Inocybe pusillima (Corner & E. Horak) Garrido (1988: 77)

Horak, Egon, Matheny, P. Brandon, Desjardin, Dennis E. & Soytong, K., 2015, The genus Inocybe (Inocybaceae, Agaricales, Basidiomycota) in Thailand and Malaysia, Phytotaxa 230 (3), pp. 201-238 : 223-224

14. Inocybe pusillima (Corner & E. Horak) Garrido (1988: 77) . Fig. 13a–f ; Pl. 8f

Basionym: Astrosporina pusillima Corner & E. Horak in Horak (1979: 163).

Etymology: pusillimus (Lat.), very small.

Pileus 3–8 mm wide, at first conical, becoming convex and plane-umbonate in age; surface dry, veil remnants absent, disc tomentose, radially fibrillose, with tiny scales near the non-striate margin; at first beige, becoming dark brown (6F6–8) at disc, brown (6E5–6) elsewhere, margin (in young specimens) with distinctive, persisting, whitish, bristle-like fibrils. Lamellae 20–24 reaching stipe, 3 (–7) lamellulae, subdistant, ascending or adnexed, up to 1 mm wide, pale brown or umber (6D4); edges entire or subfimbriate, concolorous. Stipe 5–8 (–12) × 0.5–1 mm, central, cylindrical, equal, curved; surface dry, pruinose overall, pale brown or dark brown (6F5–8) overall, weakly reddening on bruising; cortina absent; context extremely thin, concolorous, unchanging or weakly reddening upon exposure. Odor not distinctive. Taste not recorded.

Basidiospores 5.5–7 × 4.5–5 μm, nodulose, yellow-brown. Basidia 18–24 × 6–7 μm, 4-spored, clavate or subcylindrical. Cheilocystidia (32–) 40–50 × 9–15 μm, fusoid, metuloid, walls up to 2.5 μm thick at apex, hyaline, crystals present; paracystidia 10–15 × 7–9 μm, clavate or vesiculose, with thin, hyaline walls. Pleurocystidia size and shape like cheilocystidia. Caulocystidia 20–40 × 10–14 μm, shape like cheilocystidia, intermixed with clavate-vesiculose cells. Pileipellis a cutis of stiff, repent, cylindrical hyphae, 2–5 μm wide, terminal cells not differentiated, with non-gelatinized hyaline thin walls, pigment absent; subpellis hyphae slender fusoid or ovoid, 16–25 (–30) μm wide, wall encrusted with brown pigment; oleiferous hyphae absent. Clamp connections present.

Habitat: Singly on soil in tropical montane forest (dominated by Dipterocarpus , Castanopsis , with scattered Pinus kesiya ), 1000–1200 m elev.

Known distribution: Papua New Guinea (type), Singapore, northwest Thailand.

Specimens examined: PAPUA NEW GUINEA. Eastern Highlands: Ayura, on soil in tropical montane rain forest (dominated by Castanopsis ), 27 May 1973, leg. E. Horak 73-273 (ZT, holotype!). SINGAPORE. Bukit Timah: Fern Valley, on rotten wood in tropical lowland dipterocarp and fagalean forest, 19 Apr. 1941, leg. E.J.H. Corner s.n. (E; ZT78-043). THAILAND. Mae Hong Son Prov.: S of Mae Hong Son, Hwy. 108, near 235 km marker, road to TV Station 3–9, ca. 1125 m elev., on soil in tropical montane forest (dominated by Dipterocarpus obtusifolia , with scattered Pinus kesiya ), 30 Jun. 2002, leg. E. & A. Horak (ZT10093). Chiang Mai Prov.: Doi Suthep, Sangasabhasri Lane, 1200 m elev., on soil in tropical montane forest (dominated by Dipterocarpus , Castanopsis ), 4 Jul. 2002, leg. D.E. Desjardin & E. Horak (ZT10130) GenBank accession no. GQ893005, GQ892060; Hwy 1095 near 27 km marker, Pathummikaram Temple area near Ban Pha Deng, N19˚06‘28.8‘‘, E98˚44‘47.3‘‘, 1050 m elev., on soil in tropical montane forest (dominated by Dipterocarpus ), 27 Jun. 2007, leg. D.E. Desjardin (DED8145, SFSU) GenBank accession no. GQ893004, GQ892959.

Notes: Inocybe pusillima was first described from tropical montane fagalean rain forest in Papua New Guinea, and subsequently specimens collected in Singapore were discovered in the herbarium of E.J.H. Corner kept in Edinburgh, U.K. ( Horak 1980). We now report the species from Thailand. In both Papua New Guinea and Thailand, I. pusillima was gathered several times in tropical dipterocarp and fagalean forests, thus suggesting this Inocybe may be widely distributed in various habitats in southeast Asia and Australasia.

The most distinctive macroscopical character of I. pusillima is the silvery coarse fibrils on the pileus that are responsible for the conspicuous strigose „hairs“ along its margin. Further distinctive features of the basidiomes are the entirely pruinose stipe and the remarkably small nodulose basidiospores. The combination of an equal pruinose stipe, small size, and small nodulose basidiospores is consistent with other species of Inocybe section Petiginosae . Indeed, LSU sequence analysis strongly supports a phylogenetic relationship between I. pusillima , I. subexilis (Peck) Sacc. , and a sequence that is mislabeled I. petiginosa from North Carolina. Unpublished results that include French and Swedish representatives of I. petiginosa (Fr.) Gillet corroborate these findings. LSU sequences of two samples of I. pusillima exhibit substantial heterogeneity ( Fig. 1 ). Sequences of this species from elsewhere in southeast Asia, especially the type locality of Papua New Guinea, require comparison.

Figures

FIGURE 1. nLSU ML phylogeny of the Inocybaceae (with outgroups pruned) illustrating the autonomous placement and highly polyphlyetic nature of Thai and Malay species of Inocybe in bold (note that I. petchii and I. gemina are not present in the tree). Numbers above and below branches represent ML bootstrap values.

PLATE 8. SEM images. a. Basidiospores of Inocybe hydrocybiformis (ZT10077); scale bar = 2 μm. b. Basidiospores of Inocybe stellata (ZT10097, holotype); scale bar = 2 μm. c. Basidiospores of Inocybe thailandica (DED8049, holotype); scale bar = 5 μm. d. Hymenial cystidia of Inocybe angustifolia (ZT10149); scale bar = 10 μm. e. Basidiospores of Inocybe brevisquamulosa (ZT10102, holotype); scale bar = 2 μm. f. Basidiospores of Inocybe pusillima (ZT10130); scale bar = 2 μm. g. Basidiospores of I. fragilissima (DED8061, holotype); scale bar = 2 μm. h. Basidiospores of Inocybe ornata (ZT10107, holotype); scale bar = 3 μm. i. Basidiospores of Inocybe parvibulbosa (ZT10099, holotype). Scale bar = 2 μm. Photos by U. Jauch & M Kirchmair.

FIGURE 13. Macroscopic and anatomical features for Inocybe pusillima (ZT10130). a. Basidiomes. b. Basidiospores. c. Basidia. d. Cheilocystidia and pleurocytidia. e. Caulocystidia. f. Pileipellis. Scale bars: basidiomes = 20 mm, basidiospores = 10 μm, basidia and cystidia = 20 μm, pileipellis = 40 μm. Drawings by Egon Horak.