Work by Mats Wedin and colleagues highlights both the versatility of fungi and the complications these unsung organisms pose for the biologist. Two of the ways in which fungi make a living are as saprophytes, drawing sustenance from decaying matter, and as lichens, in which they form an intimate relationship with green algae or photosynthetic bacteria. Evidently, however, a single fungal species can adopt either lifestyle according to circumstance (New Phytol. 164, 459–465; 2004).

Using molecular techniques, Wedin et al. looked at fungi living on different parts of aspen (Populus tremula) in northern Sweden. On analysing four independent genomic markers, they found that three different species of a fungal genus (Stictis) grew directly on wood without bark as typical saprophytes (top photograph); but when on the bark of the trunk, the same species associated with green algae to form a whitish, crust-like lichen (bottom). In both images the scale bar is 1 mm.

The lichens had hitherto been placed in a separate genus, Conotrema. Thirty-five years ago it was realized that the fruit bodies of Stictis and Conotrema are microscopically almost indistinguishable, but Stictis was not previously recognized as a lichen-forming genus. That species classified in separate genera are in fact the same organism exhibiting different biologies is a startling discovery. In all three cases, the wood-inhabiting non-lichenized and lichenized specimens were mixed together in phylogenetic trees.

Wedin et al. point out that the ability of a fungus to opt for different nutritional modes means that individual species can exploit a wider range of habitats. If the spores that are shot from the fruits of these fungi land on wood, they establish themselves as saprophytes; if they land on bark where appropriate algae are present, they form lichens.

The frequency of this plasticity in lifestyles is unknown. But it may be common, at least in this group of fungi, as the three species able to become optionally lichenized were found on a single species of tree. The study reinforces the case for never treating lichens as anything other than a lifestyle category among the fungi. It also shows the wisdom of the 1959 decision to restrict the scientific names of lichens to the fungal partners; in consequence, the same names can be used for the fungi regardless of their lifestyle.