Figure 6.
Interpretation of the combined effects of virus accumulation and differential selection on RB. The effects of virus accumulation (VA) and differential selection (σr) on RB, the frequency of resistance breakdown, were considered for the three groups of doubled‐haploid (DH) lines defined by the regression tree (Fig. 5). Represented are the mean relative growth rates of the five Potato virus Y (PVY) variants (uppercase single or double letters) used to estimate σ r in experiment 4 (Table 1) amongst each plant group. Letters filled in black correspond to transitions and letters filled in white to transversions. Lower case letters represent, for each PVY variant, significant differences in relative growth rates amongst plant groups (Kruskal–Wallis test, P < 0.05). Double mutants GK and KN did not appear in the resistance‐breaking experiments, but were used for the estimation of σ r. When VA is high (group 1), the K mutant, which confers a high fitness gain to PVY and requires a transversion, is likely to appear. When VA is lower, resistance breakdown involves more likely mutants which require a transition (G and N), but confer lower fitness gains. As RB was evaluated in experiment 1 (Table 1) with a wild‐type PVY, its initial fitness in pvr23‐carrying plants is low (fictitious lower broken line). Fitness gains associated with resistance‐breaking mutations G and N are higher for group 3 than for group 2, despite higher σ r values in the latter group. This may explain the negative effect of σ r on RB when VA is low (Fig. 4).