Beyond gene flow: (non)-parallelism of secondary contact in a pair of highly differentiated sibling species

Replicated secondary contact zones can provide insights into the barriers to gene flow that are important during speciation and can reveal to which degree secondary contact may result in similar evolutionary outcomes. Here, we studied two secondary contact zones between highly differentiated Alpine butterflies of the genus Erebia using whole-genome resequencing data. We assessed the genomic relationships between populations and species and found hybridization to be rare, with no to little current or historical introgression in either contact zone. There are large similarities between contact zones, consistent with an allopatric origin of interspecific differentiation, with no indications for ongoing reinforcing selection. Consistent with expected reduced effective population size, we further find that scaffolds related to the Z-chromosome show increased differentiation compared to the already high levels across the entire genome, which could also hint towards a contribution of the Z chromosome to species divergence in this system. Finally, we detected the presence of the endosymbiont Wolbachia, which can cause reproductive isolation between its hosts, in all E. cassioides, while it appears to be fully or largely absent in contact zone populations of E. tyndarus. We discuss how this rare pattern may have arisen and how it may have affected the dynamics of speciation upon secondary contact.

Recent evolutionary origin and localized diversity hotspots of mammalian coronaviruses

Several coronaviruses infect humans, with three, including the SARS-CoV2, causing diseases. While coronaviruses are especially prone to induce pandemics, we know little about their evolutionary history, host-to-host transmissions, and biogeography. One of the difficulties lies in dating the origination of the family, a particularly challenging task for RNA viruses in general. Previous cophylogenetic tests of virus-host associations, including in the Coronaviridae family, have suggested a virus-host codiversification history stretching many millions of years. Here, we establish a framework for robustly testing scenarios of ancient origination and codiversification versus recent origination and diversification by host switches. Applied to coronaviruses and their mammalian hosts, our results support a scenario of recent origination of coronaviruses in bats and diversification by host switches, with preferential host switches within mammalian orders. Hotspots of coronavirus diversity, concentrated in East Asia and Europe, are consistent with this scenario of relatively recent origination and localized host switches. Spillovers from bats to other species are rare, but have the highest probability to be towards humans than to any other mammal species, implicating humans as the evolutionary intermediate host. The high host-switching rates within orders, as well as between humans, domesticated mammals, and non-flying wild mammals, indicates the potential for rapid additional spreading of coronaviruses across the world. Our results suggest that the evolutionary history of extant mammalian coronaviruses is recent, and that cases of long-term virus-host codiversification have been largely over-estimated.

Mixed Wolbachia infections resolve rapidly during in vitro evolution

The intracellular symbiont Wolbachia pipientis evolved after the divergence of arthropods and nematodes, but it reached high prevalence in many of these taxa through its abilities to infect new hosts and their germlines. Some strains exhibit long-term patterns of co-evolution with their hosts, while other strains are capable of switching hosts. This makes strain selection an important factor in symbiont-based biological control. However, little is known about the ecological and evolutionary interactions that occur when a promiscuous strain colonizes an infected host. Here, we study what occurs when two strains come into contact in host cells following horizontal transmission and infection. We focus on the faithful wMel strain from Drosophila melanogaster and the promiscuous wRi strain from Drosophila simulans using an in vitro cell culture system with multiple host cell types and combinatorial infection states. Mixing D. melanogaster cell lines stably infected with wMel and wRi revealed that wMel outcompetes wRi quickly and reproducibly. Furthermore, wMel was able to competitively exclude wRi even from minuscule starting quantities, indicating that this is a nearly deterministic outcome, independent of the starting infection frequency. This competitive advantage was not exclusive to wMel's native D. melanogaster cell background, as wMel also outgrew wRi in D. simulans cells. Overall, wRi is less adept at in vitro growth and survival than wMel and its in vivo state, revealing differences between the two strains in cellular and humoral regulation. These attributes may underlie the observed low rate of mixed infections in nature and the relatively rare rate of host-switching in most strains. Our in vitro experimental framework for estimating cellular growth dynamics of Wolbachia strains in different host species and cell types provides the first strategy for parameterizing endosymbiont and host cell biology at high resolution. This toolset will be crucial to our application of these bacteria as biological control agents in novel hosts and ecosystems.

Mixed Wolbachia infections resolve rapidly during in vitro evolution

The intracellular symbiont Wolbachia pipientis evolved after the divergence of arthropods and nematodes, but it reached high prevalence in many of these taxa through its abilities to infect new hosts and their germlines. Some strains exhibit long-term patterns of co-evolution with their hosts, while other strains are capable of switching hosts. This makes strain selection an important factor in symbiont-based biological control. However, little is known about the ecological and evolutionary interactions that occur when a promiscuous strain colonizes an infected host. Here, we study what occurs when two strains come into contact in host cells following horizontal transmission and infection. We focus on the faithful wMel strain from Drosophila melanogaster and the promiscuous wRi strain from Drosophila simulans using an in vitro cell culture system with multiple host cell types and combinatorial infection states. Mixing D. melanogaster cell lines stably infected with wMel and wRi revealed that wMel outcompetes wRi quickly and reproducibly. Furthermore, wMel was able to competitively exclude wRi even from minuscule starting quantities, indicating that this is a nearly deterministic outcome, independent of the starting infection frequency. This competitive advantage was not exclusive to wMel's native D. melanogaster cell background, as wMel also outgrew wRi in D. simulans cells. Overall, wRi is less adept at in vitro growth and survival than wMel and its in vivo state, revealing differences between cellular and humoral regulation. These attributes may underlie the observed low rate of mixed infections in nature and the relatively rare rate of host-switching in most strains. Our in vitro experimental framework for estimating cellular growth dynamics of Wolbachia strains in different host species, tissues, and cell types provides the first strategy for parameterizing endosymbiont and host cell biology at high resolution. This toolset will be crucial to our application of these bacteria as biological control agents in novel hosts and ecosystems.

Phylogenetic reconciliation: making the most of genomes to understand microbial ecology and evolution

In recent years, phylogenetic reconciliation has emerged as a promising approach for studying microbial ecology and evolution. The core idea is to model how gene trees evolve along a species tree and to explain differences between them via evolutionary events including gene duplications, transfers, and losses. Here, we describe how phylogenetic reconciliation provides a natural framework for studying genome evolution and highlight recent applications including ancestral gene content inference, the rooting of species trees, and the insights into metabolic evolution and ecological transitions they yield. Reconciliation analyses have elucidated the evolution of diverse microbial lineages, from Chlamydiae to Asgard archaea, shedding light on ecological adaptation, host-microbe interactions, and symbiotic relationships. However, there are many opportunities for broader application of the approach in microbiology. Continuing improvements to make reconciliation models more realistic and scalable, and integration of ecological metadata such as habitat, pH, temperature, and oxygen use offer enormous potential for understanding the rich tapestry of microbial life.

Wolbachia springs eternal: symbiosis in Collembola is associated with host ecology

Wolbachia are endosymbiotic alpha-proteobacteria infecting a wide range of arthropods and nematode hosts with diverse interactions, from reproductive parasites to obligate mutualists. Their taxonomy is defined by lineages called supergroups (labelled by letters of the alphabet), while their evolutionary history is complex, with multiple horizontal transfers and secondary losses. One of the least recently derived, supergroup E, infects springtails (Collembola), widely distributed hexapods, with sexual and/or parthenogenetic populations depending on species. To better characterize the diversity of Wolbachia infecting springtails, the presence of Wolbachia was screened in 58 species. Eleven (20%) species were found to be positive, with three Wolbachia genotypes identified for the first time in supergroup A. The novel genotypes infect springtails ecologically and biologically different from those infected by supergroup E. To root the Wolbachia phylogeny, rather than distant other Rickettsiales, supergroup L infecting plant-parasitic nematodes was used here. We hypothesize that the ancestor of Wolbachia was consumed by soil-dwelling nematodes, and was transferred horizontally via plants into aphids, which then infected edaphic arthropods (e.g. springtails and oribatid mites) before expanding into most clades of terrestrial arthropods and filarial nematodes.

