Gene transfer is a major factor in bacterial evolution

Lateral gene transfer in four strains of Salmonella enterica has beenassessed using genomic subtraction. Strain LT2 (subspecies I serovarTyphimurium) chromosomal DNA was used as target and subtracted by threesubspecies I strains of serovars Typhimurium (S21), Muenchen (S71), Typhi(M229), and a subspecies V strain (M321). Data from probing random cosmidsof LT2 DNA with preparations of the residual LT2 DNA after subtraction wereused to estimate the amounts of LT2 DNA not able to hybridize to strainsS21, S71, M229, and M321 to be in the range of 84-106, 191-355, 305-629,and 778-1,286 kb, respectively. Several lines of evidence indicate thatmost of this DNA is from genes not present in strain M321 and not fromgenes that have diverged in sequence. The amounts correlate with thedivergence of the four strains as revealed by multilocus enzymeelectrophoresis and sequence variation of housekeeping genes. Sequence of39 of the fragments from the M321 subtracted residual LT2 DNA revealed onlysix inserts of known gene function with evidence of both gain and loss ofgenes during the development of S. enterica clones. Sixteen of the 39segments have 45% or lower G+C content, below the species average, but overhalf are within the normal range for the species. We conclude that evenwithin a species, clones may differ by up to 20% of chromosomal DNA,indicating a major role for lateral transfer, and that on the basis of G+Ccontent, a significant proportion of the DNA is from distantly relatedspecies.