title: Multiple evolutionary origins of legume traits leading to extreme rhizobial differentiation.
abstract: *When rhizobia differentiate inside legume host nodules to become nitrogen-fixing bacteroids, they undergo a physiological as well as a morphological transformation. These transformations are more extreme in some legume species than others, leading to fundamental differences in rhizobial life history and evolution. Here, we analysed the distribution of different bacteroid morphologies over a legume phylogeny to understand the evolutionary history of this host-influenced differentiation. *Using existing electron micrographs and new flow cytometric analyses, bacteroid morphologies were categorized as swollen or nonswollen for 40 legume species in the subfamily Papilionoideae. Maximum likelihood and Bayesian frameworks were used to reconstruct ancestral states at the bases of all major subclades within the papilionoids. *Extreme bacteroid differentiation leading to swelling was found in five out of the six major papilionoid subclades. The inferred ancestral state for the Papilionoideae was hosting nonswollen bacteroids, indicating at least five independent origins of host traits leading to swollen bacteroids. *Repeated evolution of host traits causing bacteroid swelling indicates a possible fitness benefit to the plant. Furthermore, as bacteroid swelling is often correlated with loss of reproductive viability, the evolution of bacteroid cooperation or cheating strategies could be fundamentally different between the two bacteroid morphologies.
authors.affiliation: Department of Plant Biology, University of Minnesota, 250 Biological Sciences, 1445 Gortner Avenue, Saint Paul, MN 55108, USA.
authors.forename: Ryoko
authors.initials: R
authors.lastname: Oono
authors.affiliation: Department of Plant Biology and Bell Museum of Natural History, University of Minnesota, 250 Biological Sciences, 1445 Gortner Avenue, Saint Paul, MN 55108, USA.
authors.forename: Imke
authors.initials: I
authors.lastname: Schmitt
authors.affiliation: Division of Plant Sciences, University of Dundee at SCRI, Dundee, DD2 5DA, UK.
authors.forename: Janet I
authors.initials: JI
authors.lastname: Sprent
authors.affiliation: Department of Ecology, Evolution and Behavior, University of Minnesota, 1987 Buford Circle, Saint Paul, MN 55108, USA.
authors.forename: R Ford
authors.initials: RF
authors.lastname: Denison
mesh_terms: D019143:Evolution, Molecular; D007887:Fabaceae / Q000235:genetics* / Q000382:microbiology*; D005434:Flow Cytometry; D010802:Phylogeny; D019655:Quantitative Trait, Heritable; D012231:Rhizobium / Q000166:cytology / Q000502:physiology*; D053204:Root Nodules, Plant / Q000235:genetics / Q000382:microbiology; D013045:Species Specificity
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