Superinfection of five Wolbachia in the alnus ambrosia beetle, Xylosandrus germanus (Blandford) (Coleoptera: Curuculionidae)

Two sympatric lineages of Australian Cnestus solidus share Ambrosiella symbionts but not Wolbachia

Sympatric lineages of inbreeding species provide an excellent opportunity to investigate species divergence patterns and processes. Many ambrosia beetle lineages (Curculionidae: Scolytinae) reproduce by predominant inbreeding through sib mating in nests excavated in woody plant parts wherein they cultivate symbiotic ambrosia fungi as their sole source of nutrition. The Xyleborini ambrosia beetle species Cnestus solidus and Cnestus pseudosolidus are sympatrically distributed across eastern Australia and have overlapping morphological variation. Using multilocus sequencing analysis of individuals collected from 19 sites spanning their sympatric distribution, we assessed their phylogenetic relationships, taxonomic status and microbial symbionts. We found no genetic differentiation between individuals morphologically identified as C. solidus and C. pseudosolidus confirming previous suggestions that C. pseudosolidus is synonymous to C. solidus. However, within C. solidus we unexpectedly discovered the sympatric coexistence of two morphologically indistinguishable but genetically distinct lineages with small nuclear yet large mitochondrial divergence. At all sites except one, individuals of both lineages carried the same primary fungal symbiont, a new Ambrosiella species, indicating that fungal symbiont differentiation may not be involved in lineage divergence. One strain of the maternally inherited bacterial endosymbiont Wolbachia was found at high prevalence in individuals of the more common lineage but not in the other, suggesting that it may influence host fitness. Our data suggest that the two Australian Cnestus lineages diverged allopatrically, and one lineage then acquired Wolbachia. Predominant inbreeding and Wolbachia infection may have reinforced reproductive barriers between these two lineages after their secondary contact contributing to their current sympatric distribution.

Using host species traits to understand the Wolbachia infection distribution across terrestrial beetles

Knowledge of Wolbachia prevalence with respect to its hosts is restricted mainly to taxonomic/phylogenetic context. In contrast, relations between infection and most host’s ecological and biological traits are poorly understood. This study aimed to elaborate on relations between bacteria and its beetle hosts in taxonomic and the ecological contexts. In particular, the goal is to verify which ecological and biological traits of beetles could cause them to be prone to be infected. Verification of Wolbachia infection status across 297 beetle taxa showed that approximately 27% of taxa are infected by supergroups A and B. Only minor support for coevolution between bacteria and its beetle hosts was observed in some genera of beetles, but in general coevolution between beetles and Wolbachia was rejected. Some traits of beetles were found to be unrelated to Wolbachia prevalence (type of range and thermal preferences); some traits were related with ambiguous effects (habitats, distribution, mobility and body size); some were substantially related (reproduction mode and trophy). The aforementioned summary does not show obvious patterns of Wolbachia prevalence and diversity in relation to host taxonomy, biology, and ecology. As both Wolbachia and Coleoptera are diverse groups, this lack of clear patterns is probably a reflection of nature, which is characterised by highly diversified and probably unstable relations.

wsp-based analysis of Wolbachia strains associated with Phlebotomus papatasi and P. sergenti (Diptera: Psychodidae) main cutaneous leishmaniasis vectors, introduction of a new subgroup wSerg

Sand flies of Phlebotomus papatasi and P. sergenti are the main vectors of cutaneous leishmanisis (CL) in the old world. We aimed to screen Iranian P. papatasi and P. sergenti for their natural infections with Wolbachia and to determine their phylogenetic association with other species. Wolbachia surface protein (wsp) gene was PCR amplified from DNA extracted from Phlebotomus species, sequenced, and were analysed in combination with wsp sequences related to Phelebtominae and other insects. All Wolbachia-infecting Iranian sand flies of P. papatasi and P. sergenti were classified in the Supergroup A., Wolbachia isolated from P. sergenti were clustered in a new subgroup within Supergroup A so-called wSreg. The Wolbachia strains identified from the P. papatasi clustered mainly in the subgroup wPap and partly in wSerg. Multiple Wholbachia infection within a single population of P.papatasi warrants investigation on existence and intensity of cytoplasmic incompatibility between the wPap and wSerg subgroups.

