Taxonomy and Phylogeny of the Wood-Inhabiting Fungal Genus Hyphoderma with Descriptions of Three New Species from East Asia

Abstract

Three new wood-inhabiting fungi, Hyphoderma crystallinum, H. membranaceum, and H. microporoides spp. nov., are proposed based on a combination of morphological features and molecular evidence. Hyphoderma crystallinum is characterized by the resupinate basidiomata with smooth hymenial surface scattering scattered nubby crystals, a monomitic hyphal system with clamped generative hyphae, and numerous encrusted cystidia present. Hyphoderma membranaceum is characterized by the resupinate basidiomata with tuberculate hymenial surface, presence of the moniliform cystidia, and ellipsoid to cylindrical basidiospores. Hyphoderma microporoides is characterized by the resupinate, cottony basidiomata distributing the scattered pinholes visible using hand lens on the hymenial surface, presence of halocystidia, and cylindrical to allantoid basidiospores. Sequences of ITS+nLSU rRNA gene regions of the studied samples were generated, and phylogenetic analyses were performed with maximum likelihood, maximum parsimony, and Bayesian inference methods. These phylogenetic analyses showed that three new species clustered into Hyphoderma, in which H. crystallinum was sister to H. variolosum, H. membranaceum was retrieved as a sister species of H. sinense, and H. microporoides was closely grouped with H. nemorale. In addition to new species, map to show global distribution of Hyphoderma species treated in the phylogenetic tree and an identification key to Chinese Hyphoderma are provided.

1. Introduction

Fungi are an ecologically important branch of the tree of life based on its distinct and diverse characteristics, in which these organisms play a vital role in ecosystems as diverse as soil, forest, rocks, and ocean, but their roles are primarily enacted behind the scenes, literally as hidden layers within their substrate [1]. On the basis of the nature of their intertwined partners in numerous symbiotic interactions, they have mostly marched via stepwise codiversification with the plants [2]. Fungi have evolved numerous strategies to degrade hard-to-digest substrates for outcompeting with other microbes, while combating competitors using an arsenal of bioactive metabolites, such as the familiar antibiotics, ethanol, and organic acids [3]. Taxonomy plays a central role in understanding the diversity of life, discovering into systems of names that capture the relationships between species, and translating the products of biological exploration [4]. Despite the early embrace of the molecular systematics by mycologists, both the discovery and classification of fungi are still in great flux, particularly among the more basal branches of the tree, in which the true diversity is only now coming to light from genomic analyses and environmental DNA surveys [1]. The researches revealed that perhaps less than 5% of the estimated two to four million species have been formally described, therefore, the hidden and microscopic nature of many fungi also means that their diversity is undersampled [5,6].

The genus Hyphoderma Wallr. is one of the most important fungal groups because of its key role in the carbon cycle and being the most efficient wood decomposers in the forest ecosystem [7]. This genus is typified by H. setigerum (Fr.) Donk [8]. Hyphoderma is characterized by the resupinate to effuse-reflexed basidiomata with ceraceous consistency, and smooth to tuberculate or hydnoid hymenophore and a monomitic hyphal structure (rarely dimitic) with clamp connections on generative hyphae, presence of cystidia or not, basidia suburniform to subcylindrical and cylindrical, ellipsoid to subglobose, smooth, thin-walled basidiospores [9,10]. Currently, about 100 species have been accepted in Hyphoderma worldwide [8,11,12,13,14,15]. Index Fungorum (http://www.indexfungorum.org; accessed on 16 April 2021) and MycoBank (https://www.mycobank.org; accessed on 16 April 2021) register 192 specific and infraspecific names in Hyphoderma.

