Death caps (Amanita phalloides) frequently establish from sexual spores, but individuals can grow large and live for more than a decade in invaded forests

Global change is reshaping Earth's biodiversity, but the changing distributions of nonpathogenic fungi remain largely undocumented, as do mechanisms enabling invasions. The ectomycorrhizal Amanita phalloides is native to Europe and invasive in North America. Using population genetics and genomics, we sought to describe the life history traits of this successfully invading symbiotic fungus. To test whether death caps spread underground using hyphae, or aboveground using sexual spores, we mapped and genotyped mushrooms from European and US sites. Larger genetic individuals (genets) would suggest spread mediated by vegetative growth, while many small genets would suggest dispersal mediated by spores. To test whether genets are ephemeral or persistent, we also sampled from populations over time. At nearly every site and across all time points, mushrooms resolve into small genets. Individuals frequently establish from sexual spores. But at one Californian site, a single individual measuring nearly 10 m across dominated. At two Californian sites, the same genetic individuals were discovered in 2004, 2014, and 2015, suggesting single individuals (both large and small) can reproduce repeatedly over relatively long timescales. A flexible life history strategy combining both mycelial growth and spore dispersal appears to underpin the invasion of this deadly perennial ectomycorrhizal fungus.

Biogeographic history of a large clade of ectomycorrhizal fungi, the Russulaceae, in the Neotropics and adjacent regions

The biogeography of neotropical fungi remains poorly understood. Here, we reconstruct the origins and diversification of neotropical lineages in one of the largest clades of ectomycorrhizal fungi in the globally widespread family Russulaceae. We inferred a supertree of 3285 operational taxonomic units, representing worldwide internal transcribed spacer sequences. We reconstructed biogeographic history and diversification and identified lineages in the Neotropics and adjacent Patagonia. The ectomycorrhizal Russulaceae have a tropical African origin. The oldest lineages in tropical South America, most with African sister groups, date to the mid-Eocene, possibly coinciding with a boreotropical migration corridor. There were several transatlantic dispersal events from Africa more recently. Andean and Central American lineages mostly have north-temperate origins and are associated with North Andean uplift and the general north-south biotic interchange across the Panama isthmus, respectively. Patagonian lineages have Australasian affinities. Diversification rates in tropical South America and other tropical areas are lower than in temperate areas. Neotropical Russulaceae have multiple biogeographic origins since the mid-Eocene involving dispersal and co-migration. Discontinuous distributions of host plants may explain low diversification rates of tropical lowland ectomycorrhizal fungi. Deeply diverging neotropical fungal lineages need to be better documented.

Facilitation of Balsam Fir by Trembling Aspen in the Boreal Forest: Do Ectomycorrhizal Communities Matter?

Succession is generally well described above-ground in the boreal forest, and several studies have demonstrated the role of interspecific facilitation in tree species establishment. However, the role of mycorrhizal communities for tree establishment and interspecific facilitation, has been little explored. At the ecotone between the mixed boreal forest, dominated by balsam fir and hardwood species, and the boreal forest, dominated by black spruce, several stands of trembling aspen can be found, surrounded by black spruce forest. Regeneration of balsam fir seems to have increased in the recent decades within the boreal forest, and it seems better adapted to grow in trembling aspen stands than in black spruce stands, even when located in similar abiotic conditions. As black spruce stands are also covered by ericaceous shrubs, we investigated if differences in soil fungal communities and ericaceous shrubs abundance could explain the differences observed in balsam fir growth and nutrition. We conducted a study centered on individual saplings to link growth and foliar nutrient concentrations to local vegetation cover, mycorrhization rate, and mycorrhizal communities associated with balsam fir roots. We found that foliar nutrient concentrations and ramification indices (colonization by mycorrhiza per length of root) were greater in trembling aspen stands and were positively correlated to apical and lateral growth of balsam fir saplings. In black spruce stands, the presence of ericaceous shrubs near balsam fir saplings affected ectomycorrhizal communities associated with tree roots which in turn negatively correlated with N foliar concentrations. Our results reveal that fungal communities observed under aspen are drivers of balsam fir early growth and nutrition in boreal forest stands and may facilitate ecotone migration in a context of climate change.

Hidden fungal diversity from the Neotropics: Geastrum hirsutum, G. schweinitzii (Basidiomycota, Geastrales) and their allies

Taxonomy of Geastrum species in the neotropics has been subject to divergent opinions among specialists. In our study, type collections were reassessed and compared with recent collections in order to delimit species in Geastrum, sect. Myceliostroma, subsect. Epigaea. A thorough review of morphologic features combined with barcode and phylogenetic analyses (ITS and LSU nrDNA) revealed six new species (G. neoamericanum, G. rubellum, G. brunneocapillatum, G. baculicrystallum, G. rubropusillum and G. courtecuissei). In additon, the presence of hairs on the exoperidium, a commonly used feature to diagnose Geastrum species, proved to be ineffective because it is a derived character within subsect. Epigaea.

