Lichen symbioses represent one of the most successful nutritional strategies in the fungal
kingdom. The interactions between lichen-forming fungi and algae are described in terms
of specificity and selectivity and are assumed to be the triggers for the thallus formation.
On rocks, in oligotrophic conditions and under abiotic stresses, lichens are ecologically
successful and appear to be influenced particularly by the ecological specialization and the
physiological responses of their photobionts. The algal genus Trebouxia has been found to
be the most diverse, frequently occurring and globally distributed photobiont in lichens,
however its diversity is still unknown in lichens in extreme environments. Here we
describe and characterize the diversity of Trebouxia species in two cosmopolitan lichen
symbioses, Rhizoplaca melanophthalma and Tephromela atra, which were collected from
elevations over 4000 m a.s.l. in extremophilic conditions. These lichens share seven
different lineages of Trebouxia which were successfully isolated in culture. By
implementing an integrative approach based on genetic diversity of the nuclear ITS and
plastidial loci, morphological (ultrastructure in light and electron microscopy) analyses of
culture isolates we correlate the photobiont diversity with the specificity towards the
associated lichen fungus and the different ecological conditions.
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