Gymnopus omphalinoides J.P. Li, T.H. Li & Y. Li
MycoBank number: 837641
Basidiomata omphalinoid, collybioid or gymnopoid. Pileus 10–40 mm broad, membranous, hemispheric when young, becoming convex, plano-convex to applanate, generally umbilicate to sometimes slightly depressed at the centre, inflexed then straight or reflexed at margin, with a marginal zone often undulating with age, glabrous, radially striate or grooved towards the margin, orange (6B7) or reddish orange (7B7) to brown (7D8) overall when young, somewhat reddish orange (7B7) or dark brown (7F8), then paler towards the margin, white or pale orange (6A3) to light brown (6D4), often greyish orange (6B4) to dark brown (6F8) at the disc. Lamellae adnate, broad, ventricose to broadly ventricose, white when fresh, sometimes with greyish red (7B4) to brown (7E7) tint somewhere, margin entire to split and sometimes grooved, L = 12–17, l = 3–5. Stipe 10–30 mm long, 2–4 mm thick in the middle, central, cylindrical, or compressed, with dense basal mycelium when young that disappears when old, hollow, fibrous, glabrous, slightly longitudinally striate when old, rooting deep in the substrate, but eventually attaches to the stump, dull white to greyish red (7B4) when young, soon darker towards the base, white to reddish orange (7A7) at apex, finally entirely dark brown (7F8). Odour not distinctive.
Basidiospores [n=80] (3.5–) 4.0–5.5 (–6.0) × 2.5–3 (–3.5) μm (average= 4.63 × 2.93 μm, E = 1.33–1.83 (–2), Q=1.58), obovoid, ellipsoid to subellipsoid, sometimes amygdaliform. Basidia [n=20] 17–31 × 3–5 μm, clavate, 4-spored. Basidioles [n=20] 17–32 × 4–5.5 μm, clavate, cylindrical. Lamellar edge sterile. Cheilocystidia [n=20] 17–32 × 4–10 μm, irregularly clavate, sphaeropedunculate or almost so, with tendency to be inflated, with or without finger-like apical projection(s) or more or less diverticulate elements. Pileipellis a cutis composed of cylindrical, thin-walled hyphae, up to 12.5 μm wide, smooth or with scattered diverticula, hyaline to slightly brownish; Rameales-like structures present, rare to abundant; terminal cells short, broad, mostly inflated, vesiculose or pyriform to cystidioid (clavate), obtuse and sometimes diverticulate, mixed with a few irregularly branched, slightly coralloid elements and some resembling Dryophila-type structures. Stipitipellis a cutis composed of cylindrical, slightly thick to thick-walled, smooth, non-dextrinoid, parallelly arranged hyphae, up to 12 μm wide, with or without Rameales-like structure. Caulocystidia absent. Clamp connections present.
Habitat: Saprotrophic, gregarious or in small clusters, usually rooting around the roots and stumps in broadleaf forests.
Additional specimens examined: China, Guangdong Province, Guangzhou City, Tianluhu Forest Park, longitude and latitude not recorded, alt. not recorded, 4 April 2019, T.H. Li, W.Q. Deng, J.Y. Xu & J.P. Li (GDGM 44411); Guizhou Province, Tongren City, Fanjingshan National Nature Reserve, 14 July 2019, J.Z. Xu (HMJU 00506); Yunnan Province, Pu’er City, Meizihu Reservoir, alt. 1300 m, 19 September 2019, M. Zhang, T. Li & J.Y. Xu (GDGM 78483); Yunnan Province, Maguan County, Nanlao Village, 5 August 2017, X.H. Wang (KUN-HKAS 107312).
Notes: Gymnopus omphalinoides is a very distinct species due to its generally omphalinoid basidiomata, by a membranous and striate or grooved, reddish brown to brown pileus that becomes paler with age, by the broad, adnate, ventricose lamellae that are sometimes split to grooved at the edge, and by a pileipellis often with scattered cystidioid (clavate) or vesiculose to pyriform terminal elements. Collection GDGM 78318 is characterised by having cheilocystidia with more or less finger-like apical projection(s) and by a pileipellis with scattered Rameales-like structures, but the collection GDGM 44411 differs in its cheilocystidia with diverticulate elements and pileipellis with more Rameales-like structures. Among the known species of Gymnopus with a striate or grooved pileus and ventricose lamellae, G. bisporus (J. Carbó & Pérez-De-Greg.) J. Carbó & Pérez-De-Greg., G. dentatus Murrill, G. discipes (Clem.) Murrill, G. dysosmus Polemis & Noordel., G. fuscotramus Mešić, Tkalčec & Chun Y. Deng, G. pubipes Antonín, A. Ortega & Esteve-Rav. and G. volkertii are similar to the new species. However, G. bisporus, belonging to sect. Levipedes, has a brown to reddish brown pileus and larger basidiospores (9.0–11 × 4.5–5.5 μm), and true cheilocystidia are absent (Antonín and Noordeloos 2010); G. dentatus has a dentate pileus margin, a white stipe and larger basidiospores (7–8.5 × 6–7 μm), growing on lawns (Murrill 1916); G. discipes has free lamellae and a white stipe arising from a hypogaeous disk (Murrill 1916); G. dysosmus, sect. Impudicae, has garlic-smelling basidiomata, dark greyish brown lamellae, larger basidiospores (8.0–11 × 3.3–4.5 μm), and caulocystidia (Antonín and Noordeloos 2010); G. fuscotramus, belonging to sect. Vestipedes [= Marasmiellus fuscotramus (Mešić, Tkalčec & Chun Y. Deng) J.S. Oliveira], has abundant rhizomorphs, larger basidiospores (8.2–9.6 × 3.7–4.4), and pale grey-brown lamellar and pileus trama (Mešić et al. 2011); G. pubipes, sect. Levipedes, has deeply emarginate to adnexed lamellae and an entirely pubescent stipe with numerous caulocystidia (Antonín and Noordeloos 2010); and G. volkertii has a umbonate and estriate pileus, adnexed lamellae, and larger basidiospores (8.2–9.6 × 3.7–4.4 μm), growing on lawn.
Fig. 1 Phylogram generated by ML analysis of the combined dataset (ITS1-5.8S-ITS2-LSU region). ML-BP ≥ 70 % and BI-PP ≥ 0.95 are shown above and below the branches, respectively.