Evolution of specialists in an experimental microcosm

Area of Science:

• Evolutionary Biology
• Microbial Genetics
• Population Dynamics

Background:

• Balanced polymorphism maintenance is crucial for understanding evolutionary trajectories.
• The lactose operon in Escherichia coli provides a tractable model for studying adaptation and polymorphism.
• Frequency-dependent selection can stabilize genetic polymorphisms.

Purpose of the Study:

• To investigate the impact of adaptation on the persistence of balanced polymorphism.
• To model competition for substitutable resources using Escherichia coli.
• To track the fate of genetic polymorphisms over evolutionary time.

Main Methods:

• Utilized chemostat competition experiments with two distinct lactose operons (TD2 and TD10).
• Monitored a linked neutral genetic marker (fhuA-) for bacteriophage T5 resistance to infer polymorphism.
• Tracked polymorphism frequencies over 400-600 generations.

Main Results:

• Balanced lactose operon polymorphism persisted in 4 out of 9 chemostats.
• In other chemostats, linked neutral polymorphisms were lost between 86 and 219 generations.
• Evolved strain fitness was primarily driven by mutations independent of the initial lactose operon polymorphism.

Conclusions:

• Initial lactose operon polymorphism had minimal impact on long-term adaptive evolution outcomes.
• Ecological specialization, once stabilized, prevented selective sweeps and maintained linked neutral polymorphisms.
• Adaptation is influenced by genomic mutations beyond the initially polymorphic loci.