FINE-SCALE STRUCTURE OF POPULATIONS OF THE ECTOMYCORRHIZAL FUNGUS HEBELOMA-CYLINDROSPORUM IN COASTAL SAND DUNE FOREST ECOSYSTEMS

The basidiomycete mushroom Hebeloma cylindrosporum is a frequently fou nd pioneer ectomycorrhizal species naturally associated with Pinus pin aster trees growing in coastal sand dune ecosystems along the Atlantic south-west coast of France. The genotypic diversity and spatial struc ture of three populations of this fungal species have been studied. At each site the basidiocarps were mapped, sampled and propagated as pur e mycelial cultures. For each of the isolates, we have studied polymor phisms in the mitochondrial genome, polymorphisms at two different nuc lear loci and also fingerprints produced with a multicopy DNA probe. T he comparison of the different polymorphisms obtained, with each of th e four molecular methods used, allowed the identification of several o f the different genets present in each site. In two of the studied sit es most of the basidiocarps, which often occurred as dense patches of 10-30 in 1 m(2) or less, were of a unique genotype, suggesting the bel ow-ground mycelia to be of a small size (from 50 cm(2) to approx. 7 m( 2) 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 g enotypes identified in the first year of sampling was re-identified 3 years later. These results contrast with those reported for other spec ies of soilborne homobasidiomycete species, either ectomycorrhizal, pa rasitic or saprophytic, showing mostly large clones resulting from the vegetative growth and from persistence of belowground mycelia. Sexual reproduction through meiospore dispersal seems to play a key role in the structuring of the populations of a cylindrosporum. Mycelia associ ated with the root systems seem to be replaced after 1 or a few years, during which basidiocarp differentiation takes place. As opposed to t he few other studied ectomycorrhizal species, H. cylindrosporum has th e characteristics of ruderal species, with a short life-span adapted t o pioneer situations, e.g. to nutrient-poor and unstable sandy soils o f coastal sand dunes.