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 (α-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. unculturability of many of them. Molecular characterization of such organisms, bypassing the need for their cultivation (Amann et al., 1995), was facilitated to a great extent with the advent of PCR (Saiki et al., 1988) and automated sequencing technology (Embley, 1991 ; Hiraishi, 1992 Keywords T. T. M. Vandekerckhove and othersal., 1998 a ; Dojka et al., 1998). A huge number of intracellular (cytoplasmic) endosymbionts have now become readily accessible to this type of analysis (Eisen et al., 1992 ; Werren et al., 1994 ; Amann et al., 1995 Amann et al., , 1997 Bianciotto et al., 1996 ; Krueger & Cavanaugh, 1997 ; Haygood & Davidson, 1997 ;O'Neill et al., 1997 and references therein).A special class of cytoplasmic bacteria, composed of mainly weak parasites and mutualists, can be maternally transmitted. Their strategy is to invade the female's ovarial tissue at least during the time of reproduction, perpetuating the microbial population in following host generations. The microbes may also reside in male descendants and their gonadal cells ; however, nearly all cytoplasm is discarded during spermiogenesis, implying a dead end for cytoplasmic micro-organisms in male hosts. Therefore, maternall...

By performing cDNA AFLP on pre- and early parasitic juveniles, we identified genes encoding a novel type of ubiquitin extension proteins secreted by the dorsal pharyngeal gland in the cyst nematode Heterodera schachtii. The proteins consist of three domains, a signal peptide for secretion, a mono-ubiquitin domain, and a short C-terminal positively charged domain. A gfp-fusion of this protein is targeted to the nucleolus in tobacco BY-2 cells. We hypothesize that the C-terminal peptide might have a regulatory function during syncytium formation in plant roots.

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Available information on the terminology and ultrastructure of the stoma in Rhabditida is reviewed, with new data on the cephalobids Seleborca complexa, Triligulla aluta and Zeldia punctata, as well as the panagrolaimid Panagrolaimus superbus. It is shown that the stoma of most examined species can be divided into six regions rather than five, on the basis of the cuticular differentiations and especially the surrounding structures and tissues. Probable stomatal homologies between different families are used as a basis for a revised buccal terminology, in which the following three main regions are distinguished: 1) cheilostom, surrounded by labial cuticle; 2) gymnostom, surrounded by arcade epidermis; 3) stegostom, surrounded by cells that lie enclosed within the peripharyngeal basal lamina layer. The stegostom can be divided further into (usually) four more regions, respectively called pro-, meso-, meta-and telostegostom, defined by the presence of three interradial or six adradial cell. Continued use of the term "rhabdion" as well as the traditional five-part buccal terminology is strongly discouraged, because they prove to be based on incorrect and incompatible anatomical assumptions.

Fluorescent in situ hybridization with a 16S rRNA probe specific for Verrucomicrobia was used to (i) confirm the division-level identity of and (ii) study the behavior of the obligate intracellular verrucomicrobium "Candidatus Xiphinematobacter" in its nematode hosts. Endosymbionts in the egg move to the pole where the gut primordium arises; hence, they populate the intestinal epithelia of juvenile worms. During the host's molt to adult female, the endosymbionts concentrate around the developing ovaries to occupy the ovarian wall. Some bacteria are enclosed in the ripening oocytes for vertical transmission. Verrucomicrobia in males stay outside the testes because the tiny spermatozoids are not suitable for transmission of cytoplasmic bacteria.