R-049. Evolutionary History of the Salmonella Flagellin Loci, fliC and fljB

J. R. McQuiston1, P. I. Fields1, R. V. Tauxe1, J. M. Logsdon2;
1CDC, Atlanta, GA, 2Univ. of Iowa, Iowa City, IA.

The Salmonellae are a genus of motile bacteria that typically express two coordinately regulated flagellin loci, fliC and fljB, and include a large repertoire of flagellin antigen types. The assortment of the 114 serologically-determined flagellar antigen types of Salmonella, and the existence of a mechanism for flagellar phase variation, has lead researchers to hypothesize the flagellin loci are under great diversifying selection or undergo extensive lateral gene transfer and recombination. Presented here is the evolution of 23 phylogenetically determined antigen groups, represented by 374 fliC and fljB alleles across both species and all subspecies of Salmonella. Fifty-six of the 63 (89%) flagellar antigen types reported to the U.S. National Salmonella Surveillance System over the past 40 years are represented. Phylogenetic analysis of the flagellin sequences demonstrated that the appearance of similar flagellin alleles in multiple subspecies is most often the result of vertical evolution rather than lateral gene transfer. Only 45 (14%) of 323 antigen alleles from antigen groups with topologies congruent with the Salmonella phylogeny demonstrated inter-subspecies lateral gene transfer. Substitution calculations across the fliC and fljB genes of the alpha cluster antigens, demonstrated that the dN/dS ratios are well below neutral selection of 1.0 (mean of 0.231 for conserved regions, and 0.553 for the variable region) indicating that flagellin alleles are under purifying selection. Salmonella flagellin alleles are likely under selection constraints that maintain a variety of antigenic structures across subspecies for millions of years, rather than under diversifying selection.