Sample Preparation for Fungal Community Analysis by High-Throughput Sequencing of Barcode Amplicons

Fungal species participate in vast numbers of processes in the landscape around us. However, their cryptic mycelial growth, inside various substrates and in highly diverse species assemblages, has been a major obstacle to thorough analysis of fungal communities, hampering exhaustive description of the fungal kingdom. Technological developments allowing rapid, high-throughput sequencing of mixed communities from many samples at once are currently having a tremendous impact in fungal community ecology. Universal DNA extraction followed by amplification and sequencing of fungal species-level barcodes such as the nuclear internal transcribed spacer (ITS) region now enables identification and relative quantification of fungal community members across well-replicated experimental settings.Here, we present the sample preparation procedure presently used in our laboratory for fungal community analysis by high-throughput sequencing of amplified ITS2 markers. We focus on the procedure optimized for studies of total fungal communities in humus-rich soils, wood, and litter. However, this procedure can be applied to other sample types and markers. We focus on the laboratory-based part of sample preparation, i.e., the procedure from the point where samples enter the laboratory until amplicons are submitted for sequencing. Our procedure comprises four main parts: (1) universal DNA extraction, (2) optimization of PCR conditions, (3) production of tagged ITS amplicons, and (4) preparation of the multiplexed amplicon pool to be sequenced. The presented procedure is independent of the specific high-throughput sequencing technology used, which makes it highly versatile.

Comparative analyses of the Hymenoscyphus fraxineus and Hymenoscyphus albidus genomes reveals potentially adaptive differences in secondary metabolite and transposable element repertoires

Background

The dieback epidemic decimating common ash (Fraxinus excelsior) in Europe is caused by the invasive fungus Hymenoscyphus fraxineus. In this study we analyzed the genomes of H. fraxineus and H. albidus, its native but, now essentially displaced, non-pathogenic sister species, and compared them with several other members of Helotiales. The focus of the analyses was to identify signals in the genome that may explain the rapid establishment of H. fraxineus and displacement of H. albidus.

Results

The genomes of H. fraxineus and H. albidus showed a high level of synteny and identity. The assembly of H. fraxineus is 13 Mb longer than that of H. albidus’, most of this difference can be attributed to higher dispersed repeat content (i.e. transposable elements [TEs]) in H. fraxineus. In general, TE families in H. fraxineus showed more signals of repeat-induced point mutations (RIP) than in H. albidus, especially in Long-terminal repeat (LTR)/Copia and LTR/Gypsy elements.

Comparing gene family expansions and 1:1 orthologs, relatively few genes show signs of positive selection between species. However, several of those did appeared to be associated with secondary metabolite genes families, including gene families containing two of the genes in the H. fraxineus-specific, hymenosetin biosynthetic gene cluster (BGC).

Conclusion

The genomes of H. fraxineus and H. albidus show a high degree of synteny, and are rich in both TEs and BGCs, but the genomic signatures also indicated that H. albidus may be less well equipped to adapt and maintain its ecological niche in a rapidly changing environment.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-021-07837-2.

Comparative genomics highlights the importance of drug efflux transporters during evolution of mycoparasitism in Clonostachys subgenus Bionectria (Fungi, Ascomycota, Hypocreales)

