Wolbachia infection in crustaceans: novel hosts and potential routes for horizontal transmission

Wolbachia are endosymbiotic bacteria belonging to the a-Proteobacteria (O'Neill et al., 1992; Rousset et al., 1992). They are known to infect a wide range of arthropods (Werren & O'Neill, 1997) and nematodes (Bandi et al., 1998). Wolbachia endosymbionts can alter their host's reproduction (reviewed in O'Neill et al., 1997) by: (i) reproductive incompatibility (termed cytoplasmic incompatibility) in insects, mites and isopods, (ii) thelytokous parthenogenesis in haplodiploid wasps, (iii) male killing in insects (Hurst et al., 1999) and (iv) feminization of genetic males in isopods. The latter group is the most frequently infected by Wolbachia, with an estimated infection rate of 35% (Bouchon et al., 1998). Based on 16S rRNA and ftsZ sequences, all Wolbachia strains from isopods belong to the B-supergroup and most of them constitute a monophyletic group (Bouchon et al., 1998). Interestingly, some Wolbachia strains found in isopods (e.g. Sphaeroma sp., Asellus aquaticus and Porcellionides pruinosus) are more similar to the symbionts found in insects than to those found in other isopods (Bouchon et al., 1998), suggesting that horizontal transfer may have taken place. Such transfers have mainly been investigated in insect host-parasitoid communities, giving rise to phylogenetic evidence for horizontal transmission between parasitoids and hosts (Werren et al., 1995; Heath et al., 1999; Vavre et al., 1999) or not (Schiltuizen & Stouthamer, 1998; West et al., 1998). Frequent transfers could also occur from infected to uninfected larvae sharing a common food source as reported in parasitoid wasps (Huigens et al., 2000). However, few data are available in isopod hosts. Experimental transfers of Wolbachia by microinjection have been successful within terrestrial isopods (woodlice) (Juchault et al., 1994), but the natural mechanisms of horizontal transfer are still unclear. Rigaud &