Infectious speciation revisited: impact of symbiont-depletion on female fitness and mating behavior of Drosophila paulistorum

Mating receptivity mediated by endosymbiont interactions in a haplodiploid thrips species

Many arthropods carry maternally inherited endosymbionts that cause cytoplasmic incompatibility (CI), manifested as embryonic mortality in matings of infected males with uninfected females. Infected females, however, do not suffer this cost. Therefore, in populations with mixed endosymbiont infections, selection is expected to favour mechanisms that enable hosts to avoid or mitigate CI. This may include changes in mating behaviour, such as reduced female receptivity to mating and/or remating when approached by incompatible males. Here, we investigated mating behavioural traits in haplodiploid thrips naturally associated with two CI-inducing endosymbionts, Cardinium and Wolbachia. Compared with females with both endosymbionts, those with only Cardinium showed reduced receptivity to males carrying both. However, surprisingly, females without endosymbionts were not less receptive to incompatible males. Furthermore, in contrast to females without endosymbionts, females with Cardinium were far less likely to remate with incompatible than compatible males irrespective of the compatibility type of the first mating. Our results suggest that endosymbiont-specific sexual selection processes occur, whereby females carrying only Cardinium recognize Wolbachia in coinfected males to avoid CI. This may hinder a CI-driven Wolbachia spread. Endosymbiont-mediated mating behaviours may be crucial for the dynamics of CI-inducing endosymbionts and their application in pest management strategies.

Wolbachia strain diversity in a complex group of sympatric cryptic parasitoid wasp species

BACKGROUND

Maternally-inherited symbionts can induce pre-mating and/or post-mating reproductive isolation between sympatric host lineages, and speciation, by modifying host reproductive phenotypes. The large parasitoid wasp genus Cotesia (Braconidae) includes a diversity of cryptic species, each specialized in parasitizing one to few related Lepidoptera host species. Here, we characterized the infection status of an assemblage of 21 Cotesia species from 15 countries by several microbial symbionts, as a first step toward investigating whether symbionts may provide a barrier to gene flow between these parasitoid host lineages.

RESULTS

The symbiotic microbes Arsenophonus, Cardinium, Microsporidium and Spiroplasma were not detected in the Cotesia wasps. However, the endosymbiotic bacterium Wolbachia was present in at least eight Cotesia species, and hence we concentrated on it upon screening additional DNA extracts and SRAs from NCBI. Some of the closely related Cotesia species carry similar Wolbachia strains, but most Wolbachia strains showed patterns of horizontal transfer between phylogenetically distant host lineages.

CONCLUSIONS

The lack of co-phylogenetic signal between Wolbachia and Cotesia suggests that the symbiont and hosts have not coevolved to an extent that would drive species divergence between the Cotesia host lineages. However, as the most common facultative symbiont of Cotesia species, Wolbachia may still function as a key-player in the biology of the parasitoid wasps. Its precise role in the evolution of this complex clade of cryptic species remains to be experimentally investigated.

Beyond Haldane's rule: Sex-biased hybrid dysfunction for all modes of sex determination

Haldane's rule occupies a special place in biology as one of the few 'rules' of speciation, with empirical support from hundreds of species. And yet, its classic purview is restricted taxonomically to the subset of organisms with heteromorphic sex chromosomes. I propose explicit acknowledgement of generalized hypotheses about Haldane's rule that frame sex bias in hybrid dysfunction broadly and irrespective of the sexual system. The consensus view of classic Haldane's rule holds that sex-biased hybrid dysfunction across taxa is a composite phenomenon that requires explanations from multiple causes. Testing of the multiple alternative hypotheses for Haldane's rule is, in many cases, applicable to taxa with homomorphic sex chromosomes, environmental sex determination, haplodiploidy, and hermaphroditism. Integration of a variety of biological phenomena about hybrids across diverse sexual systems, beyond classic Haldane's rule, will help to derive a more general understanding of the contributing forces and mechanisms that lead to predictable sex biases in evolutionary divergence and speciation.

With a little help from my friends: the roles of microbial symbionts in insect populations and communities

To understand insect abundance, distribution and dynamics, we need to understand the relevant drivers of their populations and communities. While microbial symbionts are known to strongly affect many aspects of insect biology, we lack data on their effects on populations or community processes, or on insects' evolutionary responses at different timescales. How these effects change as the anthropogenic effects on ecosystems intensify is an area of intense research. Recent developments in sequencing and bioinformatics permit cost-effective microbial diversity surveys, tracking symbiont transmission, and identification of functions across insect populations and multi-species communities. In this review, we explore how different functional categories of symbionts can influence insect life-history traits, how these effects could affect insect populations and their interactions with other species, and how they may affect processes and patterns at the level of entire communities. We argue that insect-associated microbes should be considered important drivers of insect response and adaptation to environmental challenges and opportunities. We also outline the emerging approaches for surveying and characterizing insect-associated microbiota at population and community scales. This article is part of the theme issue 'Towards a toolkit for global insect biodiversity monitoring'.

Tangled Banks, Braided Rivers and Complex Hierarchies: Beyond Microevolution and Macroevolution

Ever since the Modern Synthesis, a debate about the relationship between microevolution and macroevolution has persisted - specifically, whether they are equivalent, distinct, or explain one another. How one answers this has become shorthand for a much broader set of theoretical debates in evolutionary biology. Here, we examine microevolution and macroevolution in the context of the vast proliferation of data, knowledge, and theory since the advent of the Modern Synthesis. We suggest that traditional views on microevolution and macroevolution are too binary and reductive. For example, patterns and processes are not confined to micro- and macro- domains; they are interconnected at various temporal and spatial scales and across hierarchical entities. Further, biological entities have variably fuzzy boundaries, and evolutionary processes that influence macroevolution occur at micro- and macro- levels. In addition, these conceptual advances in phylodynamics have yet to be fully integrated with contemporary macroevolutionary approaches. Finally, holding microevolution and macroevolution as distinct domains thwarts synthesis and collaboration on important research questions. We propose that the focal entities and processes considered by evolutionary studies be contextualized within the newfound complexity of the multidimensional, multi-modal, multi-level phylogenetic system.

Wolbachia effects on thermal preference of natural Drosophila melanogaster are influenced by host genetic background, Wolbachia type, and bacterial titer

Temperature plays a fundamental role in the fitness of all organisms. In particular, it strongly affects metabolism and reproduction in ectotherms that have limited physiological capabilities to regulate their body temperature. The influence of temperature variation on the physiology and behaviour of ectotherms is well studied but we still know little about the influence of symbiotic interactions on thermal preference (Tp ) of the host. A growing number of studies focusing on the Wolbachia-Drosophila host-symbiont system found that Wolbachia can influence Tp in Drosophila laboratory strains. Here, we investigated the effect of Wolbachia on Tp in wild-type D. melanogaster flies recently collected from nature. Consistent with previous data, we found reduced Tp compared to an uninfected control in one of two fly strains infected with the wMelCS Wolbachia type. Additionally, we, for the first time, found that Wolbachia titer variation influences the thermal preference of the host fly. These data indicate that the interaction of Wolbachia and Drosophila resulting in behavioural variation is strongly influenced by the genetic background of the host and symbiont. More studies are needed to better understand the evolutionary significance of Tp variation influenced by Wolbachia in natural Drosophila populations.

