A Phylogenetic and Taxonomic Study on Xylodon (Hymenochaetales): Focusing on Three New Xylodon Species from Southern China.

Three wood-inhabiting fungal species, Xylodon laceratus, X. montanus, and X. tropicus spp. nov., were collected from southern China, here proposed as new taxa based on a combination of morphological features and molecular evidence. Xylodon laceratus is characterized by the resupinate basidiomata with grandinioid hymenophore having cracked hymenial surface, and ellipsoid basidiospores; X. montanus is characterized by the annual basidiomata having the hard, brittle hymenophore with cream hymenial surface, and ellipsoid to broadly ellipsoid basidiospores (3.9-5.3 × 3.2-4.3 µm); and X. tropicus is characterized by its grandinioid hymenophore with buff to a pale brown hymenial surface and subglobose basidiospores measuring 2-4.8 × 1.6-4 µm. Sequences of ITS and nLSU rRNA markers of the studied samples were generated, and phylogenetic analyses were performed with maximum likelihood, maximum parsimony, and Bayesian inference methods. The ITS+nLSU analysis of the order Hymenochaetales indicated that the three new species clustered into the family Schizoporaceae, located in genus Xylodon; based on further analysis of ITS dataset, X. laceratus was a sister to X. heterocystidiatus; X. montanus closely grouped with X. subclavatus and X. xinpingensis with high support; while X.tropicus was retrieved as a sister to X. hastifer.

1. Introduction

Wood-inhabiting fungi are noteworthy components of woody plant ecosystems and take an active part in the decomposition and turnover of nutrients from wood, in which the corticioid fungi are one of the major groups of wood-inhabiting Basidiomycota [1]. Hymenochaetales is one of the most important orders in Basidiomycota because many species in this order are medicinal fungi, and some of them are forest pathogens [2]. Within Hymenochaetales, commonly accepted families Chaetoporellaceae, Coltriciaceae, Hymenochaetaceae, Hyphodontiaceae, Neoantrodiellaceae, Nigrofomitaceae, Oxyporaceae, and Schizoporaceae are supported as individual monophyletic lineages [3]. Schizoporaceae was introduced by Walter Jülich with Schizopora Velen. as the type genus and included the other three genera Fibriciellum J. Erikss. & Ryvarden, Fibricium J. Erikss. and Fibrodontia Parmasto [4].

The genus Xylodon (Pers.) Gray (Schizoporaceae, Hymenochaetales), with the generic type X. quercinus (Pers.) Gray is characterized by the basidiomata having a smooth, tuberculate, grandinioid, odontioid, coralloid, irpicoid or poroid hymenophore, and a monomitic, pseudodimitic or dimitic hyphal system with clamped generative hyphae and mostly presence of 1 to 3 types cystidia, negative in Melzer’s reagent; barreled clavate or suburniform basidia, and globose to ellipsoid to cylindrical basidiospores, additionally causing a white rot [5,6]. It is the largest segregate genus of Hyphodontia J. Erikss. s.l. (Hymenochaetales, Basidiomycota) based on the MycoBank database (http://www.MycoBank.org, accessed on 30 November 2021) and the Index Fungorum (http://www.indexfungorum.org, accessed on 30 November 2021), both have registered 206 specific and infraspecific names in Xylodon, but the actual number of species reaches eighty-six [7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32].

