"Hostbusters": The Bacterial Endosymbiont Wolbachia of the Parasitoid Wasp Habrobracon hebetor Improves Its Ability to Parasitize Lepidopteran Hosts

Habrobracon hebetor is a globally acknowledged larval ectoparasitoid that is widely used to control lepidopteran pests. Wolbachia is a natural endosymbiont that regulates various aspects of the insect host biology. The ability of H. hebetor to paralyze and develop on lepidopteran larvae from five families was tested under laboratory conditions. Two lines of the wasp were used, "W+" containing a naturally occurring Wolbachia from the supergroup B, and "W-", with the endosymbiont eradicated by antibiotic treatment, followed by propagation of 20 subsequent generations. The proportions of larvae in which host paralysis, as well as parasitoid oviposition, larval, pupal, and adult development were observed, were usually higher in W+ compared to W-. In Loxostege sticticalis, differences in these indices were not statistically significant. In Galleria mellonella, Mamestra brassicae, and Ostrinia nubilalis, some of the parasitism indices were significantly higher in W+ than in W-. In Bombyx mori and Plutella xylostella, H. hebetor could not complete its life cycle, but parasitism levels at the initial steps (from paralysis symptoms to the presence of larvae/pupae of the parasitoid) were 2-5 times lower in W- compared to W+ (p < 0.01). It can be suggested that the presence of Wolbachia is advantageous for H. hebetor, as it increases the success of parasitism in a broad range of lepidopteran hosts.

Exploiting Wolbachia as a Tool for Mosquito-Borne Disease Control: Pursuing Efficacy, Safety, and Sustainability

Despite the application of control measures, mosquito-borne diseases continue to pose a serious threat to human health. In this context, exploiting Wolbachia, a common symbiotic bacterium in insects, may offer effective solutions to suppress vectors or reduce their competence in transmitting several arboviruses. Many Wolbachia strains can induce conditional egg sterility, known as cytoplasmic incompatibility (CI), when infected males mate with females that do not harbor the same Wolbachia infection. Infected males can be mass-reared and then released to compete with wild males, reducing the likelihood of wild females encountering a fertile mate. Furthermore, certain Wolbachia strains can reduce the competence of mosquitoes to transmit several RNA viruses. Through CI, Wolbachia-infected individuals can spread within the population, leading to an increased frequency of mosquitoes with a reduced ability to transmit pathogens. Using artificial methods, Wolbachia can be horizontally transferred between species, allowing the establishment of various laboratory lines of mosquito vector species that, without any additional treatment, can produce sterilizing males or females with reduced vector competence, which can be used subsequently to replace wild populations. This manuscript reviews the current knowledge in this field, describing the different approaches and evaluating their efficacy, safety, and sustainability. Successes, challenges, and future perspectives are discussed in the context of the current spread of several arboviral diseases, the rise of insecticide resistance in mosquito populations, and the impact of climate change. In this context, we explore the necessity of coordinating efforts among all stakeholders to maximize disease control. We discuss how the involvement of diverse expertise-ranging from new biotechnologies to mechanistic modeling of eco-epidemiological interactions between hosts, vectors, Wolbachia, and pathogens-becomes increasingly crucial. This coordination is especially important in light of the added complexity introduced by Wolbachia and the ongoing challenges posed by global change.

Symbiosis of intracellular bacteria Wolbachia with insects: a hundred years of study summarized

Wolbachia pipientis is an α-proteobacterium, which is a widespread intracellular symbiont in a number of Arthropoda and some Nematoda species. With insects, W. pipientis forms a symbiont-host system characterized by very close interactions between its components. The mutual effects of Wolbachia on the host and the host on Wolbachia are important biotic factors for both components of this symbiotic system. Wolbachia is able to affect both host reproduction and somatic organ function. Due to its prevalence among insects and a wide variety of both negative (cytoplasmic incompatibility and androcide are among the most well-known examples) and positive (increasing resistance to biotic and abiotic factors, providing vitamins and metabolites) effects on the host organism, Wolbachia is of great interest for both entomologists and microbiologists. The diversity of host phenotypes induced by Wolbachia provides a broad choice of evolutionary strategies (such as reproductive parasitism or mutually beneficial symbiont-host relationships) that it utilizes. The influence of Wolbachia is to be considered in the design of any experiment conducted on insects. The application of sequencing technologies has led to new approaches being created to study the existing relationships within the Wolbachia-insect system, but interpretation of the data obtained is challenging. Nevertheless, the prospects for the use of the whole-genome analysis data to study Wolbachia-host coevolution are beyond doubt. Ongoing projects to introduce Wolbachia strains, which provide antiviral host defense, into insect populations to control the spread of RNA-viruses are actively pursued, which could result in saving many human lives. The aim of this brief review is to summarize the data collected by scientists over the past hundred years of Wolbachia studies and the current understanding of its genetic diversity and mechanisms of interaction with the host, including those based on transcriptome analysis.

