Basidiodendron arachnoideum Q. Yuan & C. L. Zhao 2026, sp. nov.

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Basidiodendron arachnoideum Q. Yuan & C. L. Zhao sp. nov. Figs 6, 7, 8 Diagnosis. Basidiodendron arachnoideum is characterized by its arachnoid to farinaceous basidiomata, grandinoid and white to slightly cream hymenial surface, as well as ellipsoid to short clavate basidia lacking longitudinal septa. Etymology. Arachnoideum (Lat.). Referring to the arachnoid hymenial surface of the type specimen. Type. CHINA • Yunnan Province, Tengchong, Tuantian Town, Gaoligong Mountain National Nature Reserve, 24°43'N, 98°33'E, elevation 1,360 m asl., on fallen angiosperm branch, leg. C. L. Zhao, 7 Jul. 2024, CLZhao 39268 (SWFC 00039268, holotype). Description. Basidiomata annual, resupinate, adnate, arachnoid to farinaceous, without odor or taste when fresh, up to 9 cm long, 4 cm wide, and up to 100 µm thick. Hymenial surface grandinoid, white when fresh, turning white to slightly cream upon drying. Sterile margin narrow, white, up to 1 mm. Hyphal system monomitic; generative hyphae with clamp connections, colorless, thin-walled, smooth, interwoven, 1–1.5 µm in diameter, IKI –, CB –; tissues unchanged in KOH. Leptocystidia clavate, colorless, thin-walled, smooth, 9.5–12 × 2.5–4 µm. Basidia ellipsoid to short clavate, with four sterigmata and a basal clamp connection, 7.5–8.5 × 5–5.5 µm, involucres absent. Basidioles dominant, similar to basidia in shape, but slightly smaller. Basidiospores subglobose, with a short, often eccentric and asymmetric apiculus, colorless, thin-walled, smooth, usually with one oil drop, IKI –, CB –, (3 –) 3.5–4 × 3–3.5 µm, L = 3.55 µm, W = 3.28 µm, Q = 1.08–1.10 (n = 60 / 2). Additional specimen examined (Paratype). CHINA • Yunnan Province, Tengchong, Tuantian Town, Gaoligong Mountain National Nature Reserve, 26°56'N, 98°42'E, elevation 1,580 m asl., on fallen angiosperm branch, leg. C. L. Zhao, 7 Jul. 2024, CLZhao 50722 (SWFC 00050722). Notes. The combined ITS + nrLSU + TEF 1 phylogenetic analysis revealed that Basidiodendron arachnoideum was resolved as sister to B. spiculosum (Fig. 2). However, B. spiculosum can be delimited from B. arachnoideum by its waxy basidiomata with a smooth hymenial surface and spiny, larger basidiospores (6.9–8.2 × 7.1–8.9 µm vs. 3.5–4 × 3–3.5 µm; Spirin et al. 2021). Morphologically, B. arachnoideum resembles several taxa, viz., B. alni, B. cinerellum, and B. glaucum Spirin & K. H. Larss., in having farinaceous basidiomata. However, B. alni is distinct from B. arachnoideum by having a smooth hymenial surface and larger basidiospores (4.1–5 × 4.2–5 µm vs. 3.5–4 × 3–3.5 µm; Spirin et al. 2020). Basidiodendron cinerellum differs from B. arachnoideum by its smooth hymenial surface and larger basidiospores (4.9–7.2 × 5.2–7.7 µm vs. 3.5–4 × 3–3.5 µm; Spirin et al. 2021). Basidiodendron glaucum can be separated from B. arachnoideum by its smooth hymenial surface and larger basidiospores (5.1–6.8 × 5.2–7 µm vs. 3.5–4 × 3–3.5 µm; Spirin et al. 2021). The generic concept of Basidiodendron has historically relied heavily on specific basidial morphologies, particularly the presence of longitudinally septate basidia. Interestingly, re-examination of B. arachnoideum revealed an absence of longitudinal septa in its mature basidia (Fig. 8). While this might traditionally exclude it from the genus, the phylogeny inferred from the ITS + nrLSU + TEF 1 sequence data securely nests B. arachnoideum within the well-supported main Basidiodendron clade (Fig. 2). As demonstrated in the phylogenetic tree, this species forms a distinct lineage separate from the type species. This finding indicates that structural features such as basidial septation may undergo secondary loss during the evolutionary radiation of specific clades within the genus. Consequently, molecular phylogeny provides a more reliable basis for the generic placement of such morphologically atypical taxa than reliance solely on individual morphological traits.

Ce résumé expose les affirmations des auteurs. BNTIC ne l’interprète pas comme une validation indépendante des résultats.