The researches of Xylodon based on molecular systematics are coming to light from genomic analyses and environmental DNA surveys [33]. Species diversity of Hyphodontia sensu lato has been systematically surveyed in Europe for the last several decades, in which almost all taxonomic studies did not try to address the taxonomic status of species of Hyphodontia sensu lato at the family level, including the most important monograph “Die Gattung Hyphodontia John Eriksson” [14]. Hyphodontia s.l. could be split into 13 genera, of which the most species-rich ones are Xylodon and Kneiffiella P. Karst. However, due to a lack of rDNA sequences for many taxa, the molecular grounds were not enough to define separate genera clearly. Therefore the mycologists adopted a broad concept of Hyphodontia s.l. [25]. Molecular phylogeny analyses were performed by maximum parsimony and Bayesian methods, in which the phylograms from both analyses confirmed that the taxa from genera of Xylodon, Schizopora, Palifer Stalpers & P.K. Buchanan, Lyomyces P. Karst. and Rogersella Liberta & A.J. Navas were mixed within Xylodon-Lyomyces-Rogersella clade and Xylodon-Schizopora-Palifer clade [34]. Based on the morphological and/or phylogenetic information on Hyphodontia s.l., it was hard to differentiate between the genera Xylodon and Schizopora, with neither morphological nor molecular data and proposed that both genera should be united in one genus [22]. Used sequences from the ITS and nuclear LSU regions to infer the phylogenetic position of the Xylodon, confirmed Lagarobasidium Jülich as a synonym of Xylodon, in which three species X. magnificus (Gresl. & Rajchenb.) K.H. Larss., X. pumilius (Gresl. & Rajchenb.) K.H. Larss., and X. rickii (Gresl. & Rajchenb.) K.H. Larss. were transferred into Xylodon from Lagarobasidium [27]. Phylogenetic analyses of ITS and 28S sequences revealed that Palifer and Odontiopsis Hjortstam & Ryvarden should be synonymized under Xylodon and proposed four species new to science as X. exilis Yurchenko, Riebesehl & Langer, X. filicinus Yurchenko & Riebesehl, X. follis Riebesehl, Yurchenko & Langer, and X. pseudolanatus Nakasone, Yurchenko & Riebesehl [31]. The comprehensive phylogenetic analyses on the basis of multiple loci on Hyphodontia s.l. showed that the six genera were separated into four clades within Hymenochaetales, in which Fasciodontia Yurchenko & Riebesehl, Lyomyces, and Xylodon were accepted as members of a previously known family Schizoporaceae [3]. Maximum parsimony strict consensus tree illustrating the phylogeny of Xylodon species based on ITS sequences showed that X. gossypinus C.L. Zhao & K.Y. Luo grouped with X. brevisetus (P. Karst.) Hjortstam & Ryvarden, and X. macrosporus C.L. Zhao & K.Y. Luo was sister to X. follis and X. sinensis C.L. Zhao & K.Y. Luo formed two sister groups to X. attenuatus Spirin & Viner and X. yarraensis Xue W. Wang & L.W. Zhou with very low supports [32].

During investigations on wood-decaying fungi in southern China, three additional Xylodon taxa were found, which could not be assigned to any described species. The present study examines the taxonomy and phylogeny of the three new species within Xylodon. The current study focuses on the phylogenetic and taxonomic study of Xylodon, based on the internal transcribed spacer (ITS) and the large subunit nuclear ribosomal RNA gene (nLSU) sequences.

2. Materials and Methods

2.1. Sample Collection

Fresh fruiting bodies of the fungi were collected from the Dali, Puer, Wenshan, Yuxi of Yunnan Province, P.R. China. The dried specimens were dried in an electric food dehydrator at 40 °C, then sealed and stored in an envelope bag and deposited in the herbarium of the Southwest Forestry University (SWFC), Kunming, Yunnan Province, P.R. China.

2.2. Morphology

Macromorphological descriptions are based on field notes and photos captured in the field and lab. Color terminology follows Petersen [35]. Micromorphological data were obtained from the dried specimens after the observation under a light microscope with a magnification 10 × 100 oil [17]. The following abbreviations were used: KOH = 5% potassium hydroxide water solution, CB– = acyanophilous, 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, and n = a/b (number of spores (a) measured from given number (b) of specimens).

2.3. Molecular Phylogeny

The CTAB rapid plant genome extraction kit-DN14 (Aidlab Biotechnologies Co., Ltd., Beijing, P.R. China) was used to obtain genomic DNA from the dried specimens using the manufacturer’s instructions. The nuclear ribosomal ITS region was amplified with primers ITS5 and ITS4 [36]. The nuclear nLSU region was amplified with primer pair LR0R and LR7 (http://lutzonilab.org/nuclear-ribosomal-dna/, accessed on 5 November 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 sequenced at Kunming Tsingke Biological Technology Limited Company (Yunnan Province, P.R. China). All newly generated sequences were deposited in NCBI GenBank (https://www.ncbi.nlm.nih.gov/genbank/, accessed on 20 October 2021) (Table 1).

