Case Studies

Stictis and Conotrema

Stictis and Conotrema were defined in 1799 and 1848, respectively, the former a genus of saprotrophic fungi and the latter a genus of lichenized fungi, and these two ascomycete genera could be found growing on trees. The apothecia of Stictis are small, typically no more than about a millimetre in diameter and bursting through the outer layer of the wood surface. To the naked eye the thalli of Conotrema looked like little more than washes of white or pale grey paint, sprinkled with tiny apothecia, again no more than about a millimetre in diameter. The year 1969 saw the publication of good evidence for considering the two genera to be identical. In that case the name Stictis, being the one published earlier, would take priority so that species of Conotrema would be re-named as species of Stictis. However, in the late 20th century this formal re-naming step was not taken. For one thing, the genus Stictis needed more study to clarify the boundaries between certain species. Another point was that there were some other, similar genera. Ideally these other ' Stictis-like' genera would be looked at as well so as to get a better understanding of all the ' Stictis-like' species.

Since then DNA studies have confirmed various conclusions based on micromorphology and there have been some taxonomic changes. For example, the species once known as Conotrema urceolatum (and which formed the basis for the definition of the genus Conotrema) is now known as Stictis urceolatum. This immediately shows that lichenized and non-lichenized species can be found in the one genus. That's neither surprising nor new since there are about 20 genera in which both lichenized and non-lichenized species are known. However DNA studies have also shown that some lichenized ' Conotrema' species are not just species of Stictis but are identical to some non-lichenized Stictis species. In other words, lichenization is 'optional' for some fungi since they can be found growing lichenized or growing as saprotrophs. Many lichens have quite obvious thalli that are internally stratified with a cortex (or two), a photobiont layer (with numerous photobiont cells) and a medulla. Near the beginning of this page ' Conotrema' thalli were described as rather nondescript. In fact the lichenized forms of the ' Stictis-like' species are rather loosely lichenized, simply with scattered groups of lichenized algal cells.

Four lichen-saprotroph ' Stictis-like' species have been recorded and thus far it is only from northern Scandinavia that both forms of each species have been reported. The species are Schizoxylon albescens, Stictis confusum, Stictis mollis and Stictis populorum. The saprotrophic form of Stictis mollis is known from elsewhere in Europe. In northern Scandinavia the four species have been found on trees, particularly a species of poplar (Populus tremulus), with the lichenized forms found on bark and the saprotrophic forms on decorticated wood. The lichenized form of Stictis mollis has been seen on thin, smooth-barked trees. Colonization can take place in saplings before the development of dead branches that would be suitable for the saprotrophic form. In the other species the lichenized forms grow in and around deep bark cracks, particularly on the lower parts of the trunks of older trees. The saprotrophic form grows on decorticated branches still attached to living trees. Microscopic examination of the dead wood on which saprotrophic forms were found showed no trace of algae, ruling out the possibility that seemingly saprotrophic mycelia were associated with algae hidden in the wood.

Flexibility in nutritional mode is advantageous in that it would allow a species to exploit a greater variety of niches than a species restricted to one mode of nutrition. The quantity of live trees, fissured bark or decorticated (but still-attached) branches in a given area is somewhat cyclic. Over time each of those substrates changes in quantity as seedlings grow into old trees. All the while there are also potential changes from agents such as wildfires, logging or tree fall induced by decay fungi. Moreover, what is happening in one area can be quite different to what is happening in a nearby area. Thus while live trees, fissured bark and decorticated branches will appear and re-appear, their occurrence is unpredictable in space or time. Each of the species mentioned earlier is able to occupy a variety of woody substrates and this ability would help a sizeable population of a given species persist in a given area. Compare this to the precarious foothold of a fungus that lived only as a saprotroph on decorticated branches still attached to living trees.

Ostensibly the genus Stictis contains about 250 species, the great majority of which are known only as saprotrophs. There is still some taxonomic uncertainty in this and similar genera. However, even if there were much synonymy in those species names the percentage of species known to be lichen-saprotrophs would still be quite small. Are there really so few lichen-saprotroph Stictis species? All that can be said for certain is that thus far the ability to exist in lichenized and non-lichenized forms has been found in only a small number of species. However, too few species have been investigated to allow any conclusions as to how rare or common this phenomenon is - either within the genus Stictis or amongst fungi in general. The authors of the 2004 paper listed below think the phenomenon could be more widespread:

In fact, this plasticity represents a strategy, which we predict to also occur among other fungi adapted to successional habitats that are unpredictable in space and time, and we suggest that it should be actively searched for among such fungi.

On the available evidence it seems that a lichen-saprotroph spore landing on decorticated branches would develop a saprotrophic mycelium whereas such a spore landing on bark would develop a lichen thallus, assuming the appropriate photobiont is present. One wonders what other factors could influence the post-germination development. Ideally some of the lichen-saprotroph species would be grown under laboratory conditions in order to better understand their biology and to see how lichenization could be switched on or off, especially as the authors of the 2004 paper opined that:

Optional lichenization indicates that the evolutionary step between fungal lifestyles is much smaller than earlier anticipated.

The past decade has seen the publication of various papers in which there have been estimates as to how often in evolutionary history lichenization has evolved or been lost. This is mentioned on the CLASSIFICATION page. The authors of the 2004 paper point out that the existence of optional lichenization, even if in only a few species today, casts some doubt on the usefulness of counting past lichenization and de-lichenization events.

References

Gilenstam, G. (1969). Studies in the lichen genus Conotrema. Arkiv für Botanik, 7, 149-179.

Wedin, M; Döring, H & Gilenstam, G. (2004). Saprotrophy and lichenization as options for the same fungal species on different substrata: environmental plasticity and fungal lifestyles in the Stictis-Conotrema complex. New Phytologist, 164, 459-465. [This paper reported the first examples of 'optional' lichenization in the genus Stictis.]

Wedin, M; Döring, H & Gilenstam, G. (2006). Stictis s. lat. (Ostropales, Ascomycota) in northern Scandinavia, with a key and notes on morphological variation in relation to lifestyle. Mycological Research, 110, 773-789. [This paper reports examples of several species with saprotrophic and lichenized forms and gives names to taxa that, in the 2004 paper, were referred to as Species 1, Species 2, etc.]

Wedin, M; Döring, H; Köneberg, K & Gilenstam, G. (2005). Generic delimitations in the family Stictidaceae (Ostropales, Ascomycota): the Stictis–Conotrema problem. The Lichenologist, 37, 67-75. [The paper in which the Conotrema urceolatum was renamed in Stictis. ]