Wolbachia is a genus of intracellular bacteria that infects mainly arthropod species, including a high proportion of insects, and also some nematodes. It is one of the 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 be mutualistic rather than parasitic. 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 genus was first identified 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 [that] apparently 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. 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 Wolbachia persica (now Francisella persica) was closely related to species in the genus Francisella and that Wolbachia melophagi (now Bartonella melophagi) was closely related to species in the genus Bartonella, leading to a transfer of these species to these respective genera. Furthermore, unlike true Wolbachia, which needs a host cell to multiply, F. persica and B. melophagi can be cultured on agar plates. Method of sexual differentiation in 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 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 significantly altering the reproductive capabilities of their hosts, with four different phenotypes: 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