Various strains of the mycoparasitic fungal species Clonostachys rosea are used commercially as biological control agents for the control of fungal plant diseases in agricultural crop production. Further improvements of the use and efficacy of C. rosea in biocontrol require a mechanistic understanding of the factors that determines the outcome of the interaction between C. rosea and plant pathogenic fungi. Here, we determined the genome sequences of 11 Clonostachys strains, representing five species in Clonostachys subgenus Bionectria, and performed a comparative genomic analysis with the aim to identify gene families evolving under selection for gene gains or losses. Several gene families predicted to encode proteins involved in biosynthesis of secondary metabolites, including polyketide synthases, nonribosomal peptide syntethases and cytochrome P450s, evolved under selection for gene gains (p ≤ .05) in the Bionectria subgenus lineage. This was accompanied with gene copy number increases (p ≤ .05) in ATP-binding cassette (ABC) transporters and major facilitator superfamily (MFS) transporters predicted to contribute to drug efflux. Most Clonostachys species were also characterized by high numbers of auxiliary activity (AA) family 9 lytic polysaccharide monooxygenases, AA3 glucose-methanol-choline oxidoreductases and additional carbohydrate-active enzyme gene families with putative activity (or binding) towards xylan and rhamnose/pectin substrates. Particular features of the C. rosea genome included expansions (p ≤ .05) of the ABC-B4 multidrug resistance transporters, the ABC-C5 multidrug resistance-related transporters and the 2.A.1.3 drug:H + antiporter-2 MFS drug resistance transporters. The ABC-G1 pleiotropic drug resistance transporter gene abcG6 in C. rosea was induced (p ≤ .009) by exposure to the antifungal Fusarium mycotoxin zearalenone (1121-fold) and various fungicides. Deletion of abcG6 resulted in mutants with reduced (p < .001) growth rates on media containing the fungicides boscalid, fenhexamid and iprodione. Our results emphasize the role of biosynthesis of, and protection against, secondary metabolites in Clonostachys subgenus Bionectria.

Estimating the Fitness Effect of Deleterious Mutations During the Two Phases of the Life Cycle: A New Method Applied to the Root-Rot Fungus Heterobasidion parviporum

Many eukaryote species, including taxa such as fungi or algae, have a lifecycle with substantial haploid and diploid phases. A recent theoretical model predicts that such haploid-diploid lifecycles are stable over long evolutionary time scales when segregating deleterious mutations have stronger effects in homozygous diploids than in haploids and when they are partially recessive in heterozygous diploids. The model predicts that effective dominance-a measure that accounts for these two effects-should be close to 0.5 in these species. It also predicts that diploids should have higher fitness than haploids on average. However, an appropriate statistical framework to conjointly investigate these predictions is currently lacking. In this study, we derive a new quantitative genetic model to test these predictions using fitness data of two haploid parents and their diploid offspring, and genome-wide genetic distance between haploid parents. We apply this model to the root-rot basidiomycete fungus Heterobasidion parviporum-a species where the heterokaryotic (equivalent to the diploid) phase is longer than the homokaryotic (haploid) phase. We measured two fitness-related traits (mycelium growth rate and the ability to degrade wood) in both homokaryons and heterokaryons, and we used whole-genome sequencing to estimate nuclear genetic distance between parents. Possibly due to a lack of power, we did not find that deleterious mutations were recessive or more deleterious when expressed during the heterokaryotic phase. Using this model to compare effective dominance among haploid-diploid species where the relative importance of the two phases varies should help better understand the evolution of haploid-diploid life cycles.

Out in the Cold: Identification of Genomic Regions Associated With Cold Tolerance in the Biocontrol Fungus Clonostachys rosea Through Genome-Wide Association Mapping

There is an increasing importance for using biocontrol agents in combating plant diseases sustainably and in the long term. As large scale genomic sequencing becomes economically viable, the impact of single nucleotide polymorphisms (SNPs) on biocontrol-associated phenotypes can be easily studied across entire genomes of fungal populations. Here, we improved a previously reported genome assembly of the biocontrol fungus Clonostachys rosea strain IK726 using the PacBio sequencing platform, which resulted in a total genome size of 70.7 Mbp and 21,246 predicted genes. We further performed whole-genome re-sequencing of 52 additional C. rosea strains isolated globally using Illumina sequencing technology, in order to perform genome-wide association studies in conditions relevant for biocontrol activity. One such condition is the ability to grow at lower temperatures commonly encountered in cryic or frigid soils in temperate regions, as these will be prevalent for protecting growing crops in temperate climates. Growth rates at 10°C on potato dextrose agar of the 53 sequenced strains of C. rosea were measured and ranged between 0.066 and 0.413 mm/day. Performing a genome wide association study, a total of 1,478 SNP markers were significantly associated with the trait and located in 227 scaffolds, within or close to (< 1000 bp distance) 265 different genes. The predicted gene products included several chaperone proteins, membrane transporters, lipases, and proteins involved in chitin metabolism with possible roles in cold tolerance. The data reported in this study provides a foundation for future investigations into the genetic basis for cold tolerance in fungi, with important implications for biocontrol.