Host phylogeny and ecological associations best explain Wolbachia host shifts in scale insects

Wolbachia are among the most prevalent and widespread endosymbiotic bacteria on Earth. Wolbachia's success in infecting an enormous number of arthropod species is attributed to two features: the range of phenotypes they induce in their hosts, and their ability to switch between host species. Whilst much progress has been made in elucidating their induced phenotypes, our understanding of Wolbachia host-shifting is still very limited: we lack answers to even fundamental questions concerning Wolbachia's routes of transfer and the importance of factors influencing host shifts. Here, we investigate the diversity and host-shift patterns of Wolbachia in scale insects, a group of arthropods with intimate associations with other insects that make them well suited to studying host shifts. Using Illumina multitarget amplicon sequencing of Wolbachia-infected scale insects and their direct associates we determined the identity of all Wolbachia strains. We then fitted a generalized additive mixed model to our data to estimate the influence of host phylogeny and the geographical distribution on Wolbachia strain sharing among scale insect species. The model predicts no significant contribution of host geography but strong effects of host phylogeny, with high rates of Wolbachia sharing among closely related species and a sudden drop-off in sharing with increasing phylogenetic distance. We also detected the same Wolbachia strain in scale insects and several intimately associated species (ants, wasps and flies). This indicates putative host shifts and potential routes of transfers via these associates and highlights the importance of ecological connectivity in Wolbachia host-shifting.

Wolbachia infection dynamics in a natural population of the pear psyllid Cacopsylla pyri (Hemiptera: Psylloidea) across its seasonal generations

Wolbachia is one of the most abundant intracellular symbionts of arthropods and has profound effects on host biology. Wolbachia transmission and host phenotypes often depend on its density within the host, which can be affected by multiple biotic and abiotic factors. However, very few studies measured Wolbachia density in natural host populations. Here, we describe Wolbachia in the pear psyllid Cacopsylla pyri from three populations in the Czech Republic. Using phylogenetic analyses based on wsp and multilocus sequence typing genes, we demonstrate that C. pyri harbours three new Wolbachia strains from supergroup B. A fourth Wolbachia strain from supergroup A was also detected in parasitised immatures of C. pyri, but likely came from a hymenopteran parasitoid. To obtain insights into natural Wolbachia infection dynamics, we quantified Wolbachia in psyllid individuals from the locality with the highest prevalence across an entire year, spanning several seasonal generations of the host. All tested females were infected and Wolbachia density remained stable across the entire period, suggesting a highly efficient vertical transmission and little influence from the environment and different host generations. In contrast, we observed a tendency towards reduced Wolbachia density in males which may suggest sex-related differences in Wolbachia-psyllid interactions.

A continental-scale survey of Wolbachia infections in blue butterflies reveals evidence of interspecific transfer and invasion dynamics

Infections by maternally inherited bacterial endosymbionts, especially Wolbachia, are common in insects and other invertebrates but infection dynamics across species ranges are largely under studied. Specifically, we lack a broad understanding of the origin of Wolbachia infections in novel hosts, and the historical and geographical dynamics of infections that are critical for identifying the factors governing their spread. We used Genotype-by-Sequencing data from previous population genomics studies for range-wide surveys of Wolbachia presence and genetic diversity in North American butterflies of the genus Lycaeides. As few as one sequence read identified by assembly to a Wolbachia reference genome provided high accuracy in detecting infections in host butterflies as determined by confirmatory PCR tests, and maximum accuracy was achieved with a threshold of only 5 sequence reads per host individual. Using this threshold, we detected Wolbachia in all but 2 of the 107 sampling localities spanning the continent, with infection frequencies within populations ranging from 0% to 100% of individuals, but with most localities having high infection frequencies (mean = 91% infection rate). Three major lineages of Wolbachia were identified as separate strains that appear to represent 3 separate invasions of Lycaeides butterflies by Wolbachia. Overall, we found extensive evidence for acquisition of Wolbachia through interspecific transfer between host lineages. Strain wLycC was confined to a single butterfly taxon, hybrid lineages derived from it, and closely adjacent populations in other taxa. While the other 2 strains were detected throughout the rest of the continent, strain wLycB almost always co-occurred with wLycA. Our demographic modeling suggests wLycB is a recent invasion. Within strain wLycA, the 2 most frequent haplotypes are confined almost exclusively to separate butterfly taxa with haplotype A1 observed largely in Lycaeides melissa and haplotype A2 observed most often in Lycaeides idas localities, consistent with either cladogenic mode of infection acquisition from a common ancestor or by hybridization and accompanying mutation. More than 1 major Wolbachia strain was observed in 15 localities. These results demonstrate the utility of using resequencing data from hosts to quantify Wolbachia genetic variation and infection frequency and provide evidence of multiple colonizations of novel hosts through hybridization between butterfly lineages and complex dynamics between Wolbachia strains.

Wolbachia in Black Spiny Whiteflies and Their New Parasitoid Wasp in Japan: Evidence of the Distinct Infection Status on Aleurocanthus camelliae Cryptic Species Complex

Wolbachia, an alphaproteobacterial reproductive parasite, can cause profound mitochondrial divergence in insects, which might eventually be a part of cryptic speciation. Aleurocanthus camelliae is a cryptic species complex consisting of several morphospecies and/or haplotypes that are genetically different but morphologically indistinctive. However, little is known about the Wolbachia infection status in these tea and Citrus pests. Thus, this study aimed to profile the diversity and phenotypic characteristics of Wolbachia natural infections in the A. camelliae cryptic species complex. A monophyletic strain of Wolbachia that infected the A. camelliae cryptic species complex (wAlec) with different patterns was discovered. Whiteflies that are morphologically identical to Aleurocanthus spiniferus (Aleurocanthus cf. A. spiniferus in Eurya japonica and A. spiniferus in Citrus) were grouped into uninfected populations, whereas the fixed infection was detected in A. camelliae B1 from Theaceae. The rapid evolution of wAlec was also found to occur through a high recombination event, which produced subgroups A and B in wAlec. It may also be associated with the non-cytoplasmic incompatibility (CI) phenotype of wAlec due to undetectable CI-related genes from phage WO (WOAlec). The current discovery of a novel cryptic species of A. camelliae led to a discussion about the oscillation hypothesis, which may provide insights on cryptic speciation, particularly on how specialization and host expansion have been recorded among these species. This study also identified a parasitoid wasp belonging to the genus Eretmocerus in A. camelliae, for the first time in Japan.

Predicting distributions of Wolbachia strains through host ecological contact—Who's manipulating whom?

