Parallel evolutionary paths to mycoheterotrophy in understorey Ericaceae and Orchidaceae: ecological evidence for mixotrophy in Pyroleae |
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Authors: | Leho Tedersoo Prune Pellet Urmas Kõljalg Marc-André Selosse |
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Institution: | (1) Institute of Botany and Ecology, University of Tartu, 40 Lai Str., 51005 Tartu, Estonia;(2) Centre d’Ecologie Fonctionnelle et Evolutive (CNRS, UMR 5175), Equipe Interactions Biotiques, 1919 Route de Mende, 34293 Montpellier Cédex 5, France |
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Abstract: | Several forest understorey achlorophyllous plants, termed mycoheterotrophs (MHs), obtain C from their mycorrhizal fungi. The
latter in turn form ectomycorrhizas with trees, the ultimate C source of the entire system. A similar nutritional strategy
occurs in some green forest orchids, phylogenetically close to MH species, that gain their C via a combination of MH and photosynthesis
(mixotrophy). In orchid evolution, mixotrophy evolved in shaded habitats and preceded MH nutrition. By generalizing and applying
this to Ericaceae, we hypothesized that green forest species phylogenetically close to MHs are mixotrophic. Using stable C
isotope analysis with fungi, autotrophic, mixotrophic and MH plants as comparisons, we found the first quantitative evidence
for substantial fungi-mediated mixotrophy in the Pyroleae, common ericaceous shrubs from boreal forests close to the MH Monotropoideae.
Orthilia secunda, Pyrola chlorantha, Pyrola rotundifolia and Chimaphila umbellata acquired between 10.3 and 67.5% of their C from fungi. High N and 15N contents also suggest that Pyroleae nutrition partly rely on fungi. Examination of root fungal internal transcribed spacer
sequences at one site revealed that 39 species of mostly endophytic or ectomycorrhizal fungi, including abundant Tricholoma spp., were associated with O. secunda, P. chlorantha and C. umbellata. These fungi, particularly ectomycorrhizal associates, could thus link mixotrophic Pyroleae spp. to surrounding trees, allowing
the C flows deduced from isotopic evidence. These data suggest that we need to reconsider ecological roles of understorey
plants, which could influence the dynamics and composition of forest communities. |
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Keywords: | Mixotrophy Pyroleae Stable isotopes Ectomycorrhizal fungi Ericaceae |
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