Tulasnellaceae form mycorrhizae with a broad spectrum of plants, including mycorrhiza-like association with Aneuraceae liverworts thallus Kottke et al. Tulasnella was also found associated with Graffenrieda emarginata Melastomatacecae forming a superficial layer on arbuscular mycorrhizas Haug et al. While only few named Tulasnella species are reported as forming mycorrhiza with orchids, molecular phylogeny revealed a large number of genotypes, but also inconsistencies in species concepts and taxonomy challenging comparative ecological studies Cruz et al.
These recent studies on fresh samples of basidiomata showed however, that molecular approach using the ITS The genera Ceratobasidium and Thanatephorus along with their Rhizoctonia anamorphs form a group of closely related fungal taxa in the family Ceratobasidiaceae. Species within the Ceratobasidium-Thanatephorus complex are known as crop pathogens but also as forming mycorrhizae with orchids and trees Tedersoo et al.
Delineating species in these groups has been problematic, efforts to solve this situation included anastomosis groups and molecular data Oberwinkler et al. Studies reporting the presence of members of Ceratobasidiaceae from tropical orchids remain scarce. Using isolation-dependent methods orchid species Coppensia doniana, Tolumnia variegata, Ionopsis utricularioides and Psygmorchis pusilla tribe Cymbidieae were shown to be predominantly associated to Ceratobasidiaceae Valadares et al. Atractiellales -. Atractiellales belong to the subphylum Pucciniomycotina Rust fungi , which comprises mainly parasites and to a lesser extent presumed saprophytes Aime et al.
So far only three genotypes operational taxonomic units, OTUs of Atractiellomycetes were shown by combined ultrastructural and molecular investigations to form mycorrhiza with terrestrial and epiphytic orchids Kottke et al. Experimental proof for nutritional support of protocorms is, however, still lacking. Orchid-mycobiont interaction with different levels of specificity in the tropical mountain forest. Although a high number of orchid species have been recorded, studies on their mycorrhizal fungi are still scarce Dearnaley et al.
Factors as orchid recruitment, seed dispersal limitations and availability of suitable fungal partner are affecting the distribution of orchids at regional and local scales and may disguise specificities among plants and fungi. Our studies were concentrated on the conditions of tropical mountain rain forest where intensive sampling was carried out on epiphytic and terrestrial species. Molecular data revealed narrow preferences to broad sharing of partners. Preferences were evident in case of S. Individuals wereassociated with only two different clades of closely related Tulasnella.
Kottke et al. Similar broad sharing was found for Tulasnella and Seredipita genotypes from the same area Kottke et al. The phylogenetic analysis of ITS Results show up to three different phylogenetic species of mycobionts associated to one Teagueia species suggesting high potential for sharing mycobionts among Teagueia spp.
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All the detected mycobionts had wide geographical distribution. The molecular phylogenetic based investigations of orchid mycobionts in the tropical mountain rain forest area of Southern Ecuador revealed the well established fungal groups, Serendipidaceae, Tulasnellaceae and Ceratobasidiaceae.
Additionally, Atractiellales were found as widespread mycobionts. The latter can be found in some data sets of previous investigations, but were not seriously considered before.
Structural plasticity in root-fungal symbioses: diverse interactions lead to improved plant fitness
Although we cannot definitely exclude that further fungal groups may be detected in future, we may ask why just members of these few fungal families are suitable mycobionts of green orchid. The vast majority of plants form mycorrhizae with Glomeromycota, obligate symbionts with no access to extraradical sugars. Thus, these mycobionts are unsuitable to feed the orchid protocorm. Mycobionts in Agaricomycetes, however, derived from saprotrophic relatives multiple times and the orchid mycorrhizal groups, in basal position of Agaricomycetes, preserved some genes for decay enzymes Hibbett et al.
Preservation of the respective genes means sufficient organic matter decay for acquiring carbon and nitrogen to feed the orchid protocorm without attacking living cells in a parasitic manner. According with these capabilities, these fungi are prepared to grow on tree bark Kartzinel et al. The particular conditions in the epiphytic habitat may have potentially supported adaptations to the specific groups of mycorrhizal fungi.
The life history sets Orchidaceae apart from all other Monocotyledonae and was obviously facilitated by switching from Glomeromycota to mainly these Agaricomycetes in Basidiomycota as mycorrhizal fungi. Another question under debate concerns about potential specificity among orchids and mycobionts.
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It was generally found that orchid rarity was unrelated to specificity, but coexisting, terrestrial orchid populations were associated with distinct mycobiont communities. Fungi may be rare to meet on tree branches and stems, potentially restricted to species adapted to these niches. Accordingly, Martos et al. We carried out a comparatively large-scale survey in the new world tropical forest of the Ecuadorian Andes on epiphytic and terrestrial orchids.
Our results showed different levels of specificity, ranging from unspecific, multispecies networks to narrow orchid clades associated with few world-wide spread Tulasnella genotypes, and to narrow species-species associations along an elevation gradient or among dense populations of an epiphytic orchid.
Species richness, environmental conditions but also sampling efforts and methods of fungal identification may still bias a clear statement on narrow specificity in the wet tropical habitat. More likely, sharing of mycobionts will improve the rate of successful germination of orchid seeds and also promote co-existence of closely related species as observed in the tropical montane rain forest Kottke et al. Aime, M. An overview of the higher level classification of Pucciniomycotina based on combined analyses of nuclear large and small subunit rDNA sequences.
Mycologia, 98 6 , Avis, P. Long-term increase in nitrogen supply alters aboveand below-ground ectomycorrhizal communities and increases the dominance of Russula spp. New Phytologist, 1 , Bidartondo, M. Specialized cheating of the ectomycorrhizal symbiosis by an epiparasitic liverwort. Cruz, D. Morphological revision of Tulasnellaceae, with two new species of Tulasnella and new records of Tulasnella spp.
Nova Hedwigia, , Cryptic species revealed by molecular phylogenetic analysis of sequences obtained from basidiomata of Tulasnella. Mycologia, 4 , Mycological Progress, 10 2 , Dearnaley, J. In: B. Hock Ed. Berlin, Heidelberg: Springer. Dodson, C. Native Ecuadorian Orchids, vol IV. Glen, M. Mycorrhiza12, Phylogenetic relationships of Rhizoctonia fungi within the Cantharellales. Fungal Biology, 4 , Haug, I. Graffenrieda emarginata Melastomataceae forms mycorrhizas with Glomeromycota and with a member of the Hymenoscyphus ericae aggregate in the organic soil of a neotropical mountain rain forest.
How old are fungi?
Canadian Journal of Botany, 82 3 , Hibbett, D. In: Systematics and Evolution pp. A higher-level phylogenetic classification of the Fungi. Mycological Research, Pt 5 , The relative ages of ectomycorrhizal mushrooms and their plant hosts estimated using Bayesian relaxed molecular clock analyses. BMC Biology, 7 1 , Jacquemyn, H. A spatially explicit analysis of seedling recruitment in the terrestrial orchid Orchis purpurea. New Phytologist, 2 , Spatial aspects of seed dispersal and seedling recruitment in orchids. New Phytologist, , Julou, T. Mixotrophy in orchids: insights from a comparative study of green individuals and nonphotosynthetic individuals of Cephalanthera damasonium.
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Highly diverse and spatially heterogeneous mycorrhizal symbiosis in a rare epiphyte is unrelated to broad biogeographic or environmental features. Molecular Ecology, 22 23 , Kennedy, P.
There is high potential for the formation of common mycorrhizal networks between understorey and canopy trees in a mixed evergreen forest.