Non-ovarian Wolbachia pipientis titer correlates with fertility rescue of a Drosophila melanogaster bag of marbles hypomorph

Bag of marbles ( bam ) is an essential gene that regulates germline stem cell maintenance and germline stem cell daughter cell differentiation in Drosophila melanogaster . When bam is partially functional (hypomorphic), the introduction of Wolbachia pipientis rescues the mutant fertility phenotype that would otherwise result in partial sterility. Infection by different W. pipientis variants results in differential rescue of the bam hypomorph fertility phenotype. We were intrigued by the varying degrees of rescue exhibited in the bam hypomorph when exposed to different W. pipientis variants, prompting us to investigate whether this phenomenon is attributable to variations in the titers of W. pipientis variants. We found no significant difference in ovarian titer between two W. pipientis variant groups, w Mel-like (low bam hypomorph fertility rescue) and w MelCS-like variants (higher bam hypomorph fertility rescue), at bam hypomorph peak fertility. However, carcass (whole flies without the ovaries) titer between w Mel-like and w MelCS-like infected bam hypomorph differed during peak fertility rescue. A positive correlation emerged between the combined titers of ovarian and carcass infections and fertility, implying a more extensive influence that extends beyond ovarian infection alone.

Wolbachia Effect on Drosophila melanogaster Lipid and Carbohydrate Metabolism

The effect of maternally inherited endosymbiotic bacteria Wolbachia on triglyceride and carbohydrate metabolism, starvation resistance and feeding behavior of Drosophila melanogaster females was studied. Eight D. melanogaster lines of the same nuclear background were investigated; one had no infection and served as the control, and seven others were infected with different Wolbachia strains pertaining to wMel and wMelCS groups of genotypes. Most of the infected lines had a higher overall lipid content and triglyceride level than the control line and their expression of the bmm gene regulating triglyceride catabolism was reduced. The glucose content was higher in the infected lines compared to that in the control, while their trehalose levels were similar. It was also found that the Wolbachia infection reduced the level of tps1 gene expression (coding for enzyme for trehalose synthesis from glucose) and had no effect on treh gene expression (coding for trehalose degradation enzyme). The infected lines exhibited lower appetite but higher survival under starvation compared to the control. The data obtained may indicate that Wolbachia foster their hosts' energy exchange through increasing its lipid storage and glucose content to ensure the host's competitive advantage over uninfected individuals. The scheme of carbohydrate and lipid metabolism regulation under Wolbachia's influence was suggested.

