Correspondence between genet diversity and spatial distribution of above- and below-ground populations of the ectomycorrhizal fungus Hebeloma cylindrosporum

Population studies of ectomycorrhizal fungal species have largely relied up on fruit body (the reproductive organ) sampling. Analysis of the fruit bodi es alone supposes that they reflect the present and spatial organization of all below-ground genets (mycorrhizas and extramatrical mycelia). The relat ion between fruit bodies and ectomycorrhizas was investigated for the basid iomycete agaric Hebeloma cylindrosporum in four Pinus pinaster stands in so uth-west France. Genet identification was based on the comparison of polymo rphisms within a hypervariable segment of the ribosomal intergenic spacer a mplified by polymerase chain reaction (PCR) using a H. cylindrosporum speci es-specific primer. Mycorrhizas were sorted from soil samples collected und erneath patches of fruit bodies or patches where fruit bodies had or had no t been observed during the years prior to mycorrhiza collection. On average 65% of the 1026 mycorrhizas collected underneath fruit bodies were formed by H. cylindrosporum, whereas only 2% of the 954 collected in places from w here fruit bodies were absent were formed by this species. All genotypes id entified above ground were also identified below ground. In patches where o ne genotype formed all or more than 90% of the fruit bodies, the same genot ype formed all or a large majority of the mycorrhizas. In patches occupied by several different fruiting genotypes, additional nonfruiting ones could be present on the root systems. In all cases, the mycorrhizas of one genoty pe were found no more than 10-20 cm away from its corresponding fruit bodie s, and fruit body disappearance at a given place was associated with the di sappearance of the corresponding mycorrhizas within 1 year. Although there was not a strict coincidence between the total numbers of genets present be low ground and of those forming fruit bodies, fruit body analysis for H. cy lindrosporum appears to reflect both the genetic diversity and the spatial structure of its below-ground populations. The results obtained also illust rate the rapid turnover of ectomycorrhizal fungal species on the root syste ms in the absence of any obvious major disturbance of the ecosystem.