Evidence for abundant slightly deleterious polymorphisms in bacterial populations

Area of Science:

• Molecular Evolution
• Population Genetics
• Bacteriology

Background:

• The nearly neutral theory posits that slightly deleterious mutations are prevalent in large populations.
• Purifying selection is expected to eliminate these mutations.

Purpose of the Study:

• To test the nearly neutral theory's prediction of widespread slightly deleterious mutations in bacteria.
• To investigate the role of purifying and balancing selection in bacterial polymorphism.

Main Methods:

• Analyzed 149 population datasets from 84 bacterial species.
• Estimated Tajima's D-statistic for synonymous (D(syn)) and nonsynonymous (D(non)) polymorphisms.
• Assessed the distribution and significance of D(syn) and D(non) values.

Main Results:

• Synonymous polymorphisms (D(syn)) were positive in 70% of datasets (median 0.873).
• Nonsynonymous polymorphisms (D(non)) were negative in 68% of datasets (median -0.656).
• Negative D(non) indicates widespread rare nonsynonymous polymorphisms undergoing purifying selection.

Conclusions:

• Results support the nearly neutral theory, demonstrating widespread purifying selection against slightly deleterious nonsynonymous mutations in bacteria.
• Exceptions were noted for surface proteins, especially in vertebrate-associated parasites, suggesting balancing selection.
• Bacterial molecular evolution is shaped by both purifying and balancing selection, with population size playing a key role.