of Wolbachia in Culicoides populations in the United States suggests the need for further investigation of Wolbachia as a strategy to limit transmission of diseases vectored by Culicoides midges.

Titer regulation in arthropod-Wolbachia symbioses

Symbiosis between intracellular bacteria (endosymbionts) and animals are widespread. The alphaproteobacterium Wolbachia pipientis is known to maintain a variety of symbiotic associations, ranging from mutualism to parasitism, with a wide range of invertebrates. Wolbachia infection might deeply affect host fitness (e.g. reproductive manipulation and antiviral protection), which is thought to explain its high prevalence in nature. Bacterial loads significantly influence both the infection dynamics and the extent of bacteria-induced host phenotypes. Hence, fine regulation of bacterial titers is considered as a milestone in host-endosymbiont interplay. Here, we review both environmental and biological factors modulating Wolbachia titers in arthropods.

Population Dynamics of Wolbachia in Laodelphax striatellus (Fallén) Under Successive Stress of Antibiotics

Wolbachia are the most common symbionts in arthropods; antibiotic treatment for eliminating the symbionts from their host is necessary to investigate the functions. Tetracycline antibiotics are widely used to remove endosymbiont Wolbachia from insect hosts. However, very little has been known on the effects of tetracycline on population size of Wolbachia in small brown planthopper (SBPH), Laodelphax striatellus (Fallén), an important insect pest of rice in Asia. Here, we investigated the dynamics of Wolbachia population density in females and males of L. striatellus by real-time fluorescent quantitative PCR method. The Wolbachia density in females and males of L. striatellus all declined sharply after treatment with 2 mg/mL tetracycline for one generation, and continued to decrease to a level which could not be detected by both qPCR and diagnostic PCR after treated for another generation, then maintained at 0 in the following three generations with continuous antibiotic treatment. Wolbachia infection did not recover in L. striatellus after stopping tetracycline treatment for ten generations. This is the first report to precisely monitor the population dynamics of Wolbachia in L. striatellus during successive tetracycline treatment and after that. The results provide a useful method for evaluating the efficiency of artificial operation of endosymbionts.

Molecular identification of Wolbachia and Sodalis glossinidius in the midgut of Glossina fuscipes quanzensis from the Democratic Republic of Congo

During the last 30 years, investigations on the microbiome of different tsetse species have generated substantial data on the bacterial flora of these cyclical vectors of African trypanosomes, with the overarching goal of improving the control of trypanosomiases. It is in this context that the presence of Wolbachia and Sodalis glossinidius was studied in wild populations of Glossina fuscipes quanzensis from the Democratic Republic of Congo. Tsetse flies were captured with pyramidal traps. Of the 700 Glossina f. quanzensis captured, 360 were dissected and their midguts collected and analyzed. Sodalis glossinidius and Wolbachia were identified by PCR. The Wolbachia-positive samples were genetically characterized with five molecular markers. PCR revealed 84.78% and 15.55% midguts infected by Wolbachia and S. glossinidius, respectively. The infection rates varied according to capture sites. Of the five molecular markers used to characterize Wolbachia, only the fructose bis-phosphate aldolase gene was amplified for about 60% of midguts previously found with Wolbachia infections. The sequencing results confirmed the presence of Wolbachia and revealed the presence of S. glossinidius in the midgut of Glossina f. quanzensis. A low level of midguts were naturally co-infected by both bacteria. The data generated in this study open a framework for investigations aimed at understanding the contribution of these symbiotic microorganisms to the vectorial competence of Glossina fuscipes quanzensis.

Detection of Wolbachia and different trypanosome species in Glossina palpalis palpalis populations from three sleeping sickness foci of southern Cameroon

BACKGROUND

African trypanosomiases are caused by trypanosomes that are cyclically transmitted by tsetse. Investigations aiming to generate knowledge on the bacterial fauna of tsetse have revealed distinct symbiotic microorganisms. Furthermore, studies addressing the tripartite association between trypanosomes-tsetse-symbionts relationship have so far been contradictory. Most studies included Sodalis glossinudius and, consequently, the association involving Wolbachia is poorly understood. Understanding the vectorial competence of tsetse requires decrypting these tripartite associations. In this study, we identified Wolbachia and trypanosomes in Glossina palpalis palpalis from three human African trypanosomiasis (HAT) foci in southern Cameroon.

METHODS

Tsetse flies were captured with pyramidal traps in the Bipindi, Campo and Fontem HAT foci. After morphological identification, DNA was extracted from whole tsetse flies and Wolbachia and trypanosomes were identified by PCR using different trypanosome-specific primers and two Wolbachia-specific primers (Wolbachia surface protein and 16S rRNA genes). Statistical analyses were performed to compare the trypanosome and Wolbachia infection rates between villages and different foci and to look for an association between these microorganisms.