Different Alleles of a Gene Encoding Leucoanthocyanidin Reductase (PaLAR3) Influence Resistance against the Fungus Heterobasidion parviporum in Picea abies

Despite the fact that fungal diseases are a growing menace for conifers in modern silviculture, only a very limited number of molecular markers for pathogen resistance have been validated in conifer species. A previous genetic study indicated that the resistance of Norway spruce (Picea abies) to Heterobasidion annosum s.l., a pathogenic basidiomycete species complex, is linked to a quantitative trait loci that associates with differences in fungal growth in sapwood (FGS) that includes a gene, PaLAR3, which encodes a leucoanthocyanidin reductase. In this study, gene sequences showed the presence of two PaLAR3 allelic lineages in P. abies. Higher resistance was associated with the novel allele, which was found in low frequency in the four P. abies populations that we studied. Norway spruce plants carrying at least one copy of the novel allele showed a significant reduction in FGS after inoculation with Heterobasidion parviporum compared to their half-siblings carrying no copies, indicating dominance of this allele. The amount of (+) catechin, the enzymatic product of PaLAR3, was significantly higher in bark of trees homozygous for the novel allele. Although we observed that the in vitro activities of the enzymes encoded by the two alleles were similar, we could show that allele-specific transcript levels were significantly higher for the novel allele, indicating that regulation of gene expression is responsible for the observed effects in resistance, possibly caused by differences in cis-acting elements that we observe in the promoter region of the two alleles.

Sample Preparation for Fungal Community Analysis by High-Throughput Sequencing of Barcode Amplicons

Fungal species participate in vast numbers of processes in the landscape around us. However, their often cryptic growth, inside various substrates and in highly diverse species assemblages, has been a major obstacle to thorough analysis of fungal communities, hampering exhaustive description of the fungal kingdom. Recent technological developments allowing rapid, high-throughput sequencing of mixed communities from many samples at once are currently having a tremendous impact in fungal community ecology. Universal DNA extraction followed by amplification and sequencing of fungal species-level barcodes such as the nuclear internal transcribed spacer (ITS) region now enable identification and relative quantification of fungal community members across well-replicated experimental settings. Here, we present the sample preparation procedure presently used in our laboratory for fungal community analysis by high-throughput sequencing of amplified ITS2 markers. We focus on the procedure optimized for studies of total fungal communities in humus-rich soils, wood, and litter. However, this procedure can be applied to other sample types and markers. We focus on the laboratory-based part of sample preparation, that is, the procedure from the point where samples enter the laboratory until amplicons are submitted for sequencing. Our procedure comprises four main parts: (1) universal DNA extraction, (2) optimization of PCR conditions, (3) production of tagged ITS amplicons, and (4) preparation of the multiplexed amplicon mix to be sequenced. The presented procedure is independent of the specific high-throughput sequencing technology used, which makes it highly versatile.

Transcriptional Responses Associated with Virulence and Defence in the Interaction between Heterobasidion annosum s.s. and Norway Spruce

