The lichenicolous basidiomycete fungus Marchandiomyces corallinus is widely distributed in North America and Europe. Unlike most lichenicolous fungi, M. corallinus attacks numerous host lichens. Theoretically, either of these characteristics, a wide geographic range or generalized host ecology, could provide ample opportunities for genetic differentiation within this species. To determine how genetic variation is partitioned in M. corallinus, fungi were isolated from several locations in North America and Europe, and also from different lichen hosts in one of these locations; levels of genetic differentiation were then estimated among these samples. Samples were obtained from four sites in North America (Arkansas, Missouri, Maryland, Virginia) and two in Europe (Ireland and Scotland). At a single site (Scotland), samples were obtained from three different lichen hosts. All isolates were used in vegetative mycelial compatibility tests, which provide evidence of genetic identity. In addition, a variety of nucleotide sequences from both nuclear and mitochondrial ribosomal genes were obtained for each isolate. Mycelial compatibility groups were recognizable based on geography, not the choice of lichen host. Sequence data tended to confirm this result; sequence differences among populations, where they existed at all, were observed among geographically distant populations, not different lichen hosts from the same site. These results suggest that genetic differentiation among populations of M. corallinus have developed as a consequence of geographic isolation, not host-switching.

Key words: basidiomycetes, lichenicolous fungi, lichens, Marchandiomyces corallinus