The Evolutionary Dynamics of Hyperparasites

Abstract

Evolutionary theory has typically focused on pairwise interactions, such as those between hosts and parasites, with relatively little work on more complex interactions including hyperparasites: parasites of parasites. Hyperparasites are common in nature, with the chestnut blight fungus virus CHV-1 a well-known natural example, but also notably include the phages of important human bacterial diseases. Theory on hyperparasitism has mostly focused on their impact on the evolution of virulence of their parasite host and relatively little is known about evolutionary trajectories of hyperparasites themselves. Our general modeling framework highlights the central role the that ability of a hyperparasite to be transmitted with its parasite plays in their evolution. Hyperparasites which transmit with their parasite hosts (hitchhike) will be selected for lower virulence, trending towards hypermutualism or hypercommensalism and select against causing a reduction in parasite virulence (hypovirulence). We examine the impact on the evolution of hyperparasite systems a of a wide range of host and parasite traits showing, for example, that high parasite virulence selects for higher hyperparasite virulence feeding back into selection for hypovirulence in the parasite. Our results have implications for hyperparasite research, both as biocontrol agents and for understanding of how hyperparasites shape community ecology and evolution.

Hyperparasitism and the evolution of parasite virulence

“…Our study is closely related to previous theoretical explorations of evolution in host-parasitehyperparasite systems (Sandhu et al 2021;Northrup et al 2021). Sandhu et al (2021) also explored the evolution of parasite virulence, but in contrast to our study found that the introduction of a hyperparasite generally selects for increased virulence and reduces the average mortality rate of the host.…”

“…We focused our investigation on the evolution of virulence, but the evolutionary dynamics of the hyperparasite are also likely to be important (Sandhu et al 2021; Northrup et al 2021). However, in certain cases the hyperparasite might behave as if it is evolutionarily static (e.g.…”

“…We focused our investigation on the evolution of virulence, but the evolutionary dynamics of the hyperparasite are also likely to be important (Sandhu et al 2021; Northrup et al 2021), especially for driving additional eco-evolutionary feedback loops (Ashby et al 2019) . However, in certain cases the hyperparasite might behave as if it is evolutionarily static (e.g., the repeated application of a particular biocontrol to an agricultural crop), in which case the model analysed here will be especially applicable.…”

“…First, Sandhu et al (2021), who considered parasite-hyperparasite coevolution, observed that the introduction of hyperparasites typically increases parasite virulence, but in almost all scenarios decreases average host mortality. Second, Northrup et al (2021) observed that when hyperparasites are more readily co-transmitted with evolutionarily static parasites, this selects for less harm by the hyperparasite due to an increased link between its fitness and parasite transmission (similar to virulence evolution in vertically transmitted parasites). These findings highlight the importance of understanding hyperparasitism in both an ecological and an evolutionary context.…”

“…Theoretical studies of hyperparasites are rare and have in the past mainly focussed on their ecological consequences (Taylor et al 1998; Morozov, Robin, and Franc 2007). Yet recently there has been renewed theoretical interest in hyperparasitism in an evolutionary context, in the form of theoretical models of hyperparasite evolution (Northrup et al 2021) and parasite-hyperparasite coevolution (Sandhu et al 2021). Two findings are of particular interest.…”