Heterobasidion annosum sensu lato is a serious pathogen causing root and stem rot to conifers in the northern hemisphere and rendering the timber defective for sawing and pulping. In this study we applied next-generation sequencing to i) identify transcriptional responses unique to Heterobasidion-inoculated Norway spruce and ii) investigate the H. annosum transcripts to identify putative virulence factors. To address these objectives we wounded or inoculated 30-year-old Norway spruce clones with H. annosum and 454-sequenced the transcriptome of the interaction at 0, 5 and 15 days post inoculation. The 491860 high-quality reads were de novo assembled and the relative expression was analysed. Overall, very few H. annosum transcripts were represented in our dataset. Three delta-12 fatty acid desaturase transcripts and one Clavaminate synthase-like transcript, both associated with virulence in other pathosystems, were found among the significantly induced transcripts. The analysis of the Norway spruce transcriptional responses produced a handful of differentially expressed transcripts. Most of these transcripts originated from genes known to respond to H. annosum. However, three genes that had not previously been reported to respond to H. annosum showed specific induction to inoculation: an oxophytodienoic acid–reductase (OPR), a beta–glucosidase and a germin-like protein (GLP2) gene. Even in a small data set like ours, five novel highly expressed Norway spruce transcripts without significant alignment to any previously annotated protein in Genbank but present in the P. abies (v1.0) gene catalogue were identified. Their expression pattern suggests a role in defence. Therefore a more complete survey of the transcriptional responses in the interactions between Norway spruce and its major pathogen H. annosum would probably provide a better understanding of gymnosperm defence than accumulated until now.

Root-associated fungi of Rosa rugosa grown on the frontal dunes of the Baltic Sea Coast in Lithuania

The aim of the present study was to assess fungal communities associated with fine living roots of Rosa rugosa Thunb grown on the frontal dunes of Curonian Spit at the Baltic Sea coast in Lithuania. The roots of R. rugosa were sampled at five sites (Nida, Preila, Pervalka, Juodkrante and Smiltyne) situated at a distance ca. 5-15 km from each other. Direct amplification, cloning and sequencing of fungal ITS rRNA from the fine roots resulted in 134 high-quality sequences, representing 31 fungal taxa among which saprotrophs and endophytes Mycena sp. (14.2 %), Tumularia sp. (14.2 %), Penicillium spinulosum (11.9 %) and Cadophora malorum (9.0 %) were most common. Arbuscular mycorrhizal fungi including Entrophospora baltica (0.7 %) and Rhizophagus irregularis (0.7 %) and potentially root pathogenic fungi--Ceratobasidium sp. (4.5 %), Fusarium oxysporum (3.0 %), Fusarium culmorum (0.7 %) and Ilyonectria crassa (0.7 %)--were also detected at low proportions. In conclusion, the results demonstrated that the fine roots of R. rugosa are inhabited by various groups of fungi. Although saprotrophs and endophytes were dominant, the detection of arbuscular mycorrhizal fungi indicated that these may be important for mineral nutrition of R. rugosa established on dry and poor fertility coastal dunes.

Extensive trans-specific polymorphism at the mating type locus of the root decay fungus Heterobasidion

Incompatibility systems in which individuals bearing identical alleles reject each other favor the maintenance of a diversity of alleles. Mushroom mating type loci (MAT) encode for dozens or hundreds of incompatibility alleles whose loss from the population is greatly restricted through negative frequency selection, leading to a system of alleles with highly divergent sequences. Here, we use DNA sequences of homeodomain (HD) encoding genes at the MAT locus of five closely related species of the root rot basidiomycete Heterobasidion annosum sensu lato to show that the extended coalescence time of MAT alleles greatly predates speciation in the group, contrasting loci outside of MAT that show allele divergences largely consistent with the species phylogeny with those of MAT, which show rampant trans-species polymorphism. We observe a roughly 6-fold greater genealogical depth and polymorphism of MAT compared with non-MAT that argues for the maintenance of balanced polymorphism for a minimum duration of 24 My based on a molecular-clock calibrated species phylogeny. As with other basidiomycete HD genes, balancing selection appears to be concentrated at the specificity-determining region in the N-terminus of the protein based on identification of codons under selection and the absence of recombination within the region. However, the elevated polymorphism extends into the nonspecificity determining regions as well as a neighboring non-MAT gene, the mitochondrial intermediate peptidase (MIP). In doing so, increased divergence should decrease recombination among alleles and as a by-product create incompatibilities in the functional domains not involved in allele recognition but in regulating sexual development.