Historic Museum Samples Provide Evidence for a Recent Replacement of Wolbachia Types in European Drosophila melanogaster

Wolbachia is one of the most common bacterial endosymbionts, which is frequently found in numerous arthropods and nematode taxa. Wolbachia infections can have a strong influence on the evolutionary dynamics of their hosts since these bacteria are reproductive manipulators that affect the fitness and life history of their host species for their own benefit. Host-symbiont interactions with Wolbachia are perhaps best studied in the model organism Drosophila melanogaster, which is naturally infected with at least 5 different variants among which wMel and wMelCS are the most frequent ones. Comparisons of infection types between natural flies and long-term lab stocks have previously indicated that wMelCS represents the ancestral type, which was only very recently replaced by the nowadays dominant wMel in most natural populations. In this study, we took advantage of recently sequenced museum specimens of D. melanogaster that have been collected 90 to 200 yr ago in Northern Europe to test this hypothesis. Our comparison to contemporary Wolbachia samples provides compelling support for the replacement hypothesis. Our analyses show that sequencing data from historic museum specimens and their bycatch are an emerging and unprecedented resource to address fundamental questions about evolutionary dynamics in host-symbiont interactions. However, we also identified contamination with DNA from crickets that resulted in co-contamination with cricket-specific Wolbachia in several samples. These results underpin the need for rigorous quality assessments of museomic data sets to account for contamination as a source of error that may strongly influence biological interpretations if it remains undetected.

The suppression of a selfish genetic element increases a male's mating success in a fly

X chromosome meiotic drive (XCMD) kills Y-bearing sperm during spermatogenesis, leading to the biased transmission of the selfish X chromosome. Despite this strong transmission, some natural XCMD systems remain at low and stable frequencies, rather than rapidly spreading through populations. The reason may be that male carriers can have reduced fitness, as they lose half of their sperm, only produce daughters, and may carry deleterious alleles associated with XCMD. Thus, females may benefit from avoiding mating with male carriers, yielding a further reduction in fitness. Genetic suppressors of XCMD, which block the killing of Y sperm and restore fair Mendelian inheritance, are also common and could prevent the spread of XCMD. However, whether suppressed males are as fit as a wild-type male remains an open question, as the effect that genetic suppressors may have on a male's mating success is rarely considered. Here, we investigate the mating ability of XCMD males and suppressed XCMD males in comparison to wild-type males in the fruit fly Drosophila subobscura, where drive remains at a stable frequency of 20% in wild populations where it occurs. We use both competitive and non-competitive mating trials to evaluate male mating success in this system. We found no evidence that unsuppressed XCMD males were discriminated against. Remarkably, however, their suppressed XCMD counterparts had a higher male mating success compared to wild-type controls. Unsuppressed XCMD males suffered 12% lower offspring production in comparison to wild-type males. This cost appears too weak to counter the transmission advantage of XCMD, and thus the factors preventing the spread of XCMD remain unclear.

Ovarian transcriptional response to Wolbachia infection in D. melanogaster in the context of between-genotype variation in gene expression

Wolbachia is a maternally transmitted endosymbiotic bacteria that infects a wide variety of arthropod and nematode hosts. The effects of Wolbachia on host biology are far-reaching and include changes in host gene expression. However, previous work on the host transcriptional response has generally been investigated in the context of a single host genotype. Thus, the relative effect of Wolbachia infection versus vs. host genotype on gene expression is unknown. Here, we explicitly test the relative roles of Wolbachia infection and host genotype on host gene expression by comparing the ovarian transcriptomes of 4 strains of Drosophila melanogaster (D. melanogaster) infected and uninfected with Wolbachia. Our data suggest that infection explains a small amount of transcriptional variation, particularly in comparison to variation in gene expression among strains. However, infection specifically affects genes related to cell cycle, translation, and metabolism. We also find enrichment of cell division and recombination processes among genes with infection-associated differential expression. Broadly, the transcriptomic changes identified in this study provide novel understanding of the relative magnitude of the effect of Wolbachia infection on gene expression in the context of host genetic variation and also point to genes that are consistently differentially expressed in response to infection among multiple genotypes.

Axenic and gnotobiotic insect technologies in research on host-microbiota interactions

Insects are one of the most important animal life forms on earth. Symbiotic microbes are closely related to the growth and development of the host insects and can affect pathogen transmission. For decades, various axenic insect-rearing systems have been developed, allowing further manipulation of symbiotic microbiota composition. Here we review the historical development of axenic rearing systems and the latest progress in using axenic and gnotobiotic approaches to study insect-microbe interactions. We also discuss the challenges of these emerging technologies, possible solutions to address these challenges, and future research directions that can contribute to a more comprehensive understanding of insect-microbe interactions.

Multiple introgressions shape mitochondrial evolutionary history in Drosophila paulistorum and the Drosophila willistoni group

Hybridization and the consequent introgression of genomic elements is an important source of genetic diversity for biological lineages. This is particularly evident in young clades in which hybrid incompatibilities are still incomplete and mixing between species is more likely to occur. Drosophila paulistorum, a representative of the Neotropical Drosophila willistoni subgroup, is a classic model of incipient speciation. The species is divided into six semispecies that show varying degrees of pre- and post-mating incompatibility with each other. In the present study, we investigate the mitochondrial evolutionary history of D. paulistorum and the willistoni subgroup. For that, we perform phylogenetic and comparative analyses of the complete mitochondrial genomes and draft nuclear assemblies of 25 Drosophila lines of the willistoni and saltans species groups. Our results show that the mitochondria of D. paulistorum are polyphyletic and form two non-sister clades that we name α and β. Identification and analyses of nuclear mitochondrial insertions further reveal that the willistoni subgroup has an α-like mitochondrial ancestor and strongly suggest that both the α and β mitochondria of D. paulistorum were acquired through introgression from unknown fly lineages of the willistoni subgroup. We also uncover multiple mitochondrial introgressions across D. paulistorum semispecies and generate novel insight into the evolution of the species.

Evidence of phylosymbiosis in Formica ants

Introduction

Insects share intimate relationships with microbes that play important roles in their biology. Yet our understanding of how host-bound microbial communities assemble and perpetuate over evolutionary time is limited. Ants host a wide range of microbes with diverse functions and are an emerging model for studying the evolution of insect microbiomes. Here, we ask whether phylogenetically related ant species have formed distinct and stable microbiomes.

Methods

To answer this question, we investigated the microbial communities associated with queens of 14 Formica species from five clades, using deep coverage 16S rRNA amplicon sequencing.

Results

We reveal that Formica species and clades harbor highly defined microbial communities that are dominated by four bacteria genera: Wolbachia, Lactobacillus, Liliensternia, and Spiroplasma. Our analysis reveals that the composition of Formica microbiomes mirrors the phylogeny of the host, i.e., phylosymbiosis, in that related hosts harbor more similar microbial communities. In addition, we find there are significant correlations between microbe co-occurrences.