Erratum: Author Correction: Evolutionary history of plant hosts and fungal symbionts predicts the strength of mycorrhizal mutualism

[This corrects the article DOI: 10.1038/s42003-018-0120-9.].

Evolutionary history of plant hosts and fungal symbionts predicts the strength of mycorrhizal mutualism

Most plants engage in symbioses with mycorrhizal fungi in soils and net consequences for plants vary widely from mutualism to parasitism. However, we lack a synthetic understanding of the evolutionary and ecological forces driving such variation for this or any other nutritional symbiosis. We used meta-analysis across 646 combinations of plants and fungi to show that evolutionary history explains substantially more variation in plant responses to mycorrhizal fungi than the ecological factors included in this study, such as nutrient fertilization and additional microbes. Evolutionary history also has a different influence on outcomes of ectomycorrhizal versus arbuscular mycorrhizal symbioses; the former are best explained by the multiple evolutionary origins of ectomycorrhizal lifestyle in plants, while the latter are best explained by recent diversification in plants; both are also explained by evolution of specificity between plants and fungi. These results provide the foundation for a synthetic framework to predict the outcomes of nutritional mutualisms.

Patterns of diversity, endemism and specialization in the root symbiont communities of alder species on the island of Corsica

We investigated whether the diversity, endemicity and specificity of alder symbionts could be changed by isolation in a Mediterranean glacial refugium. We studied both ectomycorrhizal (EM) fungi and nitrogen-fixing actinobacteria associated with alders, and compared their communities in Corsica and on the European continent. Nodules and root tips were sampled on the three alder species present in Corsica and continental France and Italy. Phylogenies based on internal transcribed spacer (ITS) and a multilocus sequence analysis approach were used to characterize fungal and Frankia species, respectively. Patterns of diversity, endemism and specialization were compared between hosts and regions for each symbiont community. In Corsica, communities were not generally richer than on the mainland. The species richness per site depended mainly on host identity: Alnus glutinosa and Alnus cordata hosted richer Frankia and EM communities, respectively. Half of the Frankia species were endemic to Corsica against only 4% of EM species. Corsica is not a hotspot of diversity for all alder symbionts but sustains an increased frequency of poor-dispersers such as hypogeous fungi. Generalist EM fungi and host-dependent profusely sporulating (Sp+) Frankia were abundantly associated with Corsican A. cordata, a pattern related to a more thermophilic and xerophylic climate and to the co-occurrence with other host trees.

Alder and the Golden Fleece: high diversity of Frankia and ectomycorrhizal fungi revealed from Alnus glutinosa subsp. barbata roots close to a Tertiary and glacial refugium

BACKGROUND

Recent climatic history has strongly impacted plant populations, but little is known about its effect on microbes. Alders, which host few and specific symbionts, have high genetic diversity in glacial refugia. Here, we tested the prediction that communities of root symbionts survived in refugia with their host populations. We expected to detect endemic symbionts and a higher species richness in refugia as compared to recolonized areas.

METHODS

We sampled ectomycorrhizal (EM) root tips and the nitrogen-fixing actinomycete Frankia communities in eight sites colonized by Alnus glutinosa subsp. barbata close to the Caucasus in Georgia. Three sites were located in the Colchis, one major Eurasian climatic refugia for Arcto-Tertiary flora and alders, and five sites were located in the recolonized zone. Endemic symbionts and plant ITS variants were detected by comparing sequences to published data from Europe and another Tertiary refugium, the Hyrcanian forest. Species richness and community structure were compared between sites from refugia and recolonized areas for each symbionts.

RESULTS

For both symbionts, most MOTUs present in Georgia had been found previously elsewhere in Europe. Three endemic Frankia strains were detected in the Colchis vs two in the recolonized zone, and the five endemic EM fungi were detected only in the recolonized zone. Frankia species richness was higher in the Colchis while the contrary was observed for EM fungi. Moreover, the genetic diversity of one alder specialist Alnicola xanthophylla was particularly high in the recolonized zone. The EM communities occurring in the Colchis and the Hyrcanian forests shared closely related endemic species.

DISCUSSION

The Colchis did not have the highest alpha diversity and more endemic species, suggesting that our hypothesis based on alder biogeography may not apply to alder's symbionts. Our study in the Caucasus brings new clues to understand symbioses biogeography and their survival in Tertiary and ice-age refugia, and reveals that isolated host populations could be of interest for symbiont diversity conservation.

