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Abstract
Phylogenetic relationships within the genus Hebeloma (Cortinariaceae, Agaricales) were determined, based on nuclear ribosomal ITS sequences, using cladistic methods. Special emphasis was on phylogenetic relationships within the H. crustuliniforme complex. In total 52 sequences were analysed, representing 51 collections and 39 taxa. Agrocybe praecox and two species of Alnicola were used as out-groups. The genus Hebeloma appears to be monophyletic. Several well supported clades could be recognized. However, many of the basal relationships are unresolved or only weakly supported. Alternative topologies could not be rejected. It is therefore impossible to derive a revised infrageneric classification of Hebeloma. The H. crustuliniforme complex appears paraphyletic, consisting of two clades with three and 17 intercompatibility groups respectively. In the second clade many of the phylogenetic relationships are also unresolved, reflecting a high rate of recent speciation events. Most of the species in this clade form ectomycorrhizae with members of the Salicaceae. The taxon that is basal to this clade, however, is not associated with these hosts. The host tree switch to Salicaceae has been followed by extensive and rapid speciation.