Molecular systematics covering Hyphoderma revealed the classification of corticioid fungi and showed that H. obtusum J. Erikss. and H. setigerum clustered into Meruliaceae Rea and then grouped with Hypochnicium polonense (Bres.) Å. Strid, based on the internal transcribed spacer (ITS) regions and the large subunit nuclear ribosomal RNA gene (nLSU) sequences [16]. Telleria et al. [17] discussed the relationships between Hyphoderma and Peniophorella P. Karst., in which some species from Hyphoderma and Peniophorella are grouped and they proposed a new species, H. macaronesicum Tellería et al. The research on studying Hyphoderma setigerum complex showed that H. pinicola Yurch. and Sheng H. Wu represented a fifth species in this complex, which revealed that this complex was a white-rot wood-decaying corticoid fungal species and occurred worldwide from tropical to temperate regions [18]. A revised family-level classification of the Polyporales revealed that four Hyphoderma species nested into the residual polyporoid clade belonging to Hyphodermataceae, and then, they were grouped with three related genera Meripilus P. Karst., Physisporinus P. Karst., and Rigidoporus Murrill [19]. Chinese Hyphoderma species were compared with closely related taxa, and two new species were proposed, H. fissuratum C.L. Zhao and X. Ma and H. mopanshanense C.L. Zhao [15].

In this study, three undescribed species of corticioid fungi were collected from Yunnan Province, China. Morphological characteristics and molecular phylogenetic analyses of combined ITS+nLSU rRNA sequences supported the recognition of three new species within Hyphoderma.

2. Materials and Methods

2.1. Morphology

The studied specimens are deposited at the herbarium of Southwest Forestry University (SWFC), Kunming, Yunnan Province, China. Macromorphological descriptions are based on field notes and photos captured in the field and laboratory. Color terminology follows Petersen [20]. Micromorphological data were obtained from the dried specimens, which were observed under a light microscope following Dai [21]. The following abbreviations are used: KOH = 5% potassium hydroxide water solution, CB = Cotton Blue, CB– = acyanophilous, IKI = Melzer’s reagent, IKI– = both inamyloid and indextrinoid, L = mean spore length (arithmetic average for all spores), W = mean spore width (arithmetic average for all spores), Q = variation in the L/W ratios between the specimens studied, n = a/b (number of spores (a) measured from given number (b) of specimens).

2.2. Molecular Phylogeny

CTAB rapid plant genome extraction kit-DN14 (Aidlab Biotechnologies Co., Ltd., Beijing, China) was used to obtain genomic DNA from dried specimens, according to the manufacturer’s instructions [22]. ITS region was amplified with primer pair ITS5 and ITS4 [23]. Nuclear nLSU region was amplified with primer pair LR0R and LR7 (http://lutzonilab.org/nuclear-ribosomal-dna/; accessed on 16 April 2021). The PCR procedure for ITS was as follows: initial denaturation at 95 °C for 3 min; followed by 35 cycles at 94 °C for 40 s, 58 °C for 45 s, and 72 °C for 1 min; and a final extension of 72 °C for 10 min. The PCR procedure for nLSU was as follows: initial denaturation at 94 °C for 1 min; followed by 35 cycles at 94 °C for 30 s, 48 °C for 1 min, and 72 °C for 1.5 min; and a final extension of 72 °C for 10 min. The PCR products were purified and directly sequenced at Kunming Tsingke Biological Technology Limited Company, Kunming, Yunnan Province, China. All newly generated sequences were deposited in NCBI GenBank (

Table 1. List of species, specimens, and GenBank accession numbers of sequences used in this study.