New primers to amplify the fungal ITS2 region--evaluation by 454-sequencing of artificial and natural communities

With recent methodological advances, molecular markers are increasingly used for semi-quantitative analyses of fungal communities. The aim to preserve quantitative relationships between genotypes through PCR places new demands on primers to accurately match target sites and provide short amplicons. The internal transcribed spacer (ITS) region of the ribosome encoding genes is a commonly used marker for many fungal groups. Here, we describe three new primers - fITS7, gITS7 and fITS9, which may be used to amplify the fungal ITS2 region by targeting sites in the 5.8S encoding gene. We evaluated the primers and compared their performance with the commonly used ITS1f primer by 454-sequencing of both artificially assembled templates and field samples. When the entire ITS region was amplified using the ITS1f/ITS4 primer combination, we found strong bias against species with longer amplicons. This problem could be overcome by using the new primers, which produce shorter amplicons and better preserve the quantitative composition of the template. In addition, the new primers yielded more diverse amplicon communities than the ITS1f primer.

Chemical and transcriptional responses of Norway spruce genotypes with different susceptibility to Heterobasidion spp. infection

BACKGROUND

Norway spruce [Picea abies (L.) Karst.] is one of the most important conifer species in Europe. The wood is economically important and infections by wood-rotting fungi cause substantial losses to the industry.The first line of defence in a Norway spruce tree is the bark. It is a very efficient barrier against infection based on its mechanical and chemical properties. Once an injury or an infection is recognized by the tree, induced defences are activated. In this study we examined transcriptional response, using 454-sequencing, and chemical profiles in bark of Norway spruce trees with different susceptibility to Heterobasidion annosum s.l. infection. The aim was to find associations between the transcriptome and chemical profiles to the level of susceptibility to Heterobasidion spp. in Norway spruce genotypes.

RESULTS

Both terpene and phenol compositions were analysed and at 28 days post inoculation (dpi) high levels of 3-carene was produced in response to H. annosum. However, significant patterns relating to inoculation or to genotypes with higher or lower susceptibility could only be found in the phenol fraction. The levels of the flavonoid catechin, which is polymerized into proanthocyanidins (PA), showed a temporal variation; it accumulated between 5 and 15 dpi in response to H. annosum infection in the less susceptible genotypes. The transcriptome data suggested that the accumulation of free catechin was preceded by an induction of genes in the flavonoid and PA biosynthesis pathway such as leucoanthocyanidin reductase. Quantitative PCR analyses verified the induction of genes in the phenylpropanoid and flavonoid pathway. The qPCR data also highlighted genotype-dependent differences in the transcriptional regulation of these pathways.

CONCLUSIONS

The varying dynamics in transcriptional and chemical patterns displayed by the less susceptible genotypes suggest that there is a genotypic variation in successful spruce defence strategies against Heterobasidion. However, both high levels of piceasides and flavonoids in the less susceptible genotypes suggested the importance of the phenolic compounds in the defence. Clearly an extended comparison of the transcriptional responses in the interaction with Heterobasidion between several independent genotypes exhibiting reduced susceptibility is needed to catalogue mechanisms of successful host defence strategies.

New genetic markers for identifying Cronartium flaccidum and Peridermium pini and examining genetic variation within and between lesions of Scots pine blister rust in Sweden

Microsatellite markers were developed as an identification tool and for analysis of the genetic variation in the pathogens Cronartium flaccidum and Peridermium pini, causing Scots pine blister rust in Pinus spp. Six reference aeciospore samples from Finland were used to examine genetic differences between the two pathogens. Genetic variation within and between 27 lesions on Scots pines from seven locations in Sweden was also investigated. Aeciospores were collected from single aecia within the lesions. Reference samples from P. pini were homozygous for all seven microsatellite loci investigated, while the three C. flaccidum samples contained heterozygous loci. These results confirm previous studies, where homozygous aeciospores were indicated to be characteristic for P. pini. The majority of aeciospores had two nuclei in both heterozygotic and homozygotic samples. Five of the Swedish lesions contained only homozygotic aecia, while the aecia in the remaining 22 lesions were heterozygotic. All lesions with homozygotic aecia contained only one single multilocus genotype, while many of the lesions with heterozygotic aecia contained several genotypes. The latter indicates the occurrence of multiple matings within a lesion between the resident spermogonia and alien fertilizing spermatia.

