Genetic diversity of green algal and cyanobacterial photobionts in Nephroma (Peltigerales) Katileena LOHTANDER, , Ilona OKSANEN and Jouko RIKKINEN Abstract: Genetic diversity of green algal and cyanobacterial photobionts in Nephroma was examined by using nucleotide sequences of the ribosomal gene cluster. The lichens studied included both bipartite and tripartite species. There was very little variation in green algal-ITS sequences of N. arcticum and N. expallidum. Almost identical sequences were obtained from all thalli analysed and also from two tripartite Peltigera species. On the basis of SSU rDNA data the green algal photobionts of N. arcticum are closely related to the primary photobiont of P. britannica, and also to an endophytic alga of Ginkgo biloba. The SSU rDNA region of lichen-forming cyanobacteria was rather variable. A phylogenetic analysis indicated that the Nostoc specimens formed a monophyletic group and the strains were divided into two main groups. One clade included only cyanobionts of lichens, including those of all bipartite Nephroma species. The second group was genetically more heterogeneous and included mainly cyanobionts of terricolous cyanolichens, including those of both tripartite Nephroma species studied. The distinction between bi- and tripartite Nephroma species is significant as the mycobionts of tripartite species are not monophyletic. It implies that within Nephroma, evolutionary transitions between symbiosis types cannot have been achieved simply via an acquisition or loss of the green algal photobiont. As the Nostoc symbionts of bi- and tripartite species belong to di#erent phylogenetic groups, an evolutionary change in green algal association has required a concurrent change in cyanobiont composition.  2003 The British Lichen Society. Published by Elsevier Ltd. All rights reserved. Key words: Coccomyxa, ITS, lichen, Nostoc, phylogeny, SSU rDNA, symbiosis. Introduction The genus Nephroma (Nephromataceae, Peltigerales) includes about 40 species of foliose, predominately epiphytic cyano- lichens (Wetmore 1960; James & White 1987; White & James 1988). Most of these lichens are bipartite, but the genus also includes some tripartite species. Some tri- partite Nephroma species can also produce photosymbiodemes (James & Henssen 1976; Tønsberg & Holtan-Hartwig 1983; Go$net & Bayer 1997). The cyanobacterial symbionts can provide both photosynthate and fixed nitrogen to their fungal partners and the relative importance of these func- tions varies between bi- and tripartite sym- bioses (Palmqvist 2002; Rai 2002). Several epiphytic Nephroma species prefer habitats of long environmental continuity and have been used as indicators of forest antiquity. In a recent study of Nephroma mycobionts we found that all species studied (11) formed a monophyletic group within the Peltigerales (Lohtander et al. 2002). We also found that tripartite species did not form a monophyletic group within the genus. For example, the tripartite N. expallidum was more closely related to several bipartite taxa K. Lohtander and J. Rikkinen: Department of Applied Biology, P.O. Box 27, FIN-00014 University of Helsinki, Finland. K. Lohtander, I. Oksanen and J. Rikkinen: Department of Ecology and Systematics, Division of Systematics, P.O. Box 65, FIN-00014 University of Helsinki, Finland. I. Oksanen: Department of Applied Chemistry and Microbiology, Division of Microbiology, P.O. Box 56, FIN-00014 University of Helsinki, Finland. Lichenologist 35(4): 325–339 (2003) doi:10.1016/S0024-2829(03)00051-3 0024-2829/03/040325+15 $30.00/0  2003 The British Lichen Society. Published by Elsevier Ltd. All rights reserved.