Durable Resistance of Crops to Disease: A Darwinian Perspective

James K.M. Brown,  2015.

This review takes an evolutionary view of breeding crops for durable resistance to disease. An understanding of coevolution between hosts and parasites leads to predictors of potentially durable resistance, such as corresponding virulence having a high fitness cost to the pathogen or resistance being common in natural populations. High partial resistance can also promote durability. Whether or not resistance is actually durable, however, depends on ecological and epidemiological processes that stabilize genetic polymorphism, many of which are absent from intensive agriculture. There continues to be no biological, genetic, or economic model for durable resistance. The analogy between plant breeding and natural selection indicates that the basic requirements are genetic variation in potentially durable resistance, effective and consistent selection for resistance, and an efficient breeding process in which trials of disease resistance are integrated with other traits. Knowledge about genetics and mechanisms can support breeding for durable resistance once these fundamentals are in place.

 

Key features of host-parasite coevolution. (a) Reciprocal effects of interactions between host and parasite gene frequencies and fitnesses (Brown & Tellier 2011, Frank 1992, Tellier & Brown 2007). Center of diagram: A higher frequency of resistance increases the advantage of virulence, but increased virulence reduces the advantage of resistance. Top arrow: A greater cost of virulence reduces the frequency of virulence, thus increasing selection for resistance, which in turn restores the frequency of virulence. The net effect of a higher cost of virulence is to reduce the frequency of resistance. Bottom arrow: By similar logic, the net effect of a higher cost of resistance is to increase the frequency of virulence. (b) Unstable (red ) and stable (blue) polymorphisms in host and parasite gene frequencies. The unstable case is the model described in the sidebar A Simple Model of Host-Parasite Coevolution with parameters cost of resistance in host (u) = cost of virulence in parasite (b) = 0.05, cost to host of being diseased s = 0.24 and cost to parasite of incompatible interaction (c) = 1. The graph of gene frequencies spirals around and away from the unstable equilibrium point (red cross) at Req = 0.050, aeq = 0.833. The stable case has the same values of u, b, s, and c; two parasite generations per host generation; and mixed auto- and alloinfection with epidemiological parameters z = 1.4, ε = 0.1, and ϕ = 0.32 (128). The graph of gene frequencies spirals inward toward the stable equilibrium point (blue cross); the equilibrium frequency of resistance in the host is displaced slightly because of direct frequency-dependent selection on parasite. Models began with R = a = 0.02 and were run for 700 generations.

https://doi.org/10.1146/annurev-phyto-102313-045914