Wolbachia

''Wolbachia'' is a genus of gram-negative bacteria infecting many species of arthropods and filarial nematodes. The symbiotic relationship ranges from parasitism to obligate mutualism. It is one of the most common parasitic microbes of arthropods, and is possibly the most widespread reproductive parasite bacterium in the biosphere. Its interactions with hosts are complex and highly diverse across different host species. Some host species cannot reproduce, or even survive, without ''Wolbachia'' colonisation. One study concluded that more than 16% of neotropical insect species carry bacteria of this genus, and as many as 25 to 70% of all insect species are estimated to be potential hosts.

History

The first organism classified as ''Wolbachia'' was discovered in 1924 by Marshall Hertig and Simeon Burt Wolbach in the common house mosquito. They described it as "a somewhat pleomorphic, rodlike, Gram-negative, intracellular organism hatapparently infects only the ovaries and testes". Hertig formally described the species in 1936, and proposed both the generic and specific names: ''Wolbachia pipientis''.

Research on ''Wolbachia'' intensified after 1971, when Janice Yen and A. Ralph Barr of UCLA discovered that ''Culex'' mosquito eggs were killed by a cytoplasmic incompatibility when the sperm of ''Wolbachia''-infected males fertilized infection-free eggs.

Since, a large number of bacteria with close phylogenetic affinity to the originally detected ''W. pipientis'' have been discovered in a variety of hosts spanning over the Arthropoda and Nematoda phyla. The taxonomic classification of the various discovered groups remains a subject of debate, with no consensus on whether these groups of ''Wolbachia pipientis''-like organisms should be categorized as the same or different species. Therefore, the strains are collectively referred to as ''Wolbachia'', with the various groups of phylogenetically closely related strains designated as supergroups rather than distinct species. In general, each supergroup corresponds to a specific host or group of hosts. The genus ''Wolbachia'' is of considerable interest today due to its ubiquitous distribution, its many different evolutionary interactions, and its potential use as a biocontrol agent.

Phylogenetic studies have shown that the closest relatives to ''Wolbachia'' are the genera '' Francisella'' and '' Bartonella''. Unlike ''Wolbachia'', which needs a host cell to multiply, relatives belonging to these genera can be cultured on agar plates.

Method of sexual differentiation in hosts

''Wolbachia'' can infect many different types of organs, but are most notable for the infections of the testes and ovaries of their hosts altering the reproduction abilities of these. ''Wolbachia'' species are ubiquitous in mature eggs, but not mature sperm. Only infected females, therefore, pass the infection on to their offspring. ''Wolbachia'' bacteria maximize their spread by altering the reproductive capabilities of their hosts, in favour for the infected females. Several different phenotypes have been observed, including:

* Male killing occurs when infected males die during larval development, which increases the rate of born, infected females.
* Feminization results in infected males that develop as females or infertile pseudofemales. This is especially prevalent in Lepidoptera species such as the adzuki bean borer ('' Ostrinia scapulalis'').
* Parthenogenesis is reproduction of infected females without males. Some scientists have suggested that parthenogenesis may always be attributable to the effects of ''Wolbachia,'' though this is not the case for the marbled crayfish. An example of parthenogenesis induced by presence of ''Wolbachia'' are some species within the '' Trichogramma'' parasitoid wasp genus, which have evolved to procreate without males due to the presence of ''Wolbachia''. Males are rare in this genus of wasp, possibly because many have been killed by that same strain of ''Wolbachia''.
* Cytoplasmic incompatibility is the inability of ''Wolbachia''-infected males to successfully reproduce with uninfected females or females infected with another ''Wolbachia'' strain. This reduces the reproductive success of those uninfected females and therefore promotes the infecting strain. In the cytoplasmic incompatibility mechanism, ''Wolbachia'' interferes with the parental chromosomes during the first mitotic divisions to the extent that they can no longer divide in synchrony.

Effects of sexual differentiation in hosts

Several host species, such as those within the genus ''Trichogramma'', are so dependent on sexual differentiation of ''Wolbachia'' that they are unable to reproduce effectively without the bacteria in their bodies, and some might even be unable to survive uninfected.

One study on infected woodlice showed the broods of infected organisms had a higher proportion of females than their uninfected counterparts.

''Wolbachia'', especially ''Wolbachia''-caused cytoplasmic incompatibility, may be important in promoting speciation. ''Wolbachia'' strains that distort the sex ratio may alter their host's pattern of sexual selection in nature, and also engender strong selection to prevent their action, leading to some of the fastest examples of natural selection in natural populations.

