The Evolution of Host‐Parasite Range

Abstract

Understanding the coevolution of hosts and parasites is one of the key challenges for evolutionary biology. In particular, it is important to understand the processes that generate and maintain variation. Here, we examine a coevolutionary model of hosts and parasites where infection does not depend on absolute rates of transmission and defense but is approximately all-or-nothing, depending on the relative levels of defense and infectivity of the host and the parasite. We show that considerable diversity can be generated and maintained because of epidemiological feedbacks, with strains differing in the range of host and parasite types they can respectively infect or resist. Parasites with broad and narrow ranges therefore coexist, as do broadly and narrowly resistant hosts, but this diversity occurs without the assumption of highly specific gene interactions. In contrast to gene-for-gene models, cycling in strain types is found only under a restrictive set of circumstances. The generation of diversity in both hosts and parasites is dependent on the shape of the trade-off relationships but is more likely in long-lived hosts and chronic disease with long infectious periods. Overall, our model shows that significant diversity in infectivity and resistance range can evolve and be maintained from initially monomorphic populations.

Host resistance and coevolution in spatially structured populations

“…More broadly, our model of within-population structuring owing to local interactions predicts that spatial structure makes diversity through evolutionarybranching less likely, contrasting with the predicted effects on diversity in gene-for-gene with metapopulation structure [27]. This emphasizes not only the importance of the infection genetics to the coevolution of diversity in hosts and parasites [41] but also the fact that different forms of spatial structure may cause different selective pressures [34]. Figure 4.…”

Aphanomyces astaciPsI-genotype isolates from different Finnish signal crayfish stocks show variation in their virulence but still kill fast

“…Gandon and Michalis, 2000;Bull and Ebert, 2008). On the other hand, the hostparasite relationship of a host with long life span and a parasite with long infective periods is supposed to create diversity among both hosts and parasites (Best et al, 2010), which could thus re-balance relationship of A. astaci and, at least, its North American hosts in Europe. Environmental conditions could, in their part, also be a driving force of the host-parasite coevolution (Wolinska and King, 2009;Biron and Loxdale, 2013) and thus might also be affecting the aforementioned balance.…”

A limited host immune range facilitates the creation and maintenance of diversity in parasite virulence

“…This in effect creates two niches for parasitism, and therefore again allows for evolutionary branching. A similar effect was shown where hosts and parasites could coevolve their respective resistance and infection 'ranges' [31]. They found the potential for multiple branching events leading to high levels of diversity in both hosts and parasites as well as for evolutionary cycles.…”

“…We note that MI is now a stability condition in its own right. As for either strain to be able to branch again it is required that it must have MI , 0, and that the trade-off curvature can always be chosen to satisfy the other conditions [31,35,36], we therefore limit our analysis to determining the sign of this condition. After some work, we find this condition reduces to (appendix B)…”

“…In fact, it will suffice to show that the MI condition is satisfied (i.e. MI , 0) for branching to be possible, as suitable trade-offs could always be chosen to satisfy the stability conditions [31,35,36]. Here, because MI ¼ 0, clearly branching cannot occur.…”