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Phylogenetic species recognition reveals host-specific lineages among poplar rust fungi
Authors:Agathe Vialle  Nicolas Feau  Pascal Frey  Louis Bernier  Richard C. Hamelin
Affiliation:1. Natural Resources Canada, Canadian Forest Service, Laurentian Forestry Centre, 1055 du PEPS, P.O. Box 10380, Stn. Sainte-Foy, Québec, QC, Canada G1V 4C7;2. Centre d’étude de la forêt (CEF) and Institut de biologie intégrative et des systèmes (IBIS), Université Laval, Québec, QC, Canada G1V 0A6;3. INRA, UMR 1202 BIOGECO, INRA 69 Route d’Arcachon, 33612 Cestas Cedex, France;4. INRA, UMR1136 Interactions Arbres – Microorganismes, F-54280 Champenoux, France;5. Université de Lorraine, UMR1136 Interactions Arbres – Microorganismes, F-54280 Champenoux, France
Abstract:Fungal species belonging to the genus Melampsora (Basidiomycota, Pucciniales) comprise rust pathogens that alternate between Salicaceae and other plant hosts. Species delineation and identification are difficult within this group due to the paucity of observable morphological features. Several Melampsora rusts are highly host-specific and this feature has been used for identification at the species level. However, this criterion is not always reliable since different Melampsora rust species can overlap on one host but specialize on a different one. To date, two different species recognition methods are used to recognize and define species within the Melampsora genus: (i) morphological species recognition, which is based solely on morphological criteria; and (ii) ecological species recognition, which combines morphological criteria with host range to recognize and define species. In order to clarify species recognition within the Melampsora genus, we applied phylogenetic species recognition to Melampsora poplar rusts by conducting molecular phylogenetic analyses on 15 Melampsora taxa using six nuclear and mitochondrial loci. By assessing the genealogical concordance between phylogenies, we identified 12 lineages that evolved independently, corresponding to distinct phylogenetic species. All 12 lineages were concordant with host specialization, but only three belonged to strictly defined morphological species. The estimation of the species tree obtained with Bayesian concordance analysis highlighted a potential co-evolutionary history between Melampsora species and their reciprocal aecial host plants. Within the Melampsora speciation process, aecial host may have had a strong effect on ancestral evolution, whereas telial host specificity seems to have evolved more recently. The morphological characters initially used to define species boundaries in the Melampsora genus are not reflective of the evolutionary and genetic relationships among poplar rusts. In order to construct a more meaningful taxonomy, host specificity must be considered an important criterion for delineating and describing species within the genus Melampsora as previously suggested by ecological species recognition.
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