MycoDB, a global database of plant response to mycorrhizal fungi

Plants form belowground associations with mycorrhizal fungi in one of the most common symbioses on Earth. However, few large-scale generalizations exist for the structure and function of mycorrhizal symbioses, as the nature of this relationship varies from mutualistic to parasitic and is largely context-dependent. We announce the public release of MycoDB, a database of 4,010 studies (from 438 unique publications) to aid in multi-factor meta-analyses elucidating the ecological and evolutionary context in which mycorrhizal fungi alter plant productivity. Over 10 years with nearly 80 collaborators, we compiled data on the response of plant biomass to mycorrhizal fungal inoculation, including meta-analysis metrics and 24 additional explanatory variables that describe the biotic and abiotic context of each study. We also include phylogenetic trees for all plants and fungi in the database. To our knowledge, MycoDB is the largest ecological meta-analysis database. We aim to share these data to highlight significant gaps in mycorrhizal research and encourage synthesis to explore the ecological and evolutionary generalities that govern mycorrhizal functioning in ecosystems.

What determines Alnus-associated ectomycorrhizal community diversity and specificity? A comparison of host and habitat effects at a regional scale

· Global-scale analyses of ectomycorrhizal (ECM) fungi communities emphasize host plant families as the main drivers of diversity. This study aims to test, on Alnus-ECM communities, which fungi are said to be 'host-specific', to what extent host species, habitat and distance explain their alpha and beta diversity variations, and their specificity. · In France, ECM communities associated with two subgenera and five species of Alnus, were sampled on 165 trees from 39 lowland to subalpine sites. In all, 1178 internal transcribed spacer (ITS) sequences of ECM fungi clustered in 86 molecular operational taxonomic units (MOTUs). · The species richness was low but still variable, and the evenness of communities was lower on organic soils and in Corsica. Similarity between communities was influenced both by host, soil parameters, altitude and longitude, but not by climate and distance. A large majority of 'specific' fungi were shared between host species within a subgenus, and showed habitat preferences within the subgenus distribution range. · Our study confirms that Alnus ECM communities are low in diversity, highly conserved at a regional scale, and partly shared between congeneric host species. A large part of alpha and beta diversity variations remained unexplained, and other processes may shape these communities.

Comparative phylogenies and host specialization in the alder ectomycorrhizal fungi Alnicola, Alpova and Lactarius (Basidiomycota) in Europe

BACKGROUND

Mycorrhizal fungi form intimate associations with their host plants that constitute their carbon resource and habitat. Alnus spp. (Betulaceae) are known to host an exceptional species-poor and specialized ectomycorrhizal (ECM) fungal community compared to other tree species, but the host-specificity pattern and its significance in terms of fungal diversification and speciation remain poorly documented. The degree of parallel speciation, host switching, and patterns of biogeography were explored in the historical associations between alders and three ECM taxa of Basidiomycetes: Alnicola (Agaricales), Alpova (Boletales), and Lactarius (Russulales). The aim was to develop an evolutionary framework on host specificity and diversification of Basidiomycetes in this highly specialized plant-fungus symbiosis.

RESULTS

Sporocarps of Alnicola (220), Lactarius (61) and Alpova (29) were collected from stands of the four European alder species (A. alnobetula including the endemic subsp. suaveolens in Corsica, A. cordata, A. glutinosa, A. incana) in Western Europe (mainly in France and Austria), from 1995 to 2009. Specimens were morphologically identified to the species level. From these, 402 sequences of four DNA regions (ITS, rpb2, gpd, and the V9 domain of the mit-SSU rDNA) were successfully obtained and analyzed in addition with 89 sequences available in GenBank and UNITE databases. Phylogenetic analyses were conducted on all sequence data sets (individual and combined) using maximum likelihood reconstruction and Bayesian inference. Fungal phylogenies are compared and discussed in relation to the host, with a focus on species boundaries by associating taxonomic, systematic and molecular information.

CONCLUSIONS

Patterns of host specificity and phylogenies of Alnicola and Lactarius suggest coevolution as a basal factor of speciation in relation with the subgeneric diversification of Alnus, possibly due to the very selective pressure of the host. A second element of the historical associations between Alnus and its fungal symbionts is a host-dependent speciation (radiation without host change), here observed in Alnicola and Alpova in relation with Alnus subgen. Alnus. Finally host shifts from Alnus subgen. Alnus to A. alnobetula are found in most lineages of Alnicola (at least four times), Alpova (twice) and Lactarius (once), but they do not represent such a common event as could be expected by geographic proximity of trees from the two subgenera. However, active or very recent host extensions clearly occurred in Corsica, where some fungi usually associated with Alnus glutinosa on mainland Europe locally extend there to A. alnobetula subsp. suaveolens without significant genetic or morphological deviation.

