Three recent papers in Science:
In Evolution, the Sum Is Less than Its Parts
Jeremy A. Draghi1, and
Joshua B. Plotkin1
Propagating bacteria in a lab for thousands of generations may seem tedious, or even irrelevant, to most evolutionary biologists. Nonetheless, such experiments provide an opportunity to deduce quantitative principles of evolution and directly test them in controlled environments. Combined with modern sequencing technologies, as well as theory, recent microbial experiments have suggested a critical role for genetic interactions among mutations, called epistasis, in determining the pace of evolution. Two papers in this issue, by Khan et al. on page 1193 (1) and Chou et al. (2) on page 1190, present precise experimental measurements of these epistatic interactions.
Diminishing Returns Epistasis Among Beneficial Mutations Decelerates Adaptation
Nigel F. Delaney1,
Daniel Segrè2,3, and
Christopher J. Marx1,4,†
Epistasis has substantial impacts on evolution, in particular, the rate of adaptation. We generated combinations of beneficial mutations that arose in a lineage during rapid adaptation of a bacterium whose growth depended on a newly introduced metabolic pathway. The proportional selective benefit for three of the four loci consistently decreased when they were introduced onto more fit backgrounds. These three alleles all reduced morphological defects caused by expression of the foreign pathway. A simple theoretical model segregating the apparent contribution of individual alleles to benefits and costs effectively predicted the interactions between them. These results provide the first evidence that patterns of epistasis may differ for within- and between-gene interactions during adaptation and that diminishing returns epistasis contributes to the consistent observation of decelerating fitness gains during adaptation.
Negative Epistasis Between Beneficial Mutations in an Evolving Bacterial Population
Aisha I. Khan1,*†,
Duy M. Dinh1,*,
Richard E. Lenski4, and
Tim F. Cooper1,‡
Epistatic interactions between mutations play a prominent role in evolutionary theories. Many studies have found that epistasis is widespread, but they have rarely considered beneficial mutations. We analyzed the effects of epistasis on fitness for the first five mutations to fix in an experimental population of Escherichia coli. Epistasis depended on the effects of the combined mutations-the larger the expected benefit, the more negative the epistatic effect. Epistasis thus tended to produce diminishing returns with genotype fitness, although interactions involving one particular mutation had the opposite effect. These data support models in which negative epistasis contributes to declining rates of adaptation over time. Sign epistasis was rare in this genome-wide study, in contrast to its prevalence in an earlier study of mutations in a single gene.
(In all cases, you have to pay to read the article.)