Species Name	Specimen No.	GenBank Accession No.		References
		ITS	LSU
Climacocystis borealis	FD-31	KP135308	KP135210	[19]
Diplomitoporus crustulinus	FD-137	KP135299	KP135211	[19]
Hyphoderma amoenum	USO 286622	HE577030		[17]
H. assimile	CBS 125852	MH863808	MH875272	[24]
H. cremeoalbum	NH 11538	DQ677492	DQ677492	[16]
H. crystallinum	CLZhao 9338	MW917161	MW913414	Present study
H. crystallinum	CLZhao 9374	MW917162	MW913415	Present study
H. crystallinum	CLZhao 10224	MW917163	MW913416	Present study
H. crystallinum	CLZhao 11723	MW917164	MW913417	Present study
H. crystallinum	CLZhao 15841	MW917165	MW913418	Present study
H. crystallinum	CLZhao 18459	MW917166	MW913419	Present study
H. definitum	GEL 2898		AJ406509	[18]
H. definitum	NH 12266	DQ677493	DQ677493	[16]
H.fissuratum	CLZhao 6731	MT791331	MT791335	[15]
H.fissuratum	CLZhao 6726	MT791330	MT791334	[15]
H. floccosum	CLZhao 17129	MW301683	MW293733	[25]
H. floccosum	CLZhao 17296	MW301686	MW293736	[25]
H. floccosum	CLZhao 16492	MW301688	MW293734	[25]
H. floccosum	CLZhao 17215	MW301687	MW293735	[25]
H. floccosum	CLZhao 17079	MW301685		[25]
H. floccosum	CLZhao 17065	MW301684		[25]
H. granuliferum	KHL 12561	JN710545	JN710545	[18]
H. incrustatum	KHL 6685		AY586668	[18]
H. litschaueri	NH 7603	DQ677496	DQ677496	[16]
H. litschaueri	FP-101740- sp	KP135295	KP135219	[26]
H. macaronesicum	MA:Fungi:16099	HE577027		[18]
H. macaronesicum	TFC:Mic.15981	HE577028		[18]
H. medioburiense	NH 10950	DQ677497	DQ677497	[16]
H. membranaceum	CLZhao 5844	MW917167	MW913420	Present study
H. membranaceum	CLZhao 6971	MW917168	MW913421	Present study
H. microporoides	CLZhao 6857	MW917169	MW913422	Present study
H. microporoides	CLZhao 8695	MW917170	MW913423	Present study
H. moniliforme	Wu 0211-42	KC928282	KC928283	[27]
H. moniliforme	Wu 0211-46	KC928284	KC928285	[27]
H. mopanshanense	CLZhao 6498	MT791329	MT791333	[15]
H. mopanshanense	CLZhao 6493	MT791328	MT791332	[15]
H. nemorale	TNM F3931	KJ885183	KJ885184	[27]
H. nemorale	Wu 9508-14	KC928280	KC928281	[27]
H. nudicephalum	Wu 9307-29	AJ534269		[28]
H. nudicephalum	Wu 9508-225	AJ534268		[28]
H. obtusiforme	KHL 1464	JN572909		[29]
H. obtusiforme	KHL 11105	JN572910		[29]
H. obtusum	JS 17804		AY586670	[29]
H. occidentale	KHL 8469		AY586674	[29]
H. occidentale	KHL 8477	DQ677499	DQ677499	[16]
H. paramacaronesicum	MA:Fungi:87736	KC984399	KF150074	[14]
H. paramacaronesicum	MA:Fungi:87737	KC984405	KF150073	[14]
H. pinicola	Wu 0108-32	KJ885181	KJ885182	[29]
H. pinicola	Wu 0108-36	KC928278	KC928279	[29]
H. prosopidis	E09/58-9	HE577029		[29]
H. roseocremeum	NH 10545		AY586672	[29]
H. setigerum	FCUG 1200	AJ534273		[28]
H. setigerum	FCUG 1688	AJ534272		[28]
H. sinense	CLZhao 7963	MW301679	MW293730	[25]
H. sinense	CLZhao 17811	MW301682	MW293732	[25]
H. sinense	CLZhao 8228	MW301681		[25]
H. sinense	CLZhao 7981	MW301680	MW293731	[25]
H. subsetigerum	Wu 9304-18	AJ534277		[28]
H. subsetigerum	Wu 9202-15	AJ534278		[28]
H. subtestaceum	HHB11620	GQ409521		[29]
H. subtestaceum	CFMR MJL1536	GQ409522		[29]
H. transiens	NH 12304	DQ677504	DQ677504	[16]
H. variolosum	CBS 734.91	MH862320	MH873992	[24]
H. variolosum	CBS 735.91	MH862321	MH873993	[24]
Hypochnicium cremicolor	CBS 208.54	MH857294	MH868826	[24]
H. erikssonii	NH 9635	DQ677508	DQ677508	[16]
H. geogenium	NH 10910	DQ677509	DQ677509	[16]
H. geogenium	MA-Fungi 48308	FN552534	JN939576	[30]
H. michelii	MA-Fungi 79155	NR119742	NG060635	[30]
H. punctulatum	FP101698sp	KY948827	KY948860	[19]
H. sphaerosporum	RLG15138sp	KY948803	KY948861	[19]
H. wakefieldiae	MA-Fungi 7675	FN552531	JN939577	[30]
Physisporinus subcrocatus	Dai 15917	KY131870	KY131926	[31]
P. subcrocatus	Dai 12800	KY131869	KY131925	[31]
P. tibeticus	Cui 9588	KY131873	KY131929	[31]
P. tibeticus	Cui 9518	KY131872	KY131928	[31]
Rigidoporus eminens	Dai 17200	MT279690	MT279911	[31]
R. undatus	Miettinen-13591	KY948731	KY948870	[19]

New sequences are shown in bold.