Phylogenetic species delimitation in ectomycorrhizal fungi and implications for barcoding: the case of the Tricholoma scalpturatum complex (Basidiomycota)

Population studies have revealed that the fungal ectomycorrhizal morphospecies Tricholoma scalpturatum consists of at least two genetically distinct groups that occur sympatrically in several geographical areas. This discovery prompted us to examine species boundaries and relationships between members formerly assigned to T. scalpturatum and allied taxa using phylogenetic analyses. Sequence data were obtained from three nuclear DNA regions [internal transcribed spacer (ITS), gpd and tef], from 101 carpophores collected over a large geographical range in Western Europe, and some reference sequences from public databases. The ITS was also tested for its applicability as DNA barcode for species delimitation. Four highly supported phylogenetic clades were detected. The two previously detected genetic groups of T. scalpturatum were assigned to the phylospecies Tricholoma argyraceum and T. scalpturatum. The two remaining clades were referred to as Tricholoma cingulatum and Tricholoma inocybeoides. Unexpectedly, T. cingulatum showed an accelerated rate of evolution that we attributed to narrow host specialization. This study also reveals recombinant ITS sequences in T. inocybeoides, suggesting a hybrid origin. The ITS was a useful tool for the determination of species boundaries: the mean value of intraspecific genetic distances in the entire ITS region (including 5.8S rDNA) was <0.2%, whereas interspecific divergence estimates ranged from 1.78% to 4.22%. Apart from giving insights into the evolution of the T. scalpturatum complex, this study contributes to the establishment of a library of taxonomically verified voucher specimens, an a posteriori correlation between phenotype and genotype, and DNA barcoding of ectomycorrhizal fungi.

Isozyme Variation among 40 Frankia Strains

Forty Frankia strains belonging to the Alnus and Elaeagnus host specificity groups and isolated from various plant species from different geographical areas were characterized by the electrophoretic separation of isozymes of eight enzymes. All the enzyme systems that were investigated showed large variation. Diaphorases and esterases gave multiple band patterns and confirmed the identification of specific Frankia strains. Less variability was observed with enzymes such as phosphoglucose isomerase, leucine aminopeptidase, and malate dehydrogenase, which allowed for the delineation of larger groups of Frankia strains. Cluster analysis, based on the pair-wise similarity coefficients calculated between strains, delineated three large, dissimilar groups of Frankia strains, although each of these groups contained a large amount of heterogeneity. However, numerous Frankia strains, mainly from the Alnus host specificity group, demonstrated a perfect homology for all the enzymes tested.

Population evidence of cryptic species and geographical structure in the cosmopolitan ectomycorrhizal fungus, Tricholoma scalpturatum

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.

Molecular evolution of mitochondrial ribosomal DNA in the fungal genus Tricholoma: barcoding implications

The molecular evolution of the V6 and V9 domains of the mitochondrial SSU-rDNA was investigated to evaluate the use of these sequences for DNA barcodes in the Basidiomycota division. The PCR products from 27 isolates belonging to 11 Tricholoma species were sequenced. Both domains in the isolates belonging to the same species had identical sequences. All the species possess distinctive V9 sequences due to point mutations and insertion/deletion events. Secondary structures revealed that the insertion-deletion events occurred in regions not directly involved in the maintenance of the standard SSU-rRNA structure. The inserted sequences possess conserved motifs that enable their alignment among phylogenetically distant species. Hence, the V9 domain by displaying identical sequences within species, an adequate divergence level, easy amplification, and alignment represents an alternative molecular marker for the Basidiomycota division and opens the way for this sequence to be used as specific molecular markers of the fungal kingdom.

Population dynamics of the ectomycorrhizal fungal species Tricholoma populinum and Tricholoma scalpturatum associated with black poplar under differing environmental conditions

Fungi combine sexual reproduction and clonal propagation. The balance between these two reproductive modes affects establishment dynamics, and ultimately the evolutionary potential of populations. The pattern of colonization was studied in two species of ectomycorrhizal fungi: Tricholoma populinum and Tricholoma scalpturatum. The former is considered to be a host specialist whereas T. scalpturatum is a generalist taxon. Fruit bodies of both basidiomycete species were mapped and collected over several years from a black poplar (Populus nigra) stand, at two different sites. Multilocus genotypes (= genets) were identified based on the analysis of random amplified polymorphic DNA (RAPD) patterns, inter-simple sequence repeat (ISSR) patterns and restriction fragment length polymorphisms (RFLPs) in the ribosomal DNA intergenic spacer (rDNA IGS). The genetic analyses revealed differences in local population dynamics between the two species. Tricholoma scalpturatum tended to capture new space through sexual spores whereas T. populinum did this by clonal growth, suggesting trade-offs in allocation of resources at the genet level. Genet numbers and sizes strongly differ between the two study sites, perhaps as a result of abiotic disturbance on mycelial establishment and genet behaviour.

