Wolbachia evolved from a ~400 My old clade of gram-negative, aerobic, alpha–proteobacteria (e.g., the Rickettsiales) that encompass obligatory intracellular, vertebrate pathogens and arthropod infections of the genera Rickettsia, Ehrlichia, Anaplasma, Orientia, Neorickettsia and Midichloria. Despite the clade’s ancient intracellular association, small genome size (0.9-1.6 million base pairs) and dependency on intracellular replication, the genus Wolbachia evolved opposing lifestyles as reproductive parasites in arthropods and mutualists in filarial nematodes. Wolbachia are distinguished from their relatives by their sequence divergence from the other Rickettsiales, ability to induce four major forms of reproductive parasitism in a wide range of arthropod species, a host range that includes filarial nematodes, and the lack of ability to be transmitted from arthropod to mammalian hosts.
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