Reproductive Manipulators in the Bark Beetle Pityogenes chalcographus (Coleoptera: Curculionidae)-The Role of Cardinium, Rickettsia, Spiroplasma, and Wolbachia

Heritable bacterial endosymbionts can alter the biology of numerous arthropods. They can influence the reproductive outcome of infected hosts, thus affecting the ecology and evolution of various arthropod species. The spruce bark beetle Pityogenes chalcographus (L.) (Coleoptera: Curculionidae: Scolytinae) was reported to express partial, unidirectional crossing incompatibilities among certain European populations. Knowledge on the background of these findings is lacking; however, bacterial endosymbionts have been assumed to manipulate the reproduction of this beetle. Previous work reported low-density and low-frequency Wolbachia infections of P. chalcographus but found it unlikely that this infection results in reproductive alterations. The aim of this study was to test the hypothesis of an endosymbiont-driven incompatibility, other than Wolbachia, reflected by an infection pattern on a wide geographic scale. We performed a polymerase chain reaction (PCR) screening of 226 individuals from 18 European populations for the presence of the endosymbionts Cardinium, Rickettsia, and Spiroplasma, and additionally screened these individuals for Wolbachia. Positive PCR products were sequenced to characterize these bacteria. Our study shows a low prevalence of these four endosymbionts in P. chalcographus. We detected a yet undescribed Spiroplasma strain in a single individual from Greece. This is the first time that this endosymbiont has been found in a bark beetle. Further, Wolbachia was detected in three beetles from two Scandinavian populations and two new Wolbachia strains were described. None of the individuals analyzed were infected with Cardinium and Rickettsia. The low prevalence of bacteria found here does not support the hypothesis of an endosymbiont-driven reproductive incompatibility in P. chalcographus.

Current state of knowledge on Wolbachia infection among Coleoptera: a systematic review

Background

Despite great progress in studies on Wolbachia infection in insects, the knowledge about its relations with beetle species, populations and individuals, and the effects of bacteria on these hosts, is still unsatisfactory. In this review we summarize the current state of knowledge about Wolbachia occurrence and interactions with Coleopteran hosts.

Methods

An intensive search of the available literature resulted in the selection of 86 publications that describe the relevant details about Wolbachia presence among beetles. These publications were then examined with respect to the distribution and taxonomy of infected hosts and diversity of Wolbachia found in beetles. Sequences of Wolbachia genes (16S rDNA, ftsZ) were used for the phylogenetic analyses.

Results

The collected publications revealed that Wolbachia has been confirmed in 204 beetle species and that the estimated average prevalence of this bacteria across beetle species is 38.3% and varies greatly across families and genera (0–88% infected members) and is much lower (c. 13%) in geographic studies. The majority of the examined and infected beetles were from Europe and East Asia. The most intensively studied have been two groups of herbivorous beetles: Curculionidae and Chrysomelidae. Coleoptera harbor Wolbachia belonging to three supergroups: F found in only three species, and A and B found in similar numbers of beetles (including some doubly infected); however the latter two were most prevalent in different families. A total of 59% of species with precise data were found to be totally infected. Single infections were found in 69% of species and others were doubly- or multiply-infected. Wolbachia caused numerous effects on its beetle hosts, including selective sweep with host mtDNA (found in 3% of species), cytoplasmic incompatibility (detected in c. 6% of beetles) and other effects related to reproduction or development (like male-killing, possible parthenogenesis or haplodiploidy induction, and egg development). Phylogenetic reconstructions for Wolbachia genes rejected cospeciation between these bacteria and Coleoptera, with minor exceptions found in some Hydraenidae, Curculionidae and Chrysomelidae. In contrast, horizontal transmission of bacteria has been suspected or proven in numerous cases (e.g., among beetles sharing habitats and/or host plants).

Discussion

The present knowledge about Wolbachia infection across beetle species and populations is very uneven. Even the basic data about infection status in species and frequency of infected species across genera and families is very superficial, as only c. 0.15% of all beetle species have been tested so far. Future studies on Wolbachia diversity in Coleoptera should still be based on the Multi-locus Sequence Typing system, and next-generation sequencing technologies will be important for uncovering Wolbachia relations with host evolution and ecology, as well as with other, co-occurring endosymbiotic bacteria.

