A panoramic path along multiple symbiotic patterns: together or not together?

Lichen symbioses are complex, structured associations between fungi (mycobionts) and algae (photobionts). The extent to which these associations are more or less specific, is still largely unknown, and likely correlated with the dispersal mode of the partners. Variable patterns of mycobiont-photobiont associations have been detected in lichens and other algae-associated fungi. Here we present studies based on molecular and culture-dependent approaches of fungi and algae which form either well developed or loose associations. The lichen Tephromela atra is a geographically widespread species complex, which present a continuum of morphological diversity and different degrees of specificity towards Trebouxia photobionts. The fungus Schizoxylon albescens shows optional lichenization with algae of the genus Coccomyxa, occurring both as lichen and as saprobe when growing on different substrates. The species of Lichenothelia and Saxomyces, black, rock inhabiting fungi, form loose association with multiple algae. For these fungi we tested the capacity to form lichen-like relationships with algae using culture experiments. The experiments show various types of interactions with Trebouxia and Coccomyxa and shed new light on the life-style flexibility of fungi colonizing multiple ecological niches. This further suggests these fungi to be easily controlled study systems for the investigation of lichen symbiosis under lab conditions.