• arbuscular mycorrhizal (AM) fungi;
  • clonality;
  • genetic variation;
  • ISSR fingerprints;
  • recombination

The genetic diversity of spores of two indigenous species of Glomus isolated from three soils of a long-term field experiment amended by different quantities of sewage sludges has been evaluated. Three populations of spores of Glomus claroideum (W2537) and three populations of spores of Glomus DAOM 225952 (W2538) were analysed using a microsatellite primer and aliquots of genomic DNA were obtained from single spores (Inter Simple Sequence Repeat (ISSR) fingerprints). 39 polymorphic bands were found for G. claroideum, and 43 in Glomus DAOM 225952. The intraspecific diversity was high, ranging from 22 to 33 different electrophoretic types for G. claroideum, and 15–27 for Glomus DAOM 225952 depending on the population. Resampling experiments showed that the number of polymorphic bands was sufficient to score all multilocus profiles in the populations and to describe the clonality structure within populations. On average, one multilocus profile was represented by about four spores whatever the population and the species. Partitioning of the within-species phenotypic variance showed that more than 92% of the variation was found within populations, while the among-population variance component accounted for less than 8%, even though it was statistically different from 0. This result is confirmed by the fact that only few multilocus profiles were shared by two populations of G. claroideum, and none by populations of Glomus DAOM 225952. In addition to the high level of diversity observed within populations, linkage disequilibria analyses and association indices calculated across loci indicates that reproduction cannot be solely clonal. Recombination or recombination-like events are likely to occur in these arbuscular mycorrhizal fungi. An ‘epidemic’ population structure was found for both fungal species in the soil that had received high amounts of sewage sludge.