Karl-Henrik Larsson

Towards a unified paradigm for sequence‐based identification of fungi

The nuclear ribosomal internal transcribed spacer (ITS) region is the formal fungal barcode and in most cases the marker of choice for the exploration of fungal diversity in environmental samples. Two problems are particularly acute in the pursuit of satisfactory taxonomic assignment of newly generated ITS sequences: (i) the lack of an inclusive, reliable public reference data set and (ii) the lack of means to refer to fungal species, for which no Latin name is available in a standardized stable way. Here, we report on progress in these regards through further development of the UNITE database (http://unite.ut.ee) for molecular identification of fungi. All fungal species represented by at least two ITS sequences in the international nucleotide sequence databases are now given a unique, stable name of the accession number type (e.g. Hymenoscyphus pseudoalbidus|GU586904|SH133781.05FU), and their taxonomic and ecological annotations were corrected as far as possible through a distributed, third‐party annotation effort. We introduce the term ‘species hypothesis’ (SH) for the taxa discovered in clustering on different similarity thresholds (97–99%). An automatically or manually designated sequence is chosen to represent each such SH. These reference sequences are released (http://unite.ut.ee/repository.php) for use by the scientific community in, for example, local sequence similarity searches and in the QIIME pipeline. The system and the data will be updated automatically as the number of public fungal ITS sequences grows. We invite everybody in the position to improve the annotation or metadata associated with their particular fungal lineages of expertise to do so through the new Web‐based sequence management system in UNITE.

Global diversity and geography of soil fungi

Introduction The kingdom Fungi is one of the most diverse groups of organisms on Earth, and they are integral ecosystem agents that govern soil carbon cycling, plant nutrition, and pathology. Fungi are widely distributed in all terrestrial ecosystems, but the distribution of species, phyla, and functional groups has been poorly documented. On the basis of 365 global soil samples from natural ecosystems, we determined the main drivers and biogeographic patterns of fungal diversity and community composition. Direct and indirect effects of climatic and edaphic variables on plant and fungal richness. Line thickness corresponds to the relative strength of the relationships between the variables that affect species richness. Dashed lines indicate negative relationships. MAP, mean annual precipitation; Fire, time since last fire; Dist. equator, distance from the equator; Ca, soil calcium concentration; P, soil phosphorus concentration; pH, soil pH. Rationale We identified soil-inhabiting fungi using 454 Life Sciences (Branford, CN) pyrosequencing and through comparison against taxonomically and functionally annotated sequence databases. Multiple regression models were used to disentangle the roles of climatic, spatial, edaphic, and floristic parameters on fungal diversity and community composition. Structural equation models were used to determine the direct and indirect effects of climate on fungal diversity, soil chemistry, and vegetation. We also examined whether fungal biogeographic patterns matched paradigms derived from plants and animals—namely, that species’ latitudinal ranges increase toward the poles (Rapoport’s rule) and diversity increases toward the equator. Last, we sought group-specific global biogeographic links among major biogeographic regions and biomes using a network approach and area-based clustering. Results Metabarcoding analysis of global soils revealed fungal richness estimates approaching the number of species recorded to date. Distance from equator and mean annual precipitation had the strongest effects on richness of fungi, including most fungal taxonomic and functional groups. Diversity of most fungal groups peaked in tropical ecosystems, but ectomycorrhizal fungi and several fungal classes were most diverse in temperate or boreal ecosystems, and many fungal groups exhibited distinct preferences for specific edaphic conditions (such as pH, calcium, or phosphorus). Consistent with Rapoport’s rule, the geographic range of fungal taxa increased toward the poles. Fungal endemicity was particularly strong in tropical regions, but multiple fungal taxa had cosmopolitan distribution. Conclusions Climatic factors, followed by edaphic and spatial patterning, are the best predictors of soil fungal richness and community composition at the global scale. Richness of all fungi and functional groups is causally unrelated to plant diversity, with the exception of ectomycorrhizal root symbionts, suggesting that plant-soil feedbacks do not influence the diversity of soil fungi at the global scale. The plant-to-fungi richness ratio declined exponentially toward the poles, indicating that current predictions—assuming globally constant ratios—overestimate fungal richness by 1.5- to 2.5-fold. Fungi follow similar biogeographic patterns as plants and animals, with the exception of several major taxonomic and functional groups that run counter to overall patterns. Strong biogeographic links among distant continents reflect relatively efficient long-distance dispersal compared with macro-organisms. Fungi play major roles in ecosystem processes, but the determinants of fungal diversity and biogeographic patterns remain poorly understood. Using DNA metabarcoding data from hundreds of globally distributed soil samples, we demonstrate that fungal richness is decoupled from plant diversity. The plant-to-fungus richness ratio declines exponentially toward the poles. Climatic factors, followed by edaphic and spatial variables, constitute the best predictors of fungal richness and community composition at the global scale. Fungi show similar latitudinal diversity gradients to other organisms, with several notable exceptions. These findings advance our understanding of global fungal diversity patterns and permit integration of fungi into a general macroecological framework. Global metagenomics detects hotspots of fungal diversity and macroecological patterns and indicates that plant and fungal diversity are uncoupled. [Also see Perspective by Wardle and Lindahl] Assessing fungal diversity worldwide Fungi are hyperdiverse but poorly known, despite their ecological and economic impacts. Tedersoo et al. collected nearly 15,000 topsoil samples from 365 sites worldwide and sequenced their genomes (see the Perspective by Wardle and Lindahl). Overall, they found a striking decline in fungal species richness with distance from the equator. For some specialist groups though, diversity depended more on the abundance of host plants than host diversity or geography. The findings reveal a huge gap between known and described species and the actual numbers of distinct fungi in the worlds soils. Science, this issue 10.1126/science.1256688; see also p. 1052

