Lichenized fungi (chiefly Hymenoascomycetes: Lecanorales)

Abstract

Introduction The dual nature of lichens was first hinted at by de Bary (1866) and clearly recognized by Schwendener (1867). A lichen is now defined as a ‘self-supporting association of a fungus ( mycobiont ) and a green alga or cyanobacterium ( photobiont )’ (Kirk et al ., 2001), ‘resulting in a stable thallus of specific structure’ (Ahmadjian, 1993). The fungal partner usually contributes most of the biomass to this symbiosis, including the external surface. It is thus termed the exhabitant , whereas the unicellular or filamentous photobiont cells are collectively called the inhabitant because they are located inside the lichen thallus (see Ahmadjian, 1993). Most lichens have a characteristic appearance which permits their identification if suitable keys are available (e.g. Purvis et al ., 1992; Wirth, 1995a, b; Brodo et al ., 2001). Since the structure of lichens is almost entirely due to the fungal partner, lichen taxonomy is synonymous with the taxonomy of the mycobiont. It is possible to grow the algal and fungal partners of many lichens separately in pure culture (Ahmadjian, 1993; Crittenden et al ., 1995). Whereas most photobionts multiply readily in pure culture, the fungal partner, if it grows at all, typically shows slow growth as a sterile leathery mycelium but does not produce the characteristic lichen thallus. This is in marked contrast to the natural thallus where the mycobiont displays its full sexual and asexual cycle, whereas the photobiont cells often appear swollen and are arrested in their cell cycle, i.e. their cell division is controlled by the mycobiont.