Fig. 1

Illustration of parasite fitness. Fitness of avirulent parasite \(\mathcal {P}_{1}\) and virulent parasite \(\mathcal {P}_{2}\) on the two hosts \(\mathcal {H}_{1}\) and \(\mathcal {H}_{2}\) for the matching-allele model (α=0, top), the gene-for-gene model (α=1, bottom), and two intermediate models (α=1/3 and α=2/3). Gray areas represent the fitness reduction for \(\mathcal {P}_{2}\) due to the cost of virulence κ=1/2, which is α κ σ in \(\mathcal {H}_{2}\) (Eq. (1b)), hence, σ/2 in GfG model. In \(\mathcal {H}_{1}\) the fitness reduction for \(\mathcal {P}_{2}\) due to the cost of virulence is α ² κ σ