Pholiota squarrosa is the iconic Pholiota species of the genus. It’s a rather showy mushroom with a scaly cap and stem, attached gills and a rust brown spore print. I truly love finding this species in the temperate coniferous forests in Western NY. It has a large distribution and can be found all over the United States, Canada and in parts of northern Mexico. You can also find this species doing well across the pond, as is flourishes throughout Europe as well. It is without a doubt successful and the species can owe its success to its intriguing ecology.
Most of the fungi within the Pholiota genus are saprotrophic species, breaking down the woody debris of fallen trees. When we examine this species in the wild, sometimes you will find it fruiting from a dead tree, but in other instances, you find it growing from living trees. This isn’t a rare pattern we find in nature, given that seemingly saprotrophic species can actually consume living plant tissue.
Ganoderma tsugae is another species of fungus I encounter nearly every time I hike. G. tsugae too carries out a similar, dual ecology. Yes, it decomposes dead Eastern Hemlocks, and yes it sometimes consumes living Eastern Hemlocks. We see this trend many times in forest ecosystems, so I see it fitting to talk about the evolutionary trajectory of parasitic fungi.
Reoccurring patterns in nature are not to be ignored. These are clear signs that we can learn from, clear signs that can be described through underlying biology. Many solely parasitic species have a saprotrophic ancestry. Saprotrophic fungi are equipped with many of the same tools that parasitic fungi utilize when they are consuming their woody hosts. These two species that are opportunistically parasitic may represent a transitional phase in the species evolution, as acquiring a parasitic ecology may happen over a series of stepwise genetic and biochemical events. Once again, limiting resources in forest ecosystems drive competition which ultimately drives evolution.
More than a year ago, I wrote an article about latently present fungi. In that article I describe that in unperturbed forests with diverse fungal assemblages, trees that die begin to be decomposed immediately. There really isn’t a lag for saprotrophic decomposition, because fungal spores have already found their way into the tree’s tissue. Many species of fungi are latently present in living trees, awaiting the tree’s imminent death. Opportunistic fungi are one step ahead of latently present saprotrophs.
Pholiota squarrosa won’t be found parasitizing healthy trees either. The only time you see it growing out of living trees, the tree’s immune system has been compromised by other parasitic fungi, bacteria or viral infections. This species gets a jump start even on latently present saprotrophs, which is probably why I find this fungus all over. Its aggressive, and doesn’t wait for tree death. It easily outcompetes other saprotrophic fungi.
If we fast forward 5 million years, it would be interesting to study this successful species ecology. I doubt it would revert back to living just a saprotrophic lifestyle. I hypothesize that even today, directional selection is involved and interacting with this species on a genomic level. Individuals infecting living trees have a higher fitness than less aggressive, more saprotrophic individuals. With this being said, the future of Pholiota squarrosa may be even more parasitic. Being less dependent on previous infections would increase its feeding niche and already large distribution. Does anyone have a time machine?