Discussion

Our results demonstrate Formica ants carry microbial communities that recapitulate the phylogeny of their hosts. Our data suggests that the co-occurrence of different bacteria genera may at least in part be due to synergistic and antagonistic interactions between microbes. Additional factors potentially contributing to the phylosymbiotic signal are discussed, including host phylogenetic relatedness, host-microbe genetic compatibility, modes of transmission, and similarities in host ecologies (e.g., diets). Overall, our results support the growing body of evidence that microbial community composition closely depends on the phylogeny of their hosts, despite bacteria having diverse modes of transmission and localization within the host.

Preliminary Characterization of Phage-Like Particles from the Male-Killing Mollicute Spiroplasma poulsonii (an Endosymbiont of Drosophila)

Bacteriophages are vastly abundant, diverse, and influential, but with few exceptions (e.g. the Proteobacteria genera Wolbachia and Hamiltonella), the role of phages in heritable bacteria-arthropod interactions, which are ubiquitous and diverse, remains largely unexplored. Despite prior studies documenting phage-like particles in the mollicute Spiroplasma associated with Drosophila flies, genomic sequences of such phage are lacking, and their effects on the Spiroplasma-Drosophila interaction have not been comprehensively characterized. We used a density step gradient to isolate phage-like particles from the male-killing bacterium Spiroplasma poulsonii (strains NSRO and MSRO-Br) harbored by Drosophila melanogaster. Isolated particles were subjected to DNA sequencing, assembly, and annotation. Several lines of evidence suggest that we recovered phage-like particles of similar features (shape, size, DNA content) to those previously reported in Drosophila-associated Spiroplasma strains. We recovered three ~ 19 kb phage-like contigs (two in NSRO and one in MSRO-Br) containing 21-24 open reading frames, a read-alignment pattern consistent with circular permutation, and terminal redundancy (at least in NSRO). Although our results do not allow us to distinguish whether these phage-like contigs represent infective phage-like particles capable of transmitting their DNA to new hosts, their encoding of several typical phage genes suggests that they are at least remnants of functional phage. We also recovered two smaller non-phage-like contigs encoding a known Spiroplasma toxin (Ribosome Inactivating Protein; RIP), and an insertion element, suggesting that they are packaged into particles. Substantial homology of our particle-derived contigs was found in the genome assemblies of members of the Spiroplasma poulsonii clade.

Attempts to use breeding approaches in Aedes aegypti to create lines with distinct and stable relative Wolbachia densities

Wolbachia is an insect endosymbiont being used for biological control in the mosquito Aedes aegypti because it causes cytoplasmic incompatibility (CI) and limits viral replication of dengue, chikungunya, and Zika viruses. While the genetic mechanism of pathogen blocking (PB) is not fully understood, the strength of both CI and PB are positively correlated with Wolbachia densities in the host. Wolbachia densities are determined by a combination of Wolbachia strain and insect genotype, as well as interactions with the environment. We employed both artificial selection and inbreeding with the goal of creating lines of Ae. aegypti with heritable and distinct Wolbachia densities so that we might better dissect the mechanism underlying PB. We were unable to shift the mean relative Wolbachia density in Ae. aegypti lines by either strategy, with relative densities instead tending to cycle over a narrow range. In lieu of this, we used Wolbachia densities in mosquito legs as predictors of relative densities in the remaining individual's carcass. Because we worked with outbred mosquitoes, our findings indicate either a lack of genetic variation in the mosquito for controlling relative density, natural selection against extreme densities, or a predominance of environmental factors affecting densities. Our study reveals that there are moderating forces acting on relative Wolbachia densities that may help to stabilize density phenotypes post field release. We also show a means to accurately bin vector carcasses into high and low categories for non-DNA omics-based studies of Wolbachia-mediated traits.

Cophylogeny and specificity between cryptic coral species (Pocillopora spp.) at Mo'orea and their symbionts (Symbiodiniaceae)

The congruence between phylogenies of tightly associated groups of organisms (cophylogeny) reflects evolutionary links between ecologically important interactions. However, despite being a classic example of an obligate symbiosis, tests of cophylogeny between scleractinian corals and their photosynthetic algal symbionts have been hampered in the past because both corals and algae contain genetically unresolved and morphologically cryptic species. Here, we studied co-occurring, cryptic Pocillopora species from Mo'orea, French Polynesia, that differ in their relative abundance across depth. We constructed new phylogenies of the host Pocillopora (using complete mitochondrial genomes, genomic loci, and thousands of single nucleotide polymorphisms) and their Symbiodiniaceae symbionts (using ITS2 and psbA^ncr markers) and tested for cophylogeny. The analysis supported the presence of five Pocillopora species on the fore reef at Mo'orea that mostly hosted either Cladocopium latusorum or C. pacificum. Only Pocillopora species hosting C. latusorum also hosted taxa from Symbiodinium and Durusdinium. In general, the Cladocopium phylogeny mirrored the Pocillopora phylogeny. Within Cladocopium species, lineages also differed in their associations with Pocillopora haplotypes, except those showing evidence of nuclear introgression, and with depth in the two most common Pocillopora species. We also found evidence for a new Pocillopora species (haplotype 10), that has so far only been sampled from French Polynesia, that warrants formal identification. The linked phylogenies of these Pocillopora and Cladocopium species and lineages suggest that symbiont speciation is driven by niche diversification in the host, but there is still evidence for symbiont flexibility in some cases.

Bacterial supergroup-specific "cost" of Wolbachia infections in Nasonia vitripennis

The maternally inherited endosymbiont, Wolbachia, is known to alter the reproductive biology of its arthropod hosts for its own benefit and can induce both positive and negative fitness effects in many hosts. Here, we describe the effects of the maintenance of two distinct Wolbachia infections, one each from supergroups A and B, on the parasitoid host Nasonia vitripennis. We compare the effect of Wolbachia infections on various traits between the uninfected, single A-infected, single B-infected, and double-infected lines with their cured versions. Contrary to some previous reports, our results suggest that there is a significant cost associated with the maintenance of Wolbachia infections where traits such as family size, fecundity, longevity, and rates of male copulation are compromised in Wolbachia-infected lines. The double Wolbachia infection has the most detrimental impact on the host as compared to single infections. Moreover, there is a supergroup-specific negative impact on these wasps as the supergroup B infection elicits the most pronounced negative effects. These negative effects can be attributed to a higher Wolbachia titer seen in the double and the single supergroup B infection lines when compared to supergroup A. Our findings raise important questions on the mechanism of survival and maintenance of these reproductive parasites in arthropod hosts.

