Cell wall surface hydrophobicity was recognized as a functionally important element in the lichen symbiosis. In the current and former projects we showed in ultrastructural studies that lichen-forming asco- and basidiomycetes cover the wall surface of their photoautotrophic partner (unicellular green algae or cyanobacterial colonies) with a hydrophobic film that has a characteristic rodlet pattern. By doing so the fungal partner prevents the photobiont layer from getting waterlogged at high levels of thalline hydration. Sandra Scherrer and Marcella Trembley successfully isolated class 1 hydrophobins from vegetative thalli of diverse lichen-forming ascomycetes and from the lichenized fruit-body of Dictyonema glabratum, a tropical basidiomycete.
This class of fungal proteins shows little sequence homology except 8 cysteine residues in a conserved pattern, and interfacial self-assembly into an amphipathic film with a characteristic rodlet pattern. In the current project Sandra Scherrer and Marcella Trembley explored the sites of hydrophobin gene expression in the symbiotic phenotype of selected taxa of lichen-forming asco- and basidiomycetes with in situ mRNA hybridisation techniques. The inter- and intra-specific variability of the hydrophobin gene (H1) was investigated in a range of Xanthoria spp. and compared to ribosomal gene data (ITS 1 and 2, 5.8 rDNA).
Lichen-forming fungi are a large group of nutritional specialists which comprise approx. 20% of all fungi. In contrast to most non-lichenized fungi, whose vegetative body is hidden within wood, soil, or host tissues, the thalli of most lichen-forming taxa are well visible above ground, yet little is known about their population genetics and no data at all are available on population genetics of their photoautotrophic partners. These are either cyanobacterial colonies or minute unicellular or filamentous green algae. Both partners have a long history of cohabitation, and we are interested in their evolution. We focus on few basic aspects of genetic structure within selected taxa of lichen-forming fungi and their photobionts. These are 1) mating systems of the fungal partner; 2) ï¿½echanical hybridizationï¿½ i.e. the merging of vicinal lichen thalli to larger units, which may comprise several fungal and algal genotypes, a probably common and widespread phenomenon among lichens; thus lichen thalli may often be consortia with unkown numbers of participants rather than individuals. 3) vegetative compatibility/incompatibility in lichen-forming fungi. Three closely related species of lichen-forming ascomycetes were selected as ï¿½odel systemsï¿½ 1) Xanthoria parietina, a common, almost cosmopolitan species, which has successfully invaded new continents, 2) the mostly european, less common X. calcicola, and 3) the ill-defined, mediterranean to atlantic X. ectaneoides complex. We currently explore fungal mating systems in single spore progeny of the meiosis (lichen-forming fungi do not normally form fruit bodies in aposymbiotic culture, thus mating can only be retrospectively analyzed), the range of compatible photobiont taxa (Trebouxia spp.), genetic variation in the fungal and algal partners within thalli and populations and among populations, and possible correlations between mechanical hybridisation and vegetative compatibility. We are grateful to numerous friends and colleagues all over the globe who kindly collected fresh specimens (and still do so) for this project. We are culturing large numbers of Xanthoria single spore isolates, grow the green algal partner (Trebouxia sp.) from each sample separately in axenic culture, and continue isolation and culturing work since our genetic analyses can only be performed with aposymbiotic sterile cultures.