).

Sequences were aligned in MAFFT 7 (https://mafft.cbrc.jp/alignment/server/; accessed on 10 April 2021) using the “G-INS-i” strategy for ITS+nLSU and manually adjusted in BioEdit [32]. The dataset was aligned first and then, ITS and nLSU sequences were combined with Mesquite. Alignment datasets were deposited in TreeBASE (submission ID 27983). Climacocystis borealis (Fr.) Kotl. and Pouzar and Diplomitoporus crustulinus (Bres.) Domański were selected as an outgroup for phylogenetic analysis of ITS+nLSU phylogenetic tree (Figure 1) following a previous study [19].

Maximum parsimony analysis was applied to the combined (ITS+nLSU) dataset. Its approaches followed previous study [22], and the tree construction procedure was performed in PAUP* version 4.0b10 [33]. All characters were equally weighted and gaps were treated as missing data. Trees were inferred using the heuristic search option with TBR branch swapping and 1000 random sequence additions. Max-trees were set to 5000, branches of zero length were collapsed, and all parsimonious trees were saved. Clade robustness was assessed using bootstrap (BT) analysis with 1000 replicates [34]. Descriptive tree statistics: tree length (TL), consistency index (CI), retention index (RI), rescaled consistency index (RC), and homoplasy index (HI) were calculated for each maximum parsimonious tree generated. Datamatrix was also analyzed using maximum likelihood (ML) approach with RAxML-HPC2 through the CIPRES Science Gateway (www.phylo.org; accessed on 8 April 2021) [35]. Branch support (BS) for ML analysis was determined by 1000 bootstrap replicates.

MrModeltest 2.3 [36] was used to determine the best-fit evolution model for each data set for Bayesian inference (BI). BI was calculated with MrBayes 3.1.2 [37]. Four Markov chains were run for 2 runs from random starting trees for 6 million generations for ITS+nLSU (Figure 1). The first one-fourth of all generations was discarded as burn-in. The majority rule consensus tree of all remaining trees was calculated. Branches were considered as significantly supported if they received maximum likelihood bootstrap value (BS) > 70%, maximum parsimony bootstrap value (BT) > 70%, or Bayesian posterior probabilities (BPP) > 0.95.

3. Results

3.1. Molecular Phylogeny

The ITS+nLSU dataset (Figure 1) included sequences from 78 fungal specimens representing 44 taxa. The dataset had an aligned length of 2086 characters, of which 1245 characters are constant, 127 are variable and parsimony-uninformative, and 714 are parsimony-informative. Maximum parsimony analysis yielded 5000 equally parsimonious trees (TL = 3441, CI = 0.3787, HI = 0.6213, RI = 0.7178, RC = 0.2718). Best model for the ITS+nLSU dataset estimated and applied in the Bayesian analysis was GTR+I+G (lset nst = 6, rates = invgamma; prset statefreqpr = dirichlet (1,1,1,1)). Bayesian analysis and ML analysis resulted in a similar topology to MP analysis with an average standard deviation of split frequencies = 0.007698 (BI).

The phylogram inferred from ITS+nLSU sequences (Figure 1) demonstrated that three new species are clustered into genus Hyphoderma, in which H. crystallinum was sister to H. variolosum Boidin, Lanq. and Gilles, H. membranaceum was retrieved as a sister species of H. sinense C.L. Zhao and Q.X. Guan, and H. microporoides was closely grouped with H. nemorale K.H. Larss. (100% BS, 100% BP, and 1.00 BPP).

3.2. Taxonomy

Hyphoderma crystallinum C.L. Zhao and Q.X. Guan, sp. nov. Figure 2 and Figure 3.

MycoBank no.: MB 839276.