Reproductive isolation in response to divergent selection is often mediated via third‐party interactions. Under these conditions, speciation is inextricably linked to ecological context. We present a novel framework for understanding arthropod speciation as mediated by Wolbachia, a microbial endosymbiont capable of causing host cytoplasmic incompatibility (CI). We predict that sympatric host sister‐species harbor paraphyletic Wolbachia strains that provide CI, while well‐defined congeners in ecological contact and recently diverged noninteracting congeners are uninfected due to Wolbachia redundancy. We argue that Wolbachia provides an adaptive advantage when coupled with reduced hybrid fitness, facilitating assortative mating between co‐occurring divergent phenotypes—the contact contingency hypothesis. To test this, we applied a predictive algorithm to empirical pollinating fig wasp data, achieving up to 91.60% accuracy. We further postulate that observed temporal decay of Wolbachia incidence results from adaptive host purging—adaptive decay hypothesis—but implementation failed to predict systematic patterns. We then account for post‐zygotic offspring mortality during CI mating, modeling fitness clines across developmental resources—the fecundity trade‐off hypothesis. This model regularly favored CI despite fecundity losses. We demonstrate that a rules‐based algorithm accurately predicts Wolbachia infection status. This has implications among other systems where closely related sympatric species encounter adaptive disadvantage through hybridization.

Remnants of horizontal transfers of Wolbachia genes in a Wolbachia-free woodwasp

Background

Wolbachia is a bacterial endosymbiont of many arthropod and nematode species. Due to its capacity to alter host biology, Wolbachia plays an important role in arthropod and nematode ecology and evolution. Sirex noctilio is a woodwasp causing economic loss in pine plantations of the Southern Hemisphere. An investigation into the genome of this wasp revealed the presence of Wolbachia sequences. Due to the potential impact of Wolbachia on the populations of this wasp, as well as its potential use as a biological control agent against invasive insects, this discovery warranted investigation.

Results

In this study we first investigated the presence of Wolbachia in S. noctilio and demonstrated that South African populations of the wasp are unlikely to be infected. We then screened the full genome of S. noctilio and found 12 Wolbachia pseudogenes. Most of these genes constitute building blocks of various transposable elements originating from the Wolbachia genome. Finally, we demonstrate that these genes are distributed in all South African populations of the wasp.

Conclusions

Our results provide evidence that S. noctilio might be compatible with a Wolbachia infection and that the bacteria could potentially be used in the future to regulate invasive populations of the wasp. Understanding the mechanisms that led to a loss of Wolbachia infection in S. noctilio could indicate which host species or host population should be sampled to find a Wolbachia strain that could be used as a biological control against S. noctilio.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12862-022-01995-x.

Limited Evidence for Microbial Transmission in the Phylosymbiosis between Hawaiian Spiders and Their Microbiota

The degree of similarity between the microbiotas of host species often mirrors the phylogenetic proximity of the hosts. This pattern, referred to as phylosymbiosis, is widespread in animals and plants. While phylosymbiosis was initially interpreted as the signal of symbiotic transmission and coevolution between microbes and their hosts, it is now recognized that similar patterns can emerge even if the microbes are environmentally acquired. Distinguishing between these two scenarios, however, remains challenging. We recently developed HOME (host-microbiota evolution), a cophylogenetic model designed to detect vertically transmitted microbes and host switches from amplicon sequencing data. Here, we applied HOME to the microbiotas of Hawaiian spiders of the genus Ariamnes, which experienced a recent radiation on the archipelago. We demonstrate that although Hawaiian Ariamnes spiders display a significant phylosymbiosis, there is little evidence of microbial vertical transmission. Next, we performed simulations to validate the absence of transmitted microbes in Ariamnes spiders. We show that this is not due to a lack of detection power because of the low number of segregating sites or an effect of phylogenetically driven or geographically driven host switches. Ariamnes spiders and their associated microbes therefore provide an example of a pattern of phylosymbiosis likely emerging from processes other than vertical transmission.

IMPORTANCE How host-associated microbiotas assemble and evolve is one of the outstanding questions of microbial ecology. Studies aiming at answering this question have repeatedly found a pattern of “phylosymbiosis,” that is, a phylogenetic signal in the composition of host-associated microbiotas. While phylosymbiosis was often interpreted as evidence for vertical transmission and host-microbiota coevolution, simulations have now shown that it can emerge from other processes, including host filtering of environmentally acquired microbes. However, distinguishing the processes driving phylosymbiosis in nature remains challenging. We recently developed a cophylogenetic method that can detect vertical transmission. Here, we applied this method to the microbiotas of recently diverged spiders from the Hawaiian archipelago, which display a clear phylosymbiosis pattern. We found that none of the bacterial operational taxonomic units is vertically transmitted. We show with simulations that this result is not due to methodological artifacts. Thus, we provide a striking empirical example of phylosymbiosis emerging from processes other than vertical transmission.

Unparalleled mitochondrial heteroplasmy and Wolbachia co-infection in the non-model bee, Amphylaeus morosus

Mitochondrial heteroplasmy is the occurrence of more than one type of mitochondrial DNA within a single individual. Although generally reported to occur in a small subset of individuals within a species, there are some instances of widespread heteroplasmy across entire populations. Amphylaeus morosus is an Australian native bee species in the diverse and cosmopolitan bee family Colletidae. This species has an extensive geographical range along the eastern Australian coast, from southern Queensland to western Victoria, covering approximately 2,000 km. Seventy individuals were collected from five localities across this geographical range and sequenced using Sanger sequencing for the mitochondrial cytochrome c oxidase subunit I (COI) gene. These data indicate that every individual had the same consistent heteroplasmic sites but no other nucleotide variation, suggesting two conserved and widespread heteroplasmic mitogenomes. Ion Torrent shotgun sequencing revealed that heteroplasmy occurred across multiple mitochondrial protein-coding genes and is unlikely explained by transposition of mitochondrial genes into the nuclear genome (NUMTs). DNA sequence data also demonstrated a consistent co-infection of Wolbachia across the A. morosus distribution with every individual infected with both bacterial strains. Our data are consistent with the presence of two mitogenomes within all individuals examined in this species and suggest a major divergence from standard patterns of mitochondrial inheritance. Because the host's mitogenome and the Wolbachia genome are genetically linked through maternal inheritance, we propose three possible hypotheses that could explain maintenance of the widespread and conserved co-occurring bacterial and mitochondrial genomes in this species.

Cytoplasmic incompatibility in hybrid zones: infection dynamics and resistance evolution

Cytoplasmic incompatibility is an endosymbiont-induced mating incompatibility common in arthropods. Unidirectional cytoplasmic incompatibility impairs crosses between infected males and uninfected females, whereas bidirectional cytoplasmic incompatibility occurs when two host lineages are infected with reciprocally in compatible endosymbionts. Bidirectional cytoplasmic incompatibility is unstable in unstructured populations, but may be stable in hybrid zones. Stable coexistence of incompatible host lineages should generate frequent incompatible crosses. Therefore, hosts are expected to be under selection to resist their endosymbionts. Here, we for mulate a mathematical model of hybrid zones where two bidirectionally incompatible host lineages meet. We expand this model to consider the invasion of a hypothetical resistance allele. To corroborate our mathematical predictions, we test each prediction with stochastic, individual-based simulations. Our models suggest that hybrid zones may sustain stable coinfections of bidirectionally incompatible endosymbiont strains. Over a range of conditions, host are under selection for resistance against cytoplasmic incompatibility. Under asymetric migration, a resistance allele can facilitate infection turnover and subsequently either persist or become lost. The predictions we present may inform our understanding of the cophylogenetic relationship between the endosym biont Wolbachia and its hosts.

