EVOLUTIONARY ADAPTATION TO TEMPERATURE. IV. ADAPTATION OF ESCHERICHIA COLI AT A NICHE BOUNDARY

Area of Science:

• Evolutionary biology
• Microbial adaptation
• Environmental stress response

Background:

• Environmental changes can shape population evolution.
• Past genetic history may constrain or influence adaptation.
• Bacterial adaptation to thermal environments provides a model system.

Purpose of the Study:

• To investigate the influence of past selection history on adaptation to a novel thermal environment.
• To assess the adaptive response of Escherichia coli to a near-lower-limit temperature (20°C).
• To determine if prior adaptation to different temperatures affects subsequent adaptation to cold.

Main Methods:

• Experimental evolution of Escherichia coli populations over 2000 generations.
• Selection at a novel temperature (20°C) and comparison with groups with prior selection histories at different temperatures (32°C, 37°C, 42°C, and alternating 32-42°C).
• Fitness assays in direct competition experiments at 20°C and across the thermal niche.

Main Results:

• All groups adapted to 20°C, with an average fitness increase of ~8% for the 20°C-selected group.
• Past selection history had no discernible effect on the rate or magnitude of adaptation to 20°C.
• Adaptation to 20°C resulted in significant fitness trade-offs at higher temperatures and a downward shift in thermal niche limits.

Conclusions:

• Evolutionary adaptation to a novel marginal environment (20°C) is not significantly influenced by prior selection history.
• Adaptation to cold environments can impose fitness costs at higher temperatures, unlike adaptation to hot environments.
• Asymmetrical adaptive responses across thermal niches have significant evolutionary implications.