Holotype—China, Yunnan Province, Puer, Jingdong County, the Forest of Pineapple, E 100°48′, N 24°21′, 2113 m asl., on fallen angiosperm branch, leg. C.L. Zhao, 4 January 2019, C.L. Zhao 9338 (SWFC).

Etymology—crystallinum (Lat.): referring to the numerous and larger crystals on the hymenial surface.

Fruiting body—Basidiomata annual, resupinate, adnate, without odor and taste when fresh, membranaceous when fresh, becoming hard membranaceous upon drying, and up to 15 cm long, 3 cm wide, and 30–100 µm thick. Hymenial surface white to pale gray when fresh, pale gray to slightly cream upon drying, with scattered nubby crystals. Margin sterile indistinct and white.

Hyphal system—Monomitic, generative hyphae with clamp connections, colorless, thin-walled, frequently branched, interwoven, 2–3.5 µm in diameter, IKI-, CB-; tissues unchanged in KOH.

Hymenium—Cystidia of two types: (1) tubular cystidia, colorless, thin-walled, 32–51 µm × 5–10 µm and (2) encrusted cystidia, numerous, colorless, encrusted by crystals, 14–46 µm × 4–11 µm. Basidia clavate to subcylindrical, slightly constricted in the middle to somewhat sinuous, with 4 sterigmata and a basal clamp connection, 21.5–31 µm × 6–8.5 µm.

Spores—Basidiospores allantoid, colorless, thin-walled, smooth, with oil drops inside, IKI-, CB-, (10.5–)11–14.5(–15) µm × 4–5.5(–6) µm, L = 12.99 µm, W = 4.81 µm, Q = 2.47–2.98 (n = 90/3).

Additional specimens examined—China, Yunnan Province, Puer, Jingdong County, the Forest of Pineapple, E 100°48′, N 24°21′, 2113 m asl., on fallen angiosperm branch, leg. C.L. Zhao, 4 January 2019, C.L. Zhao 9374 (SWFC); Dali, Nanjian County, Lingbaoshan National Forestry Park, E 100°30′, N 24°46′, 1963 m asl., on fallen angiosperm branch, leg. C.L. Zhao, 9 January 2019, C.L. Zhao 10224 (SWFC); Wenshan, Funing County, Guying village, E 105°35′, N 23°36′, 976 m asl., on fallen angiosperm branch, leg. C.L. Zhao, 20 January 2019, C.L. Zhao 11723 (SWFC); Wenshan, Xichou County, Jiguanshan Forestry Park, E 103°46′, N 23°33′, 1670 m asl., on fallen angiosperm trunk, leg. C.L. Zhao, 22 July 2019, C.L. Zhao 15841 (SWFC); Honghe, Pingbian County, Daweishan National Nature Reserve, E 103°35′, N 22°53′, 1990 m asl., on fallen angiosperm branch, leg. C.L. Zhao, 3 August 2019, C.L. Zhao 18459 (SWFC).

Hyphoderma membranaceum C.L. Zhao and Q.X. Guan sp. nov. Figure 4 and Figure 5.

MycoBank no.: MB 839278.

Holotype—China, Yunnan Province, Chuxiong, Zixishan Forestry Park, E 101°24′, N 25°01′, 2356 m asl., on fallen angiosperm branch, leg. C.L. Zhao, 1 July 2018, C.L. Zhao 6971 (SWFC).

Etymology—membranaceum (Lat.): referring to the membranous hymenophore.

Fruiting body—Basidiomata annual, resupinate, adnate, membranous, without odor and taste when fresh, and up to 15 cm long, 2 cm wide, and 30–100 µm thick. Hymenial surface tuberculate, white to pale gray when fresh, pale gray to cream on drying, with cracking. Margin sterile, narrow, and gray.

Hyphal system—Monomitic, generative hyphae with clamp connections, colorless, thin-walled, frequently branched, interwoven, 2.5–4.5 µm in diameter; IKI-, CB-; tissues unchanged in KOH.

Hymenium—Cystidia moniliform, thin-walled, 28–60 µm × 6.5–10.5 µm; basidia clavate to subcylindrical, slightly constricted in the middle to somewhat sinuous, with 4 sterigmata and a basal clamp connection, 21.5–31 µm × 5–7.5 µm.