A holobiont view of island biogeography: Unravelling patterns driving the nascent diversification of a Hawaiian spider and its microbial associates

The diversification of a host lineage can be influenced by both the external environment and its assemblage of microbes. Here, we use a young lineage of spiders, distributed along a chronologically arranged series of volcanic mountains, to investigate how their associated microbial communities have changed as the spiders colonized new locations. Using the stick spider Ariamnes waikula (Araneae, Theridiidae) on the island of Hawai'i, and outgroup taxa on older islands, we tested whether each component of the "holobiont" (spider hosts, intracellular endosymbionts and gut microbial communities) showed correlated signatures of diversity due to sequential colonization from older to younger volcanoes. To investigate this, we generated ddRAD data for the host spiders and 16S rRNA gene amplicon data from their microbiota. We expected sequential colonizations to result in a (phylo)genetic structuring of the host spiders and in a diversity gradient in microbial communities. The results showed that the host A. waikula is indeed structured by geographical isolation, suggesting sequential colonization from older to younger volcanoes. Similarly, the endosymbiont communities were markedly different between Ariamnes species on different islands, but more homogeneous among A. waikula populations on the island of Hawai'i. Conversely, the gut microbiota, which we suspect is generally environmentally derived, was largely conserved across all populations and species. Our results show that different components of the holobiont respond in distinct ways to the dynamic environment of the volcanic archipelago. This highlights the necessity of understanding the interplay between different components of the holobiont, to properly characterize its evolution.

Are ecological communities the seat of endosymbiont horizontal transfer and diversification? A case study with soil arthropod community

Maternally inherited endosymbionts of arthropods are one of the most abundant and diverse group of bacteria. These bacterial endosymbionts also show extensive horizontal transfer to taxonomically unrelated hosts and widespread recombination in their genomes. Such horizontal transfers can be enhanced when different arthropod hosts come in contact like in an ecological community. Higher rates of horizontal transfer can also increase the probability of recombination between endosymbionts, as they now share the same host cytoplasm. However, reports of community-wide endosymbiont data are rare as most studies choose few host taxa and specific ecological interactions among the hosts. To better understand endosymbiont spread within host populations, we investigated the incidence, diversity, extent of horizontal transfer, and recombination of three endosymbionts (Wolbachia, Cardinium, and Arsenophonus) in a specific soil arthropod community. Wolbachia strains were characterized with MLST genes whereas 16S rRNA gene was used for Cardinium and Arsenophonus. Among 3,509 individual host arthropods, belonging to 390 morphospecies, 12.05% were infected with Wolbachia, 2.82% with Cardinium and 2.05% with Arsenophonus. Phylogenetic incongruence between host and endosymbiont indicated extensive horizontal transfer of endosymbionts within this community. Three cases of recombination between Wolbachia supergroups and eight incidences of within-supergroup recombination were also found. Statistical tests of similarity indicated supergroup A Wolbachia and Cardinium show a pattern consistent with extensive horizontal transfer within the community but not for supergroup B Wolbachia and Arsenophonus. We highlight the importance of extensive community-wide studies for a better understanding of the spread of endosymbionts across global arthropod communities.

Wolbachia-driven selective sweep in a range expanding insect species

Background

Evolutionary processes can cause strong spatial genetic signatures, such as local loss of genetic diversity, or conflicting histories from mitochondrial versus nuclear markers. Investigating these genetic patterns is important, as they may reveal obscured processes and players. The maternally inherited bacterium Wolbachia is among the most widespread symbionts in insects. Wolbachia typically spreads within host species by conferring direct fitness benefits, and/or by manipulating its host reproduction to favour infected over uninfected females. Under sufficient selective advantage, the mitochondrial haplotype associated with the favoured maternally-inherited symbiotic strains will spread (i.e. hitchhike), resulting in low mitochondrial genetic variation across the host species range.

Method

The common bluetail damselfly (Ischnura elegans: van der Linden, 1820) has recently emerged as a model organism for genetics and genomic signatures of range expansion during climate change. Although there is accumulating data on the consequences of such expansion on the genetics of I. elegans, no study has screened for Wolbachia in the damselfly genus Ischnura. Here, we present the biogeographic variation in Wolbachia prevalence and penetrance across Europe and Japan (including samples from 17 populations), and from close relatives in the Mediterranean area (i.e. I. genei: Rambur, 1842; and I. saharensis: Aguesse, 1958).

Results

Our data reveal (a) multiple Wolbachia-strains, (b) potential transfer of the symbiont through hybridization, (c) higher infection rates at higher latitudes, and (d) reduced mitochondrial diversity in the north-west populations, indicative of hitchhiking associated with the selective sweep of the most common strain. We found low mitochondrial haplotype diversity in the Wolbachia-infected north-western European populations (Sweden, Scotland, the Netherlands, Belgium, France and Italy) of I. elegans, and, conversely, higher mitochondrial diversity in populations with low penetrance of Wolbachia (Ukraine, Greece, Montenegro and Cyprus). The timing of the selective sweep associated with infected lineages was estimated between 20,000 and 44,000 years before present, which is consistent with the end of the last glacial period about 20,000 years.

Conclusions

Our findings provide an example of how endosymbiont infections can shape spatial variation in their host evolutionary genetics during postglacial expansion. These results also challenge population genetic studies that do not consider the prevalence of symbionts in many insects, which we show can impact geographic patterns of mitochondrial genetic diversity.

Diverse wMel variants of Wolbachia pipientis differentially rescue fertility and cytological defects of the bag of marbles partial loss of function mutation in Drosophila melanogaster

In Drosophila melanogaster, the maternally inherited endosymbiont Wolbachia pipientis interacts with germline stem cell genes during oogenesis. One such gene, bag of marbles (bam) is the key switch for differentiation and also shows signals of adaptive evolution for protein diversification. These observations have led us to hypothesize that W. pipientis could be driving the adaptive evolution of bam for control of oogenesis. To test this hypothesis, we must understand the specificity of the genetic interaction between bam and W. pipientis. Previously, we documented that the W. pipientis variant, wMel, rescued the fertility of the bamBW hypomorphic mutant as a transheterozygote over a bam null. However, bamBW was generated more than 20 years ago in an uncontrolled genetic background and maintained over a balancer chromosome. Consequently, the chromosome carrying bamBW accumulated mutations that have prevented controlled experiments to further assess the interaction. Here, we used CRISPR/Cas9 to engineer the same single amino acid bam hypomorphic mutation (bamL255F) and a new bam null disruption mutation into the w1118 isogenic background. We assess the fertility of wildtype bam, bamL255F/bamnull hypomorphic, and bamL255F/bamL255F mutant females, each infected individually with 10 W. pipientis wMel variants representing three phylogenetic clades. Overall, we find that all of the W. pipientis variants tested here rescue bam hypomorphic fertility defects with wMelCS-like variants exhibiting the strongest rescue effects. In addition, these variants did not increase wildtype bam female fertility. Therefore, both bam and W. pipientis interact in genotype-specific ways to modulate female fertility, a critical fitness phenotype.

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.

Evidence for multiple colonisations and Wolbachia infections shaping the genetic structure of the widespread butterfly Polyommatus icarus in the British Isles

The paradigm of isolation in southern refugia during glacial periods followed by expansions during interglacials, producing limited genetic differentiation in northern areas, dominates European phylogeography. However, the existence of complex structured populations in formerly glaciated areas, and islands connected to mainland areas during glacial maxima, call for alternative explanations. We reconstructed the mtDNA phylogeography of the widespread Polyommatus Icarus butterfly with an emphasis on the formerly glaciated and connected British Isles. We found distinct geographical structuring of CO1 haplogroups, with an ancient lineage restricted to the marginal European areas, including Northern Scotland and Outer Hebrides. Population genomic analyses, using ddRADSeq genomic markers, also reveal substantial genetic structuring within Britain. However, there is negligble mito-nuclear concordance consistent with independent demographic histories of mitochondrial versus nuclear DNA. While mtDNA-Wolbachia associations in northern Britain could account for the geographic structuring of mtDNA across most of the British Isles, for nuclear DNA markers (derived from ddRADseq data) butterflies from France cluster between northern and southern British populations - an observation consistent with a scenario of multiple recolonisation. Taken together our results suggest that contemporary mtDNA structuring in the British Isles (and potentially elsewhere in Europe) largely results from Wolbachia infections, however, nuclear genomic structuring suggests a history of at least two distinct colonisations. This two-stage colonisation scenario has previously been put forth to explain genetic diversity and structuring in other British flora and fauna. Additionally, we also present preliminary evidence for potential Wolbachia-induced feminization in the Outer Hebrides.

