Virulence evolution via host exploitation and toxin production in spore-producing pathogens

Troy Day Department of Zoology, University of Toronto, 25 Harbord St., Toronto, ON, Canada, M5S 3G5 Correspondence: Tel. +416 946 5563. Fax: +416 978 8532. E-mail: [email protected] Abstract Many pathogens produce resilient free-living propagules that allow their dissemination in the absence of direct contact between susceptible and infected hosts. One might expect pathogens capable of producing such long-lived propagules to evolve high levels of virulence because their reproductive success is de-coupled from the survival of their host. Despite some comparative data supporting this prediction, theory has questioned its general validity. I present theoretical results that incorporate two transmission routes neglected by previous theory: death-mediated propagule production and direct host-host transmission. This theory predicts that spore-producing pathogens should evolve high levels of virulence under quite broad conditions. Moreover, a novel prediction of this theory is that the production of propagules can generate selection for the evolution of pathogen characteristics such as toxins whose sole function is to kill the host. This latter result reveals an unanticipated mechanism through which virulence is expected to evolve in spore-producing pathogens.