The male killing and feminization effects of ''Wolbachia'' infections can also lead to speciation in their hosts. For example, populations of the pill woodlouse, '' Armadillidium vulgare'' which are exposed to the feminizing effects of ''Wolbachia'', have been known to lose their female-determining chromosome. In these cases, only the presence of ''Wolbachia'' can cause an individual to develop into a female. Cryptic species of ground wētā ('' Hemiandrus maculifrons'' complex) are host to different lineages of ''Wolbachia'' which might explain their speciation without ecological or geographical separation.

Effect on aromatase

The enzyme aromatase is found to mediate sex-change in many species of fish. ''Wolbachia'' can affect the activity of aromatase in developing fish embryos.

Mechanism of host transfer

Step 1: Physical transfer

Predator-prey interactions

''Wolbachia'' may transfer from prey to predator through the digestive system. To do so, ''Wolbachia'' needs to first survive through the lumen secretion and then enter the host tissue through the gut epithelium. This route does not seem to occur frequently due to little evidence.

Host–parasitoid/parasite interactions

This may be one of the most common routes of ''Wolbachia'' host shifts. Compared to predator-prey interactions, the physical association between the host and parasites typically lasts longer, occurs at various developmental stages, and enables ''Wolbachia'' to directly contact various tissues.

Since this interaction may expose both sides to microbial exchange, one strategy for understanding the direction of transfer is to assess ''Wolbachia'''s presence in close relatives on both sides, as the donor side generally has a larger diversity of infection.

One parasitoid species can infect multiple shared hosts, and one host species can infect multiple parasitoids. For instance, parthenogenesis-inducing ''Wolbachia'' can spread between ''Trichogramma'' parasitoid wasps sharing host eggs.

Parasites can also serve as a vector between infected and uninfected hosts without being infected. When the mouthparts and ovipositors of aphelinid parasitoid wasps become contaminated through feeding ''Wolbachia''-infected '' Bemisia tabaci'', it can infect the next host.

Shared plant and other food sources

This route applies to microbes that can survive either within or on the surface of the food. Experiments demonstrated that the ''Wolbachia'' wAlbB strain can survive extracellularly for up to 7 days, and up to 50 days for some strains in cotton leaf phloem vessels.

Plants are one of the best platforms for this route. By physical contact between arthropod mouthparts and plant tissue, the ''Wolbachia'' inhabiting the salivary glands of some insects may be transferred to the plants. As a result, arthropod species feeding on the same plants may share common ''Wolbachia'' strains.

Other insect food sources may also mediate ''Wolbachia'' horizontal transfer, such as the sharing of dung patches between two Malagasy dung beetle species.

Step 2: Survival and proliferation in the new host

The pathogen-associated molecular patterns (PAMPs) in the bacteria, such as peptidoglycan, can activate the host's innate immune responses. In response, some ''Wolbachia'' strains have a unique functional peptidoglycan amidase (AmiDwol) that cleaves its bacterial cell wall so that it can escape from immune responses. Besides the peptidoglycans, cell-to-cell movements of ''Wolbachia'' can also cause oxidative stress to the host and trigger the host's immune response. Therefore, ''Wolbachia'' has a triple-layer vacuole that acts as a mechanical shield to protect it from cellular immune responses.

Step 3: Vertical transmission

Vertical transmission requires ''Wolbachia'' to reach germ line cells and maintain in the zygote. ''Wolbachia'' may initially occupy somatic stem cells as a stable reservoir and then use the host's vitellogenin transovarial transportation system to enter the oocyte. Once ''Wolbachia'' enter the zygote, they need to reach important host tissues without disrupting the embryo's development. This can be achieved using the host cytoskeleton, by bundling ''Wolbachia'' protein WD0830 to host actin filaments. They can also increase the division rate of germ-line stem cells to localize and increase their titer. Under natural conditions, successful vertical transmission of ''Wolbachia'' is challenging.

Step 4: Spread within the host population

Invasion of a new population likely stems from specific phenotypic effects, including reproductive manipulations and/or providing direct fitness benefits to their female hosts.

Upon transferring into a new host, ''Wolbachia'' may retain its original phenotypic effects, induce a different phenotype, or have no detectable effect. For instance, a strain that induces male killing in the moth '' Cadra cautella'' induced cytoplasmic incompatibility in a novel moth host '' Ephestia kuehniella''.

