Lichen phycobionts have generally been thought to have low fidelity with mycobionts at the species rank, but that assertion has been based on scant empirical evidence. Phycobionts from populations of several Umbilicaria species growing both sympatrically and in disjunct single-species populations were isolated from lichen thalli and cultured. DNA was extracted from cultures and characterized with RAPD fragment profiles. Distinct phycobiont haplotypes are identified. Phycobiont haplotype fidelity within and among mycobiont lineages is analyzed. Do mycobiont and phycobiont lineages mix at random, or is there some degree of fidelity of phycobionts within and among species of Umbilicaria, or within and among mixed and single-species populations? Does large-scale geographic structure of phycobiont haplotypes correspond with geographic distributions of species in Umbilicaria? Evidence from fidelity analysis of phycobiont and mycobiont lineages across a wide geographic sample in Umbilicaria provides an empirical foundation for generalizations about these and other questions pertaining to cospeciation, host-switching, and geographic structure of symbiont diversity in lichens.

Key words: cospeciation, lichen, phycobiont, RAPD, symbiosis, Umbilicaria