My favorite weekly ritual is to hike through dense vegetation, scouring the forest floor in hopes of finding evidence of an instrumental group of organisms that engineer ecosystems on a global scale. With the vast majority of fungal mass occurring below the soil surface, mushrooms are the tip of the iceberg, providing a moment of spatio-temporal ecological comprehension. For those of us who are interested in the interworking processes of what’s going on at the forest floor, finding a new mushroom is a thrilling, immersive, and sometimes maddening experience. A mushroom found in the wild is a pleasant surprise, but the real excitement begins once I get home to carry out research. This was the case about a year ago, when I stumbled upon an exceptional specimen emerging from a bank, off of a slow-moving stream.
Phylloporus rhodoxanthus - three of the five specimens found.
I looked at the top of this group of five mushrooms and immediately thought “Oh cool, boletes!” Boletoid mushrooms have pores instead of gills, and have thick caps and stems, making them generally sturdier. They’re always an excellent find for me and my love of symbiosis because many boletes form mycorrhizal relationships with trees. Their presence with long lived stands of hardwoods shows a certain level of ecological resistance and resilience.
As I turned this mushroom over to see its underside I was dumbfounded to find yellow gills. One of the mushrooms had also been parasitized by another fungus so this find was an even more lucrative goldmine. I have never seen a gilled mushroom like this. I had a long hike to go, so I took pictures, and placed two of the fives specimens in my mesh bag for spore prints and other analyses at home.
I hit up the internet even before I took a shower. I typed in gilled bolete, and hysterically identified my specimen as Phylloporus rhodoxanthus-the gilled bolete. This was a record. I have never identified a mushroom to species so fast. Proper identification is still only the first step, so I washed the hike off of me, and explored my college’s database.
The two P. rhodoxanthus mushrooms I collected. The white one is being parasitized by a species of Hypomyces.
In this modern era, we are shifting from categorizing species using morphological data to genetic similarities. These gilled boletes were once thought to be the missing link of bolete evolution from a gilled ancestor. Morphologically, Phylloporus rhodoxanthus looks just like Xerocomus species except for one minor discrepancy; pores instead of gills. New genetic data places the genus Phylloporus in the family Boletaceae. Without genetic tools, we may have placed this species with other gilled mushrooms in the Agaricoid clade. Phylloporus is a monophyletic and a sister group of the Xerocomus. The evolutionary trajectory of Phylloporus rhodoxanthus is an interesting one, with an ancient gilled ancestor it went through a period of evolution gaining a more boletoid morphology, and finally reverted back to producing gills.
A closely related Xerocomus species with a more boletoid morphology.
Gills are fascinating structures that work exceptionally well. A mutation that causes the fruiting body to not produce a spore bearing structure will not stand the test of time. But a mutation that activates genes once silenced that encode gill formation can evolve and become exceedingly common. Phylloporus rhodoxanthus illustrates just that, and the parasitized fruiting body I found exposes its commonness, as enough of these mushrooms must be present to drive the evolution of a specialized parasite. The gilled bolete has gills, but is not a true gilled mushroom. It’s a bolete, and don’t you forget it!