Table 1

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

Species Name	Specimen No.	GenBank Accession No.		References	Country
		ITS	nLSU
Fasciodontia brasiliensis	MSK-F 7245a	MK575201	MK598734	[37]	Brazil
F. bugellensis	KAS-FD 10705a	MK575203	MK598735	[37]	France
Hastodontia halonata	HHB-17058	MK575207	MK598738	[37]	Mexico
Hymenochaete cinnamomea	He 2074	KU975460	KU975500	Unpublished	China
Hym. rubiginosa	He 1049	JQ716407	JQ279667	[38]	China
Hyphodontia alutaria	GEL3183	DQ340318	DQ340373	Unpublished	Germany
Hyp. arguta	KHL11938	EU118632	EU118633	[39]	Sweden
Hyp. pallidula	KAS-GEL2097	DQ340317	DQ340372	Unpublished	Germany
Hyp. densispora	LWZ20170908-5	MT319426	MT319160	[3]	China
Hyp. zhixiangii	LWZ20180903-5	MT319423	MT319158	[3]	China
Kneiffiella barba-jovis	KHL 11730	DQ873609	DQ873610	[31]	Sweden
K. eucalypticola	LWZ20180515-9	MT319411	MT319143	[3]	Australia
K. palmae	KAS-GEL 3456	DQ340333	DQ340369	[37]	China
K. subalutacea	GEL2196	DQ340341	DQ340362	[37]	Norway
Lyomyces allantosporus	FR 0249548	KY800397	KY795963	[40]	Réunion
L. bambusinus	CLZhao 4831	MN945968	MW264919	[41]	China
L. fimbriatus	Wu 911204-4	MK575210	MK598740	[37]	China
L. mascarensis	KAS-GEL 4833	KY800399	KY795964	[37]	Réunion
L. orientalis	LWZ20170909-7	MT319436	MT319170	[3]	China
L. sambuci	KAS-JR 7	KY800402	KY795966	[40]	Germany
Xylodon acystidiatus	LWZ20180514-9	MT319474	MT319211	[3]	Australia
X. apacheriensis	Wu 0910-58	KX857797	KX857822	[42]	China
X. asper	KHL8530	AY463427	AY586675	[43]	Sweden
X. astrocystidiatus	Wu 9211-71	JN129972	JN129973	[16]	China
X. attenuatus	Spirin 8775	MH324476		[27]	USA
X. australis	LWZ20180509-8	MT319503		[3]	China
X. bambusinus	CLZhao 9174	MW394657	MW394650	[44]	China
X. borealis	JS26064	AY463429	AY586677	[43]	Norway
X. brevisetus	JS17863	AY463428	AY586676	[43]	Norway
X. crystalliger	LWZ20170816-33	MT319521	MT319269	[3]	China
X. cystidiatus	FR-0249200	MH880195	MH884896	[31]	Réunion
X. damansaraensis	LWZ20180417-23	MT319499		[3]	Malaysia
X. detriticus	Zíbarová 30.10.17	MH320793	MH651372	[27]	Czech Republic
X. filicinus	MSK F 12869	MH880199	NG067836	[31]	China
X. flaviporus	FR-0249797	MH880201	MH884901	[31]	Réunion
X. follis	FR-0249814	MH880204	MH884902	[31]	Réunion
X. gossypinus	CLZhao 4465	MZ663803	MZ663812	[32]	China
X. hastifer	K(M) 172400	NR166558		[22]	USA
X. heterocystidiatus	LWZ20180921-19	MT319676	MT319266	[3]	Australia
X. heterocystidiatus	Wei 17-314	MT731753	MT731754	Unpublished	China
X. hyphodontinus	KAS-GEL9222	MH880205	MH884903	[31]	Kenya
X. kunmingensis	TUB-FO 42565	MH880198	MH884898	[31]	China
X. laceratus	CLZhao 9841	OL619257	OL619265	Present study	China
X. laceratus	CLZhao 9892 [T]	OL619258	OL619266	Present study	China
X. lagenicystidiatus	LWZ20180513-16	MT319634	MT319368	[3]	Australia
X. lenis	Wu890714-3	KY081802		[22]	China
X. macrosporus	CLZhao 10226	MZ663809	MZ663817	[32]	China
X. mollissimus	LWZ20160318-3	KY007517	MT319347	[3]	China
X. montanus	CLZhao 8118	OL619259	OL619267	Present study	China
X. montanus	CLZhao 8179 [T]	OL619260	OL619268	Present study	China
X. nesporii	LWZ20180921-35	MT319655	MT319238	[3]	China
X. niemelaei	LWZ20150707-13	MT319630	MT319365	[3]	China
X. nongravis	GC1412-22	KX857801	KX857818	[42]	China
X. nothofagi	ICMP 13842	AF145583		[45]	China
X. ovisporus	LWZ20170815-31	MT319666	MT319346	[3]	China
X. papillosus	CBS 114.71	MH860026		[46]	Netherlands
X. paradoxus	Dai14983	MT319519	MT319267	[3]	China
X. pruinosus	Spirin 2877	MH332700		[27]	Estonia
X. pseudolanatus	FP-150922	MH880220	NG067837	[31]	Belize
X. pseudotropicus	Dai16167	MT319509	MT319255	[3]	China
X. quercinus	KHL11076	KT361633	AY586678	[43]	Sweden
X. ramicida	Spirin 7664	NR138013		Unpublished	USA
X. rhododendricola	LWZ20180513-9	MT319621	MT319357	[3]	Australia
X. rimosissimus	Ryberg 021031	DQ873627	DQ873628	[47]	Sweden
X. serpentiformis	LWZ20170816-15	MT319673	MT319218	[3]	China
X. sinensis	CLZhao 9197	MZ663810	MZ663818	[32]	China
X. spathulatus	LWZ20180804-10	MT319646	MT319354	[3]	China
X. subclavatus	TUB-FO 42167	MH880232		[31]	China
X. subflaviporus	Wu 0809-76	KX857803	KX857815	[42]	China
X. subserpentiformis	LWZ20180512-16	MT319486	MT319226	[3]	Australia
X. subtropicus	LWZ20180510-24	MT319541	MT319308	[3]	China
X. taiwanianus	CBS 125875	MH864080	MH875537	[46]	Netherlands
X. tropicus	CLZhao 3351 [T]	OL619261	OL619269	Present study	China
X. tropicus	CLZhao 3355	OL619262		Present study	China
X. tropicus	CLZhao 3395	OL619263	OL619270	Present study	China
X. tropicus	CLZhao 3397	OL619264	OL619271	Present study	China
X. ussuriensis	KUN 1989	NR166241		Unpublished	USA
X. verecundus	KHL 12261	DQ873642	DQ873643	[47]	Sweden
X. victoriensis	LWZ20180510-29	MT319487	MT319228	[3]	Australia
X. xinpingensis	CLZhao 11224	MW394662	MW394654	[44]	China
X. yarraensis	LWZ20180510-5	MT319639	MT319378	[3]	Australia
X. yunnanensis	LWZ20180922-47	MT319660		[3]	China

