Enzyme kinetics, substitutable resources and competition: from biochemistry to frequency-dependent selection in lac

Area of Science:

• Biochemistry
• Evolutionary Biology
• Microbial Genetics

Background:

• The lac permease in Escherichia coli exhibits trade-offs in catalytic efficiency, leading to alleles with specialized substrate utilization.
• These specialized alleles are favored differently depending on the type of galactoside present.

Purpose of the Study:

• To investigate the role of differential resource utilization in frequency-dependent selection at the lac permease.
• To determine the conditions under which genetic polymorphism is maintained in Escherichia coli populations competing for galactoside mixtures.

Main Methods:

• Experimental evolution with Escherichia coli strains possessing different lac permease alleles.
• Competition assays using mixtures of various galactosides.
• Analysis of allele frequency dynamics under varying environmental conditions.

Main Results:

• Differential resource utilization during competition for galactoside mixtures drives frequency-dependent selection at the lac locus.
• Polymorphism is protected only within a narrow range of galactoside ratios, despite strong selection on pure galactosides.
• An unpredicted source of weak selection was identified when comparing theoretical predictions with experimental results.

Conclusions:

• Stabilizing frequency-dependent selection, driven by differential resource utilization, is unlikely to consistently maintain natural allozyme polymorphisms in fluctuating environments.
• The stability of such polymorphisms is sporadic and ephemeral, limited by specific environmental conditions and resource ratios.
• Further research is needed to fully understand the complex selective pressures influencing genetic variation in microbial populations.