RESULTS

From a total of 2122 tsetse flies, 790 G. p. palpalis were analyzed. About 25.32% of flies hosted Wolbachia and 31.84% of non-teneral flies were infected by at least one trypanosome species. There was no significant difference between the global Wolbachia prevalence revealed by the two markers while some differences were observed between HAT foci. From 248 G. p. palpalis with trypanosome infections, 62.90% were with T. vivax, 34.68% with T. congolense forest, 16.13% with T. brucei (s.l.) and 2.42% with T. congolense savannah. Of all trypanosome-infected flies, 29.84% hosted Wolbachia and no association was observed between Wolbachia and trypanosome co-infections.

CONCLUSIONS

This study revealed differences in the prevalence of Wolbachia and trypanosomes in G. p. palpalis according to HAT foci. The use of only one marker has underestimated the prevalence of Wolbachia, thus more markers in subsequent studies may improve its detection. The presence of Wolbachia seems to have no impact on the establishment of trypanosomes in G. p. palpalis. The tripartite association between tsetse, Wolbachia and trypanosomes varies according to studied areas. Studies aiming to evaluate the genetic polymorphism of Wolbachia and its density in tsetse flies could help to better understand this association.

Hytrosavirus genetic diversity and eco-regional spread in Glossina species

BACKGROUND

The management of the tsetse species Glossina pallidipes (Diptera; Glossinidae) in Africa by the sterile insect technique (SIT) has been hindered by infections of G. pallidipes production colonies with Glossina pallidipes salivary gland hypertrophy virus (GpSGHV; Hytrosaviridae family). This virus can significantly decrease productivity of the G. pallidipes colonies. Here, we used three highly diverged genes and two variable number tandem repeat regions (VNTRs) of the GpSGHV genome to identify the viral haplotypes in seven Glossina species obtained from 29 African locations and determine their phylogenetic relatedness.

RESULTS

GpSGHV was detected in all analysed Glossina species using PCR. The highest GpSGHV prevalence was found in G. pallidipes colonized at FAO/IAEA Insect Pest Control Laboratory (IPCL) that originated from Uganda (100%) and Tanzania (88%), and a lower prevalence in G. morsitans morsitans from Tanzania (58%) and Zimbabwe (20%). Whereas GpSGHV was detected in 25-40% of G. fuscipes fuscipes in eastern Uganda, the virus was not detected in specimens of neighboring western Kenya. Most of the identified 15 haplotypes were restricted to specific Glossina species in distinct locations. Seven haplotypes were found exclusively in G. pallidipes. The reference haplotype H1 (GpSGHV-Uga; Ugandan strain) was the most widely distributed, but was not found in G. swynnertoni GpSGHV. The 15 haplotypes clustered into three distinct phylogenetic clades, the largest contained seven haplotypes, which were detected in six Glossina species. The G. pallidipes-infecting haplotypes H10, H11 and H12 (from Kenya) clustered with H7 (from Ethiopia), which presumably corresponds to the recently sequenced GpSGHV-Eth (Ethiopian) strain. These four haplotypes diverged the most from the reference H1 (GpSGHV-Uga). Haplotypes H1, H5 and H14 formed three main genealogy hubs, potentially representing the ancestors of the 15 haplotypes.

CONCLUSION

These data identify G. pallidipes as a significant driver for the generation and diversity of GpSGHV variants. This information may provide control guidance when new tsetse colonies are established and hence, for improved management of the virus in tsetse rearing facilities that maintain multiple Glossina species.

High-sensitivity detection of cryptic Wolbachia in the African tsetse fly (Glossina spp.)

BACKGROUND

In African tsetse flies Glossina, spp. detection of bacterial symbionts such as Wolbachia is challenging since their prevalence and distribution are patchy, and natural symbiont titers can range at levels far below detection limit of standard molecular techniques. Reliable estimation of symbiont infection frequency, especially with regard to interrelations between symbionts and their potential impact on host biology, is of pivotal interest in the context of future applications for the control and eradication of Glossina-vectored African trypanosomosis. The presence or absence of symbionts is routinely screened with endpoint polymerase chain reaction (PCR), which has numerous advantages, but reaches its limits, when detecting infections at natural low titer. To not only determine presence of native tsetse symbionts but also to localize them to specific host tissues, fluorescence in situ hybridization (FISH) can be applied. However, classic FISH assays may not detect low-titer infections due to limitations in sensitivity.

