Lichenization: The origins of a fungal life-style

Abstract

Following a discussion on the naming of lichens and a definition of “lichen”, hypotheses on the origins of lichenization and lichen-forming fungi are reviewed. It is emphasized that lichen associations strictly have no scientific name, while the partners in the symbiosis do. As fungi have a wide range of associations with algae and cyanobacteria, the definition of “lichen” must include the fungal partner enclosing the photosynthetic, and the photosynthetic partner not being incorporated into fungal cells. Hypotheses put forward to explain lichenization are examined in the context of the evidence from the fossil record and molecular biology. There are uncertainties over the interpretation of many of the pre-Devonian fossils, but stratified undisputed lichen-like associations were present in the Lower Devonian, and material referable to modern genera is preserved in Eiocene and Miocene amber. Some early molecular studies suggested that the earliest ascomycetes may have been lichenized, but as more fungi have been sequenced, it has emerged as more likely that there have been repeated lichenization and de-lichenization events in different lineages over time. Some caution is necessary as the molecular trees do not included data from extinct lineages. The possibility that there were early lichen-like fungal associations as far back as the late Pre-Cambrian or early Cambrian cannot be discounted on the basis that they are not recognizable in the fossil record.