Wolbachia

Wolbachia is a genus of inherited bacterium which infects arthropod species, including a high proportion of insects. It is one of the world's most common parasitic microbes and is potentially the most common reproductive parasite in the biosphere, for example more than 16% of neotropical insect species carry this bacterium.

Association with disease
Outside of Insecta, 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 heart worms in dogs. Not only are these disease-causing filarial 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 (Hoerauf et al. 2003). 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 (Outland 2005, Taylor et al. 2005).

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:
 * Male killing (death of infected males). This allows related infected females to be more likely to survive and reproduce.
 * feminization (infected males develop as females or infertile pseudo-females)
 * parthenogenesis(reproduction of infected females without males) and
 * Cytoplasmic incompatibility (the inability of Wolbachia-infected males to successfully reproduce with uninfected females or females infected with another Wolbachia strain). This has the advantage of making the Wolbachia strain more likely to become prevalent as opposed to other strains of Wolbachia. This can have the additional result of making Wolbachia more common as a whole.

Wolbachia are present in mature eggs, but not mature sperm. Only infected females pass the infection on to their offspring. It is thought that the phenotypes caused by Wolbachia, especially cytoplasmic incompatibility, may be important in promoting speciation. Wolbachia can also cause misleading results in molecular cladistical analyses (Johnstone & Hurst 1996).

Research history
The bacteria were first identified in 1924 by Hertig and Wolbach in Culex pipiens, a species of mosquito.

The genomes of Wolbachia from Drosophila melanogaster flies and Brugia malayi nematodes have been sequenced, and genome sequencing projects for several other Wolbachia strains are in progress.

A 2007 paper published in Science reports that a complete copy of the Wolbachia genome can be found within the genome of the fruit fly Drosophila ananassae and that Wolbachia appeared to have transmitted large segments of its genome into at least 7 other species.