RESULTS

We have compared classic endpoint PCR with high-sensitivity blot-PCR. We demonstrate that the latter technique allows for clear detection of low-titer Wolbachia in the morsitans and palpalis groups while classic endpoint PCR does not. In order to localize Wolbachia in situ in high and low-titer Glossina species, we applied high-end Stellaris® rRNA-FISH. We show that with this high sensitivity method, even low amounts of Wolbachia can be traced in specific tissues. Furthermore, we highlight that more tissues and organs than previously recorded are infested with Wolbachia in subspecies of the morsitans and palpalis groups.

CONCLUSIONS

Our results demonstrate that overall symbiont infection frequencies as well as the presence in specific host tissues may be underestimated when using low-sensitivity methods. To better understand the complex interrelation of tsetse flies and their native symbionts plus the pathogenic trypanosomes, it is important to consider application of a broader range of high-sensitivity detection tools.

Enhancing vector refractoriness to trypanosome infection: achievements, challenges and perspectives

With the absence of effective prophylactic vaccines and drugs against African trypanosomosis, control of this group of zoonotic neglected tropical diseases depends the control of the tsetse fly vector. When applied in an area-wide insect pest management approach, the sterile insect technique (SIT) is effective in eliminating single tsetse species from isolated populations. The need to enhance the effectiveness of SIT led to the concept of investigating tsetse-trypanosome interactions by a consortium of researchers in a five-year (2013-2018) Coordinated Research Project (CRP) organized by the Joint Division of FAO/IAEA. The goal of this CRP was to elucidate tsetse-symbiome-pathogen molecular interactions to improve SIT and SIT-compatible interventions for trypanosomoses control by enhancing vector refractoriness. This would allow extension of SIT into areas with potential disease transmission. This paper highlights the CRP's major achievements and discusses the science-based perspectives for successful mitigation or eradication of African trypanosomosis.

Effect of antibiotic treatment and gamma-irradiation on cuticular hydrocarbon profiles and mate choice in tsetse flies (Glossina m. morsitans)

BACKGROUND

Symbiotic microbes represent a driving force of evolutionary innovation by conferring novel ecological traits to their hosts. Many insects are associated with microbial symbionts that contribute to their host's nutrition, digestion, detoxification, reproduction, immune homeostasis, and defense. In addition, recent studies suggest a microbial involvement in chemical communication and mating behavior, which can ultimately impact reproductive isolation and, hence, speciation. Here we investigated whether a disruption of the microbiota through antibiotic treatment or irradiation affects cuticular hydrocarbon profiles, and possibly mate choice behavior in the tsetse fly, Glossina morsitans morsitans. Four independent experiments that differentially knock down the multiple bacterial symbionts of tsetse flies were conducted by subjecting tsetse flies to ampicillin, tetracycline, or gamma-irradiation and analyzing their cuticular hydrocarbon profiles in comparison to untreated controls by gas chromatography - mass spectrometry. In two of the antibiotic experiments, flies were mass-reared, while individual rearing was done for the third experiment to avoid possible chemical cross-contamination between individual flies.

RESULTS

All three antibiotic experiments yielded significant effects of antibiotic treatment (particularly tetracycline) on cuticular hydrocarbon profiles in both female and male G. m. morsitans, while irradiation itself had no effect on the CHC profiles. Importantly, tetracycline treatment reduced relative amounts of 15,19,23-trimethyl-heptatriacontane, a known compound of the female contact sex pheromone, in two of the three experiments, suggesting a possible implication of microbiota disturbance on mate choice decisions. Concordantly, both female and male flies preferred non-treated over tetracycline-treated flies in direct choice assays.

CONCLUSIONS

While we cannot exclude the possibility that antibiotic treatment had a directly detrimental effect on fly vigor as we are unable to recolonize antibiotic treated flies with individual symbiont taxa, our results are consistent with an effect of the microbiota, particularly the obligate nutritional endosymbiont Wigglesworthia, on CHC profiles and mate choice behavior. These findings highlight the importance of considering host-microbiota interactions when studying chemical communication and mate choice in insects.

Prevalence of trypanosomes, salivary gland hypertrophy virus and Wolbachia in wild populations of tsetse flies from West Africa

BACKGROUND

Tsetse flies are vectors of African trypanosomes, protozoan parasites that cause sleeping sickness (or human African trypanosomosis) in humans and nagana (or animal African trypanosomosis) in livestock. In addition to trypanosomes, four symbiotic bacteria Wigglesworthia glossinidia, Sodalis glossinidius, Wolbachia, Spiroplasma and one pathogen, the salivary gland hypertrophy virus (SGHV), have been reported in different tsetse species. We evaluated the prevalence and coinfection dynamics between Wolbachia, trypanosomes, and SGHV in four tsetse species (Glossina palpalis gambiensis, G. tachinoides, G. morsitans submorsitans, and G. medicorum) that were collected between 2008 and 2015 from 46 geographical locations in West Africa, i.e. Burkina Faso, Mali, Ghana, Guinea, and Senegal.

