An experimental test of evolutionary trade-offs during temperature adaptation

Area of Science:

• Evolutionary Biology
• Microbial Genetics
• Adaptation and Trade-offs

Background:

• The hypothesis that adaptation to one environment often leads to reduced fitness in others (trade-offs) is a cornerstone of evolutionary theory.
• Understanding the generality and necessity of these trade-offs is crucial for predicting evolutionary trajectories.

Purpose of the Study:

• To experimentally test if adaptation to low temperatures necessitates trade-offs at high temperatures in Escherichia coli.
• To investigate the universality, quantitative relationships, and historical contingency of such trade-offs.

Main Methods:

• Utilized experimental evolution with 24 lineages of Escherichia coli over 2,000 generations at 20°C.
• Measured relative fitness changes at both the adaptive temperature (20°C) and a non-selective high temperature (40°C).
• Analyzed trade-offs for generality, universality, quantitative associations, and historical environmental effects.

Main Results:

• A general decline in fitness at 40°C was observed across all lineages (mean decline of 9.4%), supporting the generality of trade-offs.
• However, trade-offs were not universal; only 15 out of 24 lineages showed a significant fitness decrease at high temperatures.
• No quantitative correlation was found between low-temperature adaptation and high-temperature fitness decline, nor were historical thermal environments influential.

Conclusions:

• While trade-offs accompanying low-temperature adaptation are common in E. coli, they are not a universal or necessary consequence.
• Approximately one-third of lineages adapted to cold without incurring high-temperature fitness costs, challenging the notion that all beneficial cold adaptations must impair warm-environment function.
• The underlying mechanisms for trade-offs are prevalent but not absolute, indicating flexibility in evolutionary adaptation.