Frequency-dependent fitness effects are ubiquitous

Area of Science:

• Evolutionary biology
• Microbial ecology
• Population genetics

Background:

• The assumption of constant fitness effects for mutations is foundational in evolutionary biology.
• This assumption impacts predictions of evolutionary trajectories, epistasis, and genetic diversity maintenance.
• The early generations of the Escherichia coli Long-Term Evolution Experiment (LTEE) provide a model system to test this assumption.

Purpose of the Study:

• To systematically investigate the frequency-dependent fitness effects of beneficial mutations in Escherichia coli.
• To determine the prevalence and nature of frequency-dependence in simple microbial populations.
• To explore the ecological mechanisms underlying observed frequency-dependent selection.

Main Methods:

• Utilized flow cytometry-based competition assays to measure fitness effects.
• Analyzed beneficial mutations from early generations of the LTEE.
• Quantified within-growth cycle dynamics to understand resource competition.

Main Results:

• Frequency-dependent fitness effects were observed in approximately 80% of tested strain pairs.
• Negative frequency-dependence, where fitness advantages decrease with increasing mutant frequency, was prevalent.
• Invasion fitness measurements could predict the strength of frequency-dependence, explaining about half of the variation.
• Violations of fitness transitivity were noted, complicating predictions based on a single reference strain.
• Resource competition was identified as a key driver of frequency-dependence, with faster-growing genotypes depleting resources.

Conclusions:

• Frequency-dependent selection is the norm, not the exception, in simple microbial populations.
• Ecological interactions, particularly resource competition, significantly shape evolutionary dynamics.
• The findings necessitate a re-evaluation of models assuming constant fitness effects in evolutionary studies.