Fig. 1

Chronological analysis of PB1–216 and PB1–336 in avian, swine, and human influenza A viruses. Amino acid sequence data of full-length PB1 of for a influenza A/H1N1 viruses and b influenza A/H3N2 viruses deposited in the Influenza Virus Database (GenBank) prior to August 31, 2016, were analyzed to determine the residue identity at positions 216 and 366 in the PB1 subunit of viral RdRp. The data excluded the 2013 avian A/H1N1 viruses and 2016 avian A/H3N2 viruses because these retrievals contained no full-length PB1 sequences. Blue and red lines represent the percentage of viruses with the residue indicated. Green dotted line denotes the presumed onset of the 2009 influenza A/H1N1 pandemic. Each purple bar represents the numbers of full-length PB1 sequences deposited in year indicated