Wolbachia endosymbionts in haplodiploid and diploid scolytine beetles (Coleoptera: Curculionidae: Scolytinae)

Haplodiploidy is a sex determination system in which fertilized diploid eggs develop into females and unfertilized haploid eggs develop into males. The evolutionary explanations for this phenomenon include the possibility that haplodiploidy can be reinforced by infection with endosymbiotic bacteria, such as Wolbachia. The subfamily Scolytinae contains species with haplodiploid and diploid sex determination systems. Thus, we studied the association with Wolbachia in 12 diploid and 11 haplodiploid scolytine beetles by analyzing wsp and multilocus sequence typing (MLST) of five loci in this endosymbiont. Wolbachia genotypes were compared with mitochondrial (COI) and nuclear (EF) genotypes in the scolytines. Eight of the 23 scolytine species were infected with Wolbachia, with haplodiploids at significantly higher rates than diploid species. Cloning and sequencing detected multiple infections with up to six Wolbachia strains in individual species. Phylogenetic analyses of wsp and five MLST genes revealed different Wolbachia strains in scolytines. Comparisons between the beetle and Wolbachia phylogenies revealed that closely related beetles were infected with genetically different Wolbachia strains. These results suggest the horizontal transmission of multiple Wolbachia strains between scolytines. We discuss these results in terms of the evolution of different sex determination systems in scolytine beetles.

Discordant phylogenies suggest repeated host shifts in the Fusarium-Euwallacea ambrosia beetle mutualism

The mutualism between xyleborine beetles in the genus Euwallacea (Coleoptera: Curculionidae: Scolytinae) and members of the Ambrosia Fusarium Clade (AFC) represents one of 11 known evolutionary origins of fungiculture by ambrosia beetles. Female Euwallacea beetles transport fusarial symbionts in paired mandibular mycangia from their natal gallery to woody hosts where they are cultivated in galleries as a source of food. Native to Asia, several exotic Euwallacea species were introduced into the United States and Israel within the past two decades and they now threaten urban landscapes, forests and avocado production. To assess species limits and to date the evolutionary diversification of the mutualists, we reconstructed the evolutionary histories of key representatives of the Fusarium and Euwallacea clades using maximum parsimony and maximum likelihood methods. Twelve species-level lineages, termed AF 1-12, were identified within the monophyletic AFC and seven among the Fusarium-farming Euwallacea. Bayesian diversification-time estimates placed the origin of the Euwallacea-Fusarium mutualism near the Oligocene-Miocene boundary ∼19-24 Mya. Most Euwallacea spp. appear to be associated with one species of Fusarium, but two species farmed two closely related fusaria. Euwallacea sp. #2 in Miami-Dade County, Florida cultivated Fusarium spp. AF-6 and AF-8 on avocado, and Euwallacea sp. #4 farmed Fusarium ambrosium AF-1 and Fusarium sp. AF-11 on Chinese tea in Sri Lanka. Cophylogenetic analyses indicated that the Euwallacea and Fusarium phylogenies were largely incongruent, apparently due to the beetles switching fusarial symbionts (i.e., host shifts) at least five times during the evolution of this mutualism. Three cospeciation events between Euwallacea and their AFC symbionts were detected, but randomization tests failed to reject the null hypothesis that the putative parallel cladogenesis is a stochastic pattern. Lastly, two collections of Euwallacea sp. #2 from Miami-Dade County, Florida shared an identical cytochrome oxidase subunit 1 (CO1) allele with Euwallacea validus, suggesting introgressive hybridization between these species and/or pseudogenous nature of this marker. Results of the present study highlight the importance of understanding the potential for and frequency of host-switching between Euwallacea and members of the AFC, and that these shifts may bring together more aggressive and virulent combinations of these invasive mutualists.

Equilibrium frequency of endosymbionts in multiple infections based on the balance between vertical transmission and cytoplasmic incompatibility

Cytoplasmic incompatibility (CI)-inducing endosymbiotic bacteria, such as Wolbachia and Cardinium, have been well studied through field data and validations on the basis of numerical simulations. However, the analytically derived equilibrium frequency of multiple infections has not yet been determined, although the equilibrium for cases of single infection has been reported. In this study, we considered the difference equation for endosymbionts using three parameters: the probability of the failure of vertical transmission (), CI strength (), and the level of host inbreeding (). To analyze this model, we particularly focused on , i.e., the frequency of host individuals completely infected with all -bacterial strains in the population. , at the equilibrium state, was analytically calculated in the cases where and is any arbitrary value. We found that can be described using two parameters: and , which is identical to . has a larger value in a system with a smaller . In addition, determines the maximum number of strains that infect a single host. Our results revealed the following: i) three parameters can be reduced to a single parameter, i.e., and ii) the threshold of the maximum number of infections is defined by , which prevents additional invasions by endosymbionts.

