Experimental evolution of gene duplicates in a bacterial plasmid model

Area of Science:

• Evolutionary biology
• Microbial genetics

Background:

• Gene duplication is a key driver of evolutionary innovation.
• Diversifying selection can favor the divergence of duplicated genes for new functions.
• The TEM-1 beta-lactamase gene provides a model for studying drug resistance evolution.

Purpose of the Study:

• To experimentally test the fate of gene duplicates under diversifying selection for dual drug resistance.
• To investigate the evolutionary trade-offs associated with maintaining gene copies under antibiotic pressure.

Main Methods:

• Bacterial system (E. coli) with two identical TEM-1 beta-lactamase genes on compatible plasmids.
• Experimental evolution under selection for both ampicillin and cefotaxime resistance.
• Quantitative assessment of plasmid maintenance and bacterial growth fitness.

Main Results:

• One beta-lactamase gene copy evolved increased cefotaxime resistance.
• The cefotaxime-resistant plasmid retained partial ampicillin resistance.
• Maintenance of both gene copies under increasing antibiotic concentrations led to plasmid loss or inhibited cell growth, contradicting models.
• Significant fitness costs were associated with carrying duplicate genes under dual drug selection.

Conclusions:

• The study highlights the critical role of gene dosage costs in limiting the evolution of functional divergence in gene duplicates.
• A quantitative trade-off exists between the benefits of duplicated genes and their associated fitness costs.
• Experimental evolution revealed constraints on the maintenance of gene duplicates under strong diversifying selection.