Archaeorhizomycetes: unearthing an ancient class of ubiquitous soil fungi

Estimates suggest that only one-tenth of the true fungal diversity has been described. Among numerous fungal lineages known only from environmental DNA sequences, Soil Clone Group 1 is the most ubiquitous. These globally distributed fungi may dominate below-ground fungal communities, but their placement in the fungal tree of life has been uncertain. Here, we report cultures of this group and describe the class, Archaeorhizomycetes, phylogenetically placed within subphylum Taphrinomycotina in the Ascomycota. Archaeorhizomycetes comprises hundreds of cryptically reproducing filamentous species that do not form recognizable mycorrhizal structures and have saprotrophic potential, yet are omnipresent in roots and rhizosphere soil and show ecosystem and host root habitat specificity.

The plant cell wall-decomposing machinery underlies the functional diversity of forest fungi

Brown rot decay removes cellulose and hemicellulose from wood--residual lignin contributing up to 30% of forest soil carbon--and is derived from an ancestral white rot saprotrophy in which both lignin and cellulose are decomposed. Comparative and functional genomics of the "dry rot" fungus Serpula lacrymans, derived from forest ancestors, demonstrated that the evolution of both ectomycorrhizal biotrophy and brown rot saprotrophy were accompanied by reductions and losses in specific protein families, suggesting adaptation to an intercellular interaction with plant tissue. Transcriptome and proteome analysis also identified differences in wood decomposition in S. lacrymans relative to the brown rot Postia placenta. Furthermore, fungal nutritional mode diversification suggests that the boreal forest biome originated via genetic coevolution of above- and below-ground biota.

Comparative molecular evolution of trichoderma chitinases in response to mycoparasitic interactions

Certain species of the fungal genus Trichoderma are potent mycoparasites and are used for biological control of fungal diseases on agricultural crops. In Trichoderma, whole-genome sequencing reveal between 20 and 36 different genes encoding chitinases, hydrolytic enzymes that are involved in the mycoparasitic attack. Sequences of Trichoderma chitinase genes chi18-5, chi18-13, chi18-15 and chi18-17, which all exhibit specific expression during mycoparasitism-related conditions, were determined from up to 13 different taxa and studied with regard to their evolutionary patterns. Two of them, chi18-13 and chi18-17, are members of the B1/B2 chitinase subgroup that have expanded significantly in paralog number in mycoparasitic Hypocrea atroviridis and H. virens. Chi18-13 contains two codons that evolve under positive selection and seven groups of co-evolving sites. Chi18-15 displays a unique codon-usage and contains five codons that evolve under positive selection and three groups of co-evolving sites. Regions of high amino acid variability are preferentially localized to substrate- or product side of the catalytic clefts. Differences in amino acid diversity/conservation patterns between different Trichoderma clades are observed. These observations show that Trichoderma chitinases chi18-13 and chi18-15 evolve in a manner consistent with rapid co-evolutionary interactions and identifies putative target regions involved in determining substrate-specificity.

