Transmission mode may explain variation in Wolbachia virulence

The reproductive parasite Wolbachia infects 20-76% of insect species. In many hosts it induces asexual reproduction, causes mating incompatibilities, or feminizes genetic males in order to enhance its own vertical transmission. However, incongruence of the Wolbachia phylogeny with that of the insect hosts suggests that on an evolutionary time scale, horizontal transmission (HT) is also common. Experiments have shown that parasitoids of insect eggs transmit parthenogenesis-inducing (PI) Wolbachia horizontally on an ecological time scale, but the ecological and evolutionary significance of this horizontal transmission is not well-understood. I present a model of virulence evolution developed to ask, are there ecological contexts in which high rates of horizontal transmission could cause PI Wolbachia to act like a virulent pathogen? Specifically, I model the case of a solitary egg parasitoid which can transmit Wolbachia from its offspring to those of conspecifics through super-parasitism. I used analytical and simulation techniques to examine the effects of varying super-parasitism rate (i.e., opportunity for HT), different models of tradeoffs between horizontal and vertical transmissibility, and Wolbachia effects on different host fitness parameters (e.g. fecundity versus longevity). The model was parameterized according to available empirical data on host availability and virulence in Anagrus sophiae (Hymenoptera: Mymaridae) (Trjapitzin and Strong).