The chemical ecology of lichen mycoparasites: a review

Abstract

Lichenicolous fungi colonize lichens to form a variety of biotrophic associations, and while some 300 genera and 1000 species have been recognized, almost nothing is known about their biology. The lichenicolous habit is an unusual one because lichens produce a variety of antibiotic secondary metabolites that are assumed to protect them from parasitic attack. This may explain why lichens are rarely parasitized in nature while nonlichen-forming Ascomycetes are frequently and extensively parasitized. Recent field and laboratory studies of lichen parasites from a number of fungal groups indicate that chemistry is indeed involved in these interactions. These results indicate that (i) lichenicolous taxa are better able to degrade lichens than related nonlichenicolous taxa, (ii) lichen parasites are generally tolerant of lichen secondary metabolites, (iii) this tolerance seems to be based in part on the ability of cell wall degrading enzymes of parasites to remain active in the presence of these lichen secondary metabolites, and (iv) the host preferences of some lichen parasites reflect these tolerances. Therefore, it appears likely that lichenicolous fungi, which have presumably had lengthy coevolutionary histories with lichens, have evolved a unique mode of nutrition that is based in part on an ability to tolerate lichen defense compounds. Key words: Hobsonia, lichens, lichenicolous fungi, Nectria, parasites.