RESULTS

The results indicated an overall low prevalence of SGHV and Wolbachia and a high prevalence of trypanosomes in the sampled wild tsetse populations. The prevalence of all three infections varied among tsetse species and sample origin. The highest trypanosome prevalence was found in Glossina tachinoides (61.1%) from Ghana and in Glossina palpalis gambiensis (43.7%) from Senegal. The trypanosome prevalence in the four species from Burkina Faso was lower, i.e. 39.6% in Glossina medicorum, 18.08%; in Glossina morsitans submorsitans, 16.8%; in Glossina tachinoides and 10.5% in Glossina palpalis gambiensis. The trypanosome prevalence in Glossina palpalis gambiensis was lowest in Mali (6.9%) and Guinea (2.2%). The prevalence of SGHV and Wolbachia was very low irrespective of location or tsetse species with an average of 1.7% for SGHV and 1.0% for Wolbachia. In some cases, mixed infections with different trypanosome species were detected. The highest prevalence of coinfection was Trypanosoma vivax and other Trypanosoma species (9.5%) followed by coinfection of T. congolense with other trypanosomes (7.5%). The prevalence of coinfection of T. vivax and T. congolense was (1.0%) and no mixed infection of trypanosomes, SGHV and Wolbachia was detected.

CONCLUSION

The results indicated a high rate of trypanosome infection in tsetse wild populations in West African countries but lower infection rate of both Wolbachia and SGHV. Double or triple mixed trypanosome infections were found. In addition, mixed trypanosome and SGHV infections existed however no mixed infections of trypanosome and/or SGHV with Wolbachia were found.

Tsetse fly evolution, genetics and the trypanosomiases - A review

This reviews work published since 2007. Relative efforts devoted to the agents of African trypanosomiasis and their tsetse fly vectors are given by the numbers of PubMed accessions. In the last 10 years PubMed citations number 3457 for Trypanosoma brucei and 769 for Glossina. The development of simple sequence repeats and single nucleotide polymorphisms afford much higher resolution of Glossina and Trypanosoma population structures than heretofore. Even greater resolution is offered by partial and whole genome sequencing. Reproduction in T. brucei sensu lato is principally clonal although genetic recombination in tsetse salivary glands has been demonstrated in T. b. brucei and T. b. rhodesiense but not in T. b. gambiense. In the past decade most genetic attention was given to the chief human African trypanosomiasis vectors in subgenus Nemorhina e.g., Glossina f. fuscipes, G. p. palpalis, and G. p. gambiense. The chief interest in Nemorhina population genetics seemed to be finding vector populations sufficiently isolated to enable efficient and long-lasting suppression. To this end estimates were made of gene flow, derived from F_ST and its analogues, and Ne, the size of a hypothetical population equivalent to that under study. Genetic drift was greater, gene flow and Ne typically lesser in savannah inhabiting tsetse (subgenus Glossina) than in riverine forms (Nemorhina). Population stabilities were examined by sequential sampling and genotypic analysis of nuclear and mitochondrial genomes in both groups and found to be stable. Gene frequencies estimated in sequential samplings differed by drift and allowed estimates of effective population numbers that were greater for Nemorhina spp than Glossina spp. Prospects are examined of genetic methods of vector control. The tsetse long generation time (c. 50 d) is a major contraindication to any suggested genetic method of tsetse population manipulation. Ecological and modelling research convincingly show that conventional methods of targeted insecticide applications and traps/targets can achieve cost-effective reduction in tsetse densities.

Environmental Transmission of the Gut Symbiont Burkholderia to Phloem-Feeding Blissus insularis

The plant-phloem-feeding Blissus insularis possesses specialized midgut crypts, which harbor a dense population of the exocellular bacterial symbiont Burkholderia. Most individual B. insularis harbor a single Burkholderia ribotype in their midgut crypts; however, a diverse Burkholderia community exists within a host population. To understand the mechanism underlying the consistent occurrence of various Burkholderia in B. insularis and their specific association, we investigated potential gut symbiont transmission routes. PCR amplification detected a low titer of Burkholderia in adult reproductive tracts; however, fluorescence in situ hybridization assays failed to produce detectable signals in these tracts. Furthermore, no Burkholderia-specific PCR signals were detected in eggs and neonates, suggesting that it is unlikely that B. insularis prenatally transmits gut symbionts via ovarioles. In rearing experiments, most nymphs reared on St. Augustinegrass treated with cultured Burkholderia harbored the cultured Burkholderia strains. Burkholderia was detected in the untreated host grass of B. insularis, and most nymphs reared on untreated grass harbored a Burkholderia ribotype that was closely related to a plant-associated Burkholderia strain. These findings revealed that B. insularis neonates acquired Burkholderia primarily from the environment (i.e., plants and soils), even though the possibility of acquisition via egg surface cannot be excluded. In addition, our study explains how the diverse Burkholderia symbiont community in B. insularis populations can be maintained.

