title: Decoupled genomic elements and the evolution of partner quality in nitrogen-fixing rhizobia.
abstract: Understanding how mutualisms evolve in response to a changing environment will be critical for predicting the long-term impacts of global changes, such as increased N (nitrogen) deposition. Bacterial mutualists in particular might evolve quickly, thanks to short generation times and the potential for independent evolution of plasmids through recombination and/or HGT (horizontal gene transfer). In a previous work using the legume/rhizobia mutualism, we demonstrated that long-term nitrogen fertilization caused the evolution of less-mutualistic rhizobia. Here, we use our 63 previously isolated rhizobium strains in comparative phylogenetic and quantitative genetic analyses to determine the degree to which variation in partner quality is attributable to phylogenetic relationships among strains versus recent genetic changes in response to N fertilization. We find evidence of distinct evolutionary relationships between chromosomal and pSym genes, and broad similarity between pSym genes. We also find that nifD has a unique evolutionary history that explains much of the variation in partner quality, and suggest MoFe subunit interaction sites in the evolution of less-mutualistic rhizobia. These results provide insight into the mechanisms behind the evolutionary response of rhizobia to long-term N fertilization, and we discuss the implications of our results for the evolution of the mutualism.
authors.affiliation: Department of Plant Biology University of Illinois Urbana-Champaign 505 S. Goodwin Ave. Urbana Illinois 61801.
authors.forename: Benjamin R
authors.initials: BR
authors.lastname: Gordon
authors.affiliation: Department of Plant Biology University of Illinois Urbana-Champaign 505 S. Goodwin Ave. Urbana Illinois 61801.
authors.forename: Christie R
authors.initials: CR
authors.lastname: Klinger
authors.affiliation: Department of Biology St. Ambrose University 518 West Locust St Davenport Iowa 52803.
authors.forename: Dylan J
authors.initials: DJ
authors.lastname: Weese
authors.affiliation: Kellogg Biological Station and Department of Plant Biology Michigan State University 3700 E. Gull Lake Drive Hickory Corners Michigan 49060.
authors.forename: Jennifer A
authors.initials: JA
authors.lastname: Lau
authors.affiliation: Department of Plant Biology University of Illinois Urbana-Champaign 505 S. Goodwin Ave. Urbana Illinois 61801.
authors.forename: Patricia V
authors.initials: PV
authors.lastname: Burke
authors.affiliation: Jodrell Laboratory Royal Botanic Gardens Kew Richmond Surrey TW9 3DS United Kingdom.
authors.forename: Bryn T M
authors.initials: BT
authors.lastname: Dentinger
authors.affiliation: Department of Plant Biology University of Illinois Urbana-Champaign 505 S. Goodwin Ave. Urbana Illinois 61801.
authors.forename: Katy D
authors.initials: KD
authors.lastname: Heath
keywords: Cheating; Rhizobium; mutualism; nitrogen deposition; partner quality; symbiosis