[T] is shown type material, holotype.

The sequences were aligned in MAFFT 7 (https://mafft.cbrc.jp/alignment/server/, accessed on 5 November 2021) using the “G-INS-i” strategy for the ITS and ITS+nLSU dataset. The alignment was adjusted manually using BioEdit [48]. The dataset was aligned first, and then ITS and nLSU sequences were combined with Mesquite version 3.51. Alignment datasets were deposited in TreeBASE (submission ID 29060). ITS + nLSU sequences and ITS-only datasets were used to infer the position of the three new species among Xylodon and related taxa. Sequences of Hymenochaete cinnamomea (Pers.) Bres. and H. rubiginosa (Dicks.) Lév. retrieved from GenBank were used as an outgroup in the ITS+nLSU analysis (Figure 1); sequences of Lyomyces mascarensis Riebesehl, Yurchenko & Langer, and L. sambuca (Pers.) P. Karst. retrieved from GenBank were used as an outgroup in the ITS-only analysis (Figure 2) [3].

Figure 1

Maximum parsimony strict consensus tree illustrating the phylogeny of Xylodon and related genera in Hymenochaetales based on ITS + nLSU sequences.

Figure 2

Maximum parsimony strict consensus tree illustrating the phylogeny of three new species in Xylodon based on ITS sequences. Branches are labeled with maximum likelihood bootstrap value >70%, parsimony bootstrap value >50%, and Bayesian posterior probabilities >0.95, respectively. The new species are in bold.

Maximum parsimony (MP), maximum likelihood (ML), and Bayesian inference (BI) analyses were applied to the combined three datasets following a previous study [49], and the tree construction procedure was performed in PAUP* version 4.0b10 [50]. All characters were equally weighted, and gaps were treated as missing data. Using the heuristic search option with TBR branch swapping and 1000 random sequence additions, trees were inferred. 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 [51]. 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. The multiple sequence alignment was also analyzed using maximum likelihood (ML) in RAxML-HPC2 through the Cipres Science Gateway [52]. Branch support (BS) for ML analysis was determined by 1000 bootstrap replicates.

MrModeltest 2.3 [53] was used to determine the best-fit evolution model for each data set for Bayesian inference (BI), which was performed using MrBayes 3.2.7a with a GTR + I + G model of DNA substitution and a gamma distribution rate variation across sites [54]. A total of 4 Markov chains were run for 2 runs from random starting trees for 6 million generations for ITS + nLSU (Figure 1) and 7.5 million generations for ITS (Figure 2) with trees and parameters sampled every 1000 generations. The first one-fourth of all generations were 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 77 fungal specimens representing 71 species. The dataset had an aligned length of 2269 characters. Maximum parsimony analysis yielded 26 equally parsimonious trees (TL = 4541, CI = 0.3510, HI = 0.6490, RI = 0.5827, and RC = 0.2045). The best model for the ITS + nLSU dataset estimated and applied in the Bayesian analysis was GTR + I + G. Bayesian analysis and ML analysis resulted in a similar topology to MP analysis with an average standard deviation of split frequencies = 0.015456 (BI), and the effective sample size (ESS) average ESS (avg ESS) = 791. The phylogram based on the ITS+nLSU rDNA gene regions (Figure 1), including the six genera, Fasciodontia, Hastodontia (Parmasto) Hjortstam & Ryvarden, Hyphodontia, Kneiffiella, Lyomyces, and Xylodon, were divided into four families within Hymenochaetales, which showed that three genera Fasciodontia, Lyomyces, and Xylodon nested into the family Schizoporaceae. Our current three new species clustered into the family Schizoporaceae separated into genus Xylodon. Xylodon laceratus was a sister to X. heterocystidiatus (H.X. Xiong, Y.C. Dai & Sheng H. Wu) Riebesehl, Yurchenko & Langer; X. montanus closely related with X. subclavatus (H.X. Xiong, Y.C. Dai & Sheng H. Wu) Riebesehl, Yurchenko & Langer and X. xinpingensis C.L. Zhao & X. Ma with high supports; X. tropicus was retrieved as a sister to X. hastifer (Hjortstam & Ryvarden) Hjortstam & Ryvarden.

