Protomerulius dubius (Bourdot & Galzin) Spirin & V. Malysheva

Basidiomycota: Agaricomycotina: Agaricomycetes: Auriculariales: incertae sedis: Protomerulius MycoBank symbolIndex Fungorum symbol


If you encounter a crust with septate basidia, there's good news and bad news. The good news is that your options dwindle considerably to species in Auriculariales, Cantharellales, and Sebacinales (assuming it's not actually a jelly in Dacrymycetes or Tremellomycetes). The bad news is that these are all difficult orders to work with and no comprehensive keys exist for species-level identification.

Given that I am inexperienced with heterobasidiomycetes and the go-to general crust references do not treat them at all, I was initially helpless in identifying this specimen even to order level. Then, browsing through Elia Martini's extraordinary illustrations, I just happened upon one that I knew instantly to be a match: Protomerulius dubius. The spiky balls are a really prominent feature in this crust and the drawing of them was a dead ringer. Crystals appear in such diverse forms across so many crusts. Why does one species coat its hyphae in sharp needle crystals or blocky chunky ones, another produce crystal enshrouded lance-shaped cystidia, and yet another make giant spiky balls? The chemical control of these fungi over the growth of crystals is exquisite, but to my knowledge we have no idea how or why they do this.

Like many crusts, we know basically nothing about this species except for its taxonomy and morphology, including its full biogeographic range. It appears that this is only the fifth collection from North America and the first since 1997.

Heterochaete dubia and Heterochaetella dubia are synonyms.


Ecology: Growing on the underside of a brown-rotted hardwood log; the nutritional mode of this species is unknown.

Basidiocarp: When young, whitish to cream-colored to ochraceous; hymenophore effused, smooth to finely reticulate or minutely granulose, pruinose to byssoid (floccose) to subceraceous in texture; as it ages, the basidiocarp turns darker brown and waxy to gelatinous (not observed); margin indeterminate.

Chemical reactions: NA

Spore print: White.

Hyphal system: Monomitic; generative hyphae reported as clamped, but very difficult to observe; subicular hyphae embedded in the substrate, giving rise to pseudocystidia; very prominent and characteristic (for the genus) spiky crystal balls (stellate agglomerations) present throughout the basidiocarp, up to 24 µm across.

Basidia: Cruciate (longitudinally septate heterobasidia), sphaeropedunculate (ellipsoid to subglobose basidia with a stalk), with two to four sterigmata depending on the number of cells per basidium; length (9.2) 9.7–12 (13) µm (length may include stalk, difficult to differentiate), width (5.6) 5.9–7.4 (7.9) µm, x̄ = 10.8 ✕ 6.6 µm; sterigmata length (3.7) 4.8–8.2 (9) µm (n = 10 basidia and 10 sterigmata).

Basidiospores: Ellipsoid to cylindrical with a flattened adaxial side, smooth, thin-walled, inamyloid, acyanophilous, repetitive; length (6) 6.3–7 (7.5) µm, width (3.7) 3.9–4.2 (4.4) µm, x̄ = 6.6 ✕ 4.1 µm, Q (1.5) 1.5–1.7 (1.9) (n = 30 basidiospores).

Sterile structures: Thick-walled pseudocystidia (tramal cystidia) arising from the subiculum, typically wrapped around each other and bundled together in fascicles as they project beyond the hymenium in the center of minute "teeth" or mounds (giving the basidiocarp its granulose appearance), becoming thin-walled and sometimes tapering at the apex and forming bristle-like tips; up to hundrds of microns long, width 3.0–4.5 (5.9) µm (n = 10 cystidia).

Sequences: NA

Notes: All microstructures were measured in 5% KOH stained with phloxine B. The specimen studied here fits well with Protomerulius dubius according to Spirin et al. (2019). Although Protomerulius dubius has been documented four times before in North America (MyCoPortal, as Heterochaete dubia), Spirin and colleagues state that this species is currently only known from Europe. Therefore, my determination is tentative, pending sequence data.

Specimens Analyzed

ACD0184, iNat33478375; 24 September 2019; Ann Arbor, Washtenaw Co., MI, USA, 42.2951 -83.7243; leg. & det. Alden C. Dirks, ref. Spirin et al. (2019); University of Michigan Fungarium ——.


Spirin, V., Malysheva, V., Miettinen, O., Vlasák, J., Alvarenga, R. L. M., Gibertoni, T. B., … Larsson, K. H. (2019). On Protomerulius and Heterochaetella (Auriculariales, Basidiomycota). Mycological Progress, 18(9), 1079–1099.


Crusts and Jells




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Protomerulius dubius on the underside of a log with brown rot.

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Young basidiocarps are whitish, creamy, or ochraceous in color.

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The basidiocarp is subceraceous when young and fresh with a granulose apperance, which is a result of the crust growing over an uneven substrate and also the presence of miniscule mounds.

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A look at the hymenophore, consisting of thick-walled pseudocystidia that are rooted in the substrate and project through the hymenium as bundles or fascicles. The trama is embedded with spiky crystal balls.

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Bundled pseudocystidia.

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While most pseudocystidia have regular hyphal ends, some narrow into frayed-looking, hyphidial tips.

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A closer look at the bundled pseudocystidia and embedded crystal balls

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The subiculum is in the wooden substrate from which the pseudocystidia arise.

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Spiky crystal ball.

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Longitudinally septate basidium.

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Basidia at the base and sides of a pseudocystidia fascicle.

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An example of a repetitive basidiospore growing a sterigma and a second basidiospore.