Occurrence of novel verrucomicrobial species, endosymbiotic and associatedwith parthenogenesis in Xiphinema americanum-group species (Nematoda, Longidoridae)

Numerous micro-organisms have been described as cytoplasmic symbionts of eu karyotes. Many so-called obligate endosymbionts rely exclusively on materna l (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 n ematodes. A potentially similar situation was discovered involving members of a new clade of Verrucomicrobia, another main line of descent in the Bact eria. Nematode species of the Xiphinema americanum group (Nematoda, Longido ridae), viz. Xiphinema americanum, Xiphinema rivesi and Xiphinema brevicoll um, each harbour their own specific verrucomicrobial endosymbionts. They ar e exclusively maternally inherited and their hosts reproduce by thelytokous (mother-to-daughter) parthenogenesis, males being extremely rare. A new ge nus, 'Candidatus Xiphinematobacter' gen. nov., along with three new candida te verrucomicrobial species,'Candidatus Xiphinematobacter americani' sp. no v.,'Candidatus Xiphinematobacter rivesi' sp. nov. and 'Candidatus Xiphinema tobacter brevicolli' sp. nov., are described on the basis of transmission e lectron microscopy, scanning electron microscopy, DAPI (4',6-diamidino-2-ph enylindole) epifluorescence microscopy and 16S rDNA sequence analysis. Thes e 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 endosy mbionts form a homogeneous clade for which the new candidate genus 'Candida tus Xiphinematobacter' gen. nov. is proposed. The type species is 'Candidat us Xiphinematobacter brevicolli' sp. nov.