Fitness advantages by ''Wolbachia'' infections

''Wolbachia'' infection has been linked to viral resistance in ''Drosophila melanogaster'', '' Drosophila simulans'', and mosquito species. Flies, including mosquitoes, infected with the bacteria are more resistant to RNA viruses such as Drosophila C virus, norovirus, flock house virus, cricket paralysis virus, chikungunya virus, and West Nile virus.

In the common house mosquito, higher levels of ''Wolbachia'' were correlated with more insecticide resistance.

In leafminers of the species '' Phyllonorycter blancardella'', ''Wolbachia'' bacteria help their hosts produce green islands on yellowing tree leaves, that is, small areas of leaf remaining fresh, allowing the hosts to continue feeding while growing to their adult forms. Larvae treated with tetracycline, which kills ''Wolbachia'', lose this ability and subsequently only 13% emerge successfully as adult moths.

'' Muscidifurax uniraptor, a'' parasitoid wasp, also benefits from hosting ''Wolbachia'' bacteria.

In the parasitic filarial nematode species responsible for elephantiasis, such as '' Brugia malayi'' and '' Wuchereria bancrofti'', ''Wolbachia'' has become an obligate endosymbiont and provides the host with chemicals necessary for its reproduction and survival. Elimination of the ''Wolbachia'' symbionts through antibiotic treatment therefore prevents reproduction of the nematode, and eventually results in its premature death.

Some ''Wolbachia'' species that infect arthropods also provide some metabolic provisioning to their hosts. In ''Drosophila melanogaster'', ''Wolbachia'' is found to mediate iron metabolism under nutritional stress and in '' Cimex lectularius'', the ''Wolbachia'' strain ''cCle'' helps the host to synthesize B vitamins.

Some ''Wolbachia'' strains have increased their prevalence by increasing their hosts' fecundity. ''Wolbachia'' strains captured from 1988 in southern California still induce a fecundity deficit, but nowadays the fecundity deficit has been replaced in the wild with a fecundity advantage such that infected '' Drosophila simulans'' produces more offspring than the uninfected ones.

Life-history consequences of ''Wolbachia'' infection

''Wolbachia'' often manipulates host reproduction and life-history in a way that favours its own propagation. In the Pharaoh ant, ''Wolbachia'' infection correlates with increased colony-level production of reproductives (i.e., greater reproductive investment), and earlier onset of reproductive production (i.e., shorter life-cycle). Infected colonies also seem to grow more rapidly. There is substantial evidence that the presence of ''Wolbachia'' that induce parthenogenesis have put pressure on species to reproduce primarily or entirely this way.

Additionally, ''Wolbachia'' has been seen to decrease the lifespan of ''Aedes aegypti'', carriers of mosquito-borne diseases, and it decreases their efficacy of pathogen transmission because older mosquitoes are more likely to have become carriers of one of those diseases. This has been exploited as a method for pest control.

Genomics

The first ''Wolbachia'' genome to be determined was that of strain wMel, which infects ''D. melanogaster'' fruit flies. This genome was sequenced at The Institute for Genomic Research in a collaboration between Jonathan Eisen and Scott O'Neill. The second ''Wolbachia'' genome to be determined was of strain wBm, which infects '' Brugia malayi'' nematodes . Since the development and release of high-throughput sequencing technologies in the mid-2000s, the number of published ''Wolbachia'' genomes has grown significantly, driven by both the decreased cost of sequencing and the expanding interest in studying this bacterium.

The genetic background to the reproductive parasitism has been extensively studied in different host systems. A key factor for the alteration of host reproduction is the presence of the bacteriophage WO, which harbours the CI inducing genes ''cifA'' and ''cifB,'' contributing to the phenotypic expression of altered reproductive success observed in infected hosts.

Horizontal gene transfer

Comparative sequence analyses of bacteriophage WO offer some of the most compelling examples of large-scale horizontal gene transfer between ''Wolbachia'' coinfections in the same host. It is the first bacteriophage implicated in frequent lateral transfer between the genomes of bacterial endosymbionts. Gene transfer by bacteriophages could drive significant evolutionary change in the genomes of intracellular bacteria that were previously considered highly stable or prone to loss of genes over time.

''Wolbachia'' also transfers genes to the host. A nearly complete copy of the ''Wolbachia'' genome sequence was found within the genome sequence of the fruit fly '' Drosophila ananassae'' and large segments were found in seven other ''Drosophila'' species.

