Gymnopilus rimopileus R.L. Zhao, B. Cao & W.Q. Yang, in Cao et al., Fungal Diversity 132: 539 (2025)

Index Fungorum number: IF 571925; Mycobank number: MB 571925; Facesoffungi number: FoF 17122; Figs. 1, 2

Etymology – ‘rimopileus’ refers to the cracks that develop along the margin of the mature pileus.

Holotype – HMAS 287453.

Pileus 4–19 mm diam, hemispherical, later shallowly convex, yellowish brown to brown (5B8–6C8), surface smooth and glabrous, with undulating and rimose margin, not scaly. Context thin, yellowish brown (1A4), unchanging upon cutting. Lamellae adnate to slightly adnexed, pale yellow to yellowish brown (2A3–2B3), crowded, finally ferruginous, edges even or entire. Stipe 0.9–1.1 × 0.1–0.2 cm, cylindrical, pale white to pale yellow at young (1A1–2A3), yellowish brown to brown when mature, hollow, terete, soft to solid, fragile, with or without cortinate veil. Odor indistinct.

Basidiospores (5.6–)6.2–7.3(–7.7) × (3.6–)4.4–5.4(–5.8) µm, Q = 1.35–1.41, ellipsoid, some amygdaliform, rarely ovoid, yellow–brown (2A7–5B7), verrucose with small warts, thick-walled, small suprahilar depression present. Basidia (16–)18–22(–24.6) × 5–6.7(–7) µm, clavate, two to four sterigmata, up to 2–4 µm in length, colorless, oil droplets present. Cheilocystidia (15–)16–34(–36) × (4–)5–6.3(–7) µm, narrowly lageniform with inflated basal part, utriform to narrowly lageniform, some cylindrical to narrowly clavate, with obtuse or subcapitate apex, base pedunculate or pedicellate. Pleurocystidia 12.6–24.8(26) × (2)3.5 6.9(7.4) µm, narrowly clavate, lecythiform to narrowly utriform, capitate to sub-capitate, with obtuse or subcapitate apex, hyaline, oil droplets present, thin-walled, with well-defined basal clamp. Stipitipellis a cutis, hyphae 4–10 µm in diameter, septate, branched, thin-walled, clamp connections present. Pileipellis acutis of filamentous hyphae with some inflated cells, 7–14 µm in diameter, with very light rusty brown plasmatic pigment, septate, crowded, branched, clamp connections present, thin-walled, some inflated hyphae observed with constrictions at septa. Smell and taste not recorded.

Material examined – CHINA, Sichuan Province, Tibetan Autonomous Prefecture of Garzê, Derong county, Xiayong Nature Reserve, 28°22’38.07”N 99°21’4.34”E, 3451.27 m asl, 22 August 2020, collected by Mao-Qiang He and Mei-Qi Wang, ZRL20201778 (HMAS 287453, holotype) and ZRL20201838 (HMAS 287997).

GenBank – HMAS 287453: OR913460 (ITS), OR915150 (LSU), OR915213 (SSU), PP058967 (rpb2).

Habitat – Summer to autumn scattered or in clusters on rotten hardwood in later stages of decay.

Notes – Gymnopilus rimopileus is easily distinguished by its brownish, rimose pileus. Phylogenetic analyses in this study revealed that G. rimopileus forms a distinct lineage with well support, positioned as a sister species to G. picreus (Fig. 2). In the field G. rimopileus closely resembles G. picreus in macromorphology; both species exhibit similarly colored pilei and grow on dead coniferous trees. However, G. rimopileus can be differentiated by its dehiscent pileus and slightly sparser lamellae. Additionally, G. picreus has larger spores (8.7–9.2 × 5.1–5.5 µm) and basidia (26.7–30.3 × 6.6–7.9 µm) (Cho et al. 2016).

Figure 1 – Gymnopilus rimopileus (HMAS 287453, holotype). a Basidiomata. b, c Basidiospores. d Basidia. e Cheilocystidia. f Pleurocystidia. g Pileipellis. h Stipitipellis. Scale bars: a = 1 cm, b, c = 3 µm, d = 6 µm, e = 8 µm, f = 6 µm, g = 4 µm, h = 5 µm.

Figure 2 – Maximum Likelihood phylogram of Gymnopilus inferred from ITS, nrLSU, nrSSU and rpb2 sequences. The tree is rooted by Galerina marginata. Maximum Likelihood support values (> 70) and posterior probabilities (> 0.90) are shown on the branch (ML/PP). The new species is in red.