Intraspecific ITS Variability in the Kingdom Fungi as Expressed in the International Sequence Databases and Its Implications for Molecular Species Identification

The internal transcribed spacer (ITS) region of the nuclear ribosomal repeat unit is the most popular locus for species identification and subgeneric phylogenetic inference in sequence-based mycological research. The region is known to show certain variability even within species, although its intraspecific variability is often held to be limited and clearly separated from interspecific variability. The existence of such a divide between intra- and interspecific variability is implicitly assumed by automated approaches to species identification, but whether intraspecific variability indeed is negligible within the fungal kingdom remains contentious. The present study estimates the intraspecific ITS variability in all fungi presently available to the mycological community through the international sequence databases. Substantial differences were found within the kingdom, and the results are not easily correlated to the taxonomic affiliation or nutritional mode of the taxa considered. No single unifying yet stringent upper limit for intraspecific variability, such as the canonical 3% threshold, appears to be applicable with the desired outcome throughout the fungi. Our results caution against simplified approaches to automated ITS-based species delimitation and reiterate the need for taxonomic expertise in the translation of sequence data into species names.

Taxonomic Reliability of DNA Sequences in Public Sequence Databases: A Fungal Perspective

Background DNA sequences are increasingly seen as one of the primary information sources for species identification in many organism groups. Such approaches, popularly known as barcoding, are underpinned by the assumption that the reference databases used for comparison are sufficiently complete and feature correctly and informatively annotated entries. Methodology/Principal Findings The present study uses a large set of fungal DNA sequences from the inclusive International Nucleotide Sequence Database to show that the taxon sampling of fungi is far from complete, that about 20% of the entries may be incorrectly identified to species level, and that the majority of entries lack descriptive and up-to-date annotations. Conclusions The problems with taxonomic reliability and insufficient annotations in public DNA repositories form a tangible obstacle to sequence-based species identification, and it is manifest that the greatest challenges to biological barcoding will be of taxonomical, rather than technical, nature.

The phylogenetic distribution of resupinate forms across the major clades of mushroom-forming fungi (Homobasidiomycetes)

