• genetic drift;
  • metapopulation;
  • natural selection;
  • resistance;
  • virulence

Plants and their parasites co-evolve at key genes of interactions following the so-called gene-for-gene (GFG) relationship. Previous models of co-evolution assume (i) single infinitely large populations of hosts and parasites and (ii) costs of resistance and infectivity. The effects of three biologically realistic characteristics of plant and parasite populations on polymorphism maintenance at GFG loci were investigated. First, two components of the cost of resistance were disentangled: the cost of harbouring the resistance allele itself, and the cost of triggering resistance when encountering a parasite. Secondly, it was assumed that plants encounter parasites depending on fixed disease prevalence in time. Thirdly, finite sizes of host and parasite populations were introduced, assuming genetic drift and mutation. In a single population, statistical polymorphism in either host or parasite can be obtained in the finite population size model if there is no cost of harbouring the resistance allele and disease prevalence is low. On the other hand, long-term polymorphism can be maintained by heterogeneity in disease prevalence and costs of resistance in a spatially structured population with two demes linked by migration. More precisely, the trench warfare co-evolutionary dynamics occurs when assuming large host and parasite population sizes, and large differences between demes for disease prevalence or costs of triggering resistance. Moreover, the resistance allele does not need to harbour a fitness cost in itself for long-term stable polymorphism to occur in the co-evolutionary models. This observation may explain the lack of empirical evidence of high costs of carrying resistance alleles.