Fungi vectored by the bark beetle Ips typographus following hibernation under the bark of standing trees and in the forest litter

The bark beetle Ips typographus has different hibernation environments, under the bark of standing trees or in the forest litter, which is likely to affect the beetle-associated fungal flora. We isolated fungi from beetles, standing I. typographus-attacked trees, and forest litter below the attacked trees. Fungal identification was done using cultural and molecular methods. The results of the two methods in detecting fungal species were compared. Fungal communities associated with I. typographus differed considerably depending on the hibernation environment. In addition to seven taxa of known ophiostomoid I. typographus-associated fungi, we detected 18 ascomycetes and anamorphic fungi, five wood-decaying basidomycetes, 11 yeasts, and four zygomycetes. Of those, 14 fungal taxa were detected exclusively from beetles that hibernated under bark, and six taxa were detected exclusively from beetles hibernating in forest litter. The spruce pathogen, Ceratocystis polonica, was detected occasionally in bark, while another spruce pathogen, Grosmannia europhioides, was detected more often from beetles hibernating under the bark as compared to litter. The identification method had a significant impact on which taxa were detected. Rapidly growing fungal taxa, e.g. Penicillium, Trichoderma, and Ophiostoma, dominated pure culture isolations; while yeasts dominated the communities detected using molecular methods. The study also demonstrated low frequencies of tree pathogenic fungi carried by I. typographus during its outbreaks and that the beetle does not require them to successfully attack and kill trees.

First Report of Canker and Dieback Disease of Grevillea robusta in East Africa Caused by Botryosphaeria spp

An outbreak of a canker and dieback disease was observed for the first time on Grevillea robusta in Uganda during October of 2001. Disease symptoms included dieback of shoots and branches, lesions and canker formation on the stems, clear or yellow-to-red exudates on stems and branches, formation of clusters of small and deformed leaves, and sapling and tree mortality. Isolations were made from the inner bark and wood of stems and branches in the Masaka and Rakai districts of Uganda in March and August to October of 2003. Samples from the border between healthy and necrotic tissues were plated on potato dextrose agar, vegetable juice agar, and water agar as well as on selective Phytophthora medium. Fungal isolates from several of the symptomatic trees were identified as belonging to the genus Botryosphaeria on the basis of morphological characteristics. Botryosphaeria-like fungi were not isolated from healthy trees. The internal transcribed spacer regions of the ribosomal DNA, as well as parts of the beta-tubulin and elongation factor 1-alpha genes, were sequenced from eight of these isolates. Five isolates matched Botryosphaeria parva in GenBank and one isolate matched B. rhodina. Two isolates matched an unidentified Botryosphaeria sp. previously isolated from Eucalyptus grandis in Uganda (GenBank Accession Nos. AY228102 and AY226852). Pathogenicity tests were performed by inoculating 10 18-month-old G. robusta seedlings with each isolate. A 10-mm³ malt agar plug with mycelium was inserted into a cut in the bark and the wound was sealed using grafting tape. One month after inoculation, the number of seedlings displaying death from the point of inoculation to the top (TM) and whole seedling mortality (SM) were recorded. Six of the isolates produced pathogenic responses in the seedlings: Botryosphaeria sp. isolate 1 (TM 10, SM 2), Botryosphaeria sp. isolate 2 (TM 10, SM 1), B. parva isolate 1 (TM 6, SM 1), B. parva isolate 2 (TM 3, SM 1), B. parva isolate 3 (TM 3, SM 0), and B. parva isolate 4 (TM1, SM 0). Inoculation with B. parva isolate 5 and B. rhodina, as well as sterile agar plugs, resulted in no TM or SM. The canker and dieback disease seems to be widespread in the East African Region since typical symptoms have been observed on G. robusta in Kenya (February 2004) and Ethiopia (February 2006). Botryosphaeria spp. are known to have multiple hosts and have been isolated from several other species in this region, including Eucalyptus spp. (1,2). From these findings, it appears that the same species of Botryosphaeria, which are previously known to be pathogenic on Eucalyptus, also cause serious disease on G. robusta. References: (1) A. Gezahgne et al. S. Afr. J. Sci. 99:29, 2003. (2) J. Roux et al. S. Afr. J. Sci. 97:16, 2001.