Detection of Low-Level Cardinium and Wolbachia Infections in Culicoides

Bacterial endosymbionts have been identified as potentially useful biological control agents for a range of invertebrate vectors of disease. Previous studies of Culicoides (Diptera: Ceratopogonidae) species using conventional PCR assays have provided evidence of Wolbachia (1/33) and Cardinium (8/33) infections. Here, we screened 20 species of Culicoides for Wolbachia and Cardinium, utilizing a combination of conventional PCR and more sensitive quantitative PCR (qPCR) assays. Low levels of Cardinium DNA were detected in females of all but one of the Culicoides species screened, and low levels of Wolbachia were detected in females of 9 of the 20 Culicoides species. Sequence analysis based on partial 16S rRNA gene and gyrB sequences identified "Candidatus Cardinium hertigii" from group C, which has previously been identified in Culicoides from Japan, Israel, and the United Kingdom. Wolbachia strains detected in this study showed 98 to 99% sequence identity to Wolbachia previously detected from Culicoides based on the 16S rRNA gene, whereas a strain with a novel wsp sequence was identified in Culicoides narrabeenensis. Cardinium isolates grouped to geographical regions independent of the host Culicoides species, suggesting possible geographical barriers to Cardinium movement. Screening also identified Asaia bacteria in Culicoides. These findings point to a diversity of low-level endosymbiont infections in Culicoides, providing candidates for further characterization and highlighting the widespread occurrence of these endosymbionts in this insect group.

Dynamics of the endosymbiont Rickettsia in an insect pest

A new heritable bacterial association can bring a fresh set of molecular capabilities, providing an insect host with an almost instantaneous genome extension. Increasingly acknowledged as agents of rapid evolution, inherited microbes remain underappreciated players in pest management programs. A Rickettsia bacterium was tracked sweeping through populations of an invasive whitefly provisionally described as the "B" or "MEAM1" of the Bemisia tabaci species complex, in the southwestern USA. In this population, Rickettsia provides strong fitness benefits and distorts whitefly sex ratios under laboratory conditions. In contrast, whiteflies in Israel show few apparent fitness benefits from Rickettsia under laboratory conditions, only slightly decreasing development time. A survey of B. tabaci B samples revealed the distribution of Rickettsia across the cotton-growing regions of Israel and the USA. Thirteen sites from Israel and 22 sites from the USA were sampled. Across the USA, Rickettsia frequencies were heterogeneous among regions, but were generally very high, whereas in Israel, the infection rates were lower and declining. The distinct outcomes of Rickettsia infection in these two countries conform to previously reported phenotypic differences. Intermediate frequencies in some areas in both countries may indicate a cost to infection in certain environments or that the frequencies are in flux. This suggests underlying geographic differences in the interactions between bacterial symbionts and this serious agricultural pest.

Microbiome frequency and their association with trypanosome infection in male Glossina morsitans centralis of Western Zambia

Tsetse flies (Diptera: Glossinidae) are considered primary cyclical vectors that transmit pathogenic trypanosomes in Africa. They harbour a variety of microbes including Wolbachia, Sodalis and the salivary gland hypertrophy virus (SGHV) which are all vertically transmitted. Knowledge on tsetse microbiome and their interactions may identify novel strategies for tsetse fly and trypanosomiasis control. Area-wide application of such strategies requires an understanding of the natural microbiome frequency in the different species and subspecies of Glossina in their geographical populations. Consequently, this study determined the prevalence of Sodalis, Wolbachia, SGHV and trypanosome infections in Glossina morsitanscentralis from two sites of Western Zambia. We also explored possible associations of the microbes with trypanosome infections. Male G. morsitanscentralis samples were collected from two sites (Lyoni and Lusinina) in Western Zambia. The age structure of the flies at each site was determined using the wing fray method. DNA was extracted from the samples and analyzed for Wolbachia, Sodalis, SGHV and trypanosome presence using PCR. Associations and measures of associations between trypanosome infection and microbes in the fly were determined. The flies from the two locations (Lusinina, n=45 and Lyoni, n=24) had a similar age structure with their median fray category not being significantly different (p=0.698). The overall prevalence of Wolbachia was 72.5% (95% CI: 61.6-83.3%), Sodalis was 15.9% (95% CI: 7.1-24.8%), SGHV was 31.9% (95% CI: 20.6-43.2%) and Trypanosoma species was 23.2% (95% CI: 13-33.4%). The prevalence of Wolbachia was significantly higher in Lusinina than Lyoni (p=0.000). However this was not the case for Sodalis, SGHV and Trypanosoma species. Despite the low number of flies that were positive for both trypanosome and Sodalis (6; 8.7%), a statistically significant association (p=0.013; AOR 6.2; 95% CI: 1.5-25.8) was observed in G. morsitanscentralis. The study showed that the prevalence of microbiota may vary within the same species of the tsetse depending on the geographical location as was the case of Wolbachia. Further it showed that infection with Sodalis could affect vector competence. The study concludes that Sodalis could be an ideal candidate for symbiont-mediated trypanosomiasis control interventions in G. morsitanscentralis.