The ITS-alone dataset (Figure 2) included sequences from 58 fungal specimens representing 53 species. The dataset had an aligned length of 875 characters. Maximum parsimony analysis yielded 5000 equally parsimonious trees (TL = 2054, CI = 0.2965, HI = 0.7035, RI = 0.4580, and RC = 0.1358). The best model for the ITS dataset estimated and applied in the Bayesian analysis was GTR+I+G. Bayesian analysis and ML analysis resulted in a similar topology to the MP analysis with an average standard deviation of split frequencies = 0.028998 (BI), and the effective sample size (ESS) of the average ESS (avg ESS) = 574.

3.2. Taxonomy

C.L. Zhao, sp. nov. Figure 3 and Figure 4.

Figure 3

Basidiomata of Xylodon laceratus (holotype): the front of the basidiomata (A), character hymenophore (B). Bars: (A) = 2 cm and (B) = 1 mm.

Figure 4

Microscopic structures of Xylodon laceratus (holotype): basidiospores (A), basidia and basidioles (B), fusiform cystidia (C), capitate cystidia (D), A section of hymenium (E). Bars: (A) = 5 μm, (B)–(E) = 10 µm.

MycoBank no.: 842068

Holotype—China. Yunnan Province, Puer, Jingdong County, Wuliangshan National Nature Reserve, GPS 22°46′ N, 100°58′ E, altitude 1400 m asl., on the trunk of angiosperm with bark, within the broad-leaved forest, leg. C.L. Zhao, 6 October 2017, CLZhao 9892 (SWFC).

Etymology— (Lat.): referring to the cracked hymenophore of the specimens.

Fruiting body—Basidiomata annual, resupinate, soft, without odor and taste when fresh, becoming coriaceous when fresh, hard coriaceous upon drying, up to 20 cm long, 3 cm wide, 50–110 µm thick. Hymenial surface grandinioid, aculei up to 0.1 mm long, cream when fresh, cream to buff upon drying, cracking. Sterile margin indistinct, cream, 0.5–1 mm wide, unattached.

Hyphal system—Monomitic, generative hyphae with clamps, colorless, thin-walled, frequently branched, interwoven, 1.8–4.4 µm in diameter, hyphae tight in aculei, IKI–, CB–; tissues unchanged in KOH; subhymenial hyphae densely covered by larger, irregular crystals; a basal layer of hyphae parallel.

Hymenium—Cystidia of two types: (1) capitate cystidia rare, smooth, colorless, thin-walled, slightly constricted at the neck, with a globose head, 15.4–24.7 × 3.8–4.7 µm; (2) fusiform cystidia frequently, smooth, colorless, thin-walled, 20.3–26.8 × 5.3–6.4 µm; basidia barreled to clavate, with four sterigmata and a basal clamp connection, 11–17.5 × 3.2–5.5 µm.

Spores—Basidiospores ellipsoid, colorless, thin-walled, smooth, with one oil drop inside, IKI–, CB–, (3.7–)3.9–5.3 × 2.6–4.1(–4.8) µm, L = 4.52 µm, W = 3.35 µm, Q = 1.28–1.43 (n = 60/2).

Additional specimen examined—China, Yunnan Province, Puer, Jingdong County, Wuliangshan National Nature Reserve, GPS 22°46′ N, 100°58′ E, altitude 1400 m asl., on the trunk of angiosperm with bark, within the broad-leaved forest, leg. C.L. Zhao, 6 October 2017, CLZhao 9841 (SWFC).

C.L. Zhao, sp. nov. Figure 5 and Figure 6.

