Fig 2. Schematic of the evolutionary history among PB2, PB1 and PA segments of H1N1 IAVs.

The schematic view is based on the phylogenetic trees of avian, swine, and human origin H1N1 IAVs and the most recent common ancestor (MRCA) for (A) PB2 gene segments, (B) PB1 gene segments, and (C) PA gene segments. North American avian (light blue) and 2009 pandemic H1N1 viruses (pink) are highlighted.