Strains used in this study (Genotyping data)1.

1Genotyping data are presented as the SNP state at the particular locus. Each locus is presented with reference to the genome position in the Ancestral Ames strain (GenBank ID: AE017334).2Sample IDs with “A” numbers indicate isolates for which we have live culture. Sample IDs bearing a 10-digit number are from an historical strain collection maintained by the Center for Disease Control and Prevention.3Canonical SNP groups as defined in Van Ert et al. (2007).4The A.Br.WNA canonical SNP was also definied in Van Ert et al. (2007).5Rules of parsimony place this isolate (A0303) in the A.Br.WNA group. This single data point is homoplastic.