More than fishing in the dark: PCR of a dispersed sequence produces simple but ultrasensitive Wolbachia detection

BACKGROUND

Detecting intracellular bacterial symbionts can be challenging when they persist at very low densities. Wolbachia, a widespread bacterial endosymbiont of invertebrates, is particularly challenging. Although it persists at high titers in many species, in others its densities are far below the detection limit of classic end-point Polymerase Chain Reaction (PCR). These low-titer infections can be reliably detected by combining PCR with DNA hybridization, but less elaborate strategies based on end-point PCR alone have proven less sensitive or less general.

RESULTS

We introduce a multicopy PCR target that allows fast and reliable detection of A-supergroup Wolbachia--even at low infection titers--with standard end-point PCR. The target is a multicopy motif (designated ARM: A-supergroup repeat motif) discovered in the genome of wMel (the Wolbachia in Drosophila melanogaster). ARM is found in at least seven other Wolbachia A-supergroup strains infecting various Drosophila, the wasp Muscidifurax and the tsetse fly Glossina. We demonstrate that end-point PCR targeting ARM can reliably detect both high- and low-titer Wolbachia infections in Drosophila, Glossina and interspecific hybrids.

CONCLUSIONS

Simple end-point PCR of ARM facilitates detection of low-titer Wolbachia A-supergroup infections. Detecting these infections previously required more elaborate procedures. Our ARM target seems to be a general feature of Wolbachia A-supergroup genomes, unlike other multicopy markers such as insertion sequences (IS).

Microsatellite and Wolbachia analysis in Rhagoletis cerasi natural populations: population structuring and multiple infections

Rhagoletis cerasi (Diptera: Tephritidae) is a major pest of sweet and sour cherries in Europe and parts of Asia. Despite its economic significance, there is a lack of studies on the genetic structure of R. cerasi populations. Elucidating the genetic structure of insects of economic importance is crucial for developing phenological-predictive models and environmental friendly control methods. All natural populations of R. cerasi have been found to harbor the endosymbiont Wolbachia pipientis, which widely affects multiple biological traits contributing to the evolution of its hosts, and has been suggested as a tool for the biological control of insect pests and disease vectors. In the current study, the analysis of 18 R. cerasi populations collected in Greece, Germany, and Russia using 13 microsatellite markers revealed structuring of R. cerasi natural populations, even at close geographic range. We also analyzed the Wolbachia infection status of these populations using 16S rRNA-, MLST- and wsp-based approaches. All 244 individuals screened were positive for Wolbachia. Our results suggest the fixation of the wCer1 strain in Greece while wCer2, wCer4, wCer5, and probably other uncharacterized strains were also detected in multiply infected individuals. The role of Wolbachia and its potential extended phenotypes needs a thorough investigation in R. cerasi. Our data suggest an involvement of this symbiont in the observed restriction in the gene flow in addition to a number of different ecological factors.

Uncovering Wolbachia diversity upon artificial host transfer

The common endosymbiotic Wolbachia bacteria influence arthropod hosts in multiple ways. They are mostly recognized for their manipulations of host reproduction, yet, more recent studies demonstrate that Wolbachia also impact host behavior, metabolic pathways and immunity. Besides their biological and evolutionary roles, Wolbachia are new potential biological control agents for pest and vector management. Importantly, Wolbachia-based control strategies require controlled symbiont transfer between host species and predictable outcomes of novel Wolbachia-host associations. Theoretically, this artificial horizontal transfer could inflict genetic changes within transferred Wolbachia populations. This could be facilitated through de novo mutations in the novel recipient host or changes of haplotype frequencies of polymorphic Wolbachia populations when transferred from donor to recipient hosts. Here we show that Wolbachia resident in the European cherry fruit fly, Rhagoletis cerasi, exhibit ancestral and cryptic sequence polymorphism in three symbiont genes, which are exposed upon microinjection into the new hosts Drosophila simulans and Ceratitis capitata. Our analyses of Wolbachia in microinjected D. simulans over 150 generations after microinjection uncovered infections with multiple Wolbachia strains in trans-infected lines that had previously been typed as single infections. This confirms the persistence of low-titer Wolbachia strains in microinjection experiments that had previously escaped standard detection techniques. Our study demonstrates that infections by multiple Wolbachia strains can shift in prevalence after artificial host transfer driven by either stochastic or selective processes. Trans-infection of Wolbachia can claim fitness costs in new hosts and we speculate that these costs may have driven the shifts of Wolbachia strains that we saw in our model system.