Phylogeny of the ectomycorrhizal mushroom genus Alnicola (Basidiomycota, Cortinariaceae) based on rDNA sequences with special emphasis on host specificity and morphological characters

Alnicola (=Naucoria, pro parte) is a mushroom genus of strictly temperate, obligately ectomycorrhizal species, traditionally included in the family Cortinariaceae. Most Alnicola spp. are primarily host specific on Alnus, although a few are mycobionts of Salix or other hosts. The different species of Alnicola exhibit unique morphological (cystidia, pileipellis) and cytological (dikaryotic or monokaryotic hyphae) characters. This makes the genus Alnicola of particular interest for studying the evolution of host specificity and morphological characters in ectomycorrhizal basidiomycetes. We used a combination of classical morphological and phylogenetic methods (rDNA ITS and LSU sequences) to address the following questions: (i) Is Alnicola monophyletic? And (ii) Are characters like host specificity or microscopical structures synapomorphic for certain clades? The study included nearly all currently known European Alnicola sp. Our results demonstrated that, on one hand, the genus Alnicola is polyphyletic, with sistergroup relationships to Hebeloma, Anamika or the clades /Hymenogaster I and /Hymenogaster II. On the other hand, Alnicola splits into three well-supported clades corresponding to the sections Alnicola, Submelinoides, and Salicicolae. The strict host-specificity to Alnus is a derived character and has occurred at least twice. The following morphological characters are synapomorphic for defined clades: the spindle-shaped hymenial cystidia for sect. Alnicola, the hymeniform pileipellis for sect. Submelinoides, and monocaryotic/clampless hyphae for sect. Salicicolae and its sistergroup /Hymenogaster II. As a taxonomical consequence, polyphyly of Alnicola implies that the sects. Submelinoides and Salicicolae need to be segregated from Alnicola.

Diversity and specificity of ectomycorrhizal fungi retrieved from an old-growth Mediterranean forest dominated by Quercus ilex

We analysed the ectomycorrhizal (ECM) fungal diversity in a Mediterranean old-growth Quercus ilex forest stand from Corsica (France), where Arbutus unedo was the only other ECM host. On a 6400 m2 stand, we investigated whether oak age and host species shaped below-ground ECM diversity. Ectomycorrhizas were collected under Q. ilex individuals of various ages (1 yr seedlings; 3-10 yr saplings; old trees) and A. unedo. They were typed by ITS-RFLP analysis and identified by match to RFLP patterns of fruitbodies, or by sequencing. A diversity of 140 taxa was found among 558 ectomycorrhizas, with many rare taxa. Cenococcum geophilum dominated (35% of ECMs), as well as Russulaceae, Cortinariaceae and Thelephoraceae. Fungal species richness was comparable above and below ground, but the two levels exhibited < 20% overlap in fungal species composition. Quercus ilex age did not strongly shape ECM diversity. The two ECM hosts, A. unedo and Q. ilex, tended to share few ECM species (< 15% of the ECM diversity). Implications for oak forest dynamics are discussed.

Diversity and fruiting patterns of ectomycorrhizal and saprobic fungi in an old-growth Mediterranean forest dominated by Quercus ilex L

We collected and mapped epigeous fruitbodies of both ectomycorrhizal (ECM) and saprobic fungi in an old-growth Quercus ilex L. Mediterranean forest within a permanent transect of 6400 m2 over three consecutive fruiting seasons. Out of 5382 fruitbodies, a total of 234 species were found, including 166 and 68 ECM and saprobic taxa, respectively. Both communities were mainly composed of rare species. Two genera, Russula and Cortinarius, accounted for 34.4% of ECM fruitbodies and 50% of species diversity. The three most abundant ECM species were Laccaria laccata (Scop.: Fr.) Berk. & Broome, Inocybe tigrina R. Heim, and Lactarius chrysorrheus Fr. The fruiting ECM community encompassed a few Mediterranean species and numerous broad host range temperate species. We also analysed the fruiting patterns in relation to forest structure, host composition, and natural canopy gaps. The results showed (i) a significant correlation of species richness to tree density, (ii) a richness decrease as the number of vegetation layers increases, and (iii) a preferential fruiting of some species near Q. ilex or Arbutus unedo L. Another noteworthy feature was that richness and production were greatly enhanced in canopy gaps. Selective fruiting was also observed among species. These results highlight the importance of forest structure and large woody debris for fungal conservation.Key words: ECM community, saprophytic fungi, holm oak, macromycete fruiting patterns, canopy gaps, fungal conservation.