Prevalence and relationship of endosymbiotic Wolbachia in the butterfly genus Erebia

BACKGROUND

Wolbachia is an endosymbiont common to most invertebrates, which can have significant evolutionary implications for its host species by acting as a barrier to gene flow. Despite the importance of Wolbachia, still little is known about its prevalence and diversification pattern among closely related host species. Wolbachia strains may phylogenetically coevolve with their hosts, unless horizontal host-switches are particularly common. We address  these issues in the genus Erebia, one of the most diverse Palearctic butterfly genera.

RESULTS

We sequenced the Wolbachia genome from a strain infecting Erebia cassioides and showed that it belongs to the Wolbachia supergroup B, capable of infecting arthropods from different taxonomic orders. The prevalence of Wolbachia across 13 closely related Erebia host species based on extensive population-level genetic data revealed that multiple Wolbachia strains jointly infect all investigated taxa, but with varying prevalence. Finally, the phylogenetic relationships of Wolbachia strains are in some cases significantly associated to that of their hosts, especially among the most closely related Erebia species, demonstrating mixed evidence for phylogenetic coevolution.

CONCLUSIONS

Closely related host species can be infected by closely related Wolbachia strains, evidencing some phylogenetic coevolution, but the actual pattern of infection more often reflects historical or contemporary geographic proximity among host species. Multiple processes, including survival in distinct glacial refugia, recent host shifts in sympatry, and a loss of Wolbachia during postglacial range expansion seem to have jointly shaped the complex interactions between Wolbachia evolution and the diversification of its host among our studied Erebia species.

Diversification, selective sweep, and body size in the invasive Palearctic alfalfa weevil infected with Wolbachia

The alfalfa weevil Hypera postica, native to the Western Palearctic, is an invasive legume pest with two divergent mitochondrial clades in its invading regions, the Western clade and the Eastern/Egyptian clade. However, knowledge regarding the native populations is limited. The Western clade is infected with the endosymbiotic bacteria Wolbachia that cause cytoplasmic incompatibility in host weevils. Our aim was to elucidate the spatial genetic structure of this insect and the effect of Wolbachia on its population diversity. We analyzed two mitochondrial and two nuclear genes of the weevil from its native ranges. The Western clade was distributed in western/central Europe, whereas the Eastern/Egyptian clade was distributed from the Mediterranean basin to central Asia. Intermediate mitotypes were found from the Balkans to central Asia. Most Western clade individuals in western Europe were infected with an identical Wolbachia strain. Mitochondrial genetic diversity of the infected individuals was minimal. The infected clades demonstrated a higher nonsynonymous/synonymous substitution rate ratio than the uninfected clades, suggesting a higher fixation of nonsynonymous mutations due to a selective sweep by Wolbachia. Trans-Mediterranean and within-European dispersal routes were supported. We suggest that the ancestral populations diversified by geographic isolation due to glaciations and that the diversity was reduced in the west by a recent Wolbachia-driven sweep(s). The intermediate clade exhibited a body size and host plant that differed from the other clades. Pros and cons of the possible use of infected-clade males to control uninfected populations are discussed.

Does getting defensive get you anywhere?-Seasonal balancing selection, temperature, and parasitoids shape real-world, protective endosymbiont dynamics in the pea aphid

Facultative, heritable endosymbionts are found at intermediate prevalence within most insect species, playing frequent roles in their hosts' defence against environmental pressures. Focusing on Hamiltonella defensa, a common bacterial endosymbiont of aphids, we tested the hypothesis that such pressures impose seasonal balancing selection, shaping a widespread infection polymorphism. In our studied pea aphid (Acyrthosiphon pisum) population, Hamiltonella frequencies ranged from 23.2% to 68.1% across a six-month longitudinal survey. Rapid spikes and declines were often consistent across fields, and we estimated that selection coefficients for Hamiltonella-infected aphids changed sign within this field season. Prior laboratory research suggested antiparasitoid defence as the major Hamiltonella benefit, and costs under parasitoid absence. While a prior field study suggested these forces can sometimes act as counter-weights in a regime of seasonal balancing selection, our present survey showed no significant relationship between parasitoid wasps and Hamiltonella prevalence. Field cage experiments provided some explanation: parasitoids drove modest ~10% boosts to Hamiltonella frequencies that would be hard to detect under less controlled conditions. They also showed that Hamiltonella was not always costly under parasitoid exclusion, contradicting another prediction. Instead, our longitudinal survey - and two overwintering studies - showed temperature to be the strongest predictor of Hamiltonella prevalence. Matching some prior lab discoveries, this suggested that thermally sensitive costs and benefits, unrelated to parasitism, can shape Hamiltonella dynamics. These results add to a growing body of evidence for rapid, seasonal adaptation in multivoltine organisms, suggesting that such adaptation can be mediated through the diverse impacts of heritable bacterial endosymbionts.

Horizontal Transmission of Microbial Symbionts Within a Guild of Fly Parasitoids

Many insects harbor facultative microbial symbionts which affect the ecology of their hosts in diverse ways. Most symbionts are transmitted vertically with high fidelity, whereas horizontal transmission occurs rarely. Parasitoid larvae feed on a single host and are in close physical contact with it, providing an ecological opportunity for symbionts' horizontal transmission, but there is little empirical evidence documenting this. Here we studied horizontal transmission of three bacterial symbionts-Rickettsia, Sodalis, and Wolbachia-between three fly pupal ectoparasitoid species: Spalangia cameroni, S. endius, and Muscidifurax raptor. Muscidifurax raptor readily parasitized and successfully developed on the Spalangia spp., while the inverse did not happen. The two Spalangia spp. attacked each other and conspecifics in very low rates. Symbiont horizontal transmissions followed by stable vertical transmission in the recipient species were achieved, in low percentages, only between conspecifics: Wolbachia from infected to uninfected M. raptor, Rickettsia in S. endius, and Sodalis in S. cameroni. Low frequency of horizontal transmissions occurred in the interspecific combinations, but none of them persisted in the recipient species beyond F₄, at most. Our study is one of few to demonstrate symbionts' horizontal transmission between hosts within the same trophic level and guild and highlights the rarity of such events.

Treerecs: an integrated phylogenetic tool, from sequences to reconciliations

MOTIVATION

Gene and species tree reconciliation methods are used to interpret gene trees, root them and correct uncertainties that are due to scarcity of signal in multiple sequence alignments. So far, reconciliation tools have not been integrated in standard phylogenetic software and they either lack performance on certain functions, or usability for biologists.

RESULTS

We present Treerecs, a phylogenetic software based on duplication-loss reconciliation. Treerecs is simple to install and to use. It is fast and versatile, has a graphic output, and can be used along with methods for phylogenetic inference on multiple alignments like PLL and Seaview.

AVAILABILITY AND IMPLEMENTATION

Treerecs is open-source. Its source code (C++, AGPLv3) and manuals are available from https://project.inria.fr/treerecs/.