Spores—Basidiospores ellipsoid to cylindrical, colorless, thin-walled, smooth, with irregular vacuole inside, IKI-, CB-, (10.5–)11–13.5(–14) µm × 4.5–5.5(–6) µm, L = 12.52 µm, W = 5.18 µm, Q = 2.42 (n = 60/2).

Additional specimens examined—China, Yunnan Province, Puer, Zhenyuan County, Heping Town, Liangzizhai, E 101°25′, N 23°56′, 2246 m asl., on fallen angiosperm branch, leg. C.L. Zhao, 15 Jan 2018, C.L. Zhao 5844 (SWFC).

Hyphoderma microporoides C.L. Zhao and Q.X. Guan sp. nov. Figure 6 and Figure 7.

MycoBank no.: MB 839277.

Holotype—China, Yunnan Province, Chuxiong, Zixishan Forestry Park, E 101°24′, N 25°01′, 2313 m asl., on fallen angiosperm trunk, leg. C.L. Zhao, 30 June 2018, C.L. Zhao 6857 (SWFC).

Etymology—microporoides (Lat.): referring to the scattered pinholes on the hymenophore that are visible under hand lens.

Fruiting body—Basidiomata annual, resupinate, adnate, without odor and taste when fresh, cottony when fresh, fragile upon drying, and up to 22 cm long, 2.5 cm wide, and 50–100 µm thick. Hymenial surface smooth with scattered pinholes visible under hand lens, cream to pale buff when fresh, and slightly buff upon drying. Margin sterile, indistinct, and white to cream.

Hyphal system—Monomitic, generative hyphae with clamp connections, colorless, thin-walled, frequently branched, interwoven, 3–5 µm in diameter, IKI-, CB-; tissues unchanged in KOH.

Hymenium—Halocystidia capitate, thin-walled, smooth, 18–51 µm × 4.5–7 µm; basidia clavate, constricted, somewhat sinuous, with 4 sterigmata and a basal clamp connection, 18.5–29.5 µm × 5–7 µm.

Spores—Basidiospores cylindrical to allantoid, colorless, thin-walled, smooth, with oil drops inside, IKI-, CB-, 8.5–10(–10.5) µm × 2.5–3.5(–4) µm, L = 9.29 µm, W = 3.24 µm, Q = 2.87 (n = 30/1).

Additional specimens examined—China, Yunnan Province, Puer, Jingdong County, Taizhong Town, Ailaoshan Ecological Station, E 100°56′, N 24°29′, 1938 m asl., on fallen angiosperm branch, leg. C.L. Zhao, 25 August 2018, C.L. Zhao 8695 (SWFC).

4. Discussion

In the present study, three new species, Hyphoderma crystallinum, H. membranaceum, and H. microporoides are described based on phylogenetic analyses and morphological characteristics.

Phylogenetically, the family-level classification of the Polyporales (Basidiomycota) amplified nLSU, nITS, and rpb1 genes across the Polyporales, was employed, in which four species Hyphoderma macaronesicum, H. medioburiense (Burt) Donk, H. mutatum (Peck) Donk, and H. setigerum, nested into family Hyphodermataceae within the residual polyporoid clade [19]. In the present study, three new taxa clustered into Hyphoderma, in which Hyphoderma crystallinum was sister to H. variolosum, H. microporoides grouped closely with H. nemorale, and H. membranaceum grouped with H. sinense and H. transiens (Bres.) Parmasto (Figure 1). However, morphologically, H. variolosum differs from H. crystallinum by its narrower tubular cystidia (40–50 µm × 4–6 µm) [38]; H. nemorale is separated from H. microporoides by having the colliculose hymenial surface, wider moniliform cystidia (35–70 µm × 7–8 µm) and basidiospores (9.5–14 µm × 4–5 µm) [27]; H. sinense differs from H. membranaceum by having the encrusted cystidia and smaller basidiospores (8–11.5 µm × 3–5 µm) [25], and another species H. transiens differs in its odontioid hymenial surface and narrower basidiospores (9–13 µm × 3–4.5 µm) [39].

