Figure 1: The phylogenetic relationships among WP isolates between 1991 and 1996 The upper panel is a schematic representation of the phylogenetic relations among type 1 WP isolated from sewage after Israel became poliomyelitis free. The short names of actual isolates appear in the ovals (see materials for the full isolate names and accession numbers). The vertical axis is proportional to the month of isolation. The lengths of the branches are scaled to the number of nucleotide substitutions. The small circles represent hypothetical intermediates along the evolutionary pathways. Very long branches have been discontinued in their middle and each such “break” has been numbered. The actual nucleotide substitutions between each branch point are listed on the tree (format: nucleotide, previous nucleotide->substituted nucleotide; example: 543a->g) for short branches. The numbers and types of substitutions for longer branches are indicated in the lower table according to the numbers that appear beside each breakpoint in the long branches. The types of substitutions listed are the total number, and the numbers of transitions, transversions, substitutions back to previous nucleotides (example A->G->A), or independent parallel substitutions in separate lineages. All isolates in the tree were rooted to the 1987 isolates from the last type 1 WP outbreak that occurred in 1987-88 [13]. The patterns of nucleotide substitutions along branches between isolates were then compared to that for short outbreaks [5] to determine whether or not the substitutions that were observed could have arisen during local endemic person- to-person transmission. Those substitutions in isolates within the gray box (Group C) were consistent with local transmission, whereas the differences in sequence between isolates in C versus D and E were not, i.e. they represent separate introductions.