FIG 1.

Phylogenetic relationships between pandemic H1 segments. The time-scaled Bayesian MCC tree was inferred for the HA (H1) sequences of 933 viruses collected from swine and humans in the United States from 2009 to 2014, including 173 viruses from swine and 760 viruses from humans. The branches are color coded by host and time (U.S. influenza epidemic) of collection (September to August). sw09-10, swine viruses collected during the 2009-2010 epidemic; sw10-11, swine viruses collected during the 2010-2011 epidemic; sw11-12, swine viruses collected during the 2011-2012 epidemic; sw12-13, swine viruses collected during the 2012-2013 epidemic; sw13-14, swine viruses collected during the 2013-2014 epidemic. The clade of human viruses from the 2013-2014 epidemic is highlighted in the box shaded orange. Posterior probabilities of >0.90 are provided for key nodes. An identical phylogeny with tip strain name labels is provided in Fig. S2 in the supplemental material.