Infection polymorphism and cytoplasmic incompatibility in Hymenoptera-Wolbachia associations.

Most cases of Wolbachia infection so far documented in haplodiploid Hymenoptera are associated with parthenogenesis induction. Only three examples of Wolbachia-mediated cytoplasmic incompatibility (CI) have been reported, resulting either in haploidisation of fertilised eggs, which develop into viable males, or in their death. To better document this variability, we studied two new Wolbachia-wasp associations involving Drosophila parasitoids. In Trichopria cf. drosophilae, individuals are infected by two different Wolbachia variants, populations are nearly totally infected, and Wolbachia induces incomplete CI resulting in death of the fertilised eggs. On the other hand, Pachycrepoideus dubius harbours only one bacterial variant, populations are polymorphic for infection, and Wolbachia has no detectable effect. These two cases show that the range of variation in Wolbachia's effects in Hymenoptera is as wide as in diploids, extending from complete CI to an undetectable effect. Cases so far studied show some parallel between the strength of incompatibility, the number of Wolbachia variants infecting each wasp, and the natural infection frequency. These empirical data support theoretical models predicting evolution of CI towards lower levels, resulting in the decline and ultimate loss of infection, and place multiple infections as being an important factor in the evolution of host-Wolbachia associations.