Molecular characterization of Wolbachia strains associated with the invasive Asian citrus psyllid Diaphorina citri in Brazil

Wolbachia is a symbiont intensively studied due to its ability to interfere with their host's reproduction, and it has been recently proposed as an alternative tool to control insect pests or vectors of diseases. The Asian citrus psyllid Diaphorina citri is an important pest of citrus since it vectors the bacterium that causes the "Huanglongbing" disease in citrus. The frequency and diversity of Wolbachia associated with D. citri is unknown, limiting the utilization of Wolbachia as an alternative strategy for insect management. Thus, we aimed to determine the natural rate of infection, to characterize the Wolbachia strains associated with this psyllid by "multilocus sequencing typing" (MLST) and wsp analysis, and to verify the association of the symbiont to particular genotypes of the host. Analysis indicated Wolbachia infects 100 % of all specimens tested from all 15 sampled populations. MLST revealed the occurrence of five new sequence types (STs) of Wolbachia, while analysis based on the wsp sequences indicated only four different types of Wolbachia. ST-173 was predominant, while the remaining STs were population specific. Analysis of the host-symbiont relationship did not reveal any particular association of Wolbachia and haplotypes or a decrease in nucleotide diversity of D. citri in populations in which more than one ST was recorded. The consequences of the diversity of STs reported are still unknown, but the fact that Wolbachia infection is fixed and that there is one ST with a broad distribution highlights the use of this symbiont as an alternative strategy to control D. citri.

Distribution patterns of Wolbachia endosymbionts in the closely related flower bugs of the genus Orius: implications for coevolution and horizontal transfer

Vertical transmission is the primary route of the endosymbiont Wolbachia for its own spread among invertebrate hosts, but horizontal transmission between different hosts is believed to have occurred multiple times. However, it is not well known how Wolbachia commonly spread among closely related hosts. We focused on the closely related species of the minute pirate bugs belonging to the genus Orius, which are important biological control agents in agricultural crops because they are the most useful natural enemy of various tiny pests, such as thrips. Here, we examined five Orius species (Orius sauteri, Orius nagaii, Orius minutus, Orius strigicollis, and Orius tantillus) from eight geographic localities in Japan for Wolbachia infection. Two distinct strains, wOus1 and wOus2, were detected based on Wolbachia surface protein (wsp) gene sequencing. Furthermore, multilocus sequence typing revealed that each of the strains comprised two variants that differed in a single nucleotide. The overall distribution patterns of the two Wolbachia strains were found to differ among host species: prevalent double infection with wOus1 and wOus2 in O. strigicollis; fixation of single infection with wOus2 in O. nagaii; occurrence of single infection with wOus1 in O. sauteri; prevalence of single infection with wOus1 in O. minutus with an exception in a single population; and lack of Wolbachia infection in O. tantillus. Such differences in the distribution patterns of Wolbachia may reflect the evolutionary history of Wolbachia infection among Orius species and/or ecological and physiological differences among the Orius species that determine the invasiveness and maintenance of the two Wolbachia strains.

Identification and biological role of the endosymbionts Wolbachia in rice water weevil (Coleoptera: Curculionidae)

Wolbachia spp. are obligate intracellular bacteria present in reproductive tissues of many arthropod species. Wolbachia infection status and roles in host reproduction were studied in the rice water weevil, Lissorhoptrus oryzophilus Kuschel (Coleoptera, Curculionidae), an introduced species in China. We examined Wolbachia infection status in five populations in China where it reproduces parthenogenetically, and one native population in Southeast Texas, where it reproduces bisexually. All populations were infected by Wolbachia, and all specimens in each population were infected by Wolbachia of a single strain. Phylogenetic analyses based on multilocus sequence typing system indicated that Wolbachia in non-native L. oryzophilus weevils diverges evidently from those in native weevils. After treatments with tetracycline, parthenogenetic weevils reduced the fecundity significantly and eggs were not viable. Our results suggest that Wolbachia are necessary for oocyte production in L oryzophilus.

What is Next in Bark Beetle Phylogeography?

