Lichenologist 33(1): 73–86 (2001) doi:10.1006/lich.2000.0298, available online at http://www.idealibrary.com on Identification of photobionts from the lichen family Physciaceae using algal-specific ITS rDNA sequencing Gert HELMS, Thomas FRIEDL, Gerhard RAMBOLD and Helmut MAYRHOFER Abstract: The identity of photobionts from 20 species of the Physciaceae from different habitats and geographical regions has been determined by ITS rDNA sequence comparisons in order to estimate the diversity of photobionts within that lichen group, to detect patterns of specificity of mycobionts towards their photobionts and as a part of an ongoing study to investigate possible parallel cladogenesis of both symbionts. Algal-specific PCR primers have been used to determine the ITS rDNA sequences from DNA extractions of dried lichens that were up to 5 years old. Direct comparisons and phylogenetic analyses allowed the assignment of Physciaceae photobionts to four distinct clades in the photobiont ITS rDNA phylogeny. The results indicate a diversity within the genus Trebouxia Puymaly and Physciaceae photobionts that is higher than expected on the basis of morphology alone. Physciaceae photobionts belonged to 12 different ITS lineages of which nine could unambiguously be assigned to six morphospecies of Trebouxia. The identity of the remaining three sequences was not clarified; they may represent new species. Specificity at the generic level was low as a whole range of photobiont species were found within a genus of Physciaceae and different ranges were detected. The photobionts of Physcia (Schreb.) Michaux were closely related and represented one morphospecies of Trebouxia, whereas the algal partners of Buellia De Not and Rinodina (Ach.) S. Gray were in distant lineages of the ITS phylogeny and from several Trebouxia morphospecies. Photobiont variation within a genus of Physciaceae may be due to phylogeny, geographical distance or because photobionts from neighbouring lichens were taken (‘algal sharing’). At the species level Physciaceae mycobionts seem to be rather selective and contained photobionts that were very closely related within one morphospecies of Trebouxia.  2001 The British Lichen Society Introduction The diversity of photobionts in any taxo- nomic group of lichens has been poorly studied so far. Records of photobionts that have been identified at the species level are limited as they refer mainly to distantly related mycobiont taxa and only few examples per group have been studied [see the lists of photobiont taxa in Tschermak- Woess (1988) and Ahmadjian (1993)]. In a recent compilation of the present knowledge of photobionts from various groups of the Lecanorales, it was suggested that the photobionts may be important markers of evolutionary relationships and, therefore, their identification should become a pre- requisite in systematic studies of lichens. The capability of lichen fungi to select an appropriate algal partner from a variety of algal taxa common in subaerial habitats may be an important phylogenetic trait (Rambold et al. 1998). The taxonomic level at which photobionts may serve as markers of evol- utionary relationships of lichens is, however, unknown. According to current taxonomic concepts, Physciaceae is a family of the G. Helms and T. Friedl: Experimentelle Phykologie und Sammlung von Algenkulturen, Albrecht-von- Haller-Institut, Universität Göttingen, Untere Karspüle 2, D-37073 Göttingen, Germany. G. Rambold: Lehrstuhl Pflanzensystematik, Universität Bayreuth, Universitätsstraße 30, D-95448 Bayreuth, Germany. H. Mayrhofer: Institut für Botanik, Karl-Franzens- Universität Graz, Holteigasse 6, A-8010 Graz, Austria. 0024–2829/01/010073+14 $35.00/0  2001 The British Lichen Society