Basidiolichens

Lichenized basidiomycetous fungi are known only from Agaricomycetes in which they originated in four or five different clades (Fig. 17), the Cantharellales, Corticiales, Atheliales, Agaricales, and possibly also the Hymenochaetales. Whether Lepidostromataceae should be integrated in the Atheliales or kept separate as their sister taxon is pending.

Fig. 54. Evolutionary trends in basidiolichens (after Oberwinkler 2012). a Fungus-alga-interaction of Dictyonema sericeum, (b) Complex globular thallus structures of the Botrydina-type, (c) Cora pavonia, upper side of thallus, (d) clavarioid basidiocarps of Multiclavula sinensis and crustous thallus, (e) basidiocarp of Lichenomphalia hudsoniana with basal Coriscium thalli. In the scheme, genera are arranged from simple to complex basidiocarps, i.e. corticioid-stereoid-clavarioid-agaricoid. Lichen thalli occur convergently with loose hyphae, globular clusters (b), and scaly structures (e). Athelia can parasitize with haustoria but also hyphae can surround algae in globular clusters. Hyphal sheaths in the Dictyonema group are always associated with endosymbiontic hyphae of the Rhizonema (formerly Scytonema) symbionts (a). Appressoria occur only in Lepidostroma. Comparatively simple globular clusters and scale-like thalli are formed by Multiclavula species (b). Complex globular structures of the Botrydina-type, and associations of globules in scale-like thalli (Coriscium) are present in Lichenomphalia (e). Bars: (a) 5 µm, (c-e) 1 cm. a from Oberwinkler (1980), b-d from Oberwinkler (2012).

Evolutionary trends in basidiolichens:

Basidiocarps resupinate > stereoid > clavarioid > agaricoid

Lichen-thalli granules > scales > leaves

Photobiont Rhizonema > green algae

Fungus-alga-interaction with haustoria > apressoria > hyphal sheaths

In a first comprehensive morphological survey, basidiolichen genera were treated comparatively (Oberwinkler 1970). Resupinate basidiocarps exist in the related Athelia and Athelopsis and convergently in Dictyonema and Cora. Multiclavula and Lepidostroma are clavarioid and evolved separately in the Cantharellales and as a sister group of the Atheliales. In the Cora-Dictyonema clade of the Hygrophoraceae, a basal family of the Agaricales, cluster also Acantholichen, Cyphellostereum, and Lichenomphalia (Lawrey et al. 2009). – Bulbils of algae and fungi may constitute convergent ancestral states in the evolution of basidiolichen thalli (Lawrey et al. 2007), as in the Multiclavula relationship, the Marchandiomphalina group, in lichenized Athelia and Atheliopsis, Cyphellostereum pusiolum, and Lichenomphalia (Fig. 54). Scales of Lichenomphalia and Lepidostroma, and finally leaf-like thalli of Cora are structurally complex and therefore certainly derived.

Photobionts in basidiolichens, as in ascolichens, are cyanobacteria and green algae. The lichenized scytonematoid cyanobacteria form a clade different from the one of free living Scytonema s.str. that is now called Rhizonema (Lücking et al. 2009b). Green algal photobionts, including Coccomyxa species, belong to the Trebouxiophyceae (Beck & Persoh 2009). – Fungus-alga-interactions (Oberwinkler 1980, 1984) with cyanobacteria are hyphal sheaths from which haustoria-like endosymbiontic hyphae originate (Fig. 54 a). Appressoria and globular clusters formed by densely arranged hyphae surrounding the phycobiont (Fig. 54 b) are the interaction structures with green algae. – An accelerated evolution as a consequence of transitions to mutualism has been calculated by Lutzoni & Pagel (1997) using lichenized Omphalina species and related non-lichenized ones.