Effect of Wolbachia Infection and Adult Food on the Sexual Signaling of Males of the Mediterranean Fruit Fly Ceratitis capitata

Sexual signaling is a fundamental component of sexual behavior of Ceratitis capitata that highly determines males' mating success. Nutritional status and age are dominant factors known to affect males' signaling performance and define the female decision to accept a male as a sexual partner. Wolbachia pipientis, a widespread endosymbiotic bacterium of insects and other arthropods, exerts several biological effects on its hosts. However, the effects of Wolbachia infection on the sexual behavior of medfly and the interaction between Wolbachia infection and adult food remain unexplored. This study was conducted to determine the effects of Wolbachia on sexual signaling of protein-fed and protein-deprived males. Our findings demonstrate that: (a) Wolbachia infection reduced male sexual signaling rates in both food regimes; (b) the negative effect of Wolbachia infection was more pronounced on protein-fed than protein-deprived males, and it was higher at younger ages, indicating that the bacterium regulates male sexual maturity; (c) Wolbachia infection alters the daily pattern of sexual signaling; and (d) protein deprivation bears significant descent on sexual signaling frequency of the uninfected males, whereas no difference was observed for the Wolbachia-infected males. The impact of our findings on the implementation of Incompatible Insect Technique (IIT) or the combined SIT/IIT towards controlling insect pests is discussed.

Symbionts on the Brain: How Wolbachia Is Strictly Corralled in Some Neotropical Drosophila spp

Wolbachia is a heritable alphaproteobacterial symbiont of arthropods and nematodes, famous for its repertoire of host manipulations, including cytoplasmic incompatibility. To be vertically transmitted, Wolbachia must efficiently colonize the female germ line, although somatic tissues outside the gonads are also infected.

Analyses of locomotion, wing morphology, and microbiome in Drosophila nigrosparsa after recovery from antibiotics

Antibiotics, such as tetracycline, have been frequently used to cure arthropods of Wolbachia endosymbionts. After the symbionts have been removed, the hosts must recover for some generations from the side effects of the antibiotics. However, most studies do not assess the direct and indirect longer-term effects of antibiotics used to remove Wolbachia, which may question the exact contribution of this endosymbiont to the effects observed. Here, we used the fly Drosophila nigrosparsa treated or not with tetracycline for three generations followed by two generations of recovery to investigate the effects of this antibiotic on the fly locomotion, wing morphology, and the gut microbiome. We found that antibiotic treatment did not affect fly locomotion two generations after being treated with the antibiotic. In addition, gut-microbiome restoration was tested as a more efficient solution to reduce the potential side effects of tetracycline on the microbiome. There was no significant difference in alpha diversity between gut restoration and other treatments, but the abundance of some bacterial taxa differed significantly between the gut-restoration treatment and the control. We conclude that in D. nigrosparsa the recovery period of two generations after being treated with the antibiotic is sufficient for locomotion, and suggest a general assessment of direct and indirect effects of antibiotics after a particular recovery time.

Complex effects of environment and Wolbachia infections on the life history of Drosophila melanogaster hosts

Wolbachia bacteria are common endosymbionts of many arthropods found in gonads and various somatic tissues. They manipulate host reproduction to enhance their transmission and confer complex effects on fitness-related traits. Some of these effects can serve to increase the survival and transmission efficiency of Wolbachia in the host population. The Wolbachia-Drosophila melanogaster system represents a powerful model to study the evolutionary dynamics of host-microbe interactions and infections. Over the past decades, there has been a replacement of the ancestral wMelCS Wolbachia variant by the more recent wMel variant in worldwide D. melanogaster populations, but the reasons remain unknown. To investigate how environmental change and genetic variation of the symbiont affect host developmental and adult life-history traits, we compared effects of both Wolbachia variants and uninfected controls in wild-caught D. melanogaster strains at three developmental temperatures. While Wolbachia did not influence any developmental life-history traits, we found that both lifespan and fecundity of host females were increased without apparent fitness trade-offs. Interestingly, wMelCS-infected flies were more fecund than uninfected and wMel-infected flies. By contrast, males infected with wMel died sooner, indicating sex-specific effects of infection that are specific to the Wolbachia variant. Our study uncovered complex temperature-specific effects of Wolbachia infections, which suggests that symbiont-host interactions in nature are strongly dependent on the genotypes of both partners and the thermal environment.

A Margulian View of Symbiosis and Speciation: the Nasonia Wasp System

Species are fundamental units of biology that exemplify lineage diversification, while symbiosis of microbes and macrobial hosts exemplify lineage unification between the domains of life. While these conceptual differences between speciation and symbiosis often dominate the narrative of the respective fields, Lynn Margulis argued for interconnection between these two subdisciplines of biology in a manner that left a legacy for scholars and students alike to pursue, detail, and discover. The Margulian perspective has always been that host evolutionary processes such as speciation are more impacted by microbial symbioses than typically appreciated. In this article, we present and review the case system that she long envisioned, one in which layers of microbial symbiosis reduce species interbreeding and assist species diversification among a closely related group of small, metallic green, parasitoid wasps from the genus Nasonia.

The relevance of studying insect-nematode interactions for human disease

Vertebrate-parasitic nematodes cause debilitating, chronic infections in millions of people worldwide. The burden of these so-called 'neglected tropical diseases' is often carried by poorer socioeconomic communities in part because research on parasitic nematodes and their vertebrate hosts is challenging and costly. However, complex biological and pathological processes can be modeled in simpler organisms. Here, we consider how insight into the interactions between entomopathogenic nematodes (EPN), their insect hosts and bacterial symbionts may reveal novel treatment targets for parasitic nematode infections. We argue that a combination of approaches that target nematodes, as well as the interaction of pathogens with insect vectors and bacterial symbionts, offer potentially effective, but underexplored opportunities.

Restriction of Wolbachia Bacteria in Early Embryogenesis of Neotropical Drosophila Species via Endoplasmic Reticulum-Mediated Autophagy

Wolbachia are maternally transmitted intracellular bacteria that are not only restricted to the reproductive organs but also found in various somatic tissues of their native hosts. The abundance of the endosymbiont in the soma, usually a dead end for vertically transmitted bacteria, causes a multitude of effects on life history traits of their hosts, which are still not well understood. Thus, deciphering the host-symbiont interactions on a cellular level throughout a host's life cycle is of great importance to understand their homeostatic nature, persistence, and spreading success. Using fluorescent and transmission electron microscopy, we conducted a comprehensive analysis of Wolbachia tropism in soma and germ line of six Drosophila species at the intracellular level during host development. Our data uncovered diagnostic patterns of infections to embryonic primordial germ cells and to particular cells of the soma in three different neotropical Drosophila species that have apparently evolved independently. We further found that restricted patterns of Wolbachia tropism are determined in early embryogenesis via selective autophagy, and their spatially restricted infection patterns are preserved in adult flies. We observed tight interactions of Wolbachia with membranes of the endoplasmic reticulum, which might play a scaffolding role for autophagosome formation and subsequent elimination of the endosymbiont. Finally, by analyzing D. simulans lines transinfected with nonnative Wolbachia, we uncovered that the host genetic background regulates tissue tropism of infection. Our data demonstrate a novel and peculiar mechanism to limit and spatially restrict bacterial infection in the soma during a very early stage of host development. IMPORTANCE All organisms are living in close and intimate interactions with microbes that cause conflicts but also cooperation between both unequal genetic partners due to their different innate interests of primarily enhancing their own fitness. However, stable symbioses often result in homeostatic interaction, named mutualism, by balancing costs and benefits, where both partners profit. Mechanisms that have evolved to balance and stably maintain homeostasis in mutualistic relationships are still quite understudied; one strategy is to "domesticate" potentially beneficial symbionts by actively controlling their replication rate below a critical and, hence, costly threshold, and/or to spatially and temporally restrict their localization in the host organism, which, in the latter case, in its most extreme form, is the formation of a specialized housing organ for the microbe (bacteriome). However, questions remain: how do these mutualistic associations become established in their first place, and what are the mechanisms for symbiont control and restriction in their early stages? Here, we have uncovered an unprecedented symbiont control mechanism in neotropical Drosophila species during early embryogenesis. The fruit fly evolved selective autophagy to restrict and control the proliferation of its intracellular endosymbiont Wolbachia in a defined subset of the stem cells as soon as the host's zygotic genome is activated.

