DETECTING SELECTIVE SWEEPS IN NATURALLY-OCCURRING ESCHERICHIA-COLI

The nucleotide sequences of the gapA and pabB genes (separated by appr oximately 32.5 kb) were determined in 12 natural isolates of Escherich ia coli. Three analyses were performed on the data. First, the levels of polymorphism at the loci were compared within and between E. coli a nd Salmonella strains relative to their degrees of constraint. Second, the gapA and pabB loci were analyzed by the Hudson-Kreitman-Aguade (H KA) test for selective neutrality. Four additional dispersed genes (cr r, putP, trp and gnd) were added to the analysis to provide the necess ary frame of reference. Finally, the gene genealogies of gapA and pabB were examined for topological consistency within and between the loci . These lines of evidence indicate that some evolutionary event has re cently purged the variability in the region surrounding the gapA and p abB loci in E. coli. This can best be explained by the spread of a sel ected allele through the global E. coli population by directional sele ction and the resulting loss in variability in the surrounding regions due to genetic hitchhiking.