The first steps of adaptation of Escherichia coli to the gut are dominated by soft sweeps.

TitleThe first steps of adaptation of Escherichia coli to the gut are dominated by soft sweeps.
Publication TypeJournal Article
Year of Publication2014
AuthorsBarroso-Batista J, Sousa A, Lourenço M, Bergman M-L, Sobral D, Demengeot J, Xavier KB, Gordo I
JournalPLoS Genet
Volume10
Issue3
Paginatione1004182
Date Published2014 Mar
ISSN1553-7404
Abstract

The accumulation of adaptive mutations is essential for survival in novel environments. However, in clonal populations with a high mutational supply, the power of natural selection is expected to be limited. This is due to clonal interference--the competition of clones carrying different beneficial mutations--which leads to the loss of many small effect mutations and fixation of large effect ones. If interference is abundant, then mechanisms for horizontal transfer of genes, which allow the immediate combination of beneficial alleles in a single background, are expected to evolve. However, the relevance of interference in natural complex environments, such as the gut, is poorly known. To address this issue, we have developed an experimental system which allows to uncover the nature of the adaptive process as Escherichia coli adapts to the mouse gut. This system shows the invasion of beneficial mutations in the bacterial populations and demonstrates the pervasiveness of clonal interference. The observed dynamics of change in frequency of beneficial mutations are consistent with soft sweeps, where different adaptive mutations with similar phenotypes, arise repeatedly on different haplotypes without reaching fixation. Despite the complexity of this ecosystem, the genetic basis of the adaptive mutations revealed a striking parallelism in independently evolving populations. This was mainly characterized by the insertion of transposable elements in both coding and regulatory regions of a few genes. Interestingly, in most populations we observed a complete phenotypic sweep without loss of genetic variation. The intense clonal interference during adaptation to the gut environment, here demonstrated, may be important for our understanding of the levels of strain diversity of E. coli inhabiting the human gut microbiota and of its recombination rate.

DOI10.1371/journal.pgen.1004182
Alternate JournalPLoS Genet.
PubMed ID24603313
PubMed Central IDPMC3945185
Grant ListHHMI-55007436 / / Howard Hughes Medical Institute / United States