Intercompatibility tests in the Hebeloma crustuliniforme complex in northwestern Europe

Sexual intercompatibility has been tested for members of the ectomycorrhizal Hebeloma crustuliniforme complex. Intercompatibility tests have been applied to 110 collections from northwestern Europe, for which monospore cultures were obtained. Crossing experiments indicated the existence of at least 20 intercompatibility groups (ICGs). All ICGs were bifactorial heterothallic with multiple mating type alleles. A low level of partial intercompatibility between ICGs was observed in two species clusters. One isolate was completely compatible with members of two ICGs (and partly compatible with one member of a third ICG) and could therefore not be assigned to an ICG. A large majority of these 20 ICGs could not be recognized by morphological characters, supporting claims that many basidiomycete morphospecies contain cryptic biological species. Possible explanations for this large number of ICGs that are not morphologically, ecologically or geographically delineated are discussed. A simplified strategy for executing intercompatibility tests with a high number of isolates is described. This procedure could substantially reduce the number of necessary matings.