Neighbor-Joining Tree of 281 MSG sequences. The relationships among the 281 non-identical MSG sequences were inferred using the Neighbor-Joining method  in MEGA4 software . Gaps were ignored. Circle symbols mark the branches containing members of the 73 core MSG sequences (see text). The tree is drawn to scale in p-distance units. The bar at the top of the tree represents a p-distance of 0.02.