Intercompatibility tests in the Hebeloma crustuliniforme complex in northwestern Europe

Sexual intercompatibility has been tested for members of the ectomycorrhiza l Hebeloma crustuliniforme complex. Intercompatibility tests have been appl ied to 110 collections from northwestern Europe, for which monospore cultur es were obtained. Crossing experiments indicated the existence of at least 20 intercompatibility groups (ICGs). All ICGs were bifactorial heterothalli c with multiple mating type alleles. A low level of partial intercompatibil ity between ICGs was observed in two species clusters. One isolate was comp letely 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 lar ge majority of these 20 ICGs could not be recognized by morphological chara cters, supporting claims that many basidiomycete morphospecies contain cryp tic biological species. Possible explanations for this large number of ICGs that are not morphologically, ecologically or geographically delineated ar e discussed. A simplified strategy for executing intercompatibility tests w ith a high number of isolates is described. This procedure could substantia lly reduce the number of necessary matings.