Wolbachia affects mitochondrial population structure in two systems of closely related Palaearctic blue butterflies

The bacterium Wolbachia infects many insect species and spreads by diverse vertical and horizontal means. As co-inherited organisms, these bacteria often cause problems in mitochondrial phylogeny inference. The phylogenetic relationships of many closely related Palaearctic blue butterflies (Lepidoptera: Lycaenidae: Polyommatinae) are ambiguous. We considered the patterns of Wolbachia infection and mitochondrial diversity in two systems: Aricia agestis/Aricia artaxerxes and the Pseudophilotes baton species complex. We sampled butterflies across their distribution ranges and sequenced one butterfly mitochondrial gene and two Wolbachia genes. Both butterfly systems had uninfected and infected populations, and harboured several Wolbachia strains. Wolbachia was highly prevalent in A. artaxerxes and the host's mitochondrial structure was shallow, in contrast to A. agestis. Similar bacterial alleles infected both Aricia species from nearby sites, pointing to a possible horizontal transfer. Mitochondrial history of the P. baton species complex mirrored its Wolbachia infection and not the taxonomical division. Pseudophilotes baton and P. vicrama formed a hybrid zone in Europe. Wolbachia could obscure mitochondrial history, but knowledge on the infection helps us to understand the observed patterns. Testing for Wolbachia should be routine in mitochondrial DNA studies.

Supergroup F Wolbachia in terrestrial isopods: Horizontal transmission from termites?

Horizontal transmission between distantly related species has been used to explain how Wolbachia infect multiple species at astonishing rates despite the selection for resistance. Recently, a terrestrial isopod species was found to be infected by an unusual strain of supergroup F Wolbachia. However, only Wolbachia of supergroup B is typically found in isopods. One possibility is that these isopods acquired the infection because of their recurrent contact with termites—a group with strong evidence of infection by supergroup F Wolbachia. Thus, our goals were: (1) check if the infection was an isolated case in isopods, or if it revealed a broader pattern; (2) search for Wolbachia infection in the termites within Brazil; and (3) look for evidence consistent with horizontal transmission between isopods and termites. We collected Neotroponiscus terrestrial isopods and termites along the Brazilian coastal Atlantic forest. We sequenced and identified the Wolbachia strains found in these groups using coxA, dnaA, and fpbA genes. We constructed phylogenies for both bacteria and host taxa and tested for coevolution. We found the supergroup F Wolbachia in other species and populations of Neotroponiscus, and also in Nasutitermes and Procornitermes termites. The phylogenies showed that, despite the phylogenetic distance between isopods and termites, the Wolbachia strains clustered together. Furthermore, cophylogenetic analyses showed significant jumps of Wolbachia between terrestrial isopods and termites. Thus, our study suggests that the horizontal transmission of supergroup F Wolbachia between termites and terrestrial isopods is likely. Our study also helps understanding the success and worldwide distribution of this symbiont.

Towards unravelling Wolbachia global exchange: a contribution from the Bicyclus and Mylothris butterflies in the Afrotropics

Background

Phylogenetically closely related strains of maternally inherited endosymbiotic bacteria are often found in phylogenetically divergent, and geographically distant insect host species. The interspecies transfer of the symbiont Wolbachia has been thought to have occurred repeatedly, facilitating its observed global pandemic. Few ecological interactions have been proposed as potential routes for the horizontal transfer of Wolbachia within natural insect communities. These routes are however likely to act only at the local scale, but how they may support the global distribution of some Wolbachia strains remains unclear.

Results

Here, we characterize the Wolbachia diversity in butterflies from the tropical forest regions of central Africa to discuss transfer at both local and global scales. We show that numerous species from both the Mylothris (family Pieridae) and Bicyclus (family Nymphalidae) butterfly genera are infected with similar Wolbachia strains, despite only minor interclade contacts across the life cycles of the species within their partially overlapping ecological niches. The phylogenetic distance and differences in resource use between these genera rule out the role of ancestry, hybridization, and shared host-plants in the interspecies transfer of the symbiont. Furthermore, we could not identify any shared ecological factors to explain the presence of the strains in other arthropod species from other habitats, or even ecoregions.

Conclusion

Only the systematic surveys of the Wolbachia strains from entire species communities may offer the material currently lacking for understanding how Wolbachia may transfer between highly different and unrelated hosts, as well as across environmental scales.

Supplementary information

Supplementary information accompanies this paper at 10.1186/s12866-020-02011-2.

Large scale genome reconstructions illuminate Wolbachia evolution

Wolbachia is an iconic example of a successful intracellular bacterium. Despite its importance as a manipulator of invertebrate biology, its evolutionary dynamics have been poorly studied from a genomic viewpoint. To expand the number of Wolbachia genomes, we screen over 30,000 publicly available shotgun DNA sequencing samples from 500 hosts. By assembling over 1000 Wolbachia genomes, we provide a substantial increase in host representation. Our phylogenies based on both core-genome and gene content provide a robust reference for future studies, support new strains in model organisms, and reveal recent horizontal transfers amongst distantly related hosts. We find various instances of gene function gains and losses in different super-groups and in cytoplasmic incompatibility inducing strains. Our Wolbachia-host co-phylogenies indicate that horizontal transmission is widespread at the host intraspecific level and that there is no support for a general Wolbachia-mitochondrial synchronous divergence.

By greatly expanding the number of assembled genomes for Wolbachia (a group of intracellular bacteria) and constructing robust phylogenies, this study finds strong rate heterogeneity among Wolbachiapopulations and no support for synchronous divergence between Wolbachia and host mitochondria.

Phylogenomic Analysis of Wolbachia Strains Reveals Patterns of Genome Evolution and Recombination

Wolbachia are widespread intracellular bacteria that mediate many important biological processes in arthropod species. In this study, we identified 210 conserved single-copy genes in 33 genome-sequenced Wolbachia strains in the A–F supergroups. Phylogenomic analyses with these core genes indicate that all 33 Wolbachia strains maintain the supergroup relationship, which was classified previously based on the multilocus sequence typing (MLST) genes. Using an interclade recombination screening method, 14 inter-supergroup recombination events were discovered in six genes (2.9%) among 210 single-copy orthologs. This finding suggests a relatively low frequency of intergroup recombination. Interestingly, they have occurred not only between A and B supergroups (nine events) but also between A and E supergroups (five events). Maintenance of such transfers suggests possible roles in Wolbachia infection-related functions. Comparisons of strain divergence using the five genes of the MLST system show a high correlation (Pearson correlation coefficient r = 0.98) between MLST and whole-genome divergences, indicating that MLST is a reliable method for identifying related strains when whole-genome data are not available. The phylogenomic analysis and the identified core gene set in our study will serve as a valuable foundation for strain identification and the investigation of recombination and genome evolution in Wolbachia.

