Alejandro Couce

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The distribution of fitness effects of new mutations (DFE) is central to predicting
many aspects of adaptive evolution. While short-term experiments offer a
snapshot of this distribution, tracking how it changes as organisms adapt has only
recently become possible. We took advantage of the possibility of creating and
characterizing libraries with >100,000 independent mutations in bacteria to
examine how the DFE changed in the multi-decade, E. coli Long-term Evolution
Experiment (LTEE). We found that the overall structure of the DFE has hardly
changed even after 50,000 generations, and that nonessential genes frequently
became essential and vice versa, often in parallel. In contrast, the beneficial
fraction declined rapidly, approximating an exponential distribution, with strong
epistasis profoundly changing the genetic identity of adaptive mutations. Despite
this volatility, many important targets of selection were predictable from the
ancestral distribution. This predictability occurs because genetic target size
contributed to the fixation of beneficial mutations as much as or more than their
effect sizes. Taken together, our results demonstrate the dynamic - but often
statistically predictable - nature of mutational fitness effects.

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University of Madrid
0.02
Contact: Gabriela Petrungaro