Evolution of pleiotropic costs in experimental populations

Area of Science:

• Evolutionary Biology
• Microbial Ecology
• Genetics

Background:

• Populations adapting to new environments are expected to incur fitness costs in ancestral environments.
• Empirical evidence for such adaptation costs is often lacking, prompting re-evaluation of experimental designs.
• The initial fitness of populations in their original environment is a critical factor in interpreting ecological trade-offs.

Purpose of the Study:

• To investigate the role of initial fitness in determining the sign and magnitude of pleiotropic effects during adaptation.
• To test whether ecological trade-offs change over time with long-term adaptation.
• To understand the relationship between direct and indirect fitness effects during environmental adaptation.

Main Methods:

• Experimental evolution of yeast Saccharomyces cerevisiae lineages over 5000 generations in a glucose-limited environment.
• Isolation of single clones and subsequent selection in a galactose-limited environment for ~120 generations.
• Estimation of single-clone fitness in both glucose and galactose environments before and after adaptation to galactose.

Main Results:

• Pleiotropic effects on glucose fitness, resulting from selection in galactose, shifted from positive to negative as glucose fitness increased.
• This shift strongly supports the hypothesis that initial fitness influences the direction and size of pleiotropic effects.
• No correlation was found between the magnitude of fitness changes in galactose and glucose; pleiotropic effects in glucose diminished as direct effects in galactose increased.

Conclusions:

• The sign of pleiotropic effects can change during long-term adaptation to an ancestral environment.
• Initial fitness is a key determinant for interpreting ecological trade-offs and pleiotropic effects in adaptive evolution.
• Understanding the dynamics of pleiotropic effects is essential for a comprehensive view of evolutionary adaptation.