Research Progress of the Gut Microbiome in Hybrid Fish

Fish, including hybrid species, are essential components of aquaculture, and the gut microbiome plays a vital role in fish growth, behavior, digestion, and immune health. The gut microbiome can be affected by various internal and/or external factors, such as host development, diet, and environment. We reviewed the effects of diet and dietary supplements on intestinal microorganisms in hybrid fish and the difference in the gut microbiome between the hybrid and their hybrids that originate. Then, we summarized the role of the gut microbiome in the speciation and ecological invasion of hybrid fish. Finally, we discussed possible future studies on the gut microbiome in hybrid fish, including the potential interaction with environmental microbiomes, the effects of the gut microbiome on population expansion, and fish conservation and management.

Environmental Temperature, but Not Male Age, Affects Wolbachia and Prophage WO Thereby Modulating Cytoplasmic Incompatibility in the Parasitoid Wasp, Habrobracon Hebetor

Wolbachia is an endosymbiotic bacterium found in many species of arthropods and manipulates its host reproduction. Cytoplasmic incompatibility (CI) is one of the most common manipulations that is induced when an uninfected female mates with a Wolbachia-infected male. The CI factors (cifA and cifB genes) are encoded by phage WO that naturally infects Wolbachia. Here, we questioned whether an environmental factor (temperature) or host factor (male age) affected the strength of the CI phenotype in the ectoparasitoid wasp, Habrobracon hebetor. We found that temperature, but not male age, results in reduced CI penetrance. Consistent with these results, we also found that the expression of the cif CI factors decreased in temperature-exposed males but was consistent across aging male wasps. Similar to studies of other insect systems, cifA showed a higher expression level than cifB, and male hosts showed increased cif expression relative to females. Our results suggest that prophage WO is present in the Wolbachia-infected wasps and expression of cif genes contributes to the induction of CI in this insect. It seems that male aging has no effect on the intensity of CI; however, temperature affects Wolbachia and prophage WO titers as well as expression levels of cif genes, which modulate the CI level.

Wolbachia promotes successful sex with siblings in the parasitoid Habrobracon hebetor

BACKGROUND

Wolbachia are intracellular α-proteobacteria that have a wide distribution among various arthropods and nematodes. They affect the host reproduction favoring their maternal transmission, which sets up a potential conflict in inbreeding situations when the host avoids sexual reproduction preventing inbreeding depression, while Wolbachia pushes it. We used the wasp Habrobracon hebetor to test the hypothesis that Wolbachia modulates inbreeding avoidance behavior and promotes sib mating.

RESULTS

Our results showed no obvious pre-copulatory inbreeding avoidance in this wasp. However, H. hebetor showed a strong post-copulatory inbreeding avoidance behavior that resulted in a low fertilization rate of uninfected siblings and therefore high rate of production of male progeny was obtained. We observed higher rates of fertilization success in the Wolbachia-infected lines that resulted in significantly higher female progeny production compared to the uninfected sib mates. Since diploid females are the result of successful fertilization due to haplodiploidy sex determination system in this insect, our results indicate that Wolbachia promoted fertile sib mating in H. hebetor. Interestingly, the rate of adult emergence in the progeny of Wolbachia-infected sib mates were almost similar to the non-sib mate crosses and significantly more than those observed in the uninfected sib mate crosses.

CONCLUSION

Our results support the idea that Wolbachia modulates inbreeding avoidance and promotes sib mating and also mitigates inbreeding depression. By promoting successful sex with siblings and increasing the probability of female progeny, Wolbachia enhances its transmission to the next generation. This is an undescribed effect of Wolbachia on the host reproduction. © 2021 Society of Chemical Industry.

The microbiome impacts host hybridization and speciation

Microbial symbiosis and speciation profoundly shape the composition of life's biodiversity. Despite the enormous contributions of these two fields to the foundations of modern biology, there is a vast and exciting frontier ahead for research, literature, and conferences to address the neglected prospects of merging their study. Here, we survey and synthesize exemplar cases of how endosymbionts and microbial communities affect animal hybridization and vice versa. We conclude that though the number of case studies remain nascent, the wide-ranging types of animals, microbes, and isolation barriers impacted by hybridization will likely prove general and a major new phase of study that includes the microbiome as part of the functional whole contributing to reproductive isolation. Though microorganisms were proposed to impact animal speciation a century ago, the weight of the evidence supporting this view has now reached a tipping point.

Testing the potential contribution of Wolbachia to speciation when cytoplasmic incompatibility becomes associated with host-related reproductive isolation

Endosymbiont‐induced cytoplasmic incompatibility (CI) may play an important role in arthropod speciation. However, whether CI consistently becomes associated or coupled with other host‐related forms of reproductive isolation (RI) to impede the transfer of endosymbionts between hybridizing populations and further the divergence process remains an open question. Here, we show that varying degrees of pre‐ and postmating RI exist among allopatric populations of two interbreeding cherry‐infesting tephritid fruit flies (Rhagoletis cingulata and R. indifferens) across North America. These flies display allochronic and sexual isolation among populations, as well as unidirectional reductions in egg hatch in hybrid crosses involving southwestern USA males. All populations are infected by a Wolbachia strain, wCin2, whereas a second strain, wCin3, only co‐infects flies from the southwest USA and Mexico. Strain wCin3 is associated with a unique mitochondrial DNA haplotype and unidirectional postmating RI, implicating the strain as the cause of CI. When coupled with nonendosymbiont RI barriers, we estimate the strength of CI associated with wCin3 would not prevent the strain from introgressing from infected southwestern to uninfected populations elsewhere in the USA if populations were to come into secondary contact and hybridize. In contrast, cytoplasmic–nuclear coupling may impede the transfer of wCin3 if Mexican and USA populations were to come into contact. We discuss our results in the context of the general paucity of examples demonstrating stable Wolbachia hybrid zones and whether the spread of Wolbachia among taxa can be constrained in natural hybrid zones long enough for the endosymbiont to participate in speciation.

