Population Evidence of Cryptic Species and Geographical Structure in the Cosmopolitan Ectomycorrhizal Fungus, Tricholoma scalpturatum

Laboratoire Evolution et Diversité Biologique, UMR 5174 CNRS-UPS-ENFA, Bât. 4R3, Université Paul Sabatier-Toulouse III, 118 route de Narbonne, 31062, Toulouse Cedex 9, France.
Microbial Ecology (Impact Factor: 2.97). 03/2008; 56(3):513-24. DOI: 10.1007/s00248-008-9370-2
Source: PubMed


Tricholoma scalpturatum is an ectomycorrhizal fungus that forms symbioses with roots of diverse trees and shrubs. It is commonly encountered in a wide range of habitats, across temperate ecosystems. A previous study has revealed a high genetic diversity at a local scale, and ruderal abilities. To examine genetic structure at a large geographical scale, a total of 164 basidiocarps were collected from 30 populations located in Western Europe, from Spain to Scandinavia. These samples were analyzed by three molecular methods with different levels of resolution: inter-simple sequence repeats (ISSRs), restriction fragment length polymorphisms (RFLPs) in the rDNA internal transcribed spacer (ITS), and ITS sequence analysis. Considerable genetic variation was found, and the morphospecies was separated into two genetic groups that were distinct from each other. The ISSR data and the relatively low percentage value (96%) of shared sequence polymorphisms in the ITS between isolates from the two groups, strongly suggest cryptic species and long-lasting separation. No geographical exclusion was detected for these two widely distributed taxa. However, high estimates of population differentiation were observed in each group, including between populations less than a few kilometers apart. This result provides evidence for limited gene flow and/or founding effects. It also indicates that T. scalpturatum does not constitute a random mating population, and the hypothesis of endemism cannot be excluded for this cosmopolitan wind-dispersed fungus.

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Available from: Jacob Heilmann-Clausen, Dec 19, 2013
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    • "In the case of microorganisms, it has been said that microbial taxa will not exhibit endemism because their enormous populations remove dispersal as an effective constraint on geographical range [25]. The results from our research support previous studies with other microorganisms, including fungi, that there are very few truly cosmopolitan species [2], [17], [18]. Because T. discophora is a species complex that includes species with restricted geography and ecology, we hypothesize that the limited effective dispersal mechanisms over long geographic distances is affecting their distribution. "
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    ABSTRACT: The distribution of microbial species, including fungi, has long been considered cosmopolitan. Recently, this perception has been challenged by molecular studies in historical biogeography, phylogeny and population genetics. Here we explore this issue using the fungal morphological species Thelonectria discophora, one of the most common species of fungi in the family Nectriaceae, encountered in almost all geographic regions and considered as a cosmopolitan taxon. In order to determine if T. discophora is a single cosmopolitan species or an assemblage of sibling species, we conducted various phylogenetic analyses, including standard gene concatenation, Bayesian concordance methods, and coalescent-based species tree reconstruction on isolates collected from a wide geographic range. Results show that diversity among isolates referred as T. discophora is greatly underestimated and that it represents a species complex. Within this complex, sixteen distinct highly supported lineages were recovered, each of which has a restricted geographic distribution and ecology. The taxonomic status of isolates regarded as T. discophora is reconsidered, and the assumed cosmopolitan distribution of this species is rejected. We discuss how assumptions about geographically widespread species have implications regarding their taxonomy, true diversity, biological diversity conservation, and ecological functions.
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    • "Although the recent estimate of Mora et al. (2011) seems to be in line with May, most studies expect high fungal diversity (O'Brien et al., 2005; Bass & Richards, 2011; Blackwell, 2011). This is supported by the recent discovery of higher fungal taxa (Jones et al., 2011) and novel lineages (Amend et al., 2012), the extent of cryptic diversity within morphospecies (Carriconde et al., 2008; Crespo & Lumbsch, 2010), fungal habitat heterogeneity, the shear amount of unexplored habitats, limited primer inclusivity and PCR stochasticity in molecular studies (Schmidt et al., 2013), etc. Estimates of high global fungal diversity are also supported by the increasingly widespread next-generation sequencing metabarcoding studies (Hibbett et al., 2011). Nextgeneration metabarcoding frequently reveals high taxonomic divergence among geographically close samples (Danielsen et al., 2012; Xu et al., 2012; Schmidt et al., 2013). "
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    • "All these estimates clearly suggest that most of the fungal species are not described as yet. Some of the reasons for the low number of described species include: numerous fungi are microscopic and therefore inconspicuous ; in many cases they show few morphological characters to distinguish among species; and it has been demonstrated that cryptic species are common among different fungal groups (Geiser et al. 1998; Cruse et al. 2002; Pringle et al. 2005; Carriconde et al. 2008; Pavlic et al. 2009). The presence of cryptic species is especially common in lichenized fungi (Crespo & Perez-Ortega 2009; Crespo & Lumbsch 2010; Lumbsch & Leavitt 2011). "
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