Phyogeography of Yersinia pestis , identifying transmission routes out of China and single introductions into the US and Madagascar . The figure represents a minimal spanning tree describing the relationship of the Y. pestis isolates. Colored circles represent genotypes, colored according to geographical location, and the size of the circle indicates the number of strains with that genotype. Letters within the circles indicate node designations. Grey text on lines between nodes shows the numbers of SNPs separating each node, except that one or two SNPs are indicated by thick and thin black lines, respectively. It is clear that the different genotypes show a strong geographical signal, and the relationships between them therefore suggest geographical transmission routes.