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Trio of Vanderbilt Researchers Identify Variable Outcomes in Experimental Evolution

Posted by on Friday, April 7, 2023 in featured.

By Dr. Andy Flick Evolutionary Studies scientific coordinator

Woman reading in a chairSarah Worthan a postdoctoral researcher led a new effort with undergraduate researcher Robert McCarthy and their advisor, assistant professor of biological sciences Megan Behringer, to understand the effects of the environment on the outcome of experimental evolution studies. The article, “Case Studies in the Assessment of Microbial Fitness: Seemingly Subtle Changes Can Have Major Effects on Phenotypic Outcomes,” was just published in the Journal of Molecular Evolution.

Experimental evolution, especially studies using microbes like E. coli, often use a fitness evaluation to report the results of evolution. A typical setup might be to freeze two strains of E. coli, which we will name red strain and blue strain, on Day 1 of the experiment and then add a selective pressure to a population of the red strain until Day 10. On Day 11, the researchers will thaw the ancestral strains to use in a pair of common fitness evaluations. First, the ancestral red strain may be grown alone in a petri dish as might the evolved red strain. This is a common approach to reveal the differences in growth rates between the ancestral and evolved strains. A second approach is for the researchers to grow both populations in a petri dish. First, they will test the ancestral red strain against the ancestral blue strain and second, they will test the evolved red strain against the ancestral blue strain. The differences in growth rate and survival of the two red strains will help identify the effects of the selective pressure over time.

In this study, the team tested small but important changes in the execution of these fitness tests to see how those changes might affect the results. What they discovered was that the environment that the experimental population was subjected to is important in determining the outcome of the fitness tests. For example, the results may be different in the fitness tests are done in sucrose rather than galactose. A more subtle difference might be if the populations were grown in petri dishes but then the fitness experiments happened in flasks.

According to Worthan, “the main takeaway from the paper is that fitness is totally relative to the environment. When evaluating the fitness of an evolved population or a derived mutation, the most important thing is that the original evolved environment must be considered. Any slight deviation from the original evolved environment can have drastic effects on a microbe’s physiology and alter the outcome of the fitness experiment. With that being said, fitness evaluations in environments that differ from the evolved environment can also be useful as well as they may provide insight into the mechanisms of adaptation and any potential trade-offs associated with adaptations.”

Funding Statement: This study was supported by the Life Sciences Division, Army Research Office,W911NF-21-1-0161 to Megan G. Behringer.

Citation: Worthan, S.B., McCarthy, R.D. and Behringer, M.G., 2023. Case studies in the assessment of microbial fitness: seemingly subtle changes can have major effects on phenotypic outcomes. Journal of Molecular Evolution, pp.1-14.