Marasmius tantulus Wannathes, Desjardin & Lumyong
Pileus < 1.5 mm diam., convex, shallowly umbilicate, without papilla, striate, glabrous, dull, greyish cream. Context cream, thin. Lamellae adnate to a poorly developed collarium, remote (3 -4), narrow, cream, nonmarginate, non-intervenose. Stipe < 1 × < 0.3 mm, eccentric, tapering upward with subbulbous at base, subvelutinous, insititious, brown; rhizomorphs absent. Odor and taste not distinctive. Basidiospores (17-)20-22 × 4 μm [xm =20.1 ± 1.3 × 4.0 ± 0 μm, Q = 4.3-5.5, Qm = 5.0, n = 9 spores, s = 1 specimen], fusoid to narrowly clavate, smooth, hyaline, inamyloid, thin-walled. Basidia not observed. Basidioles fusoid to clavate. Cheilocystidia common, of Rotalis-type broom cells; main body 18-23 ×10-18 μm, subglobose to broadly clavate, hyaline, inamyloid, thin-walled; divergent setulae 1-2 × 1 μm, knob-like to conical, obtuse, dense over upper half of cell, hyaline, thin- to thick-walled. Pleurocystidia absent. Pileipellis hymeniform, not mottled, composed of Rotalistype broom cells; main body 27-35 × 16-22 μm, broadly clavate, subglobose, pyriform, or irregular in outline, hyaline, inamyloid, thinwalled; divergent setulae 1-2 × 1 μm, knob-like to conical, obtuse, often dense over upper half bof cell, yellow, thick-walled. Pileus trama interwoven, inamyloid. Lamellar trama hyphae 3-6 μm diam., interwoven to regular, cylindrical, smooth, hyaline, inamyloid, thinwalled, non-gelatinous. Stipitipellis hyphae 5-10 μm diam., parallel, cylindrical with scattered finger-like projections, light yellow to brownish yellow, smooth, weakly dextrinoid to dextrinoid, thin-walled, non-gelatinous. Stipe trama hyphae 4-6 μm diam., subparallel, cylindrical, hyaline, smooth, inamyloid, thinwalled, non-gelatinous. Caulocystidia absent. Clamp connections present in all tissues.
Habit, habitat and known distribution: Scattered to gregarious on wood, Northern Thailand.
Material examined: Thailand. Chiang Mai Province, Doi Inthanon National Park Junction of Highway 1,009 and road to Mae Chaem, N 18º 31.6’ E 98º 29.6’ alt. 1,703 m., 28 July 2004, N. Wannathes 239 (CMU: holotype).
Notes: Marasmius tantulus is characterized by very tiny basidiomes with greyish cream pilei less than 1.5 mm diam, collariate and remote lamellae (3-4), an eccentric stipe less than 1 mm long, and very long basidiospores with mean 20.1 × 4.0 μm. Few species of sect. Marasmius subsect. Marasmius have been described with basidiospores longer than 11 μm, and none were previously known with basidiospores 20-22 μm long. The most similar species are M. arimanus Dennis from Trinidad, and M. rosulatus Desjardin & R.H. Petersen from New Zealand. Marasmius arimanus differs in forming mummy brown pilei 2-3 mm diam. and basidiospores 11-14 × 3.5-4 μm (fide Singer, 1976). Marasmius rosulatus differs in forming pure white basidiomes with basidiospores 7.8-9.6 × 4.0-4.8 μm and grows on Cyperales leaves (Desjardin and Petersen,1989).
Fig. 1 Marasmius tantulus (N. Wannathes 239). 1. Basidiomes 2. Basidiospores 3. Basidioles 4. Cheilocystidia 5. Pileipellis 6. Stipitipellis, Scale bar 1 = 2 mm, 2-6 = 10 μm.
Wannathes N, Desjardin DE, Hyde KD, Perry BA, Lumyong S. 2009 - A monograph of Marasmius (Basidiomycota) from northern Thailand based on morphological and molecular ITS sequences data. Fungal Diversity, 37, pp.209-306.