Historical Contingency Causes Divergence in Adaptive Expression of the lac Operon.

Area of Science:

• Microbial evolution
• Bacterial adaptation
• Genetics

Background:

• Escherichia coli populations adapt to lactose environments through mutations in the lacI repressor, enabling constitutive operon expression.
• These beneficial repressor mutations are expected to fix rapidly due to their high rate and significant fitness advantage.

Purpose of the Study:

• Investigate why some Escherichia coli populations did not acquire detectable repressor mutations despite prolonged evolution in lactose environments.
• Explore the evolutionary trajectories and fitness consequences in populations that bypassed repressor mutations.

Main Methods:

• Evolving populations of Escherichia coli for 8,000 generations in lactose-containing environments.
• Reconstruction experiments to assess the fitness effects of repressor mutations in evolved populations.
• Identifying alternative mutations and evolutionary paths in populations lacking repressor mutations.

Main Results:

• Eight of 24 evolved populations lacked detectable repressor mutations.
• Repressor mutations became neutral or deleterious in populations where they were not observed.
• Populations without repressor mutations achieved equivalent fitness through alternative mutations, suggesting redundant benefits.

Conclusions:

• Bacterial evolution can follow divergent paths to achieve similar fitness outcomes, influenced by historical contingency.
• A mutation in the uspB gene promoter region may direct evolutionary selection between repressor-based and alternative adaptation strategies.
• The fitness effect of a mutation can change over evolutionary time within a population.