Genetic diversity in Tetrachaetum elegans, a mitosporic aquatic fungus

Tetrachaetum elegans Ingold is a saprobic aquatic hyphomycete for which no sexual stage has yet been described. It occurs most commonly during the initial decay of tree leaves in temperate freshwater habitats and typically sporulates under water. Dispersal of the aquatic fungus takes place primarily in the water column and has a large passive component. Differences in substrate composition (e.g. quality of leaf litter) may also play a role in the distribution of different species or genotypes. The population genetic structure of T. elegans was studied using amplified fragment length polymorphism (AFLP) multilocus fingerprints. The populations were isolated from the leaf litter of three different tree genera, sampled in nine streams distributed throughout a mixed deciduous forest. Molecular markers were developed for 97 monosporic isolates using four selective primer pairs. A total of 247 fragments were scored, of which only 32 were polymorphic. Significant stream differentiation was detected for the isolates considered in this study. Analysis of molecular variance revealed that 20% of the genetic variation observed was the result of differences between streams. No correlation between genetic and geographical distances was found but a few multilocus genotypes were observed in different locations. Altogether these results suggest that environmental barriers play a role in the population structure of this aquatic fungus. No clear-cut effect of leaf litter composition on genetic variation could be demonstrated. Finally, tests of linkage disequilibrium between the 32 polymorphic AFLP loci as well as simulations did not provide a final answer regarding clonality in T. elegans. Indeed, it was possible to reject linkage equilibrium at different sampling levels and show that full linkage was unlikely.

Recent advances in exploring physiology and biodiversity of ectomycorrhizas highlight the functioning of these symbioses in ecosystems

Ectomycorrhizas, the dominating mycorrhizal symbiosis in boreal, temperate and some tropical forests, are formed by 5000-6000 species of the asco- and basidiomycetes. This high diversity of fungal partners allows optimal foraging and mobilisation of various nitrogen and phosphorus forms from organic soil layers. In this review, two approaches to study the functioning of this multitude of symbiotic associations are presented. On selected culture models, physiological and molecular investigations have shown that the supply of hexoses has a key function in controlling the plant-fungus interaction via partner-specific regulation of gene expression. Environmental factors which affect fungal carbon supply, such as increased nitrogen availability, also affect mycorrhiza formation. Based on such laboratory results, the adaptative capability of ectomycorrhizas to changing field conditions is discussed. The second approach consists of analysing the distribution of mycorrhizas in ecosystem compartments and to relate distribution patterns to variations of ecological factors. Recent advances in identification of fungal partners in ectomycorrhizas by analysing the internal transcribed spacer of ribosomal DNA are presented, which can help to resolve sampling problems in field studies. The limits of the laboratory and the field approaches are discussed. Despite some problems, this combined approach is the most promising. Direct investigation of gene expression, which has been introduced for soil bacteria, will be difficult in the case of mycorrhizal fungi which constitute organisms with functionally varying structures.

P53 germline mutations in childhood cancers and cancer risk for carrier individuals

The family history of cancer in children treated for a solid malignant tumour in the Paediatric Oncology Department at Institute Gustave-Roussy, has been investigated. In order to determine the role of germline p53 mutations in genetic predisposition to childhood cancer, germline p53 mutations were sought in individuals with at least one relative (first- or second-degree relative or first cousin) affected by any cancer before 46 years of age, or affected by multiple cancers. Screening for germline p53 mutation was possible in 268 index cases among individuals fulfilling selection criteria. Seventeen (6.3%) mutations were identified, of which 13 were inherited and four were de novo. Using maximum likelihood methods that incorporate retrospective family data and correct for ascertainment bias, the lifetime risk of cancer for mutation carriers was estimated to be 73% for males and nearly 100% for females with a high risk of breast cancer accounting for the difference. The risk of cancer associated with such mutations is very high and no evidence of low penetrance mutation was found. These mutations are frequently inherited but de novo mutations are not rare.

Community structure of ectomycorrhizal fungi in a Pinus muricata forest: above- and below-ground views

We examined the species diversity of an ectomycorrhizal community in natural stands of bishop pine (Pinus muricata D. Don) to determine the correspondence between above- and below-ground views of species composition, spatial frequency, and abundance. We addressed this question by simultaneously sampling fruit bodies and ectomycorrhizae over a 4-year period. By using molecular methods based on polymerase chain reaction, we were able to identify the fungal symbionts directly from mycorrhizae in nearly all of the mycorrhizal morphotypes we found. Most species were either rare or low in abundance. Among the common species we observed three patterns: (i) some species, such as Russula xerampelina s.l. and Amanita francheti were well represented both above- and below-ground; (ii) some common fruiting species such as Suillus pungens were rare components belowground; (iii) some species that were common as mycorrhizae were represented poorly or unrepresented in the aboveground fruiting record. The latter was the case for Russula amoenolens, thelephoroid types (i.e., Tomentella sublilacina and thelephoroid-2), and a boletoid type. These results show that (i) the pattern of resource allocation to production of fruit bodies versus ectomycorrhizae varied among species, and (ii) the correspondence between above- and below-ground is imprecise at best at the community level. Keywords: mycorrhizal types, basidiocarps, sporocarps, fungal community, ecology of fungi, PCR, molecular ecology.