Morphologically, Hyphoderma ayresii (Berk. ex Cooke) Boidin and Gilles, H. cremeum Sheng H. Wu and H. rimulosum Sheng H. Wu are similar to H. crystallinum by having encrusted cystidia. However, H. ayresii differs in its larger encrusted cystidia (70–130 µm × 13–20 µm) and wider basidiospores (9.5–12.5 µm × 6–8 µm) [38]; H. cremeum differs from H. crystallinum by having both larger encrusted cystidia (40–90 µm × 10–15 µm) and basidia (35–50 µm × 6.5–8 µm) [40]; H. rimulosum is separated from H. crystallinum by smaller basidiospores (6–7 µm × 3.9–4.1 µm) [41]. Hyphoderma incrustatum K.H. Larss., H. medioburiense, H. multicystidium (Hjortstam and Ryvarden) Hjortstam and Tellería and H. roseocremeum (Bres.) Donk are similar to H. crystallinum by having tubular cystidia. However, H. incrustatum differs from H. crystallinum by the porulose hymenial surface and the larger tubular cystidia (50–80 µm × 6–10 µm) [42]; H. medioburiense is separated from H. crystallinum by the porulose hymenial surface and the larger tubular cystidia (60–100 µm × 7–10 µm) [8]; H. multicystidium differs in its larger tubular cystidia (60–80 µm × 5–7 µm), larger basidia (35–50 µm × 5–7 µm) and smaller basidiospores (8–10 µm × 4.5–5 µm) [43]; H. roseocremeum differs from H. crystallinum by having larger tubular cystidia (80–100 × 6–9 µm) and smaller basidiospores (8–12 µm × 3–4 µm) [8].

Hyphoderma litschaueri, H. moniliforme (P.H.B. Talbot) Manjón, G. Moreno and Hjortstam, H. paramacaronesicum Tellería et al., H. prosopidis (Burds.) Tellería et al. and H. sinense are similar to H. membranaceum by having moniliform or apically moniliform cystidia. However, H. litschaueri differs from H. membranaceum by having larger moniliform cystidia (60–100 µm × 6–8 µm) and narrower basidiospores (9–12 µm × 3–4 µm) [44]; H. moniliforme differs from H. membranaceum by having smaller basidiospores (8–9 µm × 3.5–4 µm) [27]; H. paramacaronesicum differs in its having both larger moniliform cystidia (70–124 µm × 8–13 µm) and basidia (40−48 µm × 6−9 µm), and wider basidiospores (12–15 µm × 5.5–7 µm) [14]; H. prosopidis differs from H. membranaceum by the arachnoid to farinaceous hymenial surface and larger basidia (40−45 µm × 8−11 µm) [17]; and H. sinense differs in its having encrusted cystidia (18.5–38 µm × 6–11 µm) and smaller basidiospores (8–11.5 µm × 3–5 µm) [25].

Hyphoderma clavatum Sheng H. Wu, H. etruriae Bernicchia, H. incrustatum, H. orphanellum (Bourdot & Galzin) Donk, and H. subclavatum Sheng H. Wu are similar to H. microporoides by having capitate cystidia. However, H. clavatum differs from H. microporoides by the tuberculate hymenial surface and larger basidiospores (10–13 µm × 4.2–5.2 µm) [41]; H. etruriae differs from H. microporoides by the grandinioid hymenial surface and wider basidiospores (9–11 µm × 5.5–6.5 µm) [45]; H. incrustatum differs in having larger basidiospores (11–14 µm × 4–5 µm) [42]; H. orphanellum differs from H. microporoides by having larger capitate cystidia (50–80 µm × 8–10 µm) and wider basidiospores (8–10 µm × 5–6 µm) [8]; H. subclavatum is separated from H. microporoides by having both larger basidia (40–55 µm × 7–8 µm) and basidiospores (10–12 µm × 4.2–5.3 µm) [41].

