Different Strains Of Wolbachia Research Articles

Maintenance of adaptive differentiation by Wolbachia induced bidirectional cytoplasmic incompatibility: the importance of sib-mating and genetic systems

BackgroundBacteria of the genus Wolbachia are reproductive parasites widespread among arthropods. The most common effect arising from the presence of Wolbachia in a population is Cytoplasmic Incompatibility (CI), whereby postmating reproductive isolation occurs in crosses between an infected male and an uninfected female, or when a male is infected with a different strain of Wolbachia to that of the female (bidirectional CI). Previous theoretical models have demonstrated that bidirectional CI can contribute to the genetic divergence of populations in haploid and diploid organisms. However, haplodiploid organisms were not considered in these models even though they include Nasonia parasitoid wasps – the best example of the implication of Wolbachia in ongoing speciation. Moreover, previous work did not investigate inbreeding mating systems, which are frequently observed in arthropod species.ResultsWe developed a stochastic two-island model which simulated three genetic scenarios, diploidy, haploidy, and haplodiploidy, with two CI phenotypes being considered for the latter: (1) male development of female progeny; and (2) mortality of fertilized eggs. We also investigated the effect of varying the proportion of sib mating. In the model each allopatric population was initially fixed for a single allele at a nuclear locus under positive selection and infected with one strain of Wolbachia. Each simulation presupposed that the two populations were fixed for a different allele and a different strain of Wolbachia. The degree of genetic differentiation observed in the locus under selection due to bidirectional CI was much lower for the two haplodiploid phenotypes than for either diploids or haploids. Furthermore, we demonstrated that sib-mating may compensate for the lower efficiency of bidirectional CI in haplodiploids by maintaining genetic divergence.ConclusionOur model suggests that maintenance of genetic differentiation facilitated by Wolbachia is more likely to occur in diploids and haploids than in haplodiploids. However, increasing the level of sib-mating may compensate for the weak effect of bidirectional CI in haplodiploids. Our work therefore gives a potential explanation for why the haplodiploid Nasonia species, which are infected with bidirectionally incompatible Wolbachia strains and undergo sib-mating, have differentiated genetically and maintained this differentiation without premating isolation.

Wolbachia in two populations of Melittobia digitata Dahms (Hymenoptera: Eulophidae)

We investigated two populations of Melittobia digitata Dahms, a gregarious parasitoid (primarily upon a wide range of solitary bees, wasps, and flies), in search of Wolbachia infection. The first population, from Xalapa, Mexico, was originally collected from and reared on Mexican fruit fly pupae, Anastrepha ludens Loew (Diptera: Tephritidae); the other, from Athens, Georgia, was collected from and reared on prepupae of mud dauber wasps, Trypoxylon politum Say (Hymenoptera: Crabronidae). PCR studies of the ITS2 region corroborated that both parasitoid populations were the same species; this potentially provides a useful molecular taxonomic profile since females of Melittobia species are superficially similar. Amplification of the Wolbachia surface protein gene (wsp) confirmed the presence of this endosymbiont in both populations. Sequencing revealed that the Wolbachia harbored in both populations exhibited a wsp belonging to a unique subgroup (denoted here as Dig) within the B-supergroup of known wsp genes. This new subgroup of wsp may either belong to a different strain of Wolbachia from those previously found to infect Melittobia or may be the result of a recombination event. In either case, known hosts of Wolbachia with a wsp of this subgroup are only distantly related taxonomically. Reasons are advanced as to why Melittobia - an easily reared and managed parasitoid - holds promise as an instructive model organism of Wolbachia infection amenable to the investigation of Wolbachia strains among its diverse hosts.

