946 Asuncio  n de los RI  OS 1 , Carmen ASCASO 1 and Martin GRUBE 2  Centro de Ciencias Medioambientales, Serrano 115 dpdo, Madrid-28006, Spain.  Institut fu  r Botanik, Karl-Franzens-Universita  t Graz, Holteigasse 6, A-8010 Graz, Austria. E-mail : ariosccma.csic.es Received 30 April 2002 ; accepted 18 June 2002. Rimularia insularis forms an photosynthetic thallus on lichens of the Lecanora rupicola group. In the thalli of R. insularis, some hyphae of the host are detectable, and they are situated mainly in the basal part of the Rimularia’s medulla. No Lecanora hyphae were present in the Rimularia’s algal layer and they were indistinct in the upper parts of its medulla, from where we obtained clean ITS-sequences of Rimularia. Direct PCR from various sections of the algal layer detected only one photobiont genotype, although the algal cells could be assigned to two morphologically different types. Calcium deposits were found in the upper parts of the medulla, while abundant rock substrate fragments with different mineral compositions were present in the lower parts. Cavities and fissures in the rock below the infected thalli were filled by fungal cells, PCR analyses of such parts indicated that the Rimularia hyphae extend below the host, and into the rock. INTRODUCTION Associations between fungi and algae (or cyano- bacteria) can be highly complex and may involve multiple symbionts (Hawksworth 1988). Beside the balanced association of photobionts with a single species of fungi as is typical for lichens, additional fungi may occasionally be present. Those that lack a photosynthetic thallus are lichenicolous fungi, while these that form distinct photosynthetic thalli in or on their hosts are termed lichenicolous lichens (Poelt & Doppelbauer 1956, Poelt & Steiner 1971, Hafellner & Poelt 1980, Poelt 1985, Holtan-Hartwig & Timdal 1987). Around 100 species of lichenicolous lichens were listed by Poelt & Doppelbaur (1956), and the number was considerably extended by Rambold & Triebel (1992). They included 205 lichenicolous species in inter- lecanoralean associations. However, assignments to life styles are sometimes based on small samples sizes, as some species some known only from a few collections or the type. Beside this, some lichens are facultatively lichenicolous, or change with age (Hawksworth et al. 1979, Holtan-Hartwig & Timdal 1987). The number of lichenicolous species in other fungal orders is certainly much lower, but this is still poorly studied. While non-lichenized lichenicolous fungi of larger, predominantly non-lichenized lineages may be associated with the host’s mycobiont (de los Rı  os & Grube 2000, Grube & de los Rı  os 2001, de los Rı  os et al. 2002), those of lichenized groups rather have an affinity to the hosts algae. The question of algal specificity of lichenicolous lichens has been touched in a few publications. Friedl (1987a, b) found that the original alga of the host (Cladonia sp.) is present in young stages of the Diploschistes muscorum infection, but it is later replaced by another photobiont. Here, the juvenile parasite changes to a independent lichen by switching to its own photobiont. No indication for such switch is apparent in Lecanactis grumulosa which infects Roccella thalli (Feige & Lumbsch 1993) or in Blarneya hibernica infecting Enterographa and Lecanactis species (Hawksworth et al. 1979). On the other hand, Lu  cking & Grube (2002) found a switch to a different alga in lichenicolous morphs of the foliicolous species Chroodiscus coccineus. In these cases, the manifestation of the lichenicolous lichen thallus apparently occurs by association with the host lichen’s algae. It is likely that lichenicolous lichens lost the capacity to form an independent thallus and that they take advantage of the corresponding photobiont of the host, however, this has not been studied by molecular methods. To understand the interfungal relationships in lichen associations, more ultrastructural and anatomical studies are required. In this study, we focused on the Mycol. Res. 106 (8) : 946–953 (August 2002).  The British Mycological Society DOI : 10.1017S0953756202006238 Printed in the United Kingdom. An ultrastructural, anatomical and molecular study of the lichenicolous lichen Rimularia insularis