Shared geographic histories and dispersal contribute to congruent phylogenies between amphipods and their microsporidian parasites at regional and global scales

In parasites that strongly rely on a host for dispersal, geographic barriers that act on the host will simultaneously influence parasite distribution as well. If their association persists over macroevolutionary time it may result in congruent phylogenetic and phylogeographic patterns due to shared geographic histories. Here, we investigated the level of congruent evolutionary history at a regional and global scale in a highly specialised parasite taxon infecting hosts with limited dispersal abilities: the microsporidians Dictyocoela spp. and their amphipod hosts. Dictyocoela can be transmitted both vertically and horizontally and is the most common microsporidian genus occurring in amphipods in Eurasia. However, little is known about its distribution elsewhere. We started by conducting molecular screening to detect microsporidian parasites in endemic amphipod species in New Zealand; based on phylogenetic analyses, we identified nine species-level microsporidian taxa including six belonging to Dictyocoela. With a distance-based cophylogenetic analysis at the regional scale, we identified overall congruent phylogenies between Paracalliope, the most common New Zealand freshwater amphipod taxon, and their Dictyocoela parasites. Also, hosts and parasites showed similar phylogeographic patterns suggesting shared biogeographic histories. Similarly, at a global scale, phylogenies of amphipod hosts and their Dictyocoela parasites showed broadly congruent phylogenies. The observed patterns may have resulted from covicariance and/or codispersal, suggesting that the intimate association between amphipods and Dictyocoela may have persisted over macroevolutionary time. We highlight that shared biogeographic histories could play a role in the codiversification of hosts and parasites at a macroevolutionary scale.

Sensing, Signaling, and Secretion: A Review and Analysis of Systems for Regulating Host Interaction in Wolbachia

Wolbachia (Anaplasmataceae) is an endosymbiont of arthropods and nematodes that resides within host cells and is well known for manipulating host biology to facilitate transmission via the female germline. The effects Wolbachia has on host physiology, combined with reproductive manipulations, make this bacterium a promising candidate for use in biological- and vector-control. While it is becoming increasingly clear that Wolbachia's effects on host biology are numerous and vary according to the host and the environment, we know very little about the molecular mechanisms behind Wolbachia's interactions with its host. Here, I analyze 29 Wolbachia genomes for the presence of systems that are likely central to the ability of Wolbachia to respond to and interface with its host, including proteins for sensing, signaling, gene regulation, and secretion. Second, I review conditions under which Wolbachia alters gene expression in response to changes in its environment and discuss other instances where we might hypothesize Wolbachia to regulate gene expression. Findings will direct mechanistic investigations into gene regulation and host-interaction that will deepen our understanding of intracellular infections and enhance applied management efforts that leverage Wolbachia.

How consistent is RAD-seq divergence with DNA-barcode based clustering in insects?

Promoted by the barcoding approach, mitochondrial DNA is more than ever used as a molecular marker to identify species boundaries. Yet, it has been repeatedly argued that it may be poorly suited for this purpose, especially in insects where mitochondria are often associated with invasive intracellular bacteria that may promote their introgression. Here, we inform this debate by assessing how divergent nuclear genomes can be when mitochondrial barcodes indicate very high proximity. To this end, we obtained RAD-seq data from 92 barcode-based species-like units (operational taxonomic units [OTUs]) spanning four insect orders. In 100% of the cases, the observed median nuclear divergence was lower than 2%, a value that was recently estimated as one below which nuclear gene flow is not uncommon. These results suggest that although mitochondria may occasionally leak between species, this process is rare enough in insects to make DNA barcoding a reliable tool for clustering specimens into species-like units.

Wolbachia megadiversity: 99% of these microorganismic manipulators unknown

Wolbachia (Alphaproteobacteria) are the most widespread endosymbionts of arthropods, manipulating their hosts by various means to maximize the number of host individuals infected. Based on quantitative analyzes of the published literature from Web of Science® and of DNA sequences of arthropod-hosted Wolbachia from GenBank, we made plausible that less than 1% of the expected 100 000 strains of Wolbachia in arthropods is known. Our findings suggest that more and globally better coordinated efforts in screening arthropods are needed to explore the true Wolbachia diversity and to help us understand the ecology and evolution of these host-endosymbiont interactions.

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.

Evidence for Common Horizontal Transmission of Wolbachia among Ants and Ant Crickets: Kleptoparasitism Added to the List

While Wolbachia, an intracellular bacterial symbiont, is primarily transmitted maternally in arthropods, horizontal transmission between species has been commonly documented. We examined kleptoparasitism as a potential mechanism for Wolbachia horizontal transmission, using ant crickets and their host ants as the model system. We compared prevalence and diversity of Wolbachia across multiple ant cricket species with different degrees of host specificity/integration level. Our analyses revealed at least three cases of inter-ordinal Wolbachia transfer among ant and ant crickets, and also showed that ant cricket species with high host-integration and host-specificity tend to harbor a higher Wolbachia prevalence and diversity than other types of ant crickets. This study provides empirical evidence that distribution of Wolbachia across ant crickets is largely attributable to horizontal transmission, but also elucidates the role of intimate ecological association in successful Wolbachia horizontal transmission.

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.

The Diversity and Distribution of Wolbachia, Rhizobiales, and Ophiocordyceps Within the Widespread Neotropical Turtle Ant, Cephalotes atratus (Hymenoptera: Formicidae)

Ants are an ecologically and evolutionarily diverse group, and they harbor a wide range of symbiotic microbial communities that often greatly affect their biology. Turtle ants (genus Cephalotes) engage in mutualistic relationships with gut bacteria and are exploited by microbial parasites. Studies have shown that associations among these microbial lineages and the turtle ant hosts vary geographically. However, these studies have been limited, and thorough within-species analyses of the variation and structure of these microbial communities have yet to be conducted. The giant turtle ant, Cephalotes atratus (Linnaeus 1758), is a geographically widespread, genetically diverse Neotropical species that has been sampled extensively across its geographic range, making it ideal for analysis of microbial associations. In this study, we verified the presence, genetic variation, and geographic patterns at the individual, colony, and population level of three microbial groups associated with the giant turtle ant: Wolbachia, a genus of facultative bacteria which are often parasitic, affecting host reproduction; Rhizobiales, a mutualistic order of bacteria hypothesized to be an obligate nutritional symbiont in turtle ants; and Ophiocordyceps, a genus of endoparasitic fungi infecting many arthropod species by manipulating their behavior for fungal reproduction. In this study, we found varying degrees of prevalence for two distantly related genotypes (haplogroups) of Wolbachia and high degree of prevalence of Rhizobiales across colonies with little genetic variation. In addition, we found low occurrence of Ophiocordyceps. This study highlights a key first step in understanding the diversity, distribution, and prevalence of the microbial community of C. atratus.

What Goes Up Might Come Down: the Spectacular Spread of an Endosymbiont Is Followed by Its Decline a Decade Later

Facultative, intracellular bacterial symbionts of arthropods may dramatically affect host biology and reproduction. The length of these symbiont-host associations may be thousands to millions of years, and while symbiont loss is predicted, there have been very few observations of a decline of symbiont infection rates. In a population of the sweet potato whitefly species (Bemisia tabaci MEAM1) in Arizona, USA, we documented the frequency decline of a strain of Rickettsia in the Rickettsia bellii clade from near-fixation in 2011 to 36% of whiteflies infected in 2017. In previous studies, Rickettsia had been shown to increase from 1 to 97% from 2000 to 2006 and remained at high frequency for at least five years. At that time, Rickettsia infection was associated with both fitness benefits and female bias. In the current study, we established matrilines of whiteflies from the field (2016, Rickettsia infection frequency = 58%) and studied (a) Rickettsia vertical transmission, (b) fitness and sex ratios associated with Rickettsia infection, (c) symbiont titer, and (d) bacterial communities within whiteflies. The vertical transmission rate was high, approximately 98%. Rickettsia infection in the matrilines was not associated with fitness benefits or sex ratio bias and appeared to be slightly costly, as more Rickettsia-infected individuals produced non-hatching eggs. Overall, the titer of Rickettsia in the matrilines was lower in 2016 than in the whiteflies collected in 2011, but the titer distribution appeared bimodal, with high- and low-titer lines, and constancy of the average titer within lines over three generations. We found neither association between Rickettsia titer and fitness benefits or sex ratio bias nor evidence that Rickettsia was replaced by another secondary symbiont. The change in the interaction between symbiont and host in 2016 whiteflies may explain the drop in symbiont frequency we observed.

