Filarial nematodes Wolbachia

Wolbachia endosymbionts are abundant in arthropods, where they promote a variety of reproductive manipulations, including feminization of genetic males, parthenogenesis and cytoplasmic incompatibility. Wolbachia is also present in filarial nematodes and has recently attracted a great deal of attention. This chapter reviews the studies so far published and discusses potential implications and future research prospects. Since this is a relatively young field, the chapter will also refer to unpublished studies and will include some speculation. The aim is to stimulate further work on the subject. 

Intracellular symbiosis is extremely widespread in invertebrates. For example, mutualistic symbioses with intracellular bacteria can be found in almost all animal phyla, including sponges, cnidaria, nematodes, anellids, mollusca and arthropoda. Buchner (1965) thoroughly reviews most information published on bacterial symbiosis in animals up to 1964. After this monumental work, various reviews on more specific subjects have been published (e.g. Baumann, 1998, and references therein) including some recent reviews on Wolbachia (O Neill et al., 1997 Werren, 1997). In most of these papers, the term symbiosis is apparently used with a broad meaning the intracellular bacterium is usually referred to as an endosymbiont even in the absence of data on effects on host fitness. Here only key points on intracellular symbiosis and Wolbachia will be summarized, so as to put the information available on symbiosis in filarial nematodes into a broader context. 

Exhaustive surveys have not been carried out to determine the prevalence of infection within a single species of filarial nematode. However, infection was found in all eight specimens of D. immitis collected from worldwide locations, indicating that infection prevalence is likely to be close to 100% (Sironi et al., 1995). Sequences of Wolbachia ftsZ genes from... 

Table 2.1. Filarial nematodes infected with intracellular bacteria. The method of detection is shown. Neither electron microscopy nor the immuno-histochemical staining techniques used are to be regarded as Wolbachia specific (see note). Positive identification of intracellular bacteria as Wolbachia is shown only where PCR amplified products of rRNA or ftsZ genes have been sequenced. ...

Fig. 2.1. A tree representing the phylogeny of Wolbachia in arthropods (groups A and B) and filarial nematodes (groups C and D). Group designations correspond to those proposed by Werren etal. (1995) and by Bandi etal. (1998). The names at the terminal nodes are those of the host species. The tree is based on the ftsZgene sequence alignment used by Bandi etal. (1998). The tree was obtained using a distance matrix method (Jukes and Cantor correction neighbour-joining method).

Comparison of host and symbiont phylogenies is a powerful method for assessing the occurrence and frequency of horizontal transmission (Moran and Baumann, 1994). There are limited sequence data available for assessing the phylogeny of filarial nematodes. Comparison of Wolbachia... 

In view of the diversity of reproductive effects induced by arthropod Wolbachia, it would not be surprising if Wolbachia behaves in different ways in different filarial nematodes. The C and D Wolbachia groups are estimated to have diverged approximately 100 million years ago (see earlier). Given this fact, it is quite possible that Wolbachia in different lineages have followed different evolutionary trajectories. 

The effectors of the mammalian host immune attack against filaria include reactive oxygen intermediates. Filarial nematodes express glutathione peroxidase, thioredoxin peroxidase and superoxide dismutase at their surface - enzymes believed to protect the nematode from this attack (Selkirk et al., 1998). A bacterial catalase gene has been identified that most probably derives from the endosymbiont genome (Henkle-Duhrsen et al., 1998) this enzyme may contribute with other enzymes to the protection of both Wolbachia and its nematode host from oxygen radicals. 

This chapter focuses on Wolbachia in filarial nematodes. EM studies have also revealed intracellular bacteria in other nematodes (e.g. Shepperd et al., 1973 Marti et al., 1995) and the bacterial endosymbionts of plant pathogenic nematodes belonging to the genus Xiphinema have recently been identified as belonging to the verrucomicrobia group (Vandekerckhove et al., 2000). However, most nematode bacteria are still to be identified. These may also play important roles in nematode biology. 

Bandi, C., Anderson, T.J.C., Genchi, C. and Blaxter, M. (1998) Phylogeny of Wolbachia in filarial nematodes. Proceedings of the Royal Society of London B 265, 2407-2413. 

M. B., Tanya, V.N., Donnely, M.J. and Trees, A.J. (2000) Macrofilaricidal activity of tetracycline against the filarial nematode Onchocerca ochengi elimination of Wolbachia precedes worm death and suggests a dependent relationship. Proceedings of the Royal Society of London B 267, 1063-1069. 

Taylor, M.J., Cross, H.F. and Bilo, K. (2000) Inflammatory responses induced by the filarial nematode Brugia malayi are mediated by lipopolysaccharide-like activity from endosymbiotic Wolbachia bacteria. Journal of Experimental Medicine 191, 1429-1436. 

Bandi C, Anderson TJ, Genchi C, Blaxter ML. Phylogency of Wolbachia in filarial nematodes. Proc R Soc Lond B Biol Sci 265 2407-2413, 1998. 

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