Figure 5

Basidiomata of Xylodon montanus (holotype): the front of the basidiomata (A), character hymenophore (B). Bars: (A) = 2 cm and (B) = 0.5 mm.

Figure 6

Microscopic structures of Xylodon montanus (holotype): basidiospores (A), basidia and basidioles (B), moniliform cystidia (C), A section of hymenium (D). Bars: (A) = 5 μm, (B–D) = 10 µm.

MycoBank no.: 842069

Holotype—China. Yunnan Province, Yuxi, Xinping County, Chama Ancient Road Scenic spot, GPS 23°96′ N, 101°51′ E, altitude 2183 m asl., on the angiosperm fallen branch with bark, within the mixed broadleaf-conifer forest, leg. C.L. Zhao, 21 August 2018, CLZhao 8179 (SWFC).

Etymology— (Lat.): referring for species collected in montane habitat in Yunnan-Guizhou Plateau.

Fruiting body—Basidiomata annual, resupinate, woolly when fresh, hard brittle when dry, up to 15 cm long, 3.5 cm wide, 80–130 µm thick. Hymenial surface smooth, white to cream when fresh, cream upon drying. Sterile margin indistinct, white to cream, about 1 mm wide, attached.

Hyphal system—Monomitic, generative hyphae with clamps, colorless, thin-walled, frequently branched, interwoven, 1.3–3.7 µm in diameter, IKI–, CB–; tissues unchanged in KOH; a basal layer of hyphae parallel.

Hymenium—Cystidia moniliform, numerous, smooth, colorless, thin-walled, slightly constricted at the neck, with a globose head, 19.5–47.6 × 3.6–7.1 µm, rare presence of one globose head; basidia subclavate to clavate with a median constriction, with four sterigmata and a basal clamp connection, 10.1–17.8 × 3.8–5.6 µm.

Spores—Basidiospores ellipsoid to broad ellipsoid, colorless, thin-walled, smooth, IKI–, CB–, 3.9–5.3 × 3.2–4.3 µm, L = 4.68 µm, W = 3.61 µm, Q = 1.27–1.32 (n = 60/2).

Additional specimen examined—China, Yunnan Province, Puer, Zhenyuan County, Ailaoshan, Jinshan Original Forestry, GPS 23°94′ N, 101°52′ E, altitude 2310 m asl., on the angiosperm fallen branch with bark, mixed broadleaf-conifer forest, leg. C.L. Zhao, 21 August 2018, CLZhao 8118 (SWFC).

C.L. Zhao, sp. nov. Figure 7 and Figure 8.

Figure 7

Basidiomata of Xylodon tropicus (holotype): the front of the basidiomata (A), character hymenophore (B). Bars: (A) = 1 cm and (B) = 1 mm.

Figure 8

Microscopic structures of Xylodon tropicus (holotype): basidiospores (A), basidia and basidioles (B), and fusiform cystidioles (C), A section of hymenium (D). Bars: (A) = 5 μm, (B–D) = 10 µm.

MycoBank no.: 842070

Holotype—China. Yunnan Province, Puer, Zhenyuan County, Xieqipo Park, GPS 24°01′ N, 101°10′ E, altitude 1121 m asl., on the branch of dead bamboo, in the bamboo forest, leg. C.L. Zhao, 1 October 2017, CLZhao 3351 (SWFC).

Etymology— (Lat.): referring to distribution (tropical zone) of the new species.

Fruiting body—Basidiomata annual, resupinate, adnate, coriaceous, without odor and taste when fresh, up to 15 cm long, 6.5 cm wide, 110–250 µm thick. Hymenial surface grandinioid, cream when fresh, buff to pale brown on drying. Sterile margin indistinct, cream to buff, about 1 mm wide, unattached.

Hyphal system—Monomitic, generative hyphae with clamps, colorless, thick-walled, 0.4–0.75 µm thick, frequently branched, interwoven, 2.2–3.7 µm in diameter; IKI–, CB–; tissues unchanged in KOH.

Hymenium—Cystidia absent and cystidioles fusiform, rare, 7.2–15 × 2.9–4.8 µm; basidia barreled, slightly constricted in the middle to somewhat sinuous, with four sterigmata and a basal clamp, 7.1–13.5 × 2.3–4 µm.

Spores—Basidiospores subglobose, colorless, slightly thick-walled, 0.1–0.3 µm thick, smooth, IKI–, CB–, (1.7–)2–4.8 × 1.6–4(–4.4) µm, L = 3.55 µm, W = 3.06 µm, Q = 1.15–1.17 (n = 120/4).

