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Mol Ecol. 2017 May;26(10):2757-2764. doi: 10.1111/mec.14080. Epub 2017 Apr 3.

Conflicting selection alters the trajectory of molecular evolution in a tripartite bacteria-plasmid-phage interaction.

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Department of Animal and Plant Sciences, University of Sheffield, Sheffield, S10 2TN, UK.
Institute of Integrative Biology, University of Liverpool, Liverpool, L69 7ZB, UK.
SIMBIOSIS Centre, University of Abertay, Dundee, DD1 1HG, UK.


Bacteria engage in a complex network of ecological interactions, which includes mobile genetic elements (MGEs) such as phages and plasmids. These elements play a key role in microbial communities as vectors of horizontal gene transfer but can also be important sources of selection for their bacterial hosts. In natural communities, bacteria are likely to encounter multiple MGEs simultaneously and conflicting selection among MGEs could alter the bacterial evolutionary response to each MGE. Here, we test the effect of interactions with multiple MGEs on bacterial molecular evolution in the tripartite interaction between the bacterium, Pseudomonas fluorescens, the lytic bacteriophage, SBW25φ2, and conjugative plasmid, pQBR103, using genome sequencing of experimentally evolved bacteria. We show that individually, both plasmids and phages impose selection leading to bacterial evolutionary responses that are distinct from bacterial populations evolving without MGEs, but that together, plasmids and phages impose conflicting selection on bacteria, constraining the evolutionary responses observed in pairwise interactions. Our findings highlight the likely difficulties of predicting evolutionary responses to multiple selective pressures from the observed evolutionary responses to each selective pressure alone. Understanding evolution in complex microbial communities comprising many species and MGEs will require that we go beyond studies of pairwise interactions.


bacteria; coevolution; experimental evolution; microbial biology; molecular evolution; species interactions

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