title: Standing genetic variation in host preference for mutualist microbial symbionts.
abstract: Many models of mutualisms show that mutualisms are unstable if hosts lack mechanisms enabling preferential associations with mutualistic symbiotic partners over exploitative partners. Despite the theoretical importance of mutualism-stabilizing mechanisms, we have little empirical evidence to infer their evolutionary dynamics in response to exploitation by non-beneficial partners. Using a model mutualism-the interaction between legumes and nitrogen-fixing soil symbionts-we tested for quantitative genetic variation in plant responses to mutualistic and exploitative symbiotic rhizobia in controlled greenhouse conditions. We found significant broad-sense heritability in a legume host's preferential association with mutualistic over exploitative symbionts and selection to reduce frequency of associations with exploitative partners. We failed to detect evidence that selection will favour the loss of mutualism-stabilizing mechanisms in the absence of exploitation, as we found no evidence for a fitness cost to the host trait or indirect selection on genetically correlated traits. Our results show that genetic variation in the ability to preferentially reduce associations with an exploitative partner exists within mutualisms and is under selection, indicating that micro-evolutionary responses in mutualism-stabilizing traits in the face of rapidly evolving mutualistic and exploitative symbiotic bacteria can occur in natural host populations.
authors.affiliation: Department of Ecology and Evolutionary Biology, University of Toronto, 25 Willcocks Street, Toronto, Canada M5S 3B2 anna.simonsen@gmail.ca.
authors.forename: Anna K
authors.initials: AK
authors.lastname: Simonsen
authors.affiliation: Department of Ecology and Evolutionary Biology & Centre for the Analysis of Genome Evolution and Function, University of Toronto, 25 Willcocks Street, Toronto, Canada M5S 3B2.
authors.forename: John R
authors.initials: JR
authors.lastname: Stinchcombe
mesh_terms: D014644:Genetic Variation; D058507:Host Specificity / Q000235:genetics; D029902:Medicago / Q000382:microbiology*; D008969:Molecular Sequence Data; D009586:Nitrogen Fixation; D012230:Rhizobiaceae / Q000502:physiology*; D012641:Selection, Genetic; D012988:Soil Microbiology; D013559:Symbiosis / Q000235:genetics*
keywords: Medicago lupulina; genetic variation; host sanctions; mutualism; partner choice; selection