Myxarium denticulatum Spirin, 2025

Spirin, Viacheslav, Malysheva, Vera, Viner, Ilya, Alvarenga, Renato Lúcio Mendes, Grebenc, Tine, Gruhn, Gérald, Savchenko, Anton, Grootmyers, Django, Ryvarden, Leif, Vlasák, Josef, Larsson, Karl-Henrik & Nilsson, R. Henrik, 2025, Additions to the taxonomy of the Auriculariales (Basidiomycota) with pedunculate basidia, MycoKeys 120, pp. 339-392 : 339-392

Myxarium denticulatum Spirin sp. nov.

Fig. 9 C

Holotype.

Sweden. Bohuslän: Hönö, Ersdalsvägen , Sorbus sp. (recently fallen corticated branch), 12. VIII. 2024 Spirin 17365 * ( GB, isotype – H).

Etymology.

Denticulatus (Lat., adj.) – possessing small teeth.

Description.

Basidiocarps effused, small and inconspicuous, up to 8 mm in widest dimension, semitranslucent, gelatinous, greyish, adnate, almost invisible in dry condition; hymenophore hydnoid, spines rather regularly arranged, acute, single, up to 0.1 mm long, 5–6 per mm; subiculum watery greyish, semitranslucent, 0.02–0.03 mm thick; margin gradually thinning-out. Hyphal structure monomitic, hyphae hyaline, clamped; subicular hyphae thin-walled, subparallel, hardly discernible, 1–2 μm in diam., subhymenial hyphae ascending or interwoven, thin-walled, and quickly collapsing, (1.0 –) 1.1–2.5 (– 2.6) μm in diam. (n = 20 / 1). Crystals absent. Hyphidia abundant, richly branched, 1–1.5 μm in diam. in the apical part, distributed among basidia and partly covering basidial cells. Basidia two – four-celled, longitudinally septate, broadly ellipsoid to subglobose, pedunculate, (6.5 –) 7.0–8.6 (– 8.9) × (5.8 –) 6.0–7.5 (– 7.8) μm (n = 20 / 1), scattered, stalk up to 15 × 1.5–2 μm, sterigmata up to 10 × 2–2.2 μm, sometimes bifurcate. Basidiospores narrowly ellipsoid to broadly cylindrical, occasionally slightly concave on the ventral side, (5.0 –) 5.1–7.0 (– 7.3) × 3.0–4.2 (– 4.8) μm (n = 33 / 1), L = 6.18, W = 3.57, Q’ = (1.2 –) 1.4–2.0 (– 2.1), Q = 1.75, usually with a large central oil drop.

Distribution and ecology.

Europe ( Sweden); fallen angiosperm branches ( Sorbus ).

Remarks.

Myxarium denticulatum produces extremely thin basidiocarps bearing tiny spines, which are detectable only under magnification. Due to its regularly hydnoid hymenophore, it can be confused with M. legonii . However, the latter species bears more pronounced spines and has on average narrower basidiospores (see description below). Another similar species, Myxarium evanidum Spirin & K. H. Larss. , possesses irregularly arranged spine-like outgrowths on the hymenial surface. These outgrowths disappear completely after drying in M. evanidum ; in contrast, spines of M. denticulatum are visible even in dried material. Under the microscope, M. denticulatum and M. evanidum are almost indistinguishable. Phylogenetically, M. denticulatum , M. evanidum , and M. legonii are not closely related (Fig. 5 ). Myxarium denticulatum is currently known only from the type locality, but it is presumably overlooked elsewhere due to its highly diminutive basidiocarps.

Figures

Figure 5. Bayesian majority-rule consensus ITS-based tree of the Myxarium clade. Support values are given as Bayesian posterior probabilities. Myxarium crystallinum was used as an outgroup.

Figure 9. Microscopic structures of the Myxarium legonii complex: A. Myxarium legonii (specimen Spirin 16419) (hymenial cells and subhymenial hyphae); B. M. legonii (specimen Spirin 16419) (crystals); C. M. denticulatum (holotype) (basidiospores); D. M. legonii (specimen TAAM 132119) (basidiospores); E. M. spiniferum (holotype) (basidiospores). Scale bar: 10 µm.