Molecular Survey of Endosymbionts in Florida Populations of Diaphorina citri (Hemiptera: Psyllidae) and Its Parasitoids Tamarixia radiata (Hymenoptera: Eulophidae) and Diaphorencyrtus aligarhensis (Hymenoptera: Encyrtidae)

A molecular survey of endosymbionts was conducted in Florida populations of the Asian citrus psyllid, Diaphorina citri Kuwayama (Hemiptera: Psyllidae), and its parasitoids Tamarixia radiata (Waterston) (Hymenoptera: Eulophidae) and Diaphorencyrtus aligarhensis (Shafee, Alam and Agarwal) (Hymenoptera: Encyrtidae). Using a high-fidelity polymerase chain reaction (PCR), we detected 3 eubacterial species each in D. citri (the primary symbiont Candidatus Carsonella rudii, a secondary symbiont related to Oxalobacter and Herbaspirillum species, and Wolbachia), and T. radiata (Caulobacter sp., Methylobacterium sp. and a bacterium in the family Alcaligenaceae), whereas only 1 species was identified in D. aligarhensis (Wolbachia). Each eubacterial symbiont of D. citri was detected in the eggs, which is indicative of transovarial transmission. However, none of the eubacterial symbionts of T. radiata were detected in the eggs, suggesting that they are only transient associates. Stable horizontal transfer of these eubacterial species likely has not occurred between the host and its parasitoids because each insect hosts a different complement of Eubacteria. For example, different strains of Wolbachia were detected in D. citri and D. aligarhensis, based on evidence from the 16S rRNA and wsp gene sequences. Also, the orf7 gene of the bacteriophage WO associated with Wolbachia was detected in D. citri but not in D. aligarhensis. No endosymbiotic Archaea, Helicosporidia, Microsporidia, Fungi, or Yeast-like symbionts were detected with PCR in these populations of D. citri or its parasitoids.

Wolbachia-induced cytoplasmic incompatibility in Japanese populations of Tetranychus urticae (Acari: Tetranychidae)

Intracellular bacteria of the genus Wolbachia (alpha Proteobacteria) induce cytoplasmic incompatibility (CI) in many arthropod species, including spider mites, but not all Wolbachia cause CI. In spider mites CI becomes apparent by a reduced egg hatchability and a lower daughter:son ratio: CI in haplodiploid organisms in general was expected to produce all-male offspring or a male-biased sex ratio without any death of eggs. In a previous study of Japanese populations of Tetranychus urticae, two out of three green-form populations tested were infected with non-CI Wolbachia strains, whereas none of six red-form populations harbored Wolbachia. As the survey of Wolbachia infection in T. urticae is still fragmentary in Japan, we checked Wolbachia infection in thirty green-form populations and 29 red-form populations collected from a wide range of Japanese islands. For Wolbachia-infected populations, we tested the effects of Wolbachia on the reproductive traits and determined the phylogenetic relationships of the different strains of Wolbachia. All but one green-form populations were infected with Wolbachia and all strains belonged to the subgroup Ori when the wsp gene was used to determine the phylogenetic relationships of different strains of Wolbachia. Six out of 29 red-form populations harbored Wolbachia and the infected strains belonged to the subgroups Ori and Bugs. Twenty-four of 29 infected green-form populations and five of six infected red-form populations induced CI among the hosts. Thus, CI-Wolbachia strains are widespread in Japan, and no geographical trend was observed in the CI-Wolbachia. Although three red-form populations harbored other intracellular bacteria Cardinium, they did not affect host reproduction.

The invasion and coexistence of competing Wolbachia strains.

Cytoplasmic incompatibility between arthropods infected with different strains of Wolbachia has been proposed as an important mechanism for speciation. However, a basic requirement for this mechanism is the coexistence of different strains in neighbouring populations. Here we test whether this required coexistence is possible in a spatial context. Continuous-time models for the behaviour of one and two strains of Wolbachia within a single well-mixed population demonstrate the Allee effect and founder control, such that one strain is always driven extinct. In contrast, discretised spatial models show patchy persistence of the two strains although coexistence within the same habitat is rare. A simplified model of such founder control suggests that it is fragmentation of (or barriers within) the habitat rather than space itself that leads to persistence.

