The basidiomycete mushroom Hebeloma cylindrosporum is a frequently found pioneer ectomycorrhizal species naturally associated with Pinus pinaster trees growing in coastal sand dune ecosystems along the Atlantic south‐west coast of France. The genotypic diversity and spatial structure of three populations of this fungal species have been studied. At each site the basidiocarps were mapped, sampled and propagated as pure mycelial cultures. For each of the isolates, we have studied polymorphisms in the mitochondrial genome, polymorphisms at two different nuclear loci and also fingerprints produced with a multicopy DNA probe. The comparison of the different polymorphisms obtained, with each of the four molecular methods used, allowed the identification of several of the different genets present in each site. In two of the studied sites most of the basidiocarps, which often occurred as dense patches of 10–30 in 1 m2 or less, were of a unique genotype, suggesting the below‐ground mycelia to be of a small size (from 50 cm2 to approx. 7 m2 for the larger mycelia) and that the root system of a single Pinus tree can host several genets of the same symbiotic fungus. In the two sites, which were studied again after a 3‐year interval, none of the genotypes identified in the first year of sampling was re‐identified 3 years later. These results contrast with those reported for other species of soilborne homobasidiomycete species, either ectomycorrhizal, parasitic or saprophytic, showing mostly large clones resulting from the vegetative growth and from persistence of below‐ground mycelia. Sexual reproduction through meiospore dispersal seems to play a key role in the structuring of the populations of H. cylindrosporum. Mycelia associated with the root systems seem to be replaced after 1 or a few years, during which basidiocarp differentiation takes place. As opposed to the few other studied ectomycorrhizal species, H. cylindrosporum has the characteristics of ruderal species, with a short life‐span adapted to pioneer situations, e.g. to nutrient‐poor and unstable sandy soils of coastal sand dunes.

To appreciate the functional diversity of communities of soil eukaryotic micro-organisms we evaluated an experimental approach based on the construction and screening of a cDNA library using polyadenylated mRNA extracted from a forest soil. Such a library contains genes that are expressed by each of the different organisms forming the community and represents its metatranscriptome. The diversity of the organisms that contributed to this library was evaluated by sequencing a portion of the 18S rDNA gene amplified from either soil DNA or reverse-transcribed RNA. More than 70% of the sequences were from fungi and unicellular eukaryotes (protists) while the other most represented group was the metazoa. Calculation of richness estimators suggested that more than 180 species could be present in the soil samples studied. Sequencing of 119 cDNA identified genes with no homologues in databases (32%) and genes coding proteins involved in different biochemical and cellular processes. Surprisingly, the taxonomic distribution of the cDNA and of the 18S rDNA genes did not coincide, with a marked under-representation of the protists among the cDNA. Specific genes from such an environmental cDNA library could be isolated by expression in a heterologous microbial host, Saccharomyces cerevisiae. This is illustrated by the functional complementation of a histidine auxotrophic yeast mutant by two cDNA originating possibly from an ascomycete and a basidiomycete fungal species. Study of the metatranscriptome has the potential to uncover adaptations of whole microbial communities to local environmental conditions. It also gives access to an abundant source of genes of biotechnological interest.

S \; M M .\ R YThe ectomyt'orrhizal activity (expressed as the number of mycorrhizas per plant) of lndole-^-acetic acid (IAA) overproducer mutants of Hebeluma cylindrosporum Romagnesi and of their mono-and dikaryotic progenies has been studied in an attempt to clarify the role of fungal I A.\ in the establishment of ecromycorrhizal symbiosis. The original mutants obtained from the hi nionokaryon produced a higher number of mycorrhizas than did the wild type. The study of the mycorrhizal activity of synthesized dikarv'ons heterozygous for the FIR (fluoroindoieresistant) mutations, which are responsible for IAA overproduction, showed that these mutations are recessive.The in vrtro fruiting of a dikaryon heterozygous for a FIR mutation allowed the isolation of I A.A-overproducing sib-monokaryons. Their average mycorrhizal activity was higher than that of non-o\erproducers, indicating that, although the mycorrhizal activity of sib-mnnokaryons is under polygenic control, high IAA production gives an ad\antage in mycorrhiza formation. This was confirmed by the higher number of myc(irrhi7,as formed by synthesized dikaryons homozygous for a FIR mutation than hy wild type mycelia.The mycorrhizas formed by the mono-or dikaryotic wild types were characterized by a uniseriate Hartig net, whereas it was pluriseriate in the case of mycorrhizas formed by IAA overproducer mutants. The mutants stimulated the growth of the host plants to the same extent as did the wild types, indicating that growth stimulation is not a direct consequence of fungal auxin production. The usefulness of these mutants as a model to study the role of fungal IAA in mycorrhiza formation is discussed.