MycoBank no.: MB 825542, Faces of Fungi FoF 08037.
Basidiomata medium to large-sized. Pileus 60–115 mm diam, convex when young, applanate to slightly plano-concave in age; surface dry, finely tomentose, brownish grey (5C2) to black with patches of light brown (6D7) or white; margin straight to slightly reflexed; context 10–20 mm thick at center, 5–7 mm thick half-way between center and margin, white to yellowish white, bruising greyish (1B1) then slowly violaceous grey (15D2), with a yellowish to greenish tinge above stipe. Hymenophore poroid, adnate at first, then sinuate to sinuate and narrowly decurrent; tubes 3–7 mm long, white to yellowish white, bruising greyish (1B1) then slowly violaceous grey (15D2); pores 0.1–0.4 mm diam, angular, irregular surface, white, bruising dark grey (1F1). Stipe 65–85 mm long, 10–29 mm wide, central, solid, equal or tapering upwards to clavate or sub-bulbous, sometimes curved, densely reticulate over the upper 2/3; surface dry, background color cream (4A3) or white around the apex, darker towards the base, with more or less pronounced yellow to pale orange stains; reticulum pale grey (1B2), dark brown (6F7) or black; context fleshy fibrous, marbled white to yellowish white bruising greyish white (1B1) or olive green (3D7) tinged with silver white to greyish yellow (2B3–5), with some yellow or orange-yellow stains on the sides and base of stipe. Basal mycelium white. Sporeprint not obtained. Odor fungoid-sclerodermatoid. Taste slightly bitter.
Macrochemical reactions. pileipellis and stipitipellis yellow to orange-yellow in KOH solution, unchanging to yellowish in NH4OH solution; pileus context and stipe context pale yellow in KOH solution, unchanging to yellowish in NH4OH solution.
Micromorphology: Basidiospores [285/5/5] (9–)10–12(–13.5) × 4–5 µm, 10.95 × 4.52 µm on average, Q = (1.9–)2.1–2.7(–3), 2.42 on average, elliptical to subfusiform, greenish grey (1C2) in water and KOH, smooth. Basidia 26–40(–41) × (8–)9–11(–12) µm, 33.2 × 10 µm on average, clavate, four-spored, hyaline in KOH; sterigmata 3–5 μm long. Hymenophoral trama bilateral of the boletus type, composed of hyphae 4–10 μm in wide, hyaline with some rust-brown (6E8) hyphae in KOH. Cheilocystidia (35–)36–63 × (7–)8–13 µm, 50.9 × 11.4 µm on average, abundant, ventricose or subfusiform, usually with reddish brown to dark brown (7E–F8) intracellular pigments, without encrustations. Pleurocystidia same as cheilocystidia. Pileipellis a trichodermium about 100–150 μm thick, composed of hyphae 6–10 μm wide, greyish yellow (4C5) to reddish brown (8E8) in KOH; terminal cells 17–50 × 5–10 μm, subfusiform, clavate, or subcylindrical, with obtuse apex. Pileal trama made up of hyphae 5–9 μm wide, hyaline in KOH. Stipitipellis hymeniform, about 70–140 μm thick, composed of emergent hyphae, golden (4C6) to raw umber (5F8), or blackish in water and KOH, with 25–60 × 7–17 μm, clavate, ventricose, or subfusiform terminal cells. Stipe trama composed of longitudinally arranged, parallel hyphae, 4–10 μm wide, cylindrical, almost hyaline in KOH, brown to mustard brown (5E5–6) in water and ammonium solution. Clamp connections not seen in any tissue.
Habitat: Solitary to gregarious on the ground in forests with Lithocarpus spp., Castanopsis spp., Shorea spp., and Dipterocarpus spp.
Known distribution: Southern China and Thailand.
Material examined: THAILAND. Chiang Mai Province, Mae Taeng District, 30 Jun 2010, O. Raspé & S.C. Karunarathna, OR0049 (MFLU10-0881); —Pa Pae, 9 Jul 2010, O. Raspé & S.C. Karunarathna, OR0108 (MFLU10-0940); —, 12 Jul 2010, O. Raspé & S.C. Karunarathna, OR0127 (MFLU10-0959); —, 22 Jun 2018, B. Chuankid, BC0178 (MFLU18-1772) and BC0179 (MFLU18-1773); —, Mueang District, Doi Suthep-Pui National Park, 18°47’16’’N, 98°55’25’’E, elev. 860 m, 14 Aug 2015, S. Vadthanarat, SV0218 (BKF, SDBR-CMU).
Note: Macro- and micro-morphology of R. nigrogriseus specimens from Thailand match the description of the specimens from China (holotype, FHMU 2800), except for the pileipellis structure, which was reported to be a subcutis by Zeng et al. (2018), whereas we observed a trichodermium. This discrepancy may be due to observations being made on basidiomes at different stages of development. It is indeed common in boletes to have a trichodermium in young basidiomes which more or less collapses and may look like a subcutis with age. In addition, R. nigrogriseus is characterized by its pileus and stipe context changing brownish to fuliginous when injured, stipe with grey to black reticulum and white basal mycelium. When observed in KOH, greyish yellow to reddish brown pigments were found in terminal cells of the pileipellis. The hymeniform stipitipellis is composed of emergent hyphae, golden to raw umber, or blackish pigments in water and KOH. In the phylogenetic analyses of Zeng et al. (2018), a collection from Thailand (OR049) was included, which seemed to be related to R. nigrogriseus, but they did not study the morphology of this collection. The sequences of OR049 were first published in Raspé et al. (2016), who also did not give a morphological description of the material. Here, we provide both three-gene phylogenetic evidence and morphological evidence to support the identification of the original material from Thailand as R. nigrogriseus.
Fig. 1 Maximum likelihood phylogenetic tree inferred from the three-gene dataset (atp6, rpb2, tef1), including Retiboletus brevibasidiatus, R. fuscus, R. nigrogriseus and selected Retiboletus species. Four species were used as outgroup taxa: Borofutus dhakanus, Ionosporus longipes, Rhodactina rostratispora, Spongiforma thailandica. Bootstrap frequencies ≥ 70% and posterior probabilities ≥ 0.95 are shown above supported branches. The prefix “GB” before voucher numbers indicates that sequences were retrieved from GenBank.
Fig. 2 Basidiomata of Retiboletus nigrogriseus in the field. A: OR049. B, C: OR0108. D: OR0127. Bars: 2 cm.