Additional species examined—China, Yunnan Province, Puer, Zhenyuan County, Xieqipo Park, GPS 24°01′ N, 101°10′ E, altitude 1121 m asl., on the branch of dead bamboo, in the bamboo forest, leg. C.L. Zhao, 1 October 2017, CLZhao 3355, CLZhao 3395, CLZhao 3397 (SWFC).

4. Discussion

In their geographical distribution and ecological preferences, the members of Xylodon are widespread and primarily wood decomposers, causing a white-rot of woody substrates, which are widely distributed in various forest ecosystems from boreal, temperate, subtropical, to tropical zones [16,55]. Some taxa of Xylodon have been collected on rotten trunks and stumps of conifers and angiosperms, bamboo, and ferns [3,12,32,34,44,56,57,58,59,60,61,62,63,64,65,66,67,68,69,70,71,72].

Phylogenetically, the molecular relationships of species belonging to Hyphodontia s.l. within Hymenochaetales inferred from the combined dataset of ITS, nLSU, and mt-SSU regions, showed that the phylogeny employed by the dataset strongly supported Hymenochaetales as an independent order, in which seven families, Chaetoporellaceae, Coltriciaceae, Hymenochaetaceae, Neoantrodiellaceae, Nigrofomitaceae, Oxyporaceae, Schizoporaceae were as seven monophyletic lineages, that several genera nested into Hyphodontia s.l. were supported as independent genera [3]. In the present study (Figure 1), four related families in the order Hymenochaetales were carried out by the ITS+nLSU analysis, which showed that the six genera, Fasciodontia, Hastodontia, Hyphodontia, Kneiffiella, Lyomyces, and Xylodon nested into related families, and our three new taxa clustered into genus Xylodon, belonging to the family Schizoporaceae.

Molecular phylogeny of the genus Xylodon based on the ITS-alone dataset included sequences from 61 fungal specimens representing 55 species, revealed that X. gossypinus, X. macrosporus, and X. sinensis were grouped into three different subclades of Xylodon. X. gossypinus grouped closely with X. ussuriensis Viner; X. macrosporus grouped with X. follis with high supports, and X. sinensis grouped to a clade comprising X. attenuatus and X. yarraensis with lower supports [32]. In the current study (Figure 1), our three undescribed taxa nested into the genus Xylodon, in which X. laceratus was a sister to X. heterocystidiatus; X. montanus closely grouped with X. subclavatus and X. xinpingensis with a high supports; X. tropicus was retrieved as a sister to X. hastifer. The ITS-based minimum evolution phylogram for Xylodon and allied species showed that X. cystidiatus (A. David & Rajchenb.) Riebesehl & Langer, X. hyphodontinus (Hjortstam & Ryvarden) Riebesehl, Yurchenko & G. Gruhn, X. serpentiformis (Langer) Hjortstam & Ryvarden and X. subclavatus formed individual lineages in genus Xylodon, and two genera Palifer and Odontiopsis were synonymized under Xylodon based on morphological and sequence data [31]; in the same study, X. erikssonii (M. Galán & J.E. Wright) Riebesehl & Langer, X. gamundiae (Gresl. & Rajchenb.) Riebesehl & Langer, X. hjortstamii (Gresl. & Rajchenb.) Riebesehl & Langer, X. hyphodontinus, X. septocystidiatus (H.X. Xiong, Y.C. Dai & Sheng H. Wu) Riebesehl & Langer and X. verecundus (G. Cunn.) Yurchenko & Riebesehl were proposed as the new combinations [31]. Our new taxa based on the ITS dataset analysis showed that X. tropicus and X. hyphodontinus grouped within the same subclade; both X. laceratus and X. serpentiformis clustered in the same subclade, but they were distinct; X. montanus was closely grouped with X. subclavatus with a well supports. However, morphologically, Xylodon heterocystidiatus differs from X. laceratus by having the membranaceous basidiomata and cylindrical or clavate leptocystidia (34–51 × 6–9 µm) [11]. X. subclavatus is different than X. montanus by having the cracked basidiomata, odontioid hymenial surface, and subclavate, cylindrical or fusiform cystidia (12–33 × 4.5–8.5 μm) [15]. X. xinpingensis is different from X. montanus by having the reticulate hymenophore, fusiform cystidia (19.5–31 × 2–6 µm), bigger basidia (18.5–33 × 3–6.5 µm), and subglobose basidiospores (4.5–6 × 3.5–5 µm) [44]. X. hastifer could be delimited from X. tropicus by its whitish, distinctly odontioid hymenophore, and subulate cystidia (40–60 × 6–8 µm) [25].

