THE ACQUISITION OF HYPOVIRULENCE IN HOST-PATHOGEN SYSTEMS WITH 3 TROPHIC LEVELS

A major focus of research on the dynamics of host-pathogen interaction s has been the evolution of pathogen virulence, which is defined as th e loss in host fitness due to infection. It is usually assumed that ch anges in pathogen virulence are the result of selection to increase pa thogen fitness. However, in some cases, pathogens have acquired hypovi rulence by themselves becoming infected with hyperparasites. For examp le, the chestnut blight fungus Cryphonectria parasitica has become hyp ovirulent in some areas by acquiring a double-stranded RNA hyperparasi te that debilitates the pathogen, thereby reducing its virulence to th e host. In this article, we develop and analyze a mathematical model o f the dynamics of host-pathogen interactions with three trophic levels . The system may be dominated by either uninfected (virulent) or hyper parasitized (hypovirulent) pathogens, or by a mixture of the two. Hypo virulence may allow some recovery of the host population, but it can a lso harm the host population if the hyperparasite moves the transmissi on rate of the pathogen closer to its evolutionarily stable strategy. In the latter case, the hyperparasite is effectively a mutualist of th e pathogen. Selection among hyperparasites will often minimize the del eterious effects, or maximize the beneficial effects,, of the hyperpar asite on the pathogen. Increasing the frequency of multiple infections of the same host individual promotes the acquisition of hypovirulence by increasing the opportunity for horizontal transmission of the hype rparasite. This effect opposes the usual theoretical expectation that multiple injections promote the evolution of more virulent pathogens v ia selection for rapid growth within hosts.