Wolbachia is a genus of bacteria which infects arthropod species, including a high proportion of insects (~60% of species). It is one of the world’s most common parasitic microbes and possibly the most common reproductive parasite in biosphere. Studies have suggested that 25-70% of all insect species are estimated to be potential hosts.

Marshall Hertig and Burt Wolbach first identified the bacterium in 1924 in a species of mosquito. Hertig described the genus as Wolbachia pipientis. Not much interest was placed in the discovery until Janice Yen and A. Ralph Barr discovered that Culex mosquito eggs were killed by a cytoplasmic incompatibility when the sperm of Wolbachia-infected males fertilized infection-free eggs. Richard Stouthamer discovered that Wolbachia can make males dispensable in some species.

It is noted, within arthropods, that Wolbachia alters the reproductive capabilities of its hosts. The bacteria can infect many different types of organs but are most notable for infection of the testes and ovaries of their hosts. It can cause four different phenotypes: male killing in which they die during the larval development, Feminization where infected males develop as females. There is also Parthenogenesis where females reproduces without males and finally they create cytoplasmic incompatibility where infected males can’t successfully reproduce with uninfected females or females infected with other strains. Some species are so dependent that they are unable to reproduce effectively without the bacteria in their bodies.

Wolbachia is present in mature eggs but not sperm. Infected females pass the infection on to their offspring. It is thought that Wolbachia is important in promoting speciation. It has been linked to viral resistance in Drosophila melanogaster and mosquioto species. Flies that infected are more resistance to RNA viruses. Higher levels of Wolbachia in mosquitoes tend to correlate to higher insecticide resistance. Larvae treated for Wolbachia tend to only have 13% emerge successfully as adult moths.

The first genome determined was of the one that infects Drosphila melanogaster flies. The genome was sequenced at The Institute for Genomic Research in a collaboration between Jonathan Eisen and Scott O’Neill. The second was Brugia Malayi nematodes. Several other strains are in the progress of being sequenced.

Wolbachia also harbors a temperate bacteriophage called WO. Sequence analyses of bacteriophage WO offer 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.
Other than insects it infects a variety of isopod species, spiders, mites, and various species of filarial nematodes. Using Wolbachia to control mosquito populations has been a topic of research.

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