Cacaoporus tenebrosus Vadthanarat, Raspé & Lumyong, in Vadthanarat et al., MycoKeys 54: 14 (2019)
Etymology: Refers to the overall darkness of basidiomata, including the context.
Diagnosis: Cacaoporus tenebrosus is characterised by having a darker context than the other species, longer basidia and cylindrical to narrowly subclavate hymenophoral cystidia.
Basidiomata medium-sized. Pileus (2.3)3.1–5(9) cm in diameter, convex when young becoming plano-convex to slightly depressed with age; margin inflexed to deflexed, slightly exceeding (1–2 mm), surface even to subrugulose, minutely tomentose, slightly cracked at the centre, dull, greyish-brown (10F3) to dark brown to blackish-brown (8F4–5) to the margin; Context 5–10 mm thick half-way to the margin, soft, marmorated, greyish-brown to dark brown (10F3–5) with greyish-brown (9B/D3), scattered with reddish-brown to brownish-black, fine encrustations at places, slightly reddening in paler spots when cut. Stipe central, terete, cylindrical to sometimes with slightly wider base, 4.3–7.0 × 0.7–1.4 cm, surface even, minutely tomentose, dull, dark brown to greyish-brown (9F4 to 10F3), basal mycelium white to off-white becoming reddish-white to pale red (7A3–4) when bruised; Context solid, greyish-brown to dark brown (9–10F3–5) marmorated with reddish-grey (7/10B2), usually scattered with small reddish-brown to brownish-black fine encrustations, slightly reddening when cut. Hymenophore tubulate, adnate, subventricose to ventricose, slightly depressed around the stipe. Tubes (4)7–13 mm long half-way to the margin, brown to dark brown (8F3 to 9F4), not separable from the pileus context. Pores 0.8–2 mm wide at mid-radius, regularly arranged, mostly roundish at first, becoming slightly angular with age, sometime irregular, elongated around the stipe; colour distribution even, greyish-brown to dark brown (9F4) at first, becoming chocolate brown to brown (10F3 to 7–8F4–5) with age. Odour mild fungoid. Taste slightly bitter at first, then mild. Spore print dark brown (8/9F4) in mass. Macrochemical reactions. KOH, yellowish then brown to black on cap, stipe, pileus context, stipe context and hymenium; NH4OH, yellowish then orange to brown on cap, stipe, pileus context, stipe context and hymenium.
Basidiospores [290/8/6] (7.4–) 7.7–8.4–9.2(–10) × (4.5–) 5–5.3–5.7(–6.1) µm Q = (1.25–) 1.44–1.57–1.77(–2). From the type (2 basidiomata, N = 134) (7.5–) 7.7–8.2–9(–9.9) × (4.9–)5–5.4–5.7(–5.9) µm, Q = (1.41–)1.43–1.54–1.71(–1.9), ovoid, thin-walled, smooth, slightly reddish to brownish hyaline in water, slightly yellowish to greenish hyaline in KOH or NH4OH, inamyloid. Basidia 4-spored, (33.6–)34.3–38.8–45.8(–47) × (7.7–)7.8–9.5–10.8(–10.9) µm, clavate to narrowly clavate without basal clamp connection, yellowish to brownish hyaline to slightly dark in KOH or NH4OH; sterigmata up to 5 µm long. Cheilocystidia (22–)22.1–28.7–37(–37) × (3–)3.1–4.4–5(–5) µm, frequent, cylindrical with obtuse apex, sometimes bent or sinuate, thin-walled, yellowish-brown to dark brown in KOH or NH4OH, often scattered with small brownish-yellow to yellowish-brown crystals on the walls in KOH or NH4OH. Pleurocystidia (62– ) 62.5–81.5–99(–99) × (7–)7–8–9(–9) µm, frequent, cylindrical to narrowly subclavate, sometimes bent or sinuate, thin-walled, with yellowish-brown to slightly dark content in KOH or NH4OH, densely covered with small reddish-brown to brownish dark encrustations on the walls when observed in H2O, with some scattered small brownish-yellow to yellowish-brown crystals on the walls in KOH or NH4OH. Hymenophoral trama subdivergent to divergent, 80–170 µm wide, with 60–80 µm wide of subregular mediostratum, composed of cylindrical, 4–8(11) µm wide hyphae, slightly yellowish to brownish hyaline in KOH or NH4OH. Pileipellis a tangled trichoderm to tomentum at places, 70–110 µm thick, composed of moderately interwoven thin-walled hyphae; terminal cells 12–48 × 4–7 µm mostly slightly sinuate, cylindrical to irregular with rounded apex, at places clavate to elongated clavate terminal cells 18–33 × 7–9 µm, slightly dark to reddish to brownish dark in water, yellowish-brown to slightly dark in KOH or NH4OH, scattered with small brownish-yellow to yellowish-brown crystals on the walls in KOH or NH4OH. Pileus context made of moderately interwoven, thin-walled, hyaline hyphae, 7–12 µm wide. Stipitipellis a trichoderm to tangled trichoderm, 70–120 µm thick, composed of loosely to moderately interwoven cylindrical hyphae anastomosing at places, brownish dark to dark in KOH or NH4OH. Caulocystidia (17–) 17.6–29.4–46.3(–47) × (4–)4.1–5.5–6.9(–7) µm, clavate to cylindrical with obtuse apex, thin-walled, yellowish to brownish dark in KOH or NH4OH. Stipe context composed of parallel, 4–6(12) µm wide hyphae, brownish hyaline to yellowish pale brown in KOH or NH4OH. Clamp connections not seen in any tissue.
Habitat and distribution: Gregarious (up to 9 basidiomata) to fasciculate or solitary, on soil in hill evergreen forest dominated by Fagaceae trees, with a few Dipterocarpus spp. and Shorea spp. or in Dipterocarp forest dominated by Dipterocarpus spp., Shorea spp. with a few Lithocarpus sp., Castanopsis sp. and Quercus sp. Currently known only from Chiang Mai Province, Northern Thailand.
Index Fungorum number: IF 829656
Notes: There were many small yellowish to reddish to dark brownish particles or crystals on the walls of pileipellis, stipitipellis and hymenium cells, especially on the cystidia and basidia when observed in water. The small particles or crystals are somewhat dissolved and discoloured in KOH.
Microscopically, Cacaoporus tenebrosus differs from C. pallidicarneus by having a darker context, longer basidia (33.6–47 µm vs. 25.3–33.8 µm, respectively), longer and larger hymenophoral cystidia, which also differ in shape (cylindrical to narrowly subclavate in C. tenebrosus but fusiform to narrowly fusiform in C. pallidicarneus). Phylogenetically, all Cacaoporus collections with a dark context formed a clade sister to C. pallidicarneus (BS = 85% and PP = 0.88), but some (SV0224 and SV0422) were genetically somewhat distant from the other collections. However, No difference could be found in morphology. Consequently, they were considered as the same species (C. tenebrosus). Study of more collections is needed to confirm or infirm that they belong to the same species.
Figure 1: Phylogenetic tree inferred from the four-gene dataset (atp6, cox3, rpb2 and tef1), including Cacaoporus species and selected Boletaceae using Maximum Likelihood and Bayesian Inference methods (ML tree is presented). The two Buchwaldoboletus and nine Chalciporus species in subfamily Chalciporoideae were used as outgroup. Most of the taxa not belonging to the Pulveroboletus group were collapsed into subfamilies. All genera clades in Pulveroboletus group that were highly supported were also collapsed. Bootstrap support values (BS ≥ 70%) and posterior probabilities (PP ≥ 0.90) are shown above the supported branches.