Evidence for abundant slightly deleterious polymorphisms in bacterial populations

The nearly neutral theory of molecular evolution predicts that slightly deleterious mutations subject to purifying selection are widespread in natural populations, particularly those of large effective population size. To test this hypothesis, the standardized difference between pair-wise nucleotide difference and number of segregation sites (corrected for number of sequences) was estimated for 149 population data sets from 84 species of bacteria. This quantity (Tajima's D-statistic) was estimated separately for synonymous (D-syn) and nonsynonymous (D-non) polymorphisms. D-syn was positive in 70% of data sets, and the overall median D-syn (0.873) was positive. By contrast D-non was negative in 68% of data sets, and the overall median D-non (-0.656) was negative. The preponderance of negative values of D-non is evidence that there are widespread rare nonsynonymous polymorphisms in the process of being eliminated by purifying selection, as predicted to occur in populations with large effective size by the nearly neutral theory. The major exceptions to this trend were seen among surface proteins, particularly those of bacteria parasitic on vertebrates, which included a number of cases of polymorphisms apparently maintained by balancing selection.