Effects of Wolbachia on Genetic Divergence Between Populations: Mainland-Island Model

Cytoplasmic incompatibility (CI) induced by intracellular bacteria is a possible mechanism for speciation. Growing empirical evidence suggests that bacteria of the group Wolbachia may indeed act as isolating factors in recent insect speciation. Wolbachia are cytoplasmically transmitted and can cause uni- or bidirectional CI. We present a mainland-island model to investigate how much impact Wolbachia can have on genetic divergence between populations. In the first scenario we assume that the island population has diverged at a selected locus and ask whether genetic divergence will be maintained after introduction of migration from the mainland. In the second we explore whether divergence will originate under migration. For simplicity, the host organisms are modeled as haploid sexuals. Simulations show that if each population is initially infected with a different strain of Wolbachia, then higher levels of divergence occur at the locally selected locus than in the absence of Wolbachia. A weaker effect is seen when there is only unidirectional CI caused by a single strain of Wolbachia on the island. CI increases divergence because it reduces effective migration between mainland and island. Migrants suffer from being confronted with the wrong CI system and this also applies to their matrilineal descendants. Moreover, there is a strong linkage disequilibrium between host genotype and infection state, which helps to maintain Wolbachia differences between the populations in the face of migration A sex bias in migration can either increase or decrease the effect of Wolbachia on divergence. Results support the view that Wolbachia has the potential for increasing divergence between populations and thus could enhance probabilities of speciation.

Dynamics of double and single Wolbachia infections in Drosophila simulans from New Caledonia.

The bacterial symbiont Wolbachia can cause cytoplasmic incompatibility in Drosophila simulans flies: if an infected male mates with an uninfected female, or a female with a different strain of Wolbachia, there can be a dramatic reduction in the number of viable eggs produced. Here we explore the dynamics associated with double and single Wolbachia infections in New Caledonia. Doubly infected females were compatible with all males in the population, explaining the high proportion of doubly infected flies. In this study, males that carry only wHa or wNo infections showed reduced incompatibility when mated to uninfected females, compared with previous reports. These data suggest that either the DNA of these bacterial isolates have diverged from those previously collected, or the genetic background of the host has lead to a reduction in the phenotype of incompatibility. Mitochondrial sequence polymorphism at two sites within the host genome was assayed to investigate population structure related to infection types. There was no correlation between sequence polymorphism and infection type suggesting that double infections are the stable type, with singly infected and uninfected flies arising from stochastic segregation of bacterial strains. Finally, we discuss the nomenclature of Wolbachia strain designation.

Vertical transmission of Wolbachia in Tetranychus kanzawai Kishida and Panonychus mori Yokoyama (Acari: Tetranychidae).

The vertical transmission of Wolbachia in two species of spider mite was investigated and compared. One species, Tetranychus kanzawai Kishida, was infected with a modification negative strain of Wolbachia while the other species, Panonychus mori Yokoyama, was infected with a modification positive strain. The infection showed perfect maternal transmission in the laboratory population of T. kanzawai in which Wolbachia-infected females produced infected offspring regardless of whether they mated with infected or uninfected males, and uninfected females produced Wolbachia-free progenies without regard to the infection status of their mating partners. In artificial P. mori populations initiated with 50% infected and 50% uninfected female adults, the infection frequencies among progenies increased with each generation, reaching 100% at the sixth generation in the Sendai population and after the sixth generation in the Toyama population. In another experiment, in which an artificial T. kanzawai population was composed of 50% infected and 50% uninfected female adults, the infection frequency in progeny populations increased very slowly, reaching 62.5% at the 15th generation. The difference in infection frequency in the two spider mites may be due to the different strains of Wolbachia.

Multiple causes of male-killing in a single sample of the two-spot ladybird, Adalia bipunctata (Coleoptera: coccinellidae) from Moscow.

Thirty-six matrilines from a single Muscovite sample of Adalia bipunctata were assayed, using appropriate primers, for presence of the four male-killing symbionts known to infect this species of ladybird. All four, a Rickettsia, a Spiroplasma and two different strains of Wolbachia, were found to be present. Vertical transmission efficiencies were assessed from F1 and F2 families from each of the matrilines, and were found to differ significantly between symbionts. Potential explanations of the presence of four different male-killing symbionts within a single population, are considered.