Germ-line exclusion of a single p53 allele by premature termination of translation in a Li-Fraumeni syndrome family

Germline p53 mutations have been detected in approximately half of the families affected by the Li-Fraumeni syndrome (LFS), in which they are believed to represent the genetic status predisposing to multiple cancers. Failure to detect mutations in the other half of LFS families suggests that sequence analysis, which has been limited to the p53 gene coding region, have overlooked other genetic events lying outside of this region or/and that alterations in other gene(s) than p53 may also lead to the syndrome. In this report, we present the evidence that a single base pair deletion in the p53 coding sequence, leading to premature signal termination of translation, generates a null allele by preventing transport of mutant allele mRNAs into the cytoplasm. This allelic exclusion which confers a status of unizygote vis-à-vis the wild-type p53 gene to individuals who carry the mutant allele, leads to predisposition to multiple cancers in a Li-Fraumeni family. Thus, the loss of the wild-type p53 allele appears as the rate limiting step in tumor induction.

Molecular tools for the identification of ectomycorrhizal fungi--taxon-specific oligonucleotide probes for suilloid fungi

Five taxon-specific oligonucleotide probes are described that can be used to help identify the fungal components of ectomycorrhizae. Comparisons among partial sequence from the mitochondrial large subunit rRNA gene (mt-LrRNA) were used to select the probes, which were intended to be specific to several taxa within the suilloid group of the Boletales (Basidiomycota). Probes S1, R1, and G1 were targeted at the genera Suillus, Rhizopogon and Gomphidius; probe G2 was designed to recognize the family, Gomphidiaceae, and probe US1 was designed to recognize all of these taxa and any other members of the suilloid group. The specificity of each probe was determined empirically by testing their ability to hybridize to PCR amplified fragments derived from 84 species of basidiomycetes. Although none of the probes exhibited their intended specificity, all specifically hybridized to useful subsets of taxa, and collectively they can be used to identify many suilloid taxa to the generic level or below. The probes were also tested for their ability to identify field collected mycorrhizae and were found to perform well.

ITS primers with enhanced specificity for basidiomycetes--application to the identification of mycorrhizae and rusts

We have designed two taxon-selective primers for the internal transcribed spacer (ITS) region in the nuclear ribosomal repeat unit. These primers, ITS1-F and ITS4-B, were intended to be specific to fungi and basidiomycetes, respectively. We have tested the specificity of these primers against 13 species of ascomycetes, 14 of basidiomycetes, and 15 of plants. Our results showed that ITS4-B, when paired with either a 'universal' primer ITS1 or the fungal-specific primer ITS1-F, efficiently amplified DNA from all basidiomycetes and discriminated against ascomycete DNAs. The results with plants were not as clearcut. The ITS1-F/ITS4-B primer pair produced a small amount of PCR product for certain plant species, but the quantity was in most cases less than that produced by the 'universal' ITS primers. However, under conditions where both plant and fungal DNAs were present, the fungal DNA was amplified to the apparent exclusion of plant DNA. ITS1-F/ITS4-B preferential amplification was shown to be particularly useful for detection and analysis of the basidiomycete component in ectomycorrhizae and in rust-infected tissues. These primers can be used to study the structure of ectomycorrhizal communities or the distribution of rusts on alternate hosts.

Screening for germ line p53 mutations in children with malignant tumors and a family history of cancer

We have undertaken a routine investigation of the p53 status for all the children treated at our institution either affected by multiple tumors or whose family displays at least one second degree relative or less, affected by cancer before the age of 45 years. We report here on the first set of ten such families, eight of which were identified through a proband with sarcoma. p53 exons 5 to 8 have been sequenced following polymerase chain reaction amplification performed on DNA isolated from total blood. A missense mutation affecting codons 248, 273, and 282 was identified in three families. The mutation was inherited in these three families and was detected in unaffected members. In seven families no mutation was detected in exons 5 to 8.

