Figure 1

Rule of root identification. Red dot represents root of a group, and red triangle shows root of an entire tree. For a specific group, root identification depends on genetic distance of a virus strain to its father node. If there is a virus strain whose distance to the father node is (A) zero or (B) nearly zero, then this node is considered the root of the group because it has the closest distance to the father node relative to other viruses in the same group. (C) If no such node exists, the father node acts as the root, looping until the root of the entire tree is reached.