Figure 4 from: Rikkinen J (2013) Molecular studies on cyanobacterial diversity in lichen symbioses. MycoKeys 6: 3-32. https://doi.org/10.3897/mycokeys.6.3869

Figure 4 - Photobiont-mediated guilds in lichens (modified from Rikkinen 2003).The lichen-forming fungi belong to three different guilds, one centring around cyanobacterial symbionts (A), the second around green algal symbionts (B), and the third around another genus of green algal symbionts (C). The lichen in the middle of the picture houses both green algae and cyanobacteria (in cephalodia), meaning that its fungal symbiont can operate in two different guilds (C and A). As the symbiotic propagules of this lichen only contain the fungus and green algal photobiont, the fungus is a core species in guild C and a fringe species in guild A. Under certain conditions this fungus may give rise to cyanobacterial morphotypes (A4) and/or green algal thallus lobes. The core species of the lichen guilds produce innumerable symbiotic propagules, most of which will never develop into mature thalli of that lichen species. Germinating spores of fringe species (A1–A4, B1–B5) may commonly acquire their photobionts from small free-living populations that originate from disintegrating symbiotic propagules of the core species. At the latest when the thallus of a fringe species dies and disintegrates (A1), some of the photobionts are released back to the local environment for the common benefit of all fungi of the same guild. However, without the ability to produce symbiotic propagules, the fringe species cannot effectively disperse appropriate photobionts into new habitats. Some fringe species are aggressive enough to steal photobionts from juvenile stages or weakened thalli of other lichen species (A3), or live as lichenicolous lichens (B2) on other lichens of the same guild. The juvenile stages of some green algal lichens establish loose cyanotrophic associations with free-living cyanobacteria (B5) and/or cyanolichens (B4). Some lichenicolous fungi (B3) have evolved from lichen-forming ancestors and in many cases also their host ranges still appreciate guild boundaries