The variety of fungal-algal symbioses, their evolutionary significance, and the nature of lichens

Symbioses between fungi and algae (or cyanobacteria) can involve two-, three-, four-, five-, or more bionts. The range of symbioses developed is described by means of examples with an emphasis on mutualistic symbioses. Lichen associations appear to be very ancient and a study of the fungi in them is important to an understanding of ascomycete evolution. An unequivocal definition of ‘lichen’ cannot be devised as fungi appear to be repeatedly evolving and devolving with respect to forming mutualistic symbioses with algae, and strategies may vary with age or be facultative. A revised definition to exclude ‘mycophycobioses’ is proposed, and the terms ‘phycotype’ and ‘parasymbiont’ are rejected. A new combination in the genus Pyrenocollema is made.