High genetic diversity in arbuscular mycorrhizal fungi: evidence for recombination events

The genetic diversity of spores of two indigenous species of Glomus isolate d from three soils of a longterm field experiment amended by different quan tities of sewage sludges has been evaluated. Three populations of spores of Glomus claroideum (W2537) and three populations of spores of Glomus DAOM 2 25952 (W2538) were analysed using a microsatellite primer and aliquots of g enomic DNA were obtained from single spores (Inter Simple Sequence, Repeat (ISSR) fingerprints). 39 polymorphic bands were found for G. claroideum, an d 43 in Glomus DAOM 225952. The intraspecific diversity was high, ranging f rom 22 to 33 different electrophoretic types for G. claroideum. and 15-27 f or Glomus DAOM 225952 depending on the population. Resampling experiments s howed that the number of polymorphic bands was sufficient to score all mult ilocus profiles in the populations and to describe the clonality structure within populations. On average, one multilocus profile was represented by a bout four spores whatever the Population and the species. Partitioning of t he within-species phenotypic variance showed that more than 92% of the vari ation was found, within populations., while the among-population variance c omponent accounted for less than 8%, even though it was statistically diffe rent 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 popul ations of Glomus DAOM 225952. In addition to the high level of diversity ob served within populations, linkage disequilibria analyses and association i ndices calculated across loci indicates that reproduction cannot be solely clonal. Recombination or recombination-like events are likely to occur in t hese arbuscular mycorrhizal fungi. An 'epidemic' population structure was f ound for both fungal species in the soil that had received high amounts of sewage sludge.