Bark beetle species within the scolytid genera Dendroctonus, Ips, Pityogenes and Tomicus are known to cause extensive ecological and economical damage in spruce and pine forests during epidemic outbreaks all around the world. Dendroctonus ponderosae poses the most recent example having destroyed almost 100,000 km² of conifer forests in North America. The success and effectiveness of scolytid species lies mostly in strategies developed over the course of time. Among these, a complex system of semiochemicals promotes the communication and aggregation on the spot of infestation facilitating an en masse attack against a host tree's defenses; or an association with fungi that evolved either in the form of nutrition (ambrosia fungi) or even by reducing the resistance of host trees (blue-stain fungi). Although often specific to a tree genus or species, some bark beetles are polyphagous and have the ability to switch on to new hosts and extend their host range (i.e., between conifer genera such as Pityogenes chalcographus or even from conifer to deciduous trees as Polygraphus grandiclava). A combination of these capabilities in concert with life history or ecological traits explains why bark beetles are considered interesting subjects in evolutionary studies. Several bark beetle species appear in phylogeographic investigations, in an effort to improve our understanding of their ecology, epidemiology and evolution. In this paper investigations that unveil the phylogeographic history of bark beetles are reviewed. A close association between refugial areas and postglacial migration routes that insects and host trees have followed in the last 15,000 BP has been suggested in many studies. Finally, a future perspective of how next generation sequencing will influence the resolution of phylogeographic patterns in the coming years is presented. Utilization of such novel techniques will provide a more detailed insight into the genome of scolytids facilitating at the same time the application of neutral and non-neutral markers. The latter markers in particular promise to enhance the study of eco-physiological reaction types like the so-called pioneer beetles or obligate diapausing individuals.

A test using Wolbachia bacteria to identify Eurasian source populations of cabbage seedpod weevil, Ceutorhynchus obstrictus (Marsham), in North America

Previous research using mitochondrial haplotypes indicates that North American populations of cabbage seedpod weevil, Ceutorhynchus obstrictus (Marsham), originated from at least two separate introductions from source populations in Eurasia. We tested this hypothesis by comparing the genetic variation of symbiotic Wolbachia bacteria in C. obstrictus among seven North American and four European populations. Because Wolbachia are maternally inherited, infections acquired by a host species at one geographic location theoretically may be present in derivative populations that have established in new regions. Use of the conserved MLST Wolbachia genes gatB, coxA, hcpA, fbpA, and ftsZ identified one strain present in all beetles. Use of the variable wsp gene identified three distinct isolates of this strain that appear to co-occur in all populations and potentially in all individuals. Use of the variable wspB gene provided independent support for the presence of these isolates and evidence of a wspB pseudogene. The lack of genetic variation for these Wolbachia genes among host populations prevents their use to clarify the origins of C. obstrictus in North America. However, the results are an interesting example illustrating disjunction in genetic variation between mitochondria and a maternally-inherited symbiont.

Wolbachia infection in the tribe Naupactini (Coleoptera, Curculionidae): association between thelytokous parthenogenesis and infection status

Several parthenogenetic species of broad-nosed weevils exist, some of them of economic importance because of their pest status. Screening of the maternally inherited Wolbachia bacterium in 29 weevils of the tribe Naupactini, using multilocus sequence typing allowed us to assess a significant correlation between asexuality and infection, and suggests an involvement of Wolbachia in the origin of this reproductive mode. The nine Wolbachia strains retrieved from the Naupactini belong to the B supergroup. Phylogenetic analysis of these strains, along with other 23 strains obtained from arthropods and nematodes, supports previous hypotheses that horizontal transfer of Wolbachia amongst species from unrelated taxa has been pervasive.

Observations on sex ratio and behavior of males in Xyleborinus saxesenii Ratzeburg (Scolytinae, Coleoptera)

Strongly female-biased sex ratios are typical for the fungalfeeding haplodiploid Xyleborini (Scolytinae, Coleoptera), and are a result of inbreeding and local mate competition (LMC). These ambrosia beetles are hardly ever found outside of trees, and thus male frequency and behavior have not been addressed in any empirical studies to date. In fact, for most species the males remain undescribed. Data on sex ratios and male behavior could, however, provide important insights into the Xyleborini's mating system and the evolution of inbreeding and LMC in general.In this study, I used in vitro rearing methods to obtain the first observational data on sex ratio, male production, male and female dispersal, and mating behavior in a xyleborine ambrosia beetle. Females of Xyleborinus saxesenii Ratzeburg produced between 0 and 3 sons per brood, and the absence of males was relatively independent of the number of daughters to be fertilized and the maternal brood sex ratio. Both conformed to a strict LMC strategy with a relatively precise and constant number of males. If males were present they eclosed just before the first females dispersed, and stayed in the gallery until all female offspring had matured. They constantly wandered through the gallery system, presumably in search of unfertilized females, and attempted to mate with larvae, other males, and females of all ages. Copulations, however, only occurred with immature females. From galleries with males, nearly all females dispersed fertilized. Only a few left the natal gallery without being fertilized, and subsequently went on to produce large and solely male broods. If broods were male-less, dispersing females always failed to found new galleries.