Populations of golden yellow to orange lichens of the genus Xanthoria, as found from the coast to mountain areas, are often conspicuous landmarks which indicate nutrient-rich sites. In Europe, Xanthoria parietina, symbiotic with unicellular green algae of the genus Trebouxia, is one of the most common lichen-forming ascomycetes and certainly one of the best investigated (Honegger, 1990, 1996). As a heteromerous macrolichen, the symbiotic phenotype of X. parietina comprises two structurally and functionally distinct zones: hydrophilic, conglutinate pseudoparenchyma as peripheral upper and lower cortical layers, which passively absorb water and dissolved mineral nutrients and provide the thallus with elasticity and mechanical stability; and the gas-filled thalline interior, built up by a system of loosely interwoven aerial hyphae with hydrophobic wall surfaces, some of which contact the algal cells by means of intraparietal haustoria (Honegger, 1990). Sealing of the algal wall surface and thus of the apoplastic continuum with mycobiont-derived hydrophobic cell wall surface components was shown to be an essential feature for thalline water relations and for the symbiotic relationship per se (Honegger, 1993, 1997; Scherrer et al., 2002; for review see Dyer, 2002).
Previous studies focused on the fine structure, biochemistry and molecular genetics of wall surface hydrophobicity in X. parietina and in the closely related Xanthoria ectaneoides, which was shown to be hydrophobin-based (Scherrer et al., 2000, 2002). Hydrophobins are small (100 ± 25 amino acids) secreted fungal proteins with very low amino acid homology except for eight cysteines in a conserved pattern (Wessels, 1997, 1999; Wösten & Wessels, 1997; Kershaw & Talbot, 1998; Wösten, 2001) and with interfacial self-assembly into an amphiphilic protein film with distinct, semicrystalline rodlet pattern (Wösten et al., 1993).
The present study is the first approach to investigate the intra- and inter-specific variation of the hydrophobin gene H1 in a group of closely related lichen-forming ascomycetes, the focus being on the genus Xanthoria. The two class 1 hydrophobins so far known from lichen-forming ascomycetes, XPH1 (X. parietina) and XEH1 (X. ectaneoides), revealed a high homology and were shown to be encoded by the fungal, not by the algal partner of the lichen thallus (Scherrer et al., 2000); high gene expression was detected in medullary hyphae (Scherrer et al., 2002). The mature hydrophobin XEH1 of X. ectaneoides was isolated from washed cell wall fragments from specimens collected on coastal rocks in Roscoff (Brittany, France). In the present study, the XEH1 probe and primers designed for the amplification of XPH1 were used for testing the interspecific variation of the H1 gene in a range of Teloschistaceae (lichen-forming ascomycetes). Because of considerable phenotypic plasticity, the species delimitation in the type species of this common and widespread group of macrolichens is rather vague, conventional taxonomy being based on features of the symbiotic phenotype such as morphotype and chemotype. Therefore, parts of the rDNA gene region (ITS1, 5.8S rDNA, and ITS2) were analysed in a series of parallel experiments.