title: Multi-host environments select for host-generalist conjugative plasmids.
abstract: BACKGROUND
  Conjugative 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.
  
  
  RESULTS
  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.
  
  
  CONCLUSIONS
  Whereas 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.
authors.affiliation: Department of Biology, University of York, Wentworth Way, York, YO10 5DD, UK.
authors.forename: Anastasia
authors.initials: A
authors.lastname: Kottara
authors.affiliation: Department of Biology, University of York, Wentworth Way, York, YO10 5DD, UK.
authors.forename: James P J
authors.initials: JP
authors.lastname: Hall
authors.affiliation: Department of Biology, University of York, Wentworth Way, York, YO10 5DD, UK.
authors.forename: Ellie
authors.initials: E
authors.lastname: Harrison
authors.affiliation: Department of Biology, University of York, Wentworth Way, York, YO10 5DD, UK. michael.brockhurst@york.ac.uk.
authors.forename: Michael A
authors.initials: MA
authors.lastname: Brockhurst
mesh_terms: D005075:Biological Evolution; D024881:Drug Resistance, Bacterial; D022761:Gene Transfer, Horizontal; D056084:Genetic Fitness; D008628:Mercury / Q000633:toxicity; D010957:Plasmids; D011551:Pseudomonas fluorescens / Q000235:genetics*; D016958:Pseudomonas putida / Q000235:genetics*
chemical_list: D008628:Mercury