Abstract
BackgroundConjugative plasmids play an important role in bacterial evolution by transferring ecologically important genes within and between species. A key limit on interspecific horizontal gene transfer is plasmid host range. Here, we experimentally test the effect of single and multi-host environments on the host-range evolution of a large conjugative mercury resistance plasmid, pQBR57. Specifically, pQBR57 was conjugated between strains of a single host species, either P. fluorescens or P. putida, or alternating between P. fluorescens and P. putida. Crucially, the bacterial hosts were not permitted to evolve allowing us to observe plasmid evolutionary responses in isolation.ResultsIn all treatments plasmids evolved higher conjugation rates over time. Plasmids evolved in single-host environments adapted to their host bacterial species becoming less costly, but in the case of P. fluorescens-adapted plasmids, became costlier in P. putida, suggesting an evolutionary trade-off. When evolved in the multi-host environment plasmids adapted to P. fluorescens without a higher cost in P. putida.ConclusionWhereas evolution in a single-host environment selected for host-specialist plasmids due to a fitness trade-off, this trade-off could be circumvented in the multi-host environment, leading to the evolution of host-generalist plasmids.
Highlights
Conjugative plasmids play an important role in bacterial evolution by transferring ecologically important genes within and between species
We observed that plasmids from the single-host P. fluorescens treatment evolved lower costs in P. fluorescens, but that this adaptation was accompanied by an increased cost in P. putida relative to the ancestral plasmid (Welch’s ttest, t6.81 = 2.592, p = 0.036) (Fig. 1b)
Together this suggests an asymmetric trade-off, whereby pQBR57 adapted to P. fluorescens suffers a fitness trade-off in P. putida, but that there is not a corresponding fitness trade-off associated with adaptation to P. putida
Summary
In all treatments plasmids evolved higher conjugation rates over time. Plasmids evolved in single-host environments adapted to their host bacterial species becoming less costly, but in the case of P. fluorescens-adapted plasmids, became costlier in P. putida, suggesting an evolutionary trade-off. When evolved in the multi-host environment plasmids adapted to P. fluorescens without a higher cost in P. putida
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