title: A single evolutionary innovation drives the deep evolution of symbiotic N2-fixation in angiosperms.
abstract: Symbiotic associations occur in every habitat on earth, but we know very little about their evolutionary histories. Current models of trait evolution cannot adequately reconstruct the deep history of symbiotic innovation, because they assume homogenous evolutionary processes across millions of years. Here we use a recently developed, heterogeneous and quantitative phylogenetic framework to study the origin of the symbiosis between angiosperms and nitrogen-fixing (N2) bacterial symbionts housed in nodules. We compile the largest database of global nodulating plant species and reconstruct the symbiosis' evolution. We identify a single, cryptic evolutionary innovation driving symbiotic N2-fixation evolution, followed by multiple gains and losses of the symbiosis, and the subsequent emergence of 'stable fixers' (clades extremely unlikely to lose the symbiosis). Originating over 100 MYA, this innovation suggests deep homology in symbiotic N2-fixation. Identifying cryptic innovations on the tree of life is key to understanding the evolution of complex traits, including symbiotic partnerships.
authors.affiliation: Department of Ecological Science, Vrije Universiteit Amsterdam, De Boelelaan 1085, 1081 HV Amsterdam, The Netherlands.
authors.forename: Gijsbert D A
authors.initials: GD
authors.lastname: Werner
authors.affiliation: 1] Department of Ecological Science, Vrije Universiteit Amsterdam, De Boelelaan 1085, 1081 HV Amsterdam, The Netherlands [2].
authors.forename: William K
authors.initials: WK
authors.lastname: Cornwell
authors.affiliation: Division of Plant Sciences, College of Life Sciences, University of Dundee at James Hutton Institute, Dundee DD2 5DA, UK.
authors.forename: Janet I
authors.initials: JI
authors.lastname: Sprent
authors.affiliation: 1] Max Planck Institute for Biogeochemistry, Hans Knoell Strasse 10, 07743 Jena, Germany [2] German Centre for Integrative Biodiversity Research (iDiv) Halle-Jena-Leipzig, Deutscher Platz 5e, 04103 Leipzig, Germany.
authors.forename: Jens
authors.initials: J
authors.lastname: Kattge
authors.affiliation: Department of Ecological Science, Vrije Universiteit Amsterdam, De Boelelaan 1085, 1081 HV Amsterdam, The Netherlands.
authors.forename: E Toby
authors.initials: ET
authors.lastname: Kiers
mesh_terms: D005075:Biological Evolution; D019684:Magnoliopsida; D009586:Nitrogen Fixation; D013559:Symbiosis