#187: Phylloporus rhodoxanthus, the Gilled Bolete

So, is it an agaric or a bolete? At first glance, the answer seems obvious: it clearly has gills and therefore must be an agaric. However, if you spend enough time around mushrooms you might get a kind of uncanny valley feeling about this mushroom; its coloration, its stature, the way its cap looks somewhat puffy, the way the cap cracks as it dries out, and other subtleties just don’t look quite right for an agaric. When you ignore the gills, the mushroom looks for all the world like a bolete! Phylloporus rhodoxanthus, commonly called “the Gilled Bolete,” is actually closely related to the boletes and evolved gills independently of the true agaric lineages.¹

Description

The first time I found the Gilled Bolete on my own, it confused me for a long time. When I found it, my first reaction was, “There’s something odd about this mushroom’s gills.” Unfortunately, I didn’t have enough time to thoroughly examine the mushroom, so I just snapped a couple of pictures and ran off. This meant that I had to try to identify it later using only those pictures (a difficult endeavor for any mushroom). For a while, I was stumped. In my head, the mushroom just didn’t match any morphological group of agarics. It also wouldn’t key out with any agarics in my field guide. Sometime later, I was flipping through my field guide to find another mushroom and came across the page for P. rhodoxanthus. Finally, I had figured out why the mushroom didn’t fit anywhere: it wasn’t an agaric, it was a bolete with gills!

Most field guides key out the Gilled Bolete with the boletes. This can make it difficult for people just starting out, but makes sense from a taxonomic and morphological (when the gills are ignored) standpoint. The mushroom’s stature is most obvious morphological clue that it is a bolete: its size and the ratio of its pileus to its stipe are just what you would expect from a bolete.¹ Another major indicator that you should consult the bolete section of your field guide is its spore print: P. rhodoxanthus has a yellowish to brownish-yellow spore print.^1,2 Yellow is a rare spore print color in true agarics; most of them with yellow-toned spores either are pale yellow or lighter or are yellowish-brown or darker.

The pileus of P. rhodoxanthus is perhaps the most bolete-like part of the mushroom. It is 2-10cm across, circular, and starts out convex but becomes flattish by maturity.^1–3 To me, the pileus appears thicker than I would expect for most agarics and develops wrinkles or lobes around the edge that are reminiscent of the way boletes develop. As the mushroom ages and dries out, the cap often develops irregular cracks that expose the pale flesh below, in much the same way that boletes do. The pileus is variable in color. It is most distinctive when red, but it can be anywhere from red to brown to light brown, sometimes with shades of olive.^1–3

Experienced mushroom hunters immediately know when they have found a Gilled Bolete because it looks like a bolete from above but when they flip it over they are confronted with gills where they expected pores. The gills are not quite as organized as in true agarics; they are thick and widely-spaced and often have forks and cross-veins. Cross-veins are especially prominent where the gills meet the stipe. At that point, the cross-veins are sometimes dense enough to appear poroid. The gills are decurrent and colored yellow, but the color becomes marred with brownish spots as the mushroom ages. They usually stop just short of the pileus margin, leaving a sterile edge around the mushroom.^1–3 Interestingly, the gills easily separate from the pileus, much like the pore layer does in the boletes.²

The stipe of the Gilled Bolete is 2-9cm long and 0.5-1.5cm wide. It is roughly the same width for its entire length, although it is usually a bit wider at the top and a bit narrower at the bottom. The stipe is colored yellowish and is a bit darker than the gills. You usually find some flecks of reddish color in the stipe; sometimes the flecks turn a significant portion of the stipe reddish. Dig up the whole base and you will find yellow threads of mycelium attached to the mushroom just under the soil surface.^1–3

Like the boletes, P. rhodoxanthus is ectomycorrhizal. It forms relationships with hardwoods, most often with oaks and beeches. The mushrooms appear from summer through fall. Look for them on the ground in hardwood forests of North America; they often fruit alone but there may be a few in a small area.^1,2

Edibility

P. rhodoxanthus is considered edible, although I don’t know anyone who goes out of their way to eat this mushroom. Additionally, you usually find only a few mushrooms at once, so there is rarely enough to make a dish.³ The ones I find are typically buggy, dirty, dried up, or afflicted with some other condition that makes them not very appetizing. If you want to try these mushrooms, expect to do a fair amount of cleaning beforehand.

Similar Species

There are actually several species of Phylloporus in North America, although mycologists cannot agree on how many. The one described above is usually designated P. rhodoxanthus spp. americanus to distinguish it from the subspecies P. rhodoxanthus spp. pelletieri, which stains blue when bruised. P. pelletieri is the species that is most common in Eurasia. It is occasionally found in North America and can be distinguished from P. rhodoxanthus by its yellow cap and attached gills that only slightly run down the stipe. The species P. leucomycelinus grows in eastern North America; the white mycelial threads at the base of its stipe are the only visible difference between it and P. rhodoxanthus spp. americanus. P. arenicola is a rare species found on the West Coast under pines in sandy soil. Its gills are particularly well-developed for the genus and usually withdraw slightly from the stipe just before they meet it. P. boletinoides has the least developed gills of the genus. Cross-veins are abundant in that species and nearly make the mushroom look like it has pores (‘boletinoid’).^2,4

Taxonomy

Even the scientific name of this mushroom cannot decide whether it is an agaric or bolete. The genus name Phylloporus is composed of the words phyllus, meaning ‘gill’,* and porus, obviously meaning ‘pore’.⁵** So where does this mushroom actually belong?

Originally, mycologists thought the Gilled Bolete represented a transitional stage that was left behind when the true agarics diverged from the pored mushrooms.⁶ However, this idea never had much support. For one, the microscopic features of P. rhodoxanthus clearly linked it with the boletes;⁴ if the hypothesis were correct, the Gilled Bolete’s microscopic features should have been intermediate between the boletes and core agarics. Another piece of evidence against the hypothesis is the fact that P. rhodoxanthus can be parasitized by the fungus Hypomyces chrysospermus. H. chrysospermus (commonly known as “the Bolete Eater,” FFF#158) grows only on mushrooms related to boletes. It can parasitize some mushrooms from a few true agaric lineages, but – like Phylloporus – all those lineages are descended from the bolete group (Boletales) and not the core agaric group (Agaricales).

Using DNA evidence, mycologists can definitively say that P. rhodoxanthus is not related to the core agarics. The main bolete and main agaric linages diverged a long time ago; much more recently, Phylloporus split from the core group of boletes. Since the closest relatives of Phylloporus are normal boletes with pores, the group represents an independent acquisition of gills and is yet another example of convergent evolution in kingdom Fungi.⁶

However, the original hypothesis that P. rhodoxanthus represents a trasitional stage might not be completely incorrect. If evolution continues to select for traits characteristic of gilled mushrooms, Phylloporus could become considered a true agaric genus sometime in the distant future. Today, the genus is therefore a useful snapshot of the evolutionary process that other agaric lineages within the Boletales are likely to have followed.

Kingdom	Fungi
Division	Basidiomycota
Subdivision	Agaricomycotina
Class	Agaricomycetes
Subclass	Agaricomycetidae
Order	Boletales
Family	Boletaceae
Genus	Phylloporus
Species	Phylloporus rhodoxanthus (Schwein.) Bers.7

* Actually, phyllus means ‘leaf’, but it refers to gills when used in names of agarics.⁵

** As a side note, the specific epithet rhodoxanthus is composed of rhodon, meaning ‘rose-colored’, and xanthos, meaning ‘yellow’. This is apparently a reference to the Gilled Bolete’s color scheme.^5,8