In an application of DNA barcoding to the identification of species of '' Protocalliphora'' flies, several distinct morphospecies had identical cytochrome c oxidase I gene sequences, most likely through horizontal gene transfer (HGT) by ''Wolbachia'' species as they jump across host species. As a result, ''Wolbachia'' can cause misleading results in molecular cladistical analyses. It is estimated that between 20 and 50 percent of insect species have evidence of HGT from ''Wolbachia''—passing from microbes to animal (i.e. insects).

Small RNA

The small non-coding RNAs WsnRNA-46 and WsnRNA-59 in ''Wolbachia'' were detected in ''Aedes aegypti'' mosquitoes and ''Drosophila melanogaster''. The small RNAs (sRNAs) may regulate bacterial and host genes. Highly conserved intragenic region sRNA called ncrwmel02 was also identified in ''Wolbachia pipientis.'' It is expressed in four different strains in a regulated pattern that differs according to the sex of the host and the tissue localisation. This suggested that the sRNA may play important roles in the biology of ''Wolbachia.''

Relation to human-related infections

Role in parasites

Outside of insects, ''Wolbachia'' infects a variety of isopod species, spiders, mites, and many species of filarial nematodes (a type of parasitic worm), including those causing onchocerciasis (river blindness) and elephantiasis in humans, as well as heartworms in dogs. Not only are these disease-causing filarial worms infected with ''Wolbachia'', but ''Wolbachia'' also seems to play an inordinate role in these diseases.

A large part of the pathogenicity of filarial nematodes is due to host immune response toward their ''Wolbachia''. Elimination of ''Wolbachia'' from filarial nematodes generally results in either death or sterility of the nematode. Consequently, current strategies for control of filarial nematode diseases include elimination of their symbiotic ''Wolbachia'' via the simple doxycycline antibiotic, rather than directly killing the nematode with often more toxic antinematode medications.

Disease prevention

Naturally existing strains of ''Wolbachia'' have been shown to be a route for vector control strategies because of their presence in arthropod populations, such as mosquitoes. Due to the unique traits of ''Wolbachia'' that cause cytoplasmic incompatibility, some strains are useful to humans as a promoter of genetic drive within an insect population. ''Wolbachia''-infected females are able to produce offspring with uninfected and infected males; however, uninfected females are only able to produce viable offspring with uninfected males. This gives infected females a reproductive advantage that is greater the higher the frequency of ''Wolbachia'' in the population. Computational models predict that introducing ''Wolbachia'' strains into natural populations will reduce pathogen transmission and reduce overall disease burden. An example includes a life-shortening ''Wolbachia'' that can be used to control dengue virus and malaria by eliminating the older insects that contain more parasites. Promoting the survival and reproduction of younger insects lessens selection pressure for evolution of resistance.

In addition, some ''Wolbachia'' strains are able to directly reduce viral replication inside the insect. For dengue they include wAllbB and wMelPop with ''Aedes aegypti'', wMel with ''Aedes albopictus'' and ''Aedes aegypti''.

''Wolbachia'' has also been identified to inhibit replication of chikungunya virus (CHIKV) in ''A. aegypti''. The ''w''Mel strain of ''Wolbachia pipientis'' significantly reduced infection and dissemination rates of CHIKV in mosquitoes, compared to ''Wolbachia'' uninfected controls and the same phenomenon was observed in yellow fever virus infection converting this bacterium in an excellent promise for YFV and CHIKV suppression.

''Wolbachia'' also inhibits the secretion of West Nile virus (WNV) in cell line Aag2 derived from ''A. aegypti'' cells. The mechanism is somewhat novel, as the bacteria actually enhances the production of viral genomic RNA in the cell line ''Wolbachia''. Also, the antiviral effect in intrathoracically infected mosquitoes depends on the strain of ''Wolbachia'', and the replication of the virus in orally fed mosquitoes was completely inhibited in wMelPop strain of ''Wolbachia''.

The effect of ''Wolbachia'' infection on virus replication in insect hosts is complex and depends on the ''Wolbachia'' strain and virus species. While several studies have indicated consistent refractory phenotypes of ''Wolbachia'' infection on positive-sense RNA viruses in ''Drosophila melanogaster'', the yellow fever mosquito ''Aedes aegypti'' and the Asian tiger mosquito ''Aedes albopictus'', this effect is not seen in DNA virus infection and in some cases ''Wolbachia'' infection has been associated or shown to increase single stranded DNA and double-stranded DNA virus infection. There is also currently no evidence that ''Wolbachia'' infection restricts any tested negative-sense RNA viruses indicating ''Wolbachia'' would be unsuitable for restriction of negative-sense RNA arthropod borne viruses.

