Fig. 3

a Relationship between observed and expected multiplicative fitness for 25 TEV genotypes carrying pairs of nucleotide substitutions in the 5’ UTR. The dashed line represents the null hypothesis of multiplicative fitness effects. Deviations from this line arise as a consequence of the existence of epistatic fitness effects. In orange, pairs that do not deviate from the null expectation if the Holm-Bonferroni sequential correction is applied. In red, pairs that do significantly deviate even after applying the Holm-Bonferroni sequential correction level for the overall significance level. b Distribution of epistasis, ε. Epistasis was computed as the difference between the observed fitness of the double mutant (W ₀₀ W _ij ) and the value expected from subtracting the effects of each single mutant from the WT value (W _i0 W _0j )