The Endosymbionts.

The endosymbionts encompass a large group of intracellular bacteria of biomedical and veterinary relevance, closely related to , , and . This genus of Gram-negative members of the does not infect vertebrates but is instead restricted to ecdysozoan species, including terrestrial arthropods and a family of parasitic filarial nematodes, the Onchocercidae. The profoundly impact not only the ecology and evolution but also the reproductive biology of their hosts, through a wide range of symbiotic interactions. Because they are essential to the survival and reproduction of their filarial nematode hosts, they represent an attractive target to fight filariasis. Their abilities to spread through insect populations and to affect vector competence through pathogen protection have made a staple for controlling vector-borne diseases. Estimated to be present in up to 66% of insect species, the are probably the most abundant endosymbionts on earth. Their success resides in their unique capacity to infect and manipulate the host germ line to favor their vertical transmission through the maternal lineage. Because the resist genetic manipulation and growth in axenic culture, our understanding of their biology is still in its infancy. Despite these limitations, the "-omics" revolution combined with the use of well-established and emerging experimental host models is accelerating our comprehension of the host phenotypes caused by , and the identification of effectors is ongoing.