title: Association mapping reveals novel serpentine adaptation gene clusters in a population of symbiotic Mesorhizobium.
abstract: The genetic variants that underlie microbial environmental adaptation are key components of models of microbial diversification. Characterizing adaptive variants and the pangenomic context in which they evolve remains a frontier in understanding how microbial diversity is generated. The genomics of rhizobium adaptation to contrasting soil environments is ecologically and agriculturally important because these bacteria are responsible for half of all current biologically fixed nitrogen, yet they live the majority of their lives in soil. Our study uses whole-genome sequencing to describe the pan-genome of a focal clade of wild mesorhizobia that show contrasting levels of nickel adaptation despite high relatedness (99.8% identity at 16S). We observe ecotypic specialization within an otherwise genomically cohesive population, rather than finding distinct specialized bacterial lineages in contrasting soil types. This finding supports recent reports that heterogeneous environments impose selection that maintains differentiation only at a small fraction of the genome. Our work further uses a genome-wide association study to propose candidate genes for nickel adaptation. Several candidates show homology to genetic systems involved in nickel tolerance and one cluster of candidates correlates perfectly with soil origin, which validates our approach of ascribing genomic variation to adaptive divergence.
authors.affiliation: School of Biological Sciences, Washington State University, Vancouver, WA, USA.
authors.forename: Stephanie S
authors.initials: SS
authors.lastname: Porter
authors.affiliation: Section of Molecular and Computational Biology, Department of Biological Sciences, University of Southern California, Los Angeles, CA, USA.|Department of Plant Pathology and Nematology, University of California, Davis, CA, USA.
authors.forename: Peter L
authors.initials: PL
authors.lastname: Chang
authors.affiliation: Section of Molecular and Computational Biology, Department of Biological Sciences, University of Southern California, Los Angeles, CA, USA.
authors.forename: Christopher A
authors.initials: CA
authors.lastname: Conow
authors.affiliation: Section of Molecular and Computational Biology, Department of Biological Sciences, University of Southern California, Los Angeles, CA, USA.
authors.forename: Joseph P
authors.initials: JP
authors.lastname: Dunham
authors.affiliation: Department of Plant Biology, Michigan State University, East Lansing, MI, USA.
authors.forename: Maren L
authors.initials: ML
authors.lastname: Friesen
mesh_terms: D000064:Acclimatization; D000222:Adaptation, Physiological / Q000235:genetics; D001426:Bacterial Proteins / Q000235:genetics / Q000378:metabolism*; D002874:Chromosome Mapping; D060146:Ecotype; D014644:Genetic Variation; D055106:Genome-Wide Association Study; D023281:Genomics; D059128:Mesorhizobium / Q000145:classification / Q000235:genetics / Q000302:isolation & purification / Q000502:physiology*; D046948:Secologanin Tryptamine Alkaloids / Q000378:metabolism*; D012988:Soil Microbiology
chemical_list: D001426:Bacterial Proteins; D046948:Secologanin Tryptamine Alkaloids; C009244:serpentine (alkaloid)