Hyphoderma species are an extensively studied group [10,46], mainly distributed in Europe (e.g., Austria, Russia, France, Germany, Poland, UK, The Netherlands, Portugal, Sweden, Italy, Denmark, Norway, Finland, Spain) (Figure 8) and mainly found on hardwood, although a few species grow on coniferous wood. Many species of Hyphoderma were found in Europe, but most of them have not been reported in northern China (Figure 8), in which we presumed that Hyphoderma are undersampled by mycologists. Several studies on new wood-decaying fungi of Hyphoderma from China have been reported [15,40,41,46], in which 26 Hyphoderma species were reported, H. acystidiatum Sheng H. Wu, H. clavatum, H. cremeoalbum (Höhn. and Litsch.) Jülich, H. cremeum, H. definitum (H.S. Jacks.) Donk, H. densum Sheng H. Wu, H. fissuratum, H. floccosum C.L. Zhao and Q.X. Guan, H. litschaueri, H. crystallinum, H. medioburiense, H. microcystidium Sheng H. Wu, H. microporoides, H. moniliforme, H. mopanshanense, H. nemorale, H. obtusiforme J. Erikss. and Å. Strid, H. pinicola, H. rimulosum, H. setigerum, H. sibiricum (Parmasto) J. Erikss. and Å. Strid, H. sinense, H. subclavatum, H. subsetigerum Sheng H. Wu, H. transiens, and H. membranaceum [8,18,25,27,29,40,41,46]. Further studies should focus on the relationships between the host and Hyphoderma species, as well as trying to better understand the evolutionary directions between plant and Hyphoderma species. The researches on the phylogeny of Hyphoderma, as well as many fungal studies on the molecular systematics [47,48,49], will be useful to push the further research on fundamental research and applied research of fungi. More species of Hyphoderma should be found in subtropical and tropical Asia as it was shown that wood-inhabiting fungi are rich in tropical China [50,51].

Key to 26 accepted species of Hyphoderma in China

• Cystidia absent 2
• Cystidia present 5
• Hymenial surface grandinioid H. acystidiatum
• Hymenial surface smooth 3
• Basidiospores > 10.5 µm in length H. densum
• Basidiospores < 10.5 µm in length 4
• Hymenophore cracked; basidiospores > 8.5 µm in length H. fissuratum
• Hymenophore uncracked; basidiospores < 8.5 µm in length H. sibiricum
• Hymenophore smooth 6
• Hymenophore tuberculate, porulose, grandinioid, or odontoid 14
• Two types of cystidia present 7
• One type of cystidia present 8
• Moniliform cystidia absent H. microcystidium
• Moniliform cystidia present H. sinense
• Hymenophore uncracked 9
• Hymenophore cracked 10
• Basidiospores > 11 µm in length H. definitum
• Basidiospores < 11 µm in length H. microporoides
• Cystidia moniliform 11
• Cystidia cylindrical 12
• Basidiospores > 9 µm in length H. litschaueri
• Basidiospores < 9 µm in length H. moniliforme
• Basidiospores ellipsoid < 10 μm in length H. rimulosum
• Basidiospores cylindrical > 10 μm in length 13
• Basidiospores > 12 µm in length H. cremeum
• Basidiospores < 12 µm in length H. subclavatum
• Hymenophore odontoid or grandinioid 15
• Hymenophore tuberculate, porulose 16
• Hymenophore odontoid, basidiospores > 9 µm in length H. transiens
• Hymenophore grandinioid, basidiospores < 9 µm in length H. subsetigerum
• Cystidia of two types 17
• Cystidia of one type 19
• Septate cystidia absent H. crystallinum
• Septate cystidia present 18
• Basidia 2-sterigmata, basidiospores > 13 µm in length H. pinicola
• Basidia 4-sterigmata, basidiospores < 13 µm in length H. floccosum
• Septate cystidia present 20
• Septate cystidia absent 21
• Hymenophore porulose to pilose, basidia < 5 µm in width H. mopanshanense
• Hymenophore tuberculate, basidia > 5 µm in width H. setigerum
• Hymenophore porulose H. obtusiforme
• Hymenophore tuberculate, colliculose 22
• Cystidia > 30 µm in length 23
• Cystidia < 30 µm in length H. cremeoalbum
• Basidia > 30 µm in length 24
• Basidia < 30 µm in length 25
• Hymenophore cracking, cystidia < 10 µm in width H. medioburiense
• Hymenophore not cracking, cystidia > 10 µm in width H. clavatum
• Hymenophore colliculose H. nemorale
• Hymenophore tuberculate H. membranaceum