Horizontal transmission of Wolbachia by strepsipteran endoparasites? A response to Noda et al ., 2001

The noncongruence between molecular phylogenies of the endosymbiont bacteria, Wolbachia and their hosts imply frequent horizontal transfer between species (Werren et al. 1995). A suggested mechanism for this is infection by macro-parasites, notably parasitoids. A recent report in Molecular Ecology (Noda et al. 2001) noted that two planthopper species shared Wolbachia strains with those recovered from their shared strepsipteran endoparasites, and concluded that Strepsiptera were agents for horizontal transmission of Wolbachia in these planthopper species. There are a number of problems with this prima facie interpretation given certain aspects of the strepsipteran life history, which we outline here. The presented arguments have applicability to other studies, which speculate upon the role that parasitoids play in the transmission of Wolbachia , and underscore the need for a fuller consideration of parasitoid life history during hypothesis generating. We discuss the following main points, which are based upon published findings. 1) Shared strains of Wolbachia between two hosts and their common parasitoid do not necessarily support the hypothesis that the parasitoid is the agent of transfer. An equally plausible hypothesis is that Strepsiptera simply share the same Wolbachia strains as their hosts because of their biology and intimate association with the host. The majority of their lifecycle is spent in contact with the host haemo-lymph, and nutrients are ingested by larval stages through a mouth and by the neotenic adult female via a modified structure, the 'apron' (analogous to the insect gut), which lies on the outer body wall (see Kathirithamby 2000). A gut is present in Strepsiptera only in the larval stages, which becomes redundant in the neotenic adult female stage after the extrusion of the cephalothorax (the cephalothorax is extruded through the host cuticle, whereas the majority of the body remains within the host). There is no mouth opening in the extruded cephalothorax and the brood canal opening is not analogous to the mouth. The transition from the possession of a gut to nutrient absorption via the apron is in stark contrast to the feeding method of the two other parasites that were screened (a drynid wasp and mermi-thid nematode), or the planthoppers (Noda et al. 2001), and may promote the occurrence of Wolbachia from the planth-oppers being found within the strepsipteran. It has already been hypothesized that Wolbachia probably do not survive passage through the digestive tract (Johanowicz & Hoy 1996; Cordaux et al. 2001), but the apron is unlikely …