Minimum spanning tree of 174 global strains. Each circle indicates a haplotype (node), and a larger size of the circle corresponds to a larger number of strains included. The number along each edge reflects the phylogenic distance between each two neighboring nodes. In addition, a thicker edge corresponds to a shorter phylogenic distance. 165 (98 nodes) of the 174 strains are assigned into five complexes, C1 to C5, while the remaining nine strains (nine nodes) are indicated as 'unassigned'. Nodes in different complex are labeled with different colors. The 39 pandemic strains are assigned into five sub-groups, SG1 to SG5 (different SGs distribute in different nodes). The presence or absence of trh, tdh, toxRS/new sequence and VpaI-5 in various groups of strain are shown as well.