Wolbachia in the spittlebug Prosapia ignipectus: Variable infection frequencies, but no apparent effect on host reproductive isolation

Animals serve as hosts for complex communities of microorganisms, including endosymbionts that live inside their cells. Wolbachia bacteria are perhaps the most common endosymbionts, manipulating host reproduction to propagate. Many Wolbachia cause cytoplasmic incompatibility (CI), which results in reduced egg hatch when uninfected females mate with infected males. Wolbachia that cause intense CI spread to high and relatively stable frequencies, while strains that cause weak or no CI tend to persist at intermediate, often variable, frequencies. Wolbachia could also contribute to host reproductive isolation (RI), although current support for such contributions is limited to a few systems. To test for Wolbachia frequency variation and effects on host RI, we sampled several local Prosapia ignipectus (Fitch) (Hemiptera: Cercopidae) spittlebug populations in the northeastern United States over two years, including closely juxtaposed Maine populations with different monomorphic color forms, "black" and "lined." We discovered a group-B Wolbachia (wPig) infecting P. ignipectus that diverged from group-A Wolbachia-like model wMel and wRi strains in Drosophila-6 to 46 MYA. Populations of the sister species Prosapia bicincta (Say) from Hawaii and Florida are uninfected, suggesting that P. ignipectus acquired wPig after their initial divergence. wPig frequencies were generally high and variable among sites and between years. While phenotyping wPig effects on host reproduction is not currently feasible, the wPig genome contains three divergent sets of CI loci, consistent with high wPig frequencies. Finally, Maine monomorphic black and monomorphic lined populations of P. ignipectus share both wPig and mtDNA haplotypes, implying no apparent effect of wPig on the maintenance of this morphological contact zone. We hypothesize P. ignipectus acquired wPig horizontally as observed for many Drosophila species, and that significant CI and variable transmission produce high but variable wPig frequencies.

Wolbachia and host intrinsic reproductive barriers contribute additively to postmating isolation in spider mites

Wolbachia are maternally-inherited bacteria that induce cytoplasmic incompatibility in many arthropod species. However, the ubiquity of this isolation mechanism for host speciation processes remains elusive, as only few studies have examined Wolbachia-induced incompatibilities when host populations are not genetically compatible. Here, we used three populations of two genetically differentiated colour forms of the haplodiploid spider mite Tetranychus urticae to dissect the interaction between Wolbachia-induced and host-associated incompatibilities, and their relative contribution to postmating isolation. We found that these two sources of incompatibility act through different mechanisms in an additive fashion. Host-associated incompatibility contributes 1.5 times more than Wolbachia-induced incompatibility in reducing hybrid production, the former through an overproduction of haploid sons at the expense of diploid daughters (ca. 75% decrease) and the latter by increasing the embryonic mortality of daughters (by ca. 49%). Furthermore, regardless of cross direction, we observed near-complete F1 hybrid sterility and complete F2 hybrid breakdown between populations of the two forms, but Wolbachia did not contribute to this outcome. We thus show mechanistic independence and an additive nature of host-intrinsic and Wolbachia-induced sources of isolation. Wolbachia may contribute to reproductive isolation in this system, thereby potentially affecting host differentiation and distribution in the field.

Living in the endosymbiotic world of Wolbachia: A centennial review

The most widespread intracellular bacteria in the animal kingdom are maternally inherited endosymbionts of the genus Wolbachia. Their prevalence in arthropods and nematodes worldwide and stunning arsenal of parasitic and mutualistic adaptations make these bacteria a biological archetype for basic studies of symbiosis and applied outcomes for curbing human and agricultural diseases. Here, we conduct a summative, centennial analysis of living in the Wolbachia world. We synthesize literature on Wolbachia's host range, phylogenetic diversity, genomics, cell biology, and applications to filarial, arboviral, and agricultural diseases. We also review the mobilome of Wolbachia including phage WO and its essentiality to hallmark reproductive phenotypes in arthropods. Finally, the Wolbachia system is an exemplar for discovery-based science education using biodiversity, biotechnology, and bioinformatics lessons. As we approach a century of Wolbachia research, the interdisciplinary science of this symbiosis stands as a model for consolidating and teaching the integrative rules of endosymbiotic life.

Effect of Cardinium Infection on the Probing Behavior of Bemisia tabaci (Hemiptera: Aleyrodidae) MED

Facultative endosymbionts can affect the growth, physiology, and behavior of their arthropod hosts. There are several endosymbionts in the invasive whitefly Bemisia tabaci Mediterranean (MED, Q biotype) that influence host fitness by altering stylet probing behavior. We investigated the probing behavior of B. tabaci MED infected with the facultative endosymbiont Candidatus Cardinium hertigii (Cardinium (Sphingobacteriales: Flexibacteraceae)). We generated genetically similar Cardinium-infected (C*+) and uninfected (C-) clonal sublines and analyzed the probing behavior of newly emerged adult on cotton (Malvales: Malvaceae), Gossypium hirsutum L., using electropenetrography (EPG). The C- subline demonstrated a longer duration of E2 (2.81-fold) and more events of E2 (2.22-fold) than the C*+ subline, indicating a greater level of sustained ingestion of plant phloem. These findings provide insight into the fitness costs (fitness of a particular genotype is lower than the average fitness of the population) of the Cardinium-infected B. tabaci.

Coadaptation between host genome and microbiome under long-term xenobiotic-induced selection

One of the most difficult experimental challenges today is testing the evolutionary dynamics shaping complex host-microbiome interactions. We investigated host-microbiome codiversification in response to xenobiotic-induced selection using an experimental evolution approach. To this end, we exposed the parasitoid wasp Nasonia vitripennis to sublethal concentrations of the widely used herbicide atrazine for 85 generations. Our results reveal that atrazine exposure not only mediated adaptive changes within the microbiome, which conferred host resistance to atrazine toxicity, but also exerted selective pressure on the host genome and altered host gene expression and immune response. Furthermore, microbiome transplant experiments reveal a decreased survival of adults from the control population after exposure to the evolved microbiome of the atrazine-exposed population, while no such decrease occurred in the reciprocal transplant. These results indicate that xenobiotic-induced selection mediated host-microbiome coadaptation, ultimately leading to a new host genome-microbiome equilibrium.

Rapid divergence of the male reproductive proteins in the Drosophila dunni group and implications for postmating incompatibilities between species

Proteins involved in post-copulatory interactions between males and females are among the fastest evolving genes in many species, usually attributed to their involvement in reproductive conflict. As a result, these proteins are thought to often be involved in the formation of postmating-prezygotic incompatibilities between species. The Drosophila dunni subgroup consists of a dozen recently diverged species found across the Caribbean islands with varying levels of hybrid incompatibility. We performed experimental crosses between species in the dunni group and see some evidence of hybrid incompatibilities. We also find evidence of reduced survival following hybrid mating, likely due to postmating-prezygotic incompatibilities. We assessed rates of evolution between these species genomes and find evidence of rapid evolution and divergence of some reproductive proteins, specifically the seminal fluid proteins. This work suggests the rapid evolution of seminal fluid proteins may be associated with postmating-prezygotic isolation, which acts as a barrier for gene flow between even the most closely related species.

Comparative studies on speciation: 30 years since Coyne and Orr

Understanding the processes of population divergence and speciation remains a core question in evolutionary biology. For nearly a hundred years evolutionary geneticists have characterized reproductive isolation (RI) mechanisms and specific barriers to gene flow required for species formation. The seminal work of Coyne and Orr provided the first comprehensive comparative analysis of speciation. By combining phylogenetic hypotheses and species range data with estimates of genetic divergence and multiple mechanisms of RI across Drosophila, Coyne and Orr's influential meta-analyses answered fundamental questions and motivated new analyses that continue to push the field forward today. Now 30 years later, we revisit the five questions addressed by Coyne and Orr, identifying results that remain well supported and others that seem less robust with new data. We then consider the future of speciation research, with emphasis on areas where novel methods and data motivate potential progress. While the literature remains biased towards Drosophila and other model systems, we are enthusiastic about the future of the field.

