FIG. 1.

Schematic diagram of possible evolutionary lineages for the emergence of pathogenic V. cholerae strains. Lineages are based on the identification of intermediate strains carrying combination of genes for virulence or environmental fitness and other considerations (see text). Solid boxes describe strains that have been identified in this or other studies, whereas boxes with broken lines designate strains that could hypothetically exist but have not been isolated. Abbreviations: CTX, cholera toxin phage; TCP, toxin coregulated pilus gene cluster; MSHA, mannose-sensitive hemagglutinin; HLY, hemolysin; O1 and O139, various O antigen gene clusters; RTX, repeat in toxin gene cluster; VSP, Vibrio seventh pandemic island; TTSS, type III secretion system genes.