Just the two of us? A family of pseudomonas megaplasmids offers a rare glimpse into the evolution of largemobile elements

Abstract

Pseudomonads are ubiquitous group of environmental proteobacteria, well known for their roles in biogeochemical cycling, in the breakdown of xenobioticmaterials, as plant growth promoters, and as pathogens of a variety of host organisms.We have previously identified a largemegaplasmid present within one isolate of the plant pathogen Pseudomonas syringae, and here we report that a second member of this megaplasmid family is found within an environmental Pseudomonad isolate most closely related to Pseudomonas putida.Many of the shared genes are involved in critical cellular processes like replication, transcription, translation, and DNA repair. We argue that presence of these shared pathways sheds new light on discussions about the types of genes that undergo horizontal gene transfer (i.e., the complexity hypothesis) as well as the evolution of pangenomes. Furthermore, although bothmegaplasmids display a high level of synteny, genes that are shared differ by over 50%on average at the amino acid level. This combination of conservation in gene order despite divergence in gene sequence suggests that this Pseudomonad megaplasmid family is relatively old, that geneorder is under strongselectionwithinthis family, and that there are likelymanymoremembersof this megaplasmid family waiting to be found in nature.