Phylogeography of Cavernularia hultenii: evidence of slow genetic drift in a widely disjunct lichen.

Population structure and history is poorly known in most lichenized ascomycetes. Many species display large-scale infraspecific disjunctions, which have been explained alternately by range fragmentation in species of high age and widespread long-distance dispersal. Using the lichen Cavernularia hultenii, which is widely disjunct across North America and Europe, Pleistocene and Holocene population history was inferred. The internal transcribed spacer (ITS) and part of the the intergenic spacer (IGS) region of the nuclear ribosomal DNA were sequenced in 300 individuals representing 62 populations across the range of the species. While four ancestral haplotypes are found in all areas, none of the observed tip haplotypes is present in more than one of the three part ranges. Although this is evidence for a past fragmentation event, nested clade analysis (NCA) remains equivocal in the choice between allopatric fragmentation and long-distance dispersal. Mismatch distributions indicate exponential population growth, probably during postglacial invasion of C. hultenii into formerly glaciated areas of western North America. The presence of one southern and at least one northern glacial refugium in South Central Alaska is inferred. Evidence for another refugium in the Queen Charlotte Islands or Alexander Archipelago is inconclusive because of sparse sampling. However, a range expansion was not confirmed unambiguously by NCA. The limited power of NCA to infer past range fragmentations and expansions is due apparently to the shallow haplotype network and widespread ancestral haplotypes. This can be explained by slow genetic drift causing incomplete removal of ancestral haplotypes from the postfragmentation and postexpansion areas.