Wolbachia endosymbionts

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 Since this is a relatively young field, the chapter will also refer to unpublished studies and will include some speculation.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. 

Casiraghi, M., Anderson, T.J.C., Bandi, C., Bazzocchi, C. and Genchi, C. (2001) A phylogenetic analysis of filarial nematodes comparison with the phylogeny of Wolbachia endosymbionts. Parasitology (in press). 

Cross HF, Haarbrink M, Egerton G, Yazdanbakhsh M, Taylor MJ. Severe reactions to filarial chemotherapy and release of Wolbachia endosymbionts into blood. Lancet 2001 358(9296) 1873-5. 

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. 

Antibiotic curing experiments provide a powerful approach in investigating endosymbiont effects on host biology and have been widely used in work on arthropod Wolbachia. A number of such experiments have been inadvertently conducted on filarial worms, in the course of testing... 

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., McCall, J.W., Genchi, C., Corona, S., Venco, L. and Sacchi, L. (1999) Effects of tetracycline on the filarial worms Brugia pahangi and Dirofilaria immitis and their bacterial endosymbionts Wolbachia. InternationalJournalfor Parasitology 29, 357-364. 

Bazzocchi, C., Ceciliani, F., McCall, J.W., Ricci, I., Genchi, C. and Bandi, C. (2000b) Antigenic role of the endosymbionts of filarial nematodes IgG response against the Wolbachia surface protein in cats infected with Dirofilaria immitis. Proceedings of the Royal Society of London B (in press). 

Sironi, M., Bandi, C., Sacchi, L., Di Sacco, B., Damiani, G. and Genchi, C. (1995) A close relative of the arthropod endosymbiont Wolbachia in a filarial worm. Molecular and Biochemical Parasitology 74, 223-227. 

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