Abstract Phylogenetic relationships of resupinate Homobasidiomycetes (Corticiaceae s. lat. and others) were studied using ribosomal DNA (rDNA) sequences from a broad sample of resupinate and nonresupinate taxa. Two datasets were analysed using parsimony, a ‘core’ dataset of 142 species, each of which is represented by four rDNA regions (mitochondrial and nuclear large and small subunits), and a ‘full’ dataset of 656 species, most of which were represented only by nuclear large subunit rDNA sequences. Both datasets were analysed using traditional heuristic methods with bootstrapping, and the full dataset was also analysed with the Parsimony Ratchet, using equal character weights and six‐parameter weighted parsimony. Analyses of both datasets supported monophyly of the eight major clades of Homobasidiomycetes recognised by Hibbett and Thorn, as well as independent lineages corresponding to the Gloeophyllum clade, corticioid clade and Jaapia argillacea. Analyses of the full dataset resolved two additional groups, the athelioid clade and trechisporoid clade (the latter may be nested in the polyporoid clade). Thus, there are at least 12 independent clades of Homobasidiomycetes. Higher‐level relationships among the major clades are not resolved with confidence. Nevertheless, the euagarics clade, bolete clade, athelioid clade and Jaapia argillacea are consistently resolved as a monophyletic group, whereas the cantharelloid clade, gomphoid‐phalloid clade and hymenochaetoid clade are placed at the base of the Homobasidiomycetes, which is consistent with the preponderance of imperforate parenthesomes in those groups. Resupinate forms occur in each of the major clades of Homobasidiomycetes, some of which are composed mostly or exclusively of resupinate forms (athelioid clade, corticioid clade, trechisporoid clade, Jaapia). The largest concentrations of resupinate forms occur in the polyporoid clade, russuloid clade and hymenochaetoid clade. The cantharelloid clade also includes many resupinate forms, including some that have traditionally been regarded as heterobasidiomycetes (Sebacinaceae, Tulasnellales, Ceratobasidiales). The euagarics clade, which is by far the largest clade in the Homobasidiomycetes, has the smallest fraction of resupinate species. Results of the present study are compared with recent phylogenetic analyses, and a table summarising the phylogenetic distribution of resupinate taxa is presented, as well as notes on the ecology of resupinate forms and related Homobasidiomycetes.

High phylogenetic diversity among corticioid homobasidiomycetes

Homobasidiomycetes display a variety of fruit body morphologies. Examples include gilled mushrooms, coral fungi, polypores and puffballs but also species with simple crust-like basidiomata, usually called corticioid fungi. The latter group has largely been neglected in recent studies of homobasidiomycete evolution. The major goal of the present study was to explore the impact that the addition of a wide selection of species with crust-like basidiomata would have on homobasidiomycete phylogeny. Two genes, 5.8S and 28S in the nuclear rDNA repeats, were sequenced and a data set with 178 taxa analysed using neighbour-joining and maximum parsimony methods. Support for clades was evaluated by bootstrap. Basal nodes generally received weak support and branching order for major clades remained largely unresolved. Twelve major groups were recovered and corticioid fungi make up a major or important constituent in most of them. Nine groups are strongly supported but support for euagarics and polyporoid clades is poor. Phlebioid fungi were in earlier studies merged with the polyporoid clade but are here identified as a separate clade. Athelia is allied with ectomycorrhizal genera, inter alia Piloderma and Amphinema, in a separate clade forming a sister group to the boletes. We conclude that corticioid fungi hold a considerable share of the phylogenetic diversity displayed by homobasidiomycetes, and should always be considered when phylogenetic studies of larger basidiomycetes are designed.

Diversity and abundance of resupinate thelephoroid fungi as ectomycorrhizal symbionts in Swedish boreal forests.

Resupinate thelephoroid fungi (hereafter called tomentelloid fungi) have a world‐wide distribution and comprise ≈70 basidiomycete species with inconspicuous, resupinate sporocarps. It is only recently that their ability to form ectomycorrhizas (EM) has been realized, so their distribution, abundance and significance as mycobionts in forest ecosystems is still largely unexplored. In order to provide baseline data for future ecological studies of tomentelloid fungi, we explored their presence and abundance in nine Swedish boreal forests in which the EM communities had been analysed. Phylogenetic analyses were used to compare the internal transcribed spacer of nuclear ribosomal DNA (ITS rDNA) sequence data obtained from mycobionts on single ectomycorrhizal tips with that obtained from sporocarps of identified tomentelloid fungi. Five species of Tomentella and one species of Pseudotomentella were identified as ectomycorrhizal fungi. The symbiotic nature of Tomentella bryophila, T. stuposa, T. badia and T. atramentaria is demonstrated for the first time. T. stuposa and Pseudotomentellatristis were the most commonly encountered tomentelloid fungi, with the other species, including T. sublilacina, only being recorded from single stands. Overall, tomentelloid fungi were found in five of the studies, colonizing between 1 and 8% of the mycorrhizal root tips. Two of the five sites supported several tomentelloid species. Tomentelloid fungi appear to be relatively common ectomycorrhizal symbionts with a wide distribution in Swedish coniferous forests. The results are in accordance with accumulating data that fungal species which lack conspicuous sporocarps may be of considerable importance in EM communities.

