The transmission and effects of Wolbachia bacteria in parasitoids

Wolbachia bacteria are obligatory intracellular parasites of arthropods and have been detected in about 70 species of parasitic wasps and three parasi toid flies. Wolbachia are transmitted cytoplasmically (maternally) and modi fy host reproduction in different ways to enhance their own transmission: p arthenogenesis induction (PI), cytoplasmic incompatibility (CI), or feminiz ation (F) of genetic males. Only PI and CI are known in parasitoids. PI-Wol bachia cause thelytoky in otherwise arrhenotokous parasitoids by generating diploid (rather than haploid) unfertilized wasp eggs. CI-Wolbachia cause i ncompatibility of crosses between infected males and uninfected females bec ause the paternally derived chromosomes fail to decondense and are destroye d after syngamy. More complex situations arise when hosts harbor multiple i nfections, which can lead to bidirectional incompatibility and may be invol ved in parasitoid speciation. The relative fitness of infected and uninfect ed hosts is important to the population dynamics of Wolbachia, and more dat a are needed. Evolutionary conflict should be common between host genes, Wo lbachia genes, and other "selfish" genetic elements. Wolbachia-specific PCR primers are now available for several genes with different rates of evolut ion. These primers will permit rapid screening in future studies of spatial and temporal patterns of single and multiple infection. Molecular phylogen ies show that CI- and PI-Wolbachia do not form discrete clades. In combinat ion with experimental transfection data, this result suggests that host rep roductive alterations depend on the intel action between attributes of both Wolbachia and host. Moreover, Wolbachia isolates from closely related host s do not usually cluster together, and phylogenies suggest that Wolbachia m ay have radiated after their arthropod hosts. Both results support consider able horizontal transmission of Wolbachia between host species over evoluti onary time. Natural horizontal transmisson between parasitoids and their ho sts, or with entomoparasitic nematodes or ectoparasitic mites, remains a ta ntalizing but equivocal possibility.