Numerous micro-organisms have been described as cytoplasmic symbionts of eukaryotes. Many so-called obligate endosymbionts rely exclusively on maternal (vertical or transovarial) transmission to maintain themselves, rendering them dependent on the host sex ratio, which they would tend to manipulate to their own advantage. The latter phenomenon is often associated with the presence of Wolbachia pipientis (alpha-Proteobacteria) in arthropods and nematodes. A potentially similar situation was discovered involving members of a new clade of Verrucomicrobia, another main line of descent in the Bacteria. Nematode species of the Xiphinema americanum group (Nematoda, Longidoridae), viz. Xiphinema americanum, Xiphinema rivesi and Xiphinema brevicollum, each harbour their own specific verrucomicrobial endosymbionts. They are exclusively maternally inherited and their hosts reproduce by thelytokous (mother-to-daughter) parthenogenesis, males being extremely rare. A new genus, 'Candidatus Xiphinematobacter' gen. nov., along with three new candidate verrucomicrobial species, 'Candidatus Xiphinematobacter americani' sp. nov., 'Candidatus Xiphinematobacter rivesi' sp. nov. and 'Candidatus Xiphinematobacter brevicolli' sp. nov., are described on the basis of transmission electron microscopy, scanning electron microscopy, DAPI (4',6-diamidino-2-phenylindole) epifluorescence microscopy and 16S rDNA sequence analysis. These are the first endosymbiotic species described among the Verrucomicrobia. They share a mean 16S rDNA similarity of about 93%, whereas similarity to their closest relative, clone WCHD3-88, is less than 87%. Thus, the endosymbionts form a homogeneous clade for which the new candidate genus 'Candidatus Xiphinematobacter' gen. nov. is proposed. The type species is 'Candidatus Xiphinematobacter brevicolli' sp. nov.


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