Rare Wolbachia genotypes in laboratory Drosophila melanogaster strains

Symbiotic bacteria of the genus Wolbachia are widespread in Drosophila melanogaster populations. Based on the polymorphism of the Wolbachia genome, the symbionts’ diversity in D. melanogaster is presented by two groups: MEL (wMel, wMel2, wMel3 and wMel4) and CS (wMelCS and wMelCS2). The wMel genotype is predominant in natural D. melanogaster populations and is distributed all over the world. The CS genotypes, on the other hand, are of particular interest because it is unclear how they are maintained in the fruit f ly populations since they should have been eliminated from them due to their low frequency and genetic drift or been replaced by the wMel genotype. However, this is not what is really observed, which means these genotypes are supported by selection. It is known that the wMelPlus strain of the wMelCS genotype can increase the lifespan of infected f lies at high temperatures. The same genotype also increases the intensity of dopamine metabolism in Drosophila compared to the MEL-group genotypes. In the present study, we searched for the rare Wolbachia wMelCS and wMelCS2 genotypes, as well as for new genotypes in wild-type D. melanogaster strains and in several mutant laboratory strains. The symbiont was found in all populations, in 200 out of 385 wild-type strains and in 83 out of 170 mutant strains. Wolbachia diversity in D. melanogaster wild-type strains was represented by the wMel, wMelCS and wMelCS2 genotypes. More than 90 % of the infected strains carried wMel; 9 %, wMelCS2; and only two strains were found to carry wMelCS. No new Wolbachia genotypes were found. The northernmost point reported for the wMelCS2 genotype was Izhevsk city (Udmurtia, Russia). For the f irst time the wMelCS2 genotype was detected in D. melanogaster from the Sakhalin Island, and wMelCS, in the f lies from Nalchik (the North Caucasus). A comparison of Wolbachia genetic diversity between the wild-type laboratory strains and previously obtained data on mutant laboratory strains demonstrated differences in the frequencies of rare CS genotypes, which were more prevalent in mutant strains, apparently due to the breeding history of these Drosophila strains

Prevalence and genetic diversity of Wolbachia endosymbiont and mtDNA in Palearctic populations of Drosophila melanogaster

BACKGROUND

Maternally inherited Wolbachia symbionts infect D. melanogaster populations worldwide. Infection rates vary greatly. Genetic diversity of Wolbachia in D. melanogaster can be subdivided into several closely related genotypes coinherited with certain mtDNA lineages. mtDNA haplotypes have the following global distribution pattern: mtDNA clade I is mostly found in North America, II and IV in Africa, III in Europe and Africa, V in Eurasia, VI is global but very rare, and VIII is found in Asia. The wMel Wolbachia genotype is predominant in D. melanogaster populations. However, according to the hypothesis of global Wolbachia replacement, the wMelCS genotype was predominant before the XX century when it was replaced by the wMel genotype. Here we analyse over 1500 fly isolates from the Palearctic region to evaluate the prevalence, genetic diversity and distribution pattrern of the Wolbachia symbiont, occurrence of mtDNA variants, and finally to discuss the Wolbachia genotype global replacement hypothesis.

RESULTS

All studied Palearctic populations of D. melanogaster were infected with Wolbachia at a rate of 33-100%. We did not observe any significant correlation between infection rate and longitude or latitude. Five previously reported Wolbachia genotypes were found in Palearctic populations with a predominance of the wMel variant. The mtDNA haplotypes of the I_II_III clade and V clade were prevalent in Palearctic populations. To test the recent Wolbachia genotype replacement hypothesis, we examined three genomic regions of CS-like genotypes. Low genetic diversity was observed, only two haplotypes of the CS genotypes with a 'CCG' variant predominance were found.

CONCLUSION

The results of our survey of Wolbachia infection prevalence and genotype diversity in Palearctic D. melanogaster populations confirm previous studies. Wolbachia is ubiquitous in the Palearctic region. The wMel genotype is dominant with local occurrence of rare genotypes. Together with variants of the V mtDNA clade, the variants of the 'III+' clade are dominant in both infected and uninfected flies of Palearctic populations. Based on our data on Wolbachia and mtDNA in different years in some Palearctic localities, we can conclude that flies that survive the winter make the predominant symbiont contribution to the subsequent generation. A comprehensive overview of mtDNA and Wolbachia infection of D. melanogaster populations worldwide does not support the recent global Wolbachia genotype replacement hypothesis. However, we cannot exclude wMelCS genotype rate fluctuations in the past.

Drosophila female fertility and juvenile hormone metabolism depends on the type of Wolbachia infection