Early gene expression in lymphoma-associated hamster polyomavirus viral genomes

Hamster polyomavirus (HaPV) is the causal agent of hair follicle epithelioma in hamsters belonging to a colony bred in Berlin-Buch. These tumors shed virus particles that are assembled in the keratinized layer of the epidermis. By contrast, HaPV induces lymphomas after inoculation into newborn hamsters from a distinct colony bred in Potsdam. These lymphoid tumors accumulate massive amounts of episomal viral genomes characterized by deletions that alter specifically the regulatory and the late coding sequences. Assuming that these alterations of the regulatory region may affect the transcription of the viral oncogenes in the tumor cells, the transcriptional activity of the wild-type and deleted early promoters have been studied in vitro in transient chloramphenicol acetyltransferase (CAT) expression assays. These assays performed in various cell types demonstrate that both versions of the HaPV early promoter carry a weak constitutive activity. Simultaneous expression of the HaPV early gene products leads to a strong stimulation of CAT activity with a concomitant activation of the replication of the plasmid constructs. The results obtained with origin-defective CAT vectors indicate that the replication contributes significantly to the stimulating effect of the early gene products. Indeed, transfection of massive amounts of CAT vectors that are unable to replicate can simulate the dosage effect of replication and also leads to measurable CAT activities. Under these conditions, the wild-type promoter is more active than the deleted version, indicating that sequences within the deletion carry a distinct stimulatory effect on transcription. This conclusion is supported by the observation that the lymphoma cells contain a low level of early transcripts, indicating that the deleted episomal viral templates accumulated in these tumors carry a weak transcriptional activity.

Identification of indigenous and introduced symbiotic fungi in ectomycorrhizae by amplification of nuclear and mitochondrial ribosomal DNA

We used the polymerase chain reaction (PCR) to amplify specific regions of the nuclear and mitochondrial genomes of fungi using DNA extracted from pure cultures as well as that directly from ectomycorrhizal rootlets. The internal transcribed spacer (ITS) of the nuclear ribosomal repeat unit and a portion of the mitochondrial large subunit ribosomal RNA gene were chosen as target sequences because both exist in high copy number and amplification primers for both discriminate between plant and fungal DNAs. These features provided a sensitivity and specificity sufficient for detection and analysis of a single mycorrhizal rootlet. We evaluated the variations in the amplified products with regard to the length, restriction endonuclease sites, and primary sequence for use in identification of genera, species, and strains of ectomycorrhizal fungi, with particular attention to selected Laccaria species. Accidental contamination of jack pine seedlings by Telephora terrestris was easily recognized. Amplification and direct DNA sequencing of a portion of the ITS were done for three strains of L. bicolor, one of L. laccata, one of L. proximo, and one of T. terrestris. The nucleotide sequence variation was 32% between L. bicolor and T. terrestris, and it ranged from 3 to 5% among the three Laccaria species examined and from 1 to 2% within L. bicolor. The degree of variation observed is sufficient to allow the use of specific oligonucleotides to characterize amplified ITS products. To demonstrate the feasibility of this approach we designed and tested a probe that enabled two isolates of L. bicolor to be distinguished by a single base-pair difference in a filter-based hybridization assay. In combination these methods now provide an important set of tools for the study of mycorrhizal ecology. Key words: internal transcribed spacer, LrDNA gene, mycorrhizal ecology, polymerase chain reaction, rDNA.

Properties of simian virus 40 mutants lacking the Asp4-Glu-Asp stretch at the carboxyl-terminus of large T antigen

The biological activity carried by the carboxy-terminal domain of SV40 large T antigen has been investigated by isolating mutants deleted for a stretch of six acidic residues which by analogy with polyoma middle T antigen might be essential for the activity of the protein. We have constructed an "in-phase" deletion of 37 residues that includes the complete acid residues cluster. In order to parallel the polyoma hr-t mutants genotype, the deletion was introduced in virus strains either competent or defective for the small t antigen. We conclude from these experiments that the deletion of this unusual sequence does not affect per se any of the known biological properties of the virus.

Immortalization of rodent embryo fibroblasts by SV40 is maintained by the A gene

SV40 can induce proliferation of primary cells from various origins and by making them escape from senescence confer a potential of unlimited division characteristic of established cell lines. We have studied this effect called immortalization with various mutants of the SV40 early genes. The results establish that the immortalized phenotype is controlled by the A gene in a reversible fashion. Cells immortalized at 33 degrees by a tsA mutant, revert to a "mortal" or senescent-like phenotype after temperature shift up to 39 degrees. The nature of these striking phenotypic changes is discussed.

Viable deletion mutants in the simian virus 40 early region

For the purpose of isolating hr-t-like mutants of simian virus 40, we have constructed variants that have lost the unique site for the restriction enzyme Taq I at 0.565. Five mutants have been isolated and characterized by restriction enzyme analysis. All of them produce a normal size T antigen. Four produce a t antigen reduced in size as well as in amount; the fifth one does not seem to make any t antigen at all. The ability of these mutants to transform mouse cells in vitro, as tested by anchorage dependence, is clearly altered; however, the defect is only partial. In the same test, the mutants can complement a tsA mutant for transformation and therefore define a second complementation group in the simian virus 40 early region.

[Inhibiting effects of protein fractions isolated from the rat liver in compensating hypertrophy after partial hepatectomy]

We have purified from adult rat liver proteic fractions, which inhibit the cellular growth of cell cultures. These fractions, when they are injected into hepatectomised rats, were able to inhibit the entry of hepatic cells into phase S. Therefore we consider that these substances participate actively in homeostatic control in adult liver.