title: The evolution of symbiont preference traits in the model legume Medicago truncatula.
abstract: Many hosts preferentially associate with or reward better symbionts, but how these symbiont preference traits evolve is an open question. Legumes often form more nodules with or provide more resources to rhizobia that fix more nitrogen (N), but they also acquire N from soil via root foraging. It is unclear whether root responses to abiotically and symbiotically derived N evolve independently. Here, we measured root foraging and both preferential allocation of root resources to and preferential association with an effective vs an ineffective N-fixing Ensifer meliloti strain in 35 inbred lines of the model legume Medicago truncatula. We found that M. truncatula is an efficient root forager and forms more nodules with the effective rhizobium; root biomass increases with the number of effective, but not ineffective, nodules, indicating preferential allocation to roots harbouring effective rhizobia; root foraging is not genetically correlated with either preferential allocation or association; and selection favours plant genotypes that form more effective nodules. Root foraging and symbiont preference traits appear to be genetically uncoupled in M. truncatula. Rather than evolving to exclude ineffective partners, our results suggest that preference traits probably evolve to take better advantage of effective symbionts.
authors.affiliation: Department of Ecology and Evolutionary Biology, University of Toronto, Toronto, ON, M5S 3B2, Canada.
authors.forename: Rebecca T
authors.initials: RT
authors.lastname: Batstone
authors.affiliation: Department of Ecology and Evolutionary Biology, University of Toronto, Toronto, ON, M5S 3B2, Canada.
authors.forename: Emily M
authors.initials: EM
authors.lastname: Dutton
authors.affiliation: Department of Ecology and Evolutionary Biology, University of Toronto, Toronto, ON, M5S 3B2, Canada.
authors.forename: Donglin
authors.initials: D
authors.lastname: Wang
authors.affiliation: Department of Ecology and Evolutionary Biology, University of Toronto, Toronto, ON, M5S 3B2, Canada.
authors.forename: Molly
authors.initials: M
authors.lastname: Yang
authors.affiliation: Department of Ecology and Evolutionary Biology, University of Toronto, Toronto, ON, M5S 3B2, Canada.
authors.forename: Megan E
authors.initials: ME
authors.lastname: Frederickson
mesh_terms: D005075:Biological Evolution; D018533:Biomass; D005308:Fertilizers; D014644:Genetic Variation; D005838:Genotype; D046913:Medicago truncatula / Q000235:genetics / Q000382:microbiology*; D008954:Models, Biological; D018517:Plant Roots / Q000382:microbiology; D019655:Quantitative Trait, Heritable; D012231:Rhizobium / Q000502:physiology; D012641:Selection, Genetic; D013559:Symbiosis
chemical_list: D005308:Fertilizers
keywords: 
  Ensifer meliloti
  ; 
  Medicago truncatula
  ; cheating; genetic correlations; mutualism stability; partner choice; root foraging; sanctions