Saprotrophy and lichenization as options for the same fungal species on different substrata: environmental plasticity and fungal lifestyles in the <i>Stictis–Conotrema</i> complex

Abstract

• Lichenization is one of the most important fungal lifestyles and lichen fungi are assumed to form obligate symbioses with their photobionts. Here we test if lichenized and non-lichenized samples in three cases of closely related lichenized Conotrema and saprotrophic Stictis (Stictidaceae, Ostropales, Ascomycota) in northern Scandinavia, form distinct monophyletic groups (= species). • We applied phylogenetic species recognition, by analysing fungal DNA sequence data from four independent genomic markers. • Separate parsimony and parsimony jack-knifing analyses of three independent genes are congruent and result in intermixed groups of lichenized and saprotrophic specimens. The sequence variation in an intron also supports this. Our results suggest that all three cases represent independent fungal phylogenetic species, which can undergo their whole sexual lifecycle either as lichens or as saprotrophs. • The use of different nutritional modes – optional lichenization – allows separate individuals to exploit different niches during the forest succession. We suggest that this environmental plasticity may be common in the Stictidaceae, and propose that it is an overlooked strategy in fungi adapted to unpredictable successional ecosystems.