Wolbachia

Wolbachia
Transmission electron micrograph of Wolbachia within an insect cell.
Credit:Public Library of Science / Scott O'Neill
Scientific classification
Kingdom: Bacteria
Phylum: Proteobacteria
Class: Alphaproteobacteria
Order: Rickettsiales
Family: Rickettsiaceae
Genus: Wolbachia

Wolbachia is a genus of bacteria which infects arthropod species, including a high proportion of insects (~60% of species), as well as some nematodes. It is one of the world's most common parasitic microbes and is possibly the most common reproductive parasite in the biosphere. Its interactions with its hosts are often complex, and in some cases have evolved to symbiotic rather than parasitic. One study concludes that more than 16% of neotropical insect species carry this bacterium[1] and as many as 25-70% of all insect species are estimated to be potential hosts.[2]

Contents

History

The bacterium was first identified in 1924 by Marshall Hertig and S. Burt Wolbach in Culex pipiens, a species of mosquito. Hertig formally described the genus in 1936 as Wolbachia pipientis.[3] There was little interest after the discovery until 1971 when Janice Yen and A. Ralph Barr of the University of California, Los Angeles discovered that Culex mosquito eggs were killed by a cytoplasmic incompatibility when the sperm of Wolbachia-infected males fertilized infection-free eggs.[4] In 1990, Richard Stouthamer of the University of California, Riverside discovered that Wolbachia can make males dispensable in some species.[5] It is today of considerable interest due to its ubiquitous distribution and many different evolutionary interactions.

Role in sexual differentiation of hosts

Within arthropods, Wolbachia is notable for significantly altering the reproductive capabilities of its hosts. These bacteria can infect many different types of organs, but are most notable for the infections of the testes and ovaries of their hosts.

Wolbachia are known to cause four different phenotypes:

Several species are so dependent on Wolbachia that they are unable to reproduce effectively without the bacteria in their bodies.[9]

Wolbachia are present in mature eggs, but not mature sperm. Only infected females pass the infection on to their offspring. One study on infected woodlice showed that the broods of infected organisms had a higher proportion of females than their uninfected counterparts.[10] It is thought that Wolbachia, especially Wolbachia-caused cytoplasmic incompatibility, may be important in promoting speciation.[11][12][13] Wolbachia strains that distort the sex ratio may alter their host's pattern of sexual selection in nature,[14][15] and also engender strong selection to prevent their action, leading to some of the fastest examples of natural selection in natural populations[16]

Wolbachia infections confer fitness advantages

Wolbachia has been linked to viral resistance in Drosophila melanogaster and mosquito species. Flies infected with the bacteria are more resistant to RNA viruses such as Drosophila C Virus, Nora Virus, Flock House virus, cricket paralysis virus, Chikungunya virus, and West Nile Virus [17][18][19] In the common house mosquito, higher levels of Wolbachia density were correlated with more insecticide resistance.[20] Another interesting interaction is seen in leaf-miners Phyllonorycter blancardella where Wolbachia help their host produce green islands on yellowing tree leaves which allows the host to continue feeding while growing to their adult form. Larvae treated with tetracycline, which kills Wolbachia, lose this ability and subsequently only 13% emerge successfully as adult moths.[21] Lastly, in the nematode species Brugia malayi, Wolbachia has become an obligate endosymbiont and provides the host with chemicals necessary to its survival.[22]

Horizontal gene transfer and genomics

The first Wolbachia genome to be determined was that of one that infects Drosophila melanogaster flies.[23] 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 one that infects Brugia malayi nematodes.[24] Genome sequencing projects for several other Wolbachia strains are in progress. A 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 7 other Drosophila species.[25]

In an application of DNA barcoding to the identification of species of Protocalliphora flies, it was found that several distinct morphospecies had identical cytochrome c oxidase I gene sequences, most likely through horizontal gene transfer by Wolbachia species as they jump across host species.[26] As a result, Wolbachia can cause misleading results in molecular cladistical analyses.[27]

Wolbachia has been found to confer Drosophila hosts with resistance against certain RNA virus infections.[28]

Wolbachia also harbor a temperate bacteriophage called WO.[29] 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.[30] 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 are typically considered highly stable and prone to genomic degradation.

Applications to human health

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 seem 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.[31] Consequently, current strategies for control of filarial nematode diseases include elimination of Wolbachia via the simple doxycycline antibiotic rather than far more toxic anti-nematode medications.[32]

The use of the naturally existing strains of Wolbachia to control mosquito populations has also been a topic of research.[33][34] Wolbachia can be used to control dengue and malaria by eliminating older insects that contain more parasites. Allowing younger insects to survive lessens selection pressure for evolution of resistance.[35][36]

See also

References

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Further reading

External links