Maternally inherited intracellular bacteria Wolbachia cause both parasitic and mutualistic effects on their numerous insect hosts, including manipulating the host reproductive system in order to increase the bacteria spreading in a host population, and increasing the host fitness. Here, we demonstrate that the type of Wolbachia infection determines the effect on Drosophila melanogaster egg production as a proxy for fecundity, and metabolism of juvenile hormone (JH), which acts as gonadotropin in adult insects. For this study, we used six D. melanogaster lineages carrying the nuclear background of interbred Bi90 lineage and cytoplasmic backgrounds with or without Wolbachia of different genotype variants. The wMelCS genotype of Wolbachia decreases egg production in infected D. melanogaster females in the beginning of oviposition and increases it later (from the sixth day after eclosion), whereas the wMelPop Wolbachia strain causes the opposite effect, and the wMel, wMel2 and wMel4 genotypes of Wolbachia do not show any effect on these traits compared with uninfected Bi90 D. melanogaster females. The intensity of JH catabolism negatively correlates with the fecundity level in the flies carrying both wMelCS and wMelPop Wolbachia The JH catabolism in females infected with genotypes of the wMel group does not differ from that in uninfected females. The effects of wMelCS and wMelPop infection on egg production can be levelled by the modulation of JH titre (via precocene/JH treatment of the flies). Thus, at least one of the mechanisms promoting the effect of Wolbachia on D. melanogaster female fecundity is mediated by JH.

Various Wolbachia genotypes differently influence host Drosophila dopamine metabolism and survival under heat stress conditions

BACKGROUND

One of the most widespread prokaryotic symbionts of invertebrates is the intracellular bacteria of Wolbachia genus which can be found in about 50% of insect species. Wolbachia causes both parasitic and mutualistic effects on its host that include manipulating the host reproductive systems in order to increase their transmission through the female germline, and increasing the host fitness. One of the mechanisms, promoting adaptation in biological organisms, is a non-specific neuroendocrine stress reaction. In insects, this reaction includes catecholamines, dopamine, serotonin and octopamine, which act as neurotransmitters, neuromodulators and neurohormones. The level of dopamine metabolism correlates with heat stress resistance in Drosophila adults.

RESULTS

To examine Wolbachia effect on Drosophila survival under heat stress and dopamine metabolism we used five strains carrying the nuclear background of interbred Bi90 strain and cytoplasmic backgrounds with different genotype variants of Wolbachia (produced by 20 backcrosses of Bi90 males with appropriate source of Wolbachia). Non-infected Bi90 strain (treated with tetracycline for 3 generations) was used as a control group. We demonstrated that two of five investigated Wolbachia variants promote changes in Drosophila heat stress resistance and activity of enzymes that produce and degrade dopamine, alkaline phosphatase and dopamine-dependent arylalkylamine N-acetyltransferase. What is especially interesting, wMelCS genotype of Wolbachia increases stress resistance and the intensity of dopamine metabolism, whereas wMelPop strain decreases them. wMel, wMel2 and wMel4 genotypes of Wolbachia do not show any effect on the survival under heat stress or dopamine metabolism. L-DOPA treatment, known to increase the dopamine content in Drosophila, levels the difference in survival under heat stress between all studied groups.

CONCLUSIONS

The genotype of symbiont determines the effect that the symbiont has on the stress resistance of the host insect.

Wolbachia variants induce differential protection to viruses in Drosophila melanogaster: a phenotypic and phylogenomic analysis

Wolbachia are intracellular bacterial symbionts that are able to protect various insect hosts from viral infections. This tripartite interaction was initially described in Drosophila melanogaster carrying wMel, its natural Wolbachia strain. wMel has been shown to be genetically polymorphic and there has been a recent change in variant frequencies in natural populations. We have compared the antiviral protection conferred by different wMel variants, their titres and influence on host longevity, in a genetically identical D. melanogaster host. The phenotypes cluster the variants into two groups--wMelCS-like and wMel-like. wMelCS-like variants give stronger protection against Drosophila C virus and Flock House virus, reach higher titres and often shorten the host lifespan. We have sequenced and assembled the genomes of these Wolbachia, and shown that the two phenotypic groups are two monophyletic groups. We have also analysed a virulent and over-replicating variant, wMelPop, which protects D. melanogaster even better than the closely related wMelCS. We have found that a ~21 kb region of the genome, encoding eight genes, is amplified seven times in wMelPop and may be the cause of its phenotypes. Our results indicate that the more protective wMelCS-like variants, which sometimes have a cost, were replaced by the less protective but more benign wMel-like variants. This has resulted in a recent reduction in virus resistance in D. melanogaster in natural populations worldwide. Our work helps to understand the natural variation in wMel and its evolutionary dynamics, and inform the use of Wolbachia in arthropod-borne disease control.