Evolutionary instability of symbiotic function in Bradyrhizobium japonicum

Bacterial mutualists are often acquired from the environment by eukaryotic hosts. However, both theory and empirical work suggest that this bacterial lifestyle is evolutionarily unstable. Bacterial evolution outside of the host is predicted to favor traits that promote an independent lifestyle in th...

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Bibliographic Details
Published in:PloS one 2011-11, Vol.6 (11), p.e26370-e26370
Main Authors: Sachs, Joel L, Russell, James E, Hollowell, Amanda C
Format: Article
Language:English
Subjects:
Bacteria
Biological Evolution
Biology
Bradyrhizobium
Bradyrhizobium - classification
Bradyrhizobium - genetics
Bradyrhizobium japonicum
Deoxyribonucleic acid
DNA
E coli
Erosion
Escherichia coli
Evolution
Evolution & development
Fitness
Gene deletion
Genomes
Genotypes
Hypotheses
Islands
Legumes
Loci
Lotus strigosus
Mutation
Mutualism
Nitrogen
Nodulation
Phylogenetics
Phylogeny
Plasmids
Polymerase Chain Reaction
Predictions
Reproductive fitness
Soil erosion
Stability
Symbiosis
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Summary:Bacterial mutualists are often acquired from the environment by eukaryotic hosts. However, both theory and empirical work suggest that this bacterial lifestyle is evolutionarily unstable. Bacterial evolution outside of the host is predicted to favor traits that promote an independent lifestyle in the environment at a cost to symbiotic function. Consistent with these predictions, environmentally-acquired bacterial mutualists often lose symbiotic function over evolutionary time. Here, we investigate the evolutionary erosion of symbiotic traits in Bradyrhizobium japonicum, a nodulating root symbiont of legumes. Building on a previous published phylogeny we infer loss events of nodulation capability in a natural population of Bradyrhizobium, potentially driven by mutation or deletion of symbiosis loci. Subsequently, we experimentally evolved representative strains from the symbiont population under host-free in vitro conditions to examine potential drivers of these loss events. Among Bradyrhizobium genotypes that evolved significant increases in fitness in vitro, two exhibited reduced symbiotic quality, but no experimentally evolved strain lost nodulation capability or evolved any fixed changes at six sequenced loci. Our results are consistent with trade-offs between symbiotic quality and fitness in a host free environment. However, the drivers of loss-of-nodulation events in natural Bradyrhizobium populations remain unknown.
ISSN:1932-6203
1932-6203
DOI:10.1371/journal.pone.0026370