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

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

The Gut-Brain-Microbiome Axis in Bumble Bees

The brain-gut–microbiome axis is an emerging area of study, particularly in vertebrate systems. Existing evidence suggests that gut microbes can influence basic physiological functions and that perturbations to the gut microbiome can have deleterious effects on cognition and lead to neurodevelopmental disorders. While this relationship has been extensively studied in vertebrate systems, little is known about this relationship in insects. We hypothesized that because of its importance in bee health, the gut microbiota influences learning and memory in adult bumble bees. As an initial test of whether there is a brain-gut–microbiome axis in bumble bees, we reared microbe-inoculated and microbe-depleted bees from commercial Bombus impatiens colonies. We then conditioned experimental bees to associate a sucrose reward with a color and tested their ability to learn and remember the rewarding color. We found no difference between microbe-inoculated and microbe-depleted bumble bees in performance during the behavioral assay. While these results suggest that the brain-gut–microbiome axis is not evident in Bombus impatiens, future studies with different invertebrate systems are needed to further investigate this phenomenon.

Wolbachia pipientis Associated With Tephritid Fruit Fly Pests: From Basic Research to Applications

Members of the true fruit flies (family Tephritidae) are among the most serious agricultural pests worldwide, whose control and management demands large and costly international efforts. The need for cost-effective and environmentally friendly integrated pest management (IPM) has led to the development and implementation of autocidal control strategies. These approaches include the widely used sterile insect technique and the incompatible insect technique (IIT). IIT relies on maternally transmitted bacteria (namely Wolbachia) to cause a conditional sterility in crosses between released mass-reared Wolbachia-infected males and wild females, which are either uninfected or infected with a different Wolbachia strain (i.e., cytoplasmic incompatibility; CI). Herein, we review the current state of knowledge on Wolbachia-tephritid interactions including infection prevalence in wild populations, phenotypic consequences, and their impact on life history traits. Numerous pest tephritid species are reported to harbor Wolbachia infections, with a subset exhibiting high prevalence. The phenotypic effects of Wolbachia have been assessed in very few tephritid species, due in part to the difficulty of manipulating Wolbachia infection (removal or transinfection). Based on recent methodological advances (high-throughput DNA sequencing) and breakthroughs concerning the mechanistic basis of CI, we suggest research avenues that could accelerate generation of necessary knowledge for the potential use of Wolbachia-based IIT in area-wide integrated pest management (AW-IPM) strategies for the population control of tephritid pests.

The gut microbiota - brain axis of insects

Research on the connections between gut microbes and the neurophysiology and behavior of their animal hosts has grown exponentially in just a few years. Most studies have focused on mammalian models as their relevance to human health is widely established. However, evidence is accumulating that insect behavior may be governed by molecular mechanisms that are partly homologous to those of mammals, and therefore relevant for the understanding of their behavioral dysfunctions. Social insects in particular may provide experimentally amenable models to disentangle the contributions of individual bacterial symbionts to the gut microbiota - brain axis. In this review, we summarize findings from recent research on the neurological and behavioral effects of the gut microbiota of insects and propose an integrated approach to unravel the extended behavioral phenotypes of gut microbes in the honey bee.

Genomic differentiation and intercontinental population structure of mosquito vectors Culex pipiens pipiens and Culex pipiens molestus

Understanding the population structure and mechanisms of taxa diversification is important for organisms responsible for the transmission of human diseases. Two vectors of West Nile virus, Culex pipiens pipiens and Cx. p. molestus, exhibit epidemiologically important behavioral and physiological differences, but the whole-genome divergence between them was unexplored. The goal of this study is to better understand the level of genomic differentiation and population structures of Cx. p. pipiens and Cx. p. molestus from different continents. We sequenced and compared the whole genomes of 40 individual mosquitoes from two locations in Eurasia and two in North America. Principal Component, ADMIXTURE, and neighbor joining analyses of the nuclear genomes identified two major intercontinental, monophyletic clusters of Cx. p. pipiens and Cx. p. molestus. The level of genomic differentiation between the subspecies was uniform along chromosomes. The ADMIXTURE analysis determined signatures of admixture in Cx. p. pipens populations but not in Cx. p. molestus populations. Comparison of mitochondrial genomes among the specimens showed a paraphyletic origin of the major haplogroups between the subspecies but a monophyletic structure between the continents. Thus, our study identified that Cx. p. molestus and Cx. p. pipiens represent different evolutionary units with monophyletic origin that have undergone incipient ecological speciation.

Wolbachia affect behavior and possibly reproductive compatibility but not thermoresistance, fecundity, and morphology in a novel transinfected host, Drosophila nigrosparsa

Wolbachia, intracellular endosymbionts, are estimated to infect about half of all arthropod species. These bacteria manipulate their hosts in various ways for their maximum benefits. The rising global temperature may accelerate species migration, and thus, horizontal transfer of Wolbachia may occur across species previously not in contact. We transinfected and then cured the alpine fly Drosophila nigrosparsa with Wolbachia strain wMel to study its effects on this species. We found low Wolbachia titer, possibly cytoplasmic incompatibility, and an increase in locomotion of both infected larvae and adults compared with cured ones. However, no change in fecundity, no impact on heat and cold tolerance, and no change in wing morphology were observed. Although Wolbachia increased locomotor activities in this species, we conclude that D. nigrosparsa may not benefit from the infection. Still, D. nigrosparsa can serve as a host for Wolbachia because vertical transmission is possible but may not be as high as in the native host of wMel, Drosophila melanogaster.

Microorganisms in the reproductive tissues of arthropods

Microorganisms that reside within or transmit through arthropod reproductive tissues have profound impacts on host reproduction, health and evolution. In this Review, we discuss select principles of the biology of microorganisms in arthropod reproductive tissues, including bacteria, viruses, protists and fungi. We review models of specific symbionts, routes of transmission, and the physiological and evolutionary outcomes for both hosts and microorganisms. We also identify areas in need of continuing research, to answer the fundamental questions that remain in fields within and beyond arthropod-microorganism associations. New opportunities for research in this area will drive a broader understanding of major concepts as well as the biodiversity, mechanisms and translational applications of microorganisms that interact with host reproductive tissues.

Wolbachia infection in Argentinean populations of Anastrepha fraterculus sp1: preliminary evidence of sex ratio distortion by one of two strains

Background

Wolbachia, one of the most abundant taxa of intracellular Alphaproteobacteria, is widespread among arthropods and filarial nematodes. The presence of these maternally inherited bacteria is associated with modifications of host fitness, including a variety of reproductive abnormalities, such as cytoplasmic incompatibility, thelytokous parthenogenesis, host feminization and male-killing. Wolbachia has attracted much interest for its role in biological, ecological and evolutionary processes as well as for its potential use in novel and environmentally-friendly strategies for the control of insect pests and disease vectors including a major agricultural pest, the South American fruit fly, Anastrepha fraterculus Wiedemann (Diptera: Tephritidae).