Spatial separation of litter decomposition and mycorrhizal nitrogen uptake in a boreal forest

Our understanding of how saprotrophic and mycorrhizal fungi interact to re-circulate carbon and nutrients from plant litter and soil organic matter is limited by poor understanding of their spatiotemporal dynamics. In order to investigate how different functional groups of fungi contribute to carbon and nitrogen cycling at different stages of decomposition, we studied changes in fungal community composition along vertical profiles through a Pinus sylvestris forest soil. We combined molecular identification methods with 14C dating of the organic matter, analyses of carbon:nitrogen (C:N) ratios and 15N natural abundance measurements. Saprotrophic fungi were primarily confined to relatively recently (< 4 yr) shed litter components on the surface of the forest floor, where organic carbon was mineralized while nitrogen was retained. Mycorrhizal fungi dominated in the underlying, more decomposed litter and humus, where they apparently mobilized N and made it available to their host plants. Our observations show that the degrading and nutrient-mobilizing components of the fungal community are spatially separated. This has important implications for biogeochemical studies of boreal forest ecosystems.

Wood-inhabiting fungal communities in woody debris of Norway spruce (Picea abies (L.) Karst.), as reflected by sporocarps, mycelial isolations and T-RFLP identification

Wood-inhabiting fungi play a key role in forest ecosystems and constitute an essential part of forest biodiversity. We therefore examined the composition and abundance of wood-inhabiting fungi by three methods: sporocarp counts, mycelial culturing and direct amplification of internal transcribed spacer terminal restriction fragment length polymorphism from wood combined with sequencing of reference rDNA. Seven-year-old slash piles left after a thinning were analyzed in a 50-year-old Norway spruce plantation. Fifty-eight fungal species were detected from the piled branches and treetops. More species were revealed by sporocarp counts and cultured mycelia than by direct amplification from wood. In principle, sporocarp monitoring may reveal all fruiting taxa, but it poorly reflects their relative abundance in the wood. In contrast, terminal restriction fragment length polymorphism will record the most frequent fungal taxa in the wood, but it may overlook uncommon taxa. Culturing mycelia from wood gives a bias towards species favoured by the cultural medium. The results demonstrate the advantage and the limitations of these methods to be considered in analyses of fungal communities in wood.

Double-stranded RNA transmission through basidiospores of Heterobasidion annosum

A search for double-stranded RNA (dsRNA) was conducted among 106 isolates of the pathogenic basidiomycete Heterobasidion annosum. Of these isolates, 47 were tissue isolates from fruit bodies and 59 were isolated from decayed wood. Nucleic acids were extracted from freeze-dried mycelia and dsRNA was separated by cellulose CF-11 chromatography and confirmed by digestions with specific nucleases. dsRNA was present in 19 and 14% of the tissue and wood isolates, respectively. From five of the fruit bodies containing dsRNA basidiospores were investigated and 10-84% of the germinated basidiospores contained dsRNA. On high nutrient media, the germination frequency of basidiospores was reduced by presence of dsRNA in the fruit body (P < 0.05); germination frequencies were 34 and 78% for spores from fruit bodies with and without dsRNA, respectively. The same trend was present also on low nutrient media, although not statistically significant; germination was 3 and 10% for spores from infected and dsRNA free fruit bodies, respectively. Transmission of dsRNA in H. annosum from mycelia into basidiospores together with the lowered germination frequency are likely to play a significant role in the life cycle of the fungus. The relative importance of different transmission routes for dsRNA in H. annosum is discussed.

Transmission of double-stranded RNA in Heterobasidion annosum

Transmission of dsRNA viruses between homo- and heterokaryotic mycelia paired on agar plates and into conidia has been studied in Heterobasidion annosum. Horizontal transmission of dsRNA occurred between both homo- and heterokaryotic isolates, as well as between isolates belonging to different intersterility groups. The proportions of vertical transmission into conidia were 3% and 55%, respectively, for the two isolates included in the study. RT-PCR of dsRNA and PCR-RFLP of mitochondrial markers were used to confirm transmission of dsRNA between the cytoplasms of different mycelia. The identity of nuclei and nuclear migration during experiments were verified using PCR-RFLP of several nuclear markers.