Assessment of the role of Wolbachia in mtDNA paraphyly and the evolution of unisexuality in Calligrapha (Coleoptera: Chrysomelidae)

Calligrapha is a New World leaf beetle genus that includes several unisexual species in northeastern North America. Each unisexual species had an independent hybrid origin involving different combinations of bisexual species. However, surprisingly, they all cluster in a single mtDNA clade and with some individuals of their parental species, which are in turn deeply polyphyletic for mtDNA. This pattern is suggestive of a selective sweep which, together with mtDNA taxonomic incongruence and occurrence of unisexuality in Calligrapha, led to hypothesize that Wolbachia might be responsible. I tested this hypothesis studying the correlation between diversity of Wolbachia and well-established mtDNA lineages in >500 specimens of two bisexual species of Calligrapha and their derived unisexual species. Wolbachia appears highly prevalent (83.4%), and fifteen new supergroup-A strains of the bacteria are characterized, belonging to three main classes: wCallA, occupying the whole species ranges, and wCallB and wCallC, narrowly parapatric, infecting beetles with highly divergent mtDNAs where they coexist. Most beetles (71.6%) carried double infections of wCallA with another sequence class. Bayesian inference of ancestral character states and association tests between bacterial diversity and the mtDNA genealogy show that each mtDNA lineage of Calligrapha has specific types of infection. Moreover, shifts can be explained by horizontal or vertical transfer from local populations to an expanding lineage and cytoplasmic incompatibility between wCallB and wCallC types, suggesting that the symbionts hitchhike with the host and are not responsible for selective mtDNA sweeps. Lack of evidence for sweeps and the fact that individuals in the unisexual clade are uninfected or infected by the widespread wCallA type indicate that Wolbachia does not induce unisexuality in Calligrapha, although they may manipulate host reproduction through cytoplasmic incompatibility.

Global genetic diversity, lineage distribution, and Wolbachia infection of the alfalfa weevil Hypera postica (Coleoptera: Curculionidae)

The alfalfa weevil (Hypera postica) is a well-known example of a worldwide-distributed pest with high genetic variation. Based on the mitochondrial genes, the alfalfa weevil clusters into two main mitochondrial lineages. However, there is no clear picture of the global diversity and distribution of these lineages; neither the drivers of its diversification are known. However, it appears likely that historic demographic events including founder effects played a role. In addition, Wolbachia, a widespread intracellular parasite/symbiont, likely played an important role in the evolution of the species. Wolbachia infection so far was only detected in the Western lineage of H. postica with no information on the infecting strain, its frequency, and its consequences on the genetic diversity of the host. We here used a combination of mitochondrial and nuclear sequences of the host and sequence information on Wolbachia to document the distribution of strains and the degree of infection. The Eastern lineage has a higher genetic diversity and is found in the Mediterranean, the Middle East, Eastern Europe, and eastern America, whereas the less diverse Western lineage is found in Central Europe and the western America. Both lineages are infected with the same common strain of Wolbachia belonging to Supergroup B. Based on neutrality tests, selection tests, and the current distribution and diversification of Wolbachia in H. postica, we suggested the Wolbachia infection did not shape genetic diversity of the host. The introduced populations in the United States are generally genetically less diverse, which is in line with founder effects.

Limited female dispersal predicts the incidence of Wolbachia across ants (Hymenoptera: Formicidae)

The endosymbiotic bacterium Wolbachia is perhaps the greatest panzootic in the history of life on Earth, yet remarkably little is known regarding the factors that determine its incidence across species. One possibility is that Wolbachia more easily invades species with structured populations, due to the increased strength of genetic drift and higher initial frequency of infection. This should enable strains that induce mating incompatibilities to more easily cross the threshold prevalence above which they spread to either fixation or a stable equilibrium infection prevalence. Here, we provide empirical support for this hypothesis by analysing the relationship between female dispersal (as a proxy for population structure) and the incidence of Wolbachia across 250 species of ants. We show that species in which the dispersal of reproductive females is limited are significantly more likely to be infected with Wolbachia than species whose reproductive ecology is consistent with significant dispersal of females, and that this relationship remains after controlling for host phylogeny. We suggest that structured host populations, in this case resulting from limited female dispersal, may be an important feature determining how easily Wolbachia becomes successfully established in a novel host, and thus its occurrence across a wide diversity of invertebrate hosts.

Widespread Wolbachia infection in an insular radiation of damselflies (Odonata, Coenagrionidae)

Wolbachia is one of the most common endosymbionts found infecting arthropods. Theory predicts symbionts like Wolbachia will be more common in species radiations, as host shift events occur with greatest frequency between closely related species. Further, the presence of Wolbachia itself may engender reproductive isolation, and promote speciation of their hosts. Here we screened 178 individuals belonging to 30 species of the damselfly genera Nesobasis and Melanesobasis - species radiations endemic to the Fiji archipelago in the South Pacific - for Wolbachia, using multilocus sequence typing to characterize bacterial strains. Incidence of Wolbachia was 71% in Nesobasis and 40% in Melanesobasis, and prevalence was also high, with an average of 88% in the Nesobasis species screened. We identified a total of 25 Wolbachia strains, belonging to supergroups A, B and F, with some epidemic strains present in multiple species. The occurrence of Wolbachia in both males and females, and the similar global prevalence found in both sexes rules out any strong effect of Wolbachia on the primary sex-ratio, but are compatible with the phenotype of cytoplasmic incompatibility. Nesobasis has higher species richness than most endemic island damselfly genera, and we discuss the potential for endosymbiont-mediated speciation within this group.

Wolbachia Acquisition by Drosophila yakuba-Clade Hosts and Transfer of Incompatibility Loci Between Distantly Related Wolbachia

Maternally transmitted Wolbachia infect about half of insect species, yet the predominant mode(s) of Wolbachia acquisition remains uncertain. Species-specific associations could be old, with Wolbachia and hosts codiversifying (i.e., cladogenic acquisition), or relatively young and acquired by horizontal transfer or introgression. The three Drosophila yakuba-clade hosts [(D. santomea, D. yakuba) D. teissieri] diverged ∼3 MYA and currently hybridize on the West African islands Bioko and São Tomé. Each species is polymorphic for nearly identical Wolbachia that cause weak cytoplasmic incompatibility (CI)-reduced egg hatch when uninfected females mate with infected males. D. yakuba-clade Wolbachia are closely related to wMel, globally polymorphic in D. melanogaster We use draft Wolbachia and mitochondrial genomes to demonstrate that D. yakuba-clade phylogenies for Wolbachia and mitochondria tend to follow host nuclear phylogenies. However, roughly half of D. santomea individuals, sampled both inside and outside of the São Tomé hybrid zone, have introgressed D. yakuba mitochondria. Both mitochondria and Wolbachia possess far more recent common ancestors than the bulk of the host nuclear genomes, precluding cladogenic Wolbachia acquisition. General concordance of Wolbachia and mitochondrial phylogenies suggests that horizontal transmission is rare, but varying relative rates of molecular divergence complicate chronogram-based statistical tests. Loci that cause CI in wMel are disrupted in D. yakuba-clade Wolbachia; but a second set of loci predicted to cause CI are located in the same WO prophage region. These alternative CI loci seem to have been acquired horizontally from distantly related Wolbachia, with transfer mediated by flanking Wolbachia-specific ISWpi1 transposons.