PlutoF—a Web Based Workbench for Ecological and Taxonomic Research, with an Online Implementation for Fungal ITS Sequences

DNA sequences accumulating in the International Nucleotide Sequence Databases (INSD) form a rich source of information for taxonomic and ecological meta-analyses. However, these databases include many erroneous entries, and the data itself is poorly annotated with metadata, making it difficult to target and extract entries of interest with any degree of precision. Here we describe the web-based workbench PlutoF, which is designed to bridge the gap between the needs of contemporary research in biology and the existing software resources and databases. Built on a relational database, PlutoF allows remote-access rapid submission, retrieval, and analysis of study, specimen, and sequence data in INSD as well as for private datasets though web-based thin clients. In contrast to INSD, PlutoF supports internationally standardized terminology to allow very specific annotation and linking of interacting specimens and species. The sequence analysis module is optimized for identification and analysis of environmental ITS sequences of fungi, but it can be modified to operate on any genetic marker and group of organisms. The workbench is available at http://plutof.ut.ee.

Phylogenetic and phylogenomic overview of the Polyporales

We present a phylogenetic and phylogenomic overview of the Polyporales. The newly sequenced genomes of Bjerkandera adusta, Ganoderma sp., and Phlebia brevispora are introduced and an overview of 10 currently available Polyporales genomes is provided. The new genomes are 39 500 000–49 900 00 bp and encode for 12 910–16 170 genes. We searched available genomes for single-copy genes and performed phylogenetic informativeness analyses to evaluate their potential for phylogenetic systematics of the Polyporales. Phylogenomic datasets (25, 71, 356 genes) were assembled for the 10 Polyporales species with genome data and compared with the most comprehensive dataset of Polyporales to date (six-gene dataset for 373 taxa, including taxa with missing data). Maximum likelihood and Bayesian phylogenetic analyses of genomic datasets yielded identical topologies, and the corresponding clades also were recovered in the 373-taxa dataset although with different support values in some datasets. Three previously recognized lineages of Polyporales, antrodia, core polyporoid and phlebioid clades, are supported in most datasets, while the status of the residual polyporoid clade remains uncertain and certain taxa (e.g. Gelatoporia, Grifola, Tyromyces) apparently do not belong to any of the major lineages of Polyporales. The most promising candidate single-copy genes are presented, and nodes in the Polyporales phylogeny critical for the suprageneric taxonomy of the order are identified and discussed.

Hymenochaetales: a molecular phylogeny for the hymenochaetoid clade.

The hymenochaetoid clade is dominated by wood-decaying species previously classified in the artificial families Corticiaceae, Polyporaceae and Stereaceae. The majority of these species cause a white rot. The polypore Bridgeoporus and several corticioid species with inconspicuous basidiomata live in association with brown-rotted wood, but their nutritional strategy is not known. Mycorrhizal habit is reported for Coltricia perennis but needs confirmation. A surprising element in the hymenochaetoid clade is a group of small white to brightly pigmented agarics earlier classified in Omphalina. They form a subclade together with some similarly colored stipitate stereoid and corticioid species. Several are associated with living mosses or one-celled green algae. Hyphoderma pratermissum and some related corticioid species have specialized organs for trapping and killing nematodes as a source of nitrogen. There are no unequivocal morphological synapomorphies known for the hymenochaetoid clade. However almost all species examined ultrastructurally have dolipore septa with continuous parenthesomes while perforate parenthesomes is the normal condition for other homobasidiomycete clades. The agaricoid Hymenochaetales have not been examined. Within Hymenochaetales the Hymenochaetaceae forms a distinct clade but unfortunately all morphological characters supporting Hymenochaetaceae also are found in species outside the clade. Other subclades recovered by the molecular phylogenetic analyses are less uniform, and the overall resolution within the nuclear LSU tree presented here is still unsatisfactory.