Wolbachia infections mimic cryptic speciation in two parasitic butterfly species, Phengaris teleius and P. nausithous (Lepidoptera: Lycaenidae)

Deep mitochondrial divergence within species may result from cryptic speciation, from phylogeographic isolation or from endosymbiotic bacteria like Wolbachia that manipulate host reproduction. Phengaris butterflies are social parasites that spend most of their life in close relationship with ants. Previously, cryptic speciation has been hypothesised for two Phengaris species based on divergent mtDNA sequences. Since Phengaris species are highly endangered, the existence of cryptic species would have drastic consequences for conservation and management. We tested for cryptic speciation and alternative scenarios in P. teleius and P. nausithous based on a comprehensive sample across their Palaearctic ranges using COI gene sequences, nuclear microsatellites and tests for Wolbachia. In both species a deep mitochondrial split occurring 0.65-1.97 myrs ago was observed that did not correspond with microsatellite data but was concordant with Wolbachia infection. Haplotypes previously attributed to cryptic species were part of the Wolbachia-infected clades. In both species remaining phylogeographic structure was largely consistent between mitochondrial and nuclear genomes. In P. teleius several mitochondrial and nuclear groups were observed in East Asia while a single haplogroup and nuclear cluster prevailed across continental Eurasia. Neutrality tests suggested rapid demographic expansion into that area. In contrast, P. nausithous had several mitochondrial and nuclear groups in Europe, suggesting a complex phylogeographic history in the western part of the species range. We conclude that deep intraspecific divergences found in DNA barcode studies do not necessarily need to represent cryptic speciation but instead can be due to both infection by Wolbachia and phylogeographic structure.

Tsetse fly microbiota: form and function

Tsetse flies are the primary vectors of African trypanosomes, which cause Human and Animal African trypanosomiasis in 36 countries in sub-Saharan Africa. These flies have also established symbiotic associations with bacterial and viral microorganisms. Laboratory-reared tsetse flies harbor up to four vertically transmitted organisms—obligate Wigglesworthia, commensal Sodalis, parasitic Wolbachia and Salivary Gland Hypertrophy Virus (SGHV). Field-captured tsetse can harbor these symbionts as well as environmentally acquired commensal bacteria. This microbial community influences several aspects of tsetse's physiology, including nutrition, fecundity and vector competence. This review provides a detailed description of tsetse's microbiome, and describes the physiology underlying host-microbe, and microbe-microbe, interactions that occur in this fly.

Transgenerational transmission of the Glossina pallidipes hytrosavirus depends on the presence of a functional symbiome

The vertically transmitted endosymbionts (Sodalis glossinidius and Wigglesworthia glossinidia) of the tsetse fly (Diptera: Glossinidae) are known to supplement dietary deficiencies and modulate the reproductive fitness and the defense system of the fly. Some tsetse fly species are also infected with the bacterium, Wolbachia and with the Glossina hytrosavirus (GpSGHV). Laboratory-bred G. pallidipes exhibit chronic asymptomatic and acute symptomatic GpSGHV infection, with the former being the most common in these colonies. However, under as yet undefined conditions, the asymptomatic state can convert to the symptomatic state, leading to detectable salivary gland hypertrophy (SGH(+)) syndrome. In this study, we investigated the interplay between the bacterial symbiome and GpSGHV during development of G. pallidipes by knocking down the symbionts with antibiotic. Intrahaemocoelic injection of GpSGHV led to high virus titre (10(9) virus copies), but was not accompanied by either the onset of detectable SGH(+), or release of detectable virus particles into the blood meals during feeding events. When the F1 generations of GpSGHV-challenged mothers were dissected within 24 h post-eclosion, SGH(+) was observed to increase from 4.5% in the first larviposition cycle to >95% in the fourth cycle. Despite being sterile, these F1 SGH(+) progeny mated readily. Removal of the tsetse symbiome, however, suppressed transgenerational transfer of the virus via milk secretions and blocked the ability of GpSGHV to infect salivary glands of the F1 progeny. Whereas GpSGHV infects and replicates in salivary glands of developing pupa, the virus is unable to induce SGH(+) within fully differentiated adult salivary glands. The F1 SGH(+) adults are responsible for the GpSGHV-induced colony collapse in tsetse factories. Our data suggest that GpSGHV has co-evolved with the tsetse symbiome and that the symbionts play key roles in the virus transmission from mother to progeny.