Morphologically, Xylodon laceratus is similar to X. australis (Berk.) Hjortstam & Ryvarden, X, gamundiae, X. macrosporus, X. rhododendricola Xue W. Wang & L.W. Zhou and X. subserpentiformis Xue W. Wang & L.W. Zhou by the cracked hymenophore. However, Xylodon australis is diverse from X. laceratus by its membranous basidiomata with a brown hymenial surface with an olive tint and subulate, sinuous, or moniliform cystidia (40–60 × 10–20 µm) [22]; X. gamundiae is different than X. laceratus by its suburniform to sinuous basidia (20–30 × 4–5 µm) and cylindrical to subcylindrical basidiospores (6–6.5 × 3–3.5 µm) [56]; X. macrosporus differs from X. laceratus by its ellipsoid to broad ellipsoid, larger basidiospores (8–10.5 × 7.5–9 µm) [32]; X. rhododendricola could be delimited from X. laceratus by its odontioid, buff-yellow hymenophore, and usually encrusted with crystals leptocystidia (30–35 × 3–3.5 µm) [3]; X. subserpentiformis is distinguishable from X. laceratus by its snake-like sinuous tramacystidia (45–50 × 4.5–5.5 µm) and utriform or subclavate basidia (20–25 × 4.5–5.5 µm) [3].

Xylodon montanus is similar to X. anmashanensis (Yurchenko, H.X. Xiong & Sheng H. Wu) Riebesehl, Yurchenko & Langer, X. brevisetus (P. Karst.) Hjortstam & Ryvarden, X. crassisporus (Gresl. & Rajchenb.) Hjortstam & Ryvarden, X. gossypinus and X. pumilius in having the moniliform cystidia. However, Xylodon anmashanensis differentiates from X. montanus by its cracked, odontioid hymenophore and three kinds of cystidia: hyphoid or subulate cystidia (22–53 × 2.5–4.5 μm), moniliform and submoniliform cystidia (20–60 × 3–5 μm) [15]; X. brevisetus is diverse from X. montanus by its odontioid hymenophore, clavate to cylindrical basidia (20–25 × 4–5 μm) [25]; X. crassisporus could be delimited from X. montanus by its odontioid, pale yellow or very pale brown hymenophore, and the capitate cystidia (27–75 × 2–5 μm) [56]; X. gossypinus differs from X. montanus by its flocculent basidiomata and capitate cystidia (16–23.5 × 2.5–5 µm) [32]; X. pumilius is distinguishable from X. montanus by its ochraceous hymenophore, capitate cystidia (30–45 × 4–5 μm) and broadly ellipsoid to broadly, thick-walled basidiospores (5–6 × 4–5 μm) [56].

Xylodontropicus is similar to Xylodon damansaraensis Xue W. Wang & L.W. Zhou, X. septocystidiatus, X. tenellus Hjortstam & Ryvarden, X. ussuriensis Viner and X. yarraensis in having the grandinioid hymenophore. However, Xylodon damansaraensis distinguishes from X. tropicus by its white hymenophore, clavate-sinuous to submoniliform cystidia (35–40 × 6–7 µm) and narrowly ellipsoid basidiospores (5.2–5.7 × 2.3–3.1 µm) [3]; X. septocystidiatus differentiates from X. tropicus by its pale buff hymenophore and subcylindrical basidia with subuniform constriction (16–22 × 3 4.5–5 µm) [12]; X. tenellus is diverse from X. tropicus by its whitish hymenophore, capitate cystidia (30–60 µm long) and thin-walled basidiospores (4 × 4.5 µm) [28]; X. ussuriensis could be delimited from X. tropicus by its pale ochraceous hymenophore, three types of cystidia: capitate cystidia (71.0–188.9 × 5.7–9.4 μm), astrocystidia (15–17 × 4.5–4.8 μm), fusoid to cylindrical or submoniliform cystidia (40.0–84.0 × 5.0–9.0 μm), and ellipsoid to broadly ellipsoid basidiospores (5.1–6.0 × 3.8–4.6 μm) [27]; X. yarraensis is different from X. tropicus by its capitate cystidia (25–30 × 2.5–3.5 µm) and ellipsoid basidiospores (4.5–5.3) × 3.1–3.8 µm) [3].

Miettinen et al. [71] analyzed a higher-level phylogenetic classification of polypores and showed that the macromorphology of fruiting bodies and hymenophore construction did not reflect monophyletic groups. The current phylogeny shows that the morphological characters do not follow the phylogenetic group on different taxa in this genus. However, several characters lead the key role, e.g., the hyphal system monomitic, basidiospores ellipsoid, and thin-walled (Figure 2).

So far, 33 taxa of Xylodon were recorded in China [3,22,24,27,31,32,37,44,72]. Based on the morphological and phylogenetic study of Xylodon presented here, all of these can be delimited from our three new species (Figure 1 and Figure 2).