Contributions of alleles to the second principal component of the DAPC of seasonal (H3N2) influenza data. The height of each bar is proportional to the contribution (Equation 10) of the corresponding allele to the second principal component of the analysis, which isolated the strains from the 2006 influenza epidemic from all others (see Figure 8). Only alleles whose contribution is above an arbitrary threshold (grey horizontal line) are indicated for the sake of clarity. Alleles are labeled by their position in the original alignment, and the corresponding nucleotide, separated by a dot. Position 384 and 906 correspond respectively to residue 144 and 318 in the complete hemagglutinin (HA) protein CDS. Polymorphism at position 384 leads to a mutation from Asparagine to Lysine, present in 32.1% of strains sampled in 2006 while virtually absent before 2006. Polymorphism at position 906 is synonymous.