Interactions between the Parasite’s Previous and Current Environment Mediate the Outcome of Parasite Infection

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Abstract
The study of parasite virulence has generally focused on the conditions under which virulence is expected to increase or decrease over time and how the interactions between hosts and their environments may mediate the outcome of infection. Recently, parasite traits such as transmission, offspring production, and development have also been shown to be influenced by environmental variation. What is unclear is how variation in the parasite's environment may impact virulence. Recent theory demonstrates that plasticity can promote the evolution of decreased virulence; thus, understanding whether the parasite's environment can mediate virulence can improve predictions regarding the outcome of parasite infection. Here, an obligate mosquito parasite was reared in hosts fed high or low levels of food. Parasite oocysts (offspring) produced in these two host environments were subsequently fed to uninfected hosts. Parasites originating from well-fed hosts were found to be more virulent to these subsequent hosts compared to parasites originating from poorly fed hosts. Additionally, this effect was apparent only when current hosts were food deprived. These results demonstrate that parasite virulence was mediated by a cross-generational effect of the environment and that the overall outcome of infection was modified by variation in both the parasite's and host's environments.