I have undertaken an evolutionary and systematic study of fungi in the Collybia dryophila group (Basidiomycetes: Tricholomataceae). Species in the C. dryophila group occur worldwide as primary litter decomposers, yet little is known about their systematics.

In a previous study, the C. dryophila group in North America was shown to consist of four biological species, identifiable on the basis of mating compatibility and certain taxonomic features (Vilgalys, 1983). In this study, crosses made using isolates from over 80 individuals of the C. dryophila group from Europe indicate that there are four species from that continent as well. All four species are rigorously intersterile, while intra-specific matings occur freely. One exceptional case of reduced intra-group mating compatibility is reported, which may be the result of divergence due to geographic isolation.

Morphological studies indicate the four European biological species are all characterized by a high level of polymorphism, frequently involving features previously used to distinguish taxa. Because of this, diagnostic taxonomic features of the European species were reevaluated, and a revised taxonomy presented.

Representative tester strains from North America, Europe, Asia, and Australia were crossed to determine world-wide mating relationships in the C. dryophila group. Six major compatibility groups are identified on the basis of frequent mating compatibility. In several instances, populations of different geographic origin are mating-compatible, suggesting that these allopatric populations might be conspecific. Intercompatible allopatric populations are not always similar in their fruit body morphology, however, suggesting that extensive geographic differentiation has occurred in the C. dryophila group.

Genetic similarity of different populations was measured by the method of DNA hybridization. In general, there was good correspondence between mating compatibility and base sequence homology in the C. dryophila group. Allopatric, intercompatible populations, however, showed reduced homology when compared to that observed within sympatric populations. This suggests that geographic isolation may play a primary role in the evolution of different species in the C. dryophila group. Finally, a phylogeny based on DNA hybridization data is presented.

In summary, several lines of evidence suggest an allopatric mode for speciation in the C. dryophila group.