Results

We used wsp, 16S rRNA and a multilocus sequence typing (MLST) scheme including gatB, coxA, hcpA, fbpA, and ftsZ genes to detect and characterize the Wolbachia infection in laboratory strains and wild populations of A. fraterculus from Argentina. Wolbachia was found in all A. fraterculus individuals studied. Nucleotide sequences analysis of wsp gene allowed the identification of two Wolbachia nucleotide variants (named wAfraCast1_A and wAfraCast2_A). After the analysis of 76 individuals, a high prevalence of the wAfraCast2_A variant was found both, in laboratory (82%) and wild populations (95%). MLST analysis identified both Wolbachia genetic variants as sequence type 13. Phylogenetic analysis of concatenated MLST datasets clustered wAfraCast1/2_A in the supergroup A. Paired-crossing experiments among single infected laboratory strains showed a phenotype specifically associated to wAfraCast1_A that includes slight detrimental effects on larval survival, a female-biased sex ratio; suggesting the induction of male-killing phenomena, and a decreased proportion of females producing descendants that appears attributable to the lack of sperm in their spermathecae.

Conclusions

We detected and characterized at the molecular level two wsp gene sequence variants of Wolbachia both in laboratory and wild populations of A. fraterculus sp.1 from Argentina. Crossing experiments on singly-infected A. fraterculus strains showed evidence of a male killing-like mechanism potentially associated to the wAfraCast1_A - A. fraterculus interactions. Further mating experiments including antibiotic treatments and the analysis of early and late immature stages of descendants will contribute to our understanding of the phenotypes elicited by the Wolbachia variant wAfraCast1_A in A. fraterculus sp.1.

Medfly-Wolbachia symbiosis: genotype x genotype interactions determine host's life history traits under mass rearing conditions

BACKGROUND

Wolbachia pipientis is a widespread, obligatory intracellular and maternally inherited bacterium, that induces a wide range of reproductive alterations to its hosts. Cytoplasmic Incompatibility (CI) is causing embryonic lethality, the most common of them. Despite that Wolbachia-borne sterility has been proposed as an environmental friendly pest control method (Incompatible Insect Technique, IIT) since 1970s, the fact that Wolbachia modifies important fitness components of its hosts sets severe barriers to IIT implementation. Mass rearing of Mediterranean fruit fly, Ceratitis capitata (medfly), is highly optimized given that this pest is a model species regarding the implementation of another sterility based pest control method, the Sterile Insect Technique (SIT). We used the medfly-Wolbachia symbiotic association, as a model system, to study the effect of two different Wolbachia strains, on the life history traits of 2 C. capitata lines with different genomic background.

RESULTS

Wolbachia effects are regulated by both C. capitata genetic background and the Wolbachia strain. Wolbachia infection reduces fertility rates in both C. capitata genetic backgrounds and shortens the pre-pupa developmental duration in the GSS strain. On the other hand, regardless of the strain of Wolbachia (wCer2, wCer4) infection does not affect either the sex ratio or the longevity of adults. wCer4 infection imposed a reduction in females' fecundity but wCer2 did not. Male mating competitiveness, adults flight ability and longevity under water and food deprivation were affected by both the genetic background of medfly and the strain of Wolbachia (genotype by genotype interaction).

CONCLUSION

Wolbachia infection could alter important life history traits of mass-reared C. capitata lines and therefore the response of each genotype on the Wolbachia infection should be considered toward ensuring the productivity of the Wolbachia-infected insects under mass-rearing conditions.

Paternal Grandmother Age Affects the Strength of Wolbachia-Induced Cytoplasmic Incompatibility in Drosophila melanogaster

Wolbachia are obligate intracellular bacteria that are globally distributed in half of all arthropod species. As the most abundant maternally inherited microbe in animals, Wolbachia manipulate host reproduction via reproductive parasitism strategies, including cytoplasmic incompatibility (CI). CI manifests as embryonic death when Wolbachia-modified sperm fertilize uninfected eggs but not maternally infected eggs. Thus, CI can provide a relative fitness advantage to Wolbachia-infected females and drive the infection through a population. In the genetic model Drosophila melanogaster, the Wolbachia strain wMel induces variable CI, making mechanistic studies in D. melanogaster cumbersome. Here, we demonstrate that sons of older paternal D. melanogaster grandmothers induce stronger CI than sons of younger paternal grandmothers, and we term this relationship the "paternal grandmother age effect" (PGAE). Moreover, the embryos and adult sons of older D. melanogaster grandmothers have higher Wolbachia densities, correlating with their ability to induce stronger CI. In addition, we report that Wolbachia density positively correlates with female age and decreases after mating, suggesting that females transmit Wolbachia loads that are proportional to their own titers. These findings reveal a transgenerational impact of age on wMel-induced CI, elucidate Wolbachia density dynamics in D. melanogaster, and provide a methodological advance to studies aimed at understanding wMel-induced CI in the D. melanogaster model.IMPORTANCE Unidirectional cytoplasmic incompatibility (CI) results in a postfertilization incompatibility between Wolbachia-infected males and uninfected females. CI contributes to reproductive isolation between closely related species and is used in worldwide vector control programs to drastically lower arboviral vector population sizes or to replace populations that transmit arboviruses with those resistant to transmission. Despite decades of research on the factors that influence CI, penetrance is often variable under controlled laboratory conditions in various arthropods, suggesting that additional variables influence CI strength. Here, we demonstrate that paternal D. melanogaster grandmother age influences the strength of CI induced by their sons. Older D. melanogaster females have higher Wolbachia densities and produce offspring with higher Wolbachia densities that associate with stronger CI. This work reveals a multigenerational impact of age on CI and expands our understanding of host-Wolbachia interactions and the biology of CI induced by the Wolbachia strain infecting the most widely used arthropod model, D. melanogaster.

Even obligate symbioses show signs of ecological contingency: Impacts of symbiosis for an invasive stinkbug are mediated by host plant context

ABSTRACT

Many species interactions are dependent on environmental context, yet the benefits of obligate, mutualistic microbial symbioses to their hosts are typically assumed to be universal across environments. We directly tested this assumption, focusing on the symbiosis between the sap-feeding insect Megacopta cribraria and its primary bacterial symbiont Candidatus Ishikawaella capsulata. We assessed host development time, survival, and body size in the presence and absence of the symbiont on two alternative host plants and in the insects' new invasive range. We found that association with the symbiont was critical for host survival to adulthood when reared on either host plant, with few individuals surviving in the absence of symbiosis. Developmental differences between hosts with and without microbial symbionts, however, were mediated by the host plants on which the insects were reared. Our results support the hypothesis that benefits associated with this host-microbe interaction are environmentally contingent, though given that few individuals survive to adulthood without their symbionts, this may have minimal impact on ecological dynamics and current evolutionary trajectories of these partners.

OPEN RESEARCH BADGES

This article has earned an Open Data Badge for making publicly available the digitally-shareable data necessary to reproduce the reported results. The data is available at https://doi.org/10.5061/dryad.kg4bc56.