title: Bigger is not always better: transmission and fitness burden of ∼1MB Pseudomonas syringae megaplasmid pMPPla107.
abstract: BACKGROUND
  Horizontal gene transfer (HGT) is a widespread process that enables the acquisition of genes and metabolic pathways in single evolutionary steps. Previous reports have described fitness costs of HGT, but have largely focused on the acquisition of relatively small plasmids. We have previously shown that a Pseudomonas syringae pv. lachrymans strain recently acquired a cryptic megaplasmid, pMPPla107. This extrachromosomal element contributes hundreds of new genes to P. syringae and increases total genomic content by approximately 18%. However, this early work did not directly explore transmissibility, stability, or fitness costs associated with acquisition of pMPPla107.
  
  
  RESULTS
  Here, we show that pMPPla107 is self-transmissible across a variety of diverse pseudomonad strains, on both solid agar and within shaking liquid cultures, with conjugation dependent on a type IV secretion system. To the best of our knowledge, this is the largest self-transmissible megaplasmid known outside of Sinorhizobium. This megaplasmid can be lost from all novel hosts although the rate of loss depends on medium type and genomic background. However, in contrast, pMPPla107 is faithfully maintained within the original parent strain (Pla107) even under direct negative selection during laboratory assays. These results suggest that Pla107 specific stabilizing mutations have occurred either on this strain's chromosome or within the megaplasmid. Lastly, we demonstrate that acquisition of pMPPla107 by strains other than Pla107 imparts severe (20%) fitness costs under competitive conditions in vitro.
  
  
  CONCLUSIONS
  We show that pMPPla107 is capable of transmitting and maintaining itself across multiple Pseudomonas species, rendering it one of the largest conjugative elements discovered to date. The relative stability of pMPPla107, coupled with extensive fitness costs, makes it a tractable model system for investigating evolutionary and genetic mechanisms of megaplasmid maintenance and a unique testing ground to explore evolutionary dynamics after HGT of large secondary elements.
authors.affiliation: University of North Carolina - Chapel Hill, United States. Electronic address: romanchk@email.unc.edu.
authors.forename: Artur
authors.initials: A
authors.lastname: Romanchuk
authors.affiliation: University of North Carolina - Chapel Hill, United States.
authors.forename: Corbin D
authors.initials: CD
authors.lastname: Jones
authors.affiliation: University of North Carolina - Chapel Hill, United States.
authors.forename: Kedar
authors.initials: K
authors.lastname: Karkare
authors.affiliation: University of Arizona, United States.
authors.forename: Autumn
authors.initials: A
authors.lastname: Moore
authors.affiliation: University of Arizona, United States.
authors.forename: Brian A
authors.initials: BA
authors.lastname: Smith
authors.affiliation: University of Arizona, United States.
authors.forename: Chelsea
authors.initials: C
authors.lastname: Jones
authors.affiliation: University of Arizona, United States.
authors.forename: Kevin
authors.initials: K
authors.lastname: Dougherty
authors.affiliation: University of Arizona, United States.
authors.forename: David A
authors.initials: DA
authors.lastname: Baltrus
mesh_terms: D005075:Biological Evolution; D003227:Conjugation, Genetic; D010935:Plant Diseases / Q000235:genetics* / Q000382:microbiology; D010957:Plasmids / Q000235:genetics*; D011549:Pseudomonas / Q000145:classification / Q000235:genetics* / Q000472:pathogenicity; D011552:Pseudomonas Infections / Q000235:genetics / Q000635:transmission*; D044224:Pseudomonas syringae / Q000235:genetics* / Q000472:pathogenicity; D014774:Virulence / Q000235:genetics*
keywords: Fitness cost; HGT; Megaplasmid; Plasmid transfer; Pseudomonas