Improving Sterile Insect Technique (SIT) for tsetse flies through research on their symbionts and pathogens

Tsetse flies (Diptera: Glossinidae) are the cyclical vectors of the trypanosomes, which cause human African trypanosomosis (HAT) or sleeping sickness in humans and African animal trypanosomosis (AAT) or nagana in animals. Due to the lack of effective vaccines and inexpensive drugs for HAT, and the development of resistance of the trypanosomes against the available trypanocidal drugs, vector control remains the most efficient strategy for sustainable management of these diseases. Among the control methods used for tsetse flies, Sterile Insect Technique (SIT), in the frame of area-wide integrated pest management (AW-IPM), represents an effective tactic to suppress and/or eradicate tsetse flies. One constraint in implementing SIT is the mass production of target species. Tsetse flies harbor obligate bacterial symbionts and salivary gland hypertrophy virus which modulate the fecundity of the infected flies. In support of the future expansion of the SIT for tsetse fly control, the Joint FAO/IAEA Programme of Nuclear Techniques in Food and Agriculture implemented a six year Coordinated Research Project (CRP) entitled "Improving SIT for Tsetse Flies through Research on their Symbionts and Pathogens". The consortium focused on the prevalence and the interaction between the bacterial symbionts and the virus, the development of strategies to manage virus infections in tsetse colonies, the use of entomopathogenic fungi to control tsetse flies in combination with SIT, and the development of symbiont-based strategies to control tsetse flies and trypanosomosis. The results of the CRP and the solutions envisaged to alleviate the constraints of the mass rearing of tsetse flies for SIT are presented in this special issue.

Survey of endosymbionts in the Diaphorina citri metagenome and assembly of a Wolbachia wDi draft genome

Diaphorina citri (Hemiptera: Psyllidae), the Asian citrus psyllid, is the insect vector of Ca. Liberibacter asiaticus, the causal agent of citrus greening disease. Sequencing of the D. citri metagenome has been initiated to gain better understanding of the biology of this organism and the potential roles of its bacterial endosymbionts. To corroborate candidate endosymbionts previously identified by rDNA amplification, raw reads from the D. citri metagenome sequence were mapped to reference genome sequences. Results of the read mapping provided the most support for Wolbachia and an enteric bacterium most similar to Salmonella. Wolbachia-derived reads were extracted using the complete genome sequences for four Wolbachia strains. Reads were assembled into a draft genome sequence, and the annotation assessed for the presence of features potentially involved in host interaction. Genome alignment with the complete sequences reveals membership of Wolbachia wDi in supergroup B, further supported by phylogenetic analysis of FtsZ. FtsZ and Wsp phylogenies additionally indicate that the Wolbachia strain in the Florida D. citri isolate falls into a sub-clade of supergroup B, distinct from Wolbachia present in Chinese D. citri isolates, supporting the hypothesis that the D. citri introduced into Florida did not originate from China.

Enhancing tsetse fly refractoriness to trypanosome infection--a new IAEA coordinated research project

To date, IAEA-supported Sterile Insect Technique (SIT) projects for tsetse and trypanosomiasis control have been in areas without human sleeping sickness, but future projects could include areas of actual or potential human disease transmission. In this context it would be imperative that released sterile tsetse flies are incompetent to transmit the disease-causing trypanosome parasite. Therefore, development of tsetse fly strains refractory to trypanosome infection is highly desirable as a simple and effective method of ensuring vector incompetence of the released flies. This new IAEA Coordinated Research Project (CRP) focuses on gaining a deeper knowledge of the tripartite interactions between the tsetse fly vectors, their symbionts and trypanosome parasites. The objective of this CRP is to acquire a better understanding of mechanisms that limit the development of trypanosome infections in tsetse and how these may be enhanced.

Tsetse-Wolbachia symbiosis: comes of age and has great potential for pest and disease control

Tsetse flies (Diptera: Glossinidae) are the sole vectors of African trypanosomes, the causative agent of sleeping sickness in human and nagana in animals. Like most eukaryotic organisms, Glossina species have established symbiotic associations with bacteria. Three main symbiotic bacteria have been found in tsetse flies: Wigglesworthia glossinidia, an obligate symbiotic bacterium, the secondary endosymbiont Sodalis glossinidius and the reproductive symbiont Wolbachia pipientis. In the present review, we discuss recent studies on the detection and characterization of Wolbachia infections in Glossina species, the horizontal transfer of Wolbachia genes to tsetse chromosomes, the ability of this symbiont to induce cytoplasmic incompatibility in Glossina morsitans morsitans and also how new environment-friendly tools for disease control could be developed by harnessing Wolbachia symbiosis.