Population dynamics of the symbiotic mushroom Hebeloma cylindrosporum: mycelial persistence and inbreeding.

The pattern of colonization of a forest site by the ectomycorrhizal basidiomycete fungus Hebeloma cylindrosporum Romagnesi was followed from 1993 until 1997. Fruit-bodies of this tetrapolar heterothallic species were mapped, collected and propagated as pure mycelial cultures. Isolates were analysed for their mating-types and molecular markers (rDNA polymorphism and RAPD). Dedikaryotization of the 26 isolates collected in 1993 and the separate analysis of each individual haploid nucleus established that two fully compatible genets, which occupied two nonoverlapping territories, were present. Isolates belonging to the same genet could nevertheless be distinguished from each other based on Southern hybridization using hyperpolymorphic DNA probes. A majority of the 143 isolates collected from 1994 to 1997 belonged to either of the two genets identified in 1993, whose territories extended at a rate of about 0. 45-0.60 m per year. Selfing of the two 1993 genets, rather than outcrossing, was the most likely explanation for the origin of additional genotypes identified between 1995 and 1997. The spatial distribution of fruit-bodies and genets of H. cylindrosporum suggested that only a fraction of the sampled area was favourable to colonization and that genetic diversification through meiospore dispersal may be inhibited by the presence of resident genets, possibly via a somatic incompatibility system.