''Wolbachia'' infection can also increase mosquito resistance to malaria, as shown in ''Anopheles stephensi'' where the ''w''AlbB strain of ''Wolbachia'' hindered the lifecycle of ''Plasmodium falciparum''.

However, ''Wolbachia'' infections can also enhance pathogen transmission. ''Wolbachia'' has enhanced multiple arboviruses in '' Culex tarsalis'' mosquitoes. In another study, West Nile virus (WNV) infection rate was significantly higher in ''Wolbachia'' (strain wAlbB)-infected ''C. tarsalis'' compared to controls.

''Wolbachia'' may induce reactive oxygen species–dependent activation of the Toll (gene family) pathway, which is essential for activation of antimicrobial peptides, defensins, and cecropins that help to inhibit virus proliferation. Conversely, certain strains actually dampen the pathway, leading to higher replication of viruses. One example is with strain wAlbB in ''Culex tarsalis'', where infected mosquitoes actually carried the West Nile virus (WNV) more frequently. This is because wAlbB inhibits REL1, an activator of the antiviral Toll immune pathway. As a result, careful studies of the ''Wolbachia'' strain and ecological consequences must be done before releasing artificially-infected mosquitoes in the environment.

Techniques and deployments

= Strain wMel, mixed-sex

=
The World Mosquito Program (WMP) uses ''Wolbachia'' strain wMel to infect ''Aedes'' mosquitos. The mixed-sex mosquitos are intended to infect the local population with wMel, giving them transmission resistance.

In 2014, WMP released infected mosquitos in Townsville, an Australia city with 187,000 inhabitants plagued by dengue. For four years after introduction, no cases of dengue were reported. Trials in much smaller areas had been carried out, but a larger area had not been tested. No environmental ill-effects were reported. The cost was A$15 per inhabitant, but it was hoped that it could be reduced to US$1 in poorer countries with lower labor costs.

In 2016, WMP scientist Scott Ritchie proposed using wMel mosquitos to combat the spread of the Zika virus. A study reported that ''Wolbachia'' wMel has the ability to block Zika in Brazil. In October 2016, it was announced that US$18 million in funding was being allocated for the use of ''Wolbachia''-infected mosquitoes to fight Zika and dengue viruses. Deployment was slated for early 2017 in Colombia and Brazil.

Between 2016 and 2020, WMP conducted its first randomized controlled trial in Yogyakarta, an Indonesian city of about 400,000 inhabitants. In August 2020, the trial's Indonesian lead scientist Adi Utarini announced that the trial showed a 77% reduction in dengue cases compared to the control areas. This trial was the "strongest evidence yet" for the technique.

In 2017–2019, WMP released mosquitos in Niterói, Brazil.

In March 2023, Brazil's Oswaldo Cruz Foundation signed an agreement with WMP to provide funds for a large "mosquito factory" producing infected insects.

= Male incompatibility

=
Another method to use ''Wolbachia'' in mosquitos exploits the cytoplamic incompatibility between infected males and uninfected females. If an uninfected female mates with an infected male, her eggs become infertile. With enough infected males released, the mosquito population would be reduced temporarily.

Verily, the life sciences arm of Google's parent company Alphabet Inc., uses this method. In July 2017, it announced a plan to release about 20 million ''Wolbachia''-infected male ''Aedes aegypti'' mosquitoes in Fresno, California, in an attempt to combat the Zika virus. Singapore's National Environment Agency has teamed up with Verily to come up with an advanced, more efficient way to release male ''Wolbachia'' mosquitoes for Phase 2 of its study to suppress the urban ''Aedes aegypti'' mosquito population and fight dengue.

On November 3, 2017, the US Environmental Protection Agency (EPA) registered Mosquito Mate, Inc. to release ''Wolbachia'' strain "ZAP"-infected male mosquitoes in 20 US states and the District of Columbia.

See also

* Intragenomic conflict
* Quorum sensing
* '' Delftia tsuruhatensis'' a bacterium that naturally prevent malaria.
* '' Serratia'' a genus of bacteria that can be genetically modified to prevent malaria.

References

Further reading

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External links

Virtual Museum of Bacteria



at the National Science Foundation
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The ''Wolbachia Project''
at Vanderbilt University

Images of Wolbachia

{{Authority control

Bacteria genera
Endosymbiotic events
Gram-negative bacteria
Rickettsiales