Entoloma sequestratum T.F. Elliott, S.L. Stephenson, Karun. & D. Nelsen.
Fresh Sporocarps up to 15 mm × 10 mm, globose to irregularly globose to somewhat oblong. Stipe absent. Peridium/Pileus with occasional irregular pits, overall smooth, sometimes with small invaginated pit or stub at the base, overall white to off-white and thin in section (< 1 mm), white and appearing solid in color throughout. Gleba/Hymenophore loculate to labyrinthiform, when young off-white to faintly pinkish but darkening with maturation, lamellae fused to form compact and stuffed locules with whitish hyphae when young, but with maturation locules becoming empty and labyrinthiform and reaching nearly 0.5 mm broad, hymenophoral trama darker in color than the hymenial layer. Overall odor not distinctive. Peridiopellis/Pileipellis 75–162.5 μm thick, with two layers, outer layer thinner (28–50 μm), reddish brown, composed of compacted interwoven hyphae up to 5 μm broad, thin-walled, not gelatinized, with intermixed irregular granules, inner layer hyaline to pale yellow, interwoven with irregular hyphae up to 5 μm broad, thin-walled, not gelatinized. Hymenophoral trama 12.5–27.5 μm thick, consisting of hyaline to pale yellow hyphae, densely interwoven and irregularly shaped hyphae up to 5 μm broad, thin-walled, not gelatinized, with occasional clusters of inflated thin-walled globose cells 2.5–12.5 μm broad. Clamp connections not observed. Basidia irregularly to broadly clavate, up to 7.5 × 12.5 μm, infrequent when mature, two sterigmata observed per basidium; however, mature basidia almost entirely absent, apparently collapsing after producing spores. Basidiospores hyaline, complex heterodiametrical, cuboid, most often 4–5 angles in side-view, 7.5–12.5 × 7.5–11(−12) μm excluding the apiculus (up to 2.5 μm), spore outer wall nearly 2.5 μm thick, average spore size 11.75 × 11μm (n = 20).
Species examine: Thailand, ca. 50 km north of the city of Chiang Mai between Pa Pae and Mae Taeng, on the grounds of the Mushroom Research Centre, just past the dining hall (19° 07.200′ N, 98° 44.044′ E), 17 Jun. 2012, T.F. Elliott (holotype MFLU 12-2045).
Additional materials examined: All collections examined were made by the first author within a few meters of the type, but all were collected on different days. Thailand, ca. 50 km north of the city of Chiang Mai, between Pa Pae and Mae Taeng on the grounds of the Mushroom Research Centre, just past the dining hall (19° 07.200′ N, 98° 44.044′ E). 1 Jun. 2012, T.F. Elliott, MFLU 12-2085; idem., 1 Jun. 2012, T.F. Elliott, MFLU 12-2088; idem., 27 Jun. 2012, T.F. Elliott, MFLU 12-2080.
Notes: We have no direct evidence of the role of this fungus in forest ecosystems and whether or not it is mycorrhizal. There is not a lot known about the ecology of its close genetic relatives, so we can only hypothesize about its ecological function. It may be a decomposer or it may form ectomycorrhizal associations with trees in one or more of the following genera: Lithocarpus, Dipterocarpus, or Castanopsis (all of which occur in the area around the type collection). Many of the sequestrate species in the genus Entoloma are relatively similar; however, there are several factors that make this novel species distinct. The most morphologically similar species to E. sequestratum is E. gasteromycetoides (syn. Richoniella pumila and R. pumila f. bispora). These two species are most easily separated on the basis of geography, with E. gasteromycetoides known only from Australia and New Zealand and E. sequestratum only from northern Thailand. They also differ genetically and in morphology. Entoloma gasteromycetoides has larger sporocarps that can be up to 25 mm diam, whereas E. sequestratum has sporocarps no bigger than 15 mm × 10 mm. In Cunningham’s original description of E. gasteromycetoides, he describes that the peridium/pileus often disappears with maturation, leaving the gleba/hymenophore exposed; this is a feature never observed in E. sequestratum. Cunningham also reports that the tramal plates of E. gasteromycetoides were 75–150 μm, whereas in E. sequestratum they are 12.5–27.5 μm. For further clarification on species differences, see the key and discussion provided in the following section.
Fig. 1 Maximum Likelihood (ML) tree generated using RAxML based on the combined dataset of ITS, LSU, RPB2 and mtSSU sequences. The analysis ran for 1 000 bootstrap replications; ML bootstrap support values ≥ 50 % are given above each of the branches. The new species Entoloma sequestratum is indicated in red bold font.