Holarctic Species in the Pluteus romellii Clade. Five New Species Described and Old Names Reassessed

We studied the taxonomy of Pluteus romellii, and morphologically similar Holarctic species in the/romellii clade of section Celluloderma, using morphological and molecular data (nrITS, TEF1-α). Pluteus romellii is lectotypified and epitypified and accepted as an exclusively Eurasian species. Pluteus lutescens and P. pallescens are considered synonyms of P. romellii. Pluteus fulvibadius is accepted as a related, but separate, North American species. Five species in the/romellii clade are described as new to science: two from North America (P. austrofulvus and P. parvisporus), one from Asia (P. parvicarpus), one from Europe (P. siccus), and one widely distributed across the Holarctic region (P. vellingae). Basidioma size, pileus color, lamellae color, basidiospore size, hymenial cystidia shape and size, habitat and geographical distribution help separate the species described here, but in some instances only molecular data allows for confident identification. The current status of P. californicus, P. melleipes, P. romellii var. luteoalbus, P. splendidus, P. sternbergii and P. sulphureus is discussed.


Introduction
Pluteus Fr. is a genus of saprotrophic mushroom-forming fungi belonging to the family Pluteaceae Kotl. & Pouzar [1]. Within Pluteaceae, the genus Pluteus is easily recognizable from related genera Volvariella Speg. and Volvopluteus Vizzini, Contu & Justo by lacking a volva, but some species historically classified in the genus Chamaeota (W.G. Sm.) Earle possess partial veil [2], represented in the form of an annulus. These genera are also Macroscopic descriptions of newly collected specimens are based on fresh basidiomata, and microscopic descriptions are based on dried specimens; except for the holotypes of P. sternbergii and P. sulphureus which were preserved in Velenovský's solution [61]. Color abbreviations follow RAL Design color range system (https://www.ralcolorchart. com/ral-design; accessed on 3 April 2022) or Munsell Soil-Color charts [62]; herbarium abbreviations are those according to [63]. Microscopic features were described from dried material mounted in 10% KOH and Congo Red with a magnification of 500×, 600× and 1000×. Terminology follows [24]. Abbreviations: avl = average length, avw = average width, Q = quotient of length and width, avQ = average Q. Average values for each species are given as intervals of the individual average values for each collection examined. The notation [X, Y, Z] indicates that measurements were made on X basidiospores, in Y basidiomata from Z collections. The newly collected specimens are deposited in the herbaria BRNM, LE, NBM, PUL and the personal herbaria of GF, OK and GM. All holotypes are stored in public herbaria. MycoBank numbers were used as unique identifiers for each new species.

Molecular Phylogeny 2.2.1. DNA Extraction, Amplification, Sequencing and Sequence Alignment
For DNA extraction, small fragments of dried basidiomata were used. Sequencing of the collections deposited in the herbaria BRNM, PRM and P was performed by M. Sochor, following the molecular methods described by Ševčíková et al. [64]. The type material of P. pallescens, deposited in K, was studied by B. Dima, and the molecular methods followed [65,66]. The total genomic DNA of OK collections was extracted in accordance with Kaygusuz et al. [67]. For collections in LE, the DNA extraction procedure completely corresponded to the manufacturing protocol of the Phytosorb Kit (ZAO Syntol). New molecular sequences for collections at NBM were generated at ALVALAB (http://www. alvalab.es/index.html. Last accessed 1 July 2022).

Phylogenetic Analyses
Raw data were edited and assembled in MEGA 10 [71], with the CodonCode Aligner package (CodonCode Corp., Centerville, MA, USA) and ChromasPro (Technelysium). We assembled a nrITS dataset of all available sequences phylogenetically close to P. romellii ("/romellii-aurantiorugosus clade" in [2], "/aurantiorugosus clade" in [39]. This includes 57 newly generated nrITS sequences for this study, and 56 sequences generated in previous studies or available in public databases and biodiversity repositories (GenBank, UNITE, BOLD, iNaturalist). A total of 113 nrITS sequences were used in the final datasets, including voucher-based and environmental sequences. We assembled a TEF1-α dataset of 40 sequences, 35 of them newly generated for this study. In all datasets, we included P. phlebophorus and P. rugosidiscus as outgroup taxa, based on previous phylogenetic work on Pluteus [2,40,50]. All sequences used in the analyses are listed in Table S1. Sequences were aligned using MAFFT version 7 [72] and the strategy FFT-NS-i. The alignment was inspected and manually corrected in AliView [73]. No topological conflicts were detected in the phylogenetic analyses of the nrITS and TEF1-α datasets (detailed below), so a combined dataset was created by concatenating the nrITS and TEF1-α matrices.
For all three datasets (nrITS, TEF1-α and nrITS + TEF1-α), two separate phylogenetic analyses were run: (i) maximum likelihood (ML) analyses using RAxML 8.2.10 under a GTRGAMMAI model as recommended [74], with 100 rapid bootstrap (BS) replicates; (ii) Bayesian inference (BI) analyses using MrBayes 3.2.7 [75] for 10 million generations under a GTRGAMMAI model with four chains, and trees sampled every 1000 generations. The initial burn-in phase was set to 2.5 million generations, and this value was confirmed to be adequate by checking the graphic representation of the likelihood scores of the sampled trees. Additionally, we also confirmed that the standard deviation of split frequencies was <0.05, and that potential scale reduction factor (PRSF) values were close to 1, as detailed in Ronquist et al. [76]. All analyses were run using resources at the CIPRES Science Gateway [77]. All phylogenetic trees were initially visualized using FigTree (http: //tree.bio.ed.ac.uk/software/figtree/. Last Accessed 1 July 2022). Trees were exported from FigTree as SVG files and edited in Adobe Illustrator for final presentation.

Phylogeny
The nrITS dataset comprises 113 sequences and 760 characters (gaps included). The TEF1-α dataset comprises 40 sequences and 855 characters (gaps included). The combined nrITS + TEF1-α dataset consisted of 115 combined sequences and a total of 1615 characters (gaps included). There were no major differences in the overall topologies of the best tree from the ML analysis and the consensus tree from the BI analysis for any of the datasets.
In Figure 1, we present the best tree from the ML analysis of the nrITS + TEF1-α dataset, which will be the main reference point for the taxonomic discussion. The individual nrITS and TEF1-α trees (ML analyses) are presented in Figures 2 and 3, respectively, as some aspects of these single-gene phylogenies are relevant to the taxonomic discussions.
For clarity, throughout the paper, the term "/romellii clade" is used for the clade previously recognized as "/romellii-aurantiorugosus clade" [2] and "aurantiorugosus clade" [39]. This clade always appears as a distinct and well-supported lineage in broader phylogenetic analyses of sect. Celluloderma (data not shown). The term "romellii species complex" is reserved for the least inclusive clade that includes P. romellii sensu stricto and closely related species, but not P. aurantiorugosus. Thus, some of the species discussed here are part of both the/romellii clade and the romellii species complex, while others are part of one but not the other.
A clade or relation is considered to be strongly supported if it receives BS ≥ 90% and posterior probability/probabilities (PP) = 1, and well supported if it receives BS ≥ 70% and PP ≥ 0.95.
Pluteus sternbergii appears as sister to the/romellii clade. The topology of the nrITS tree is shown in Figure 2. The overall topology is very similar to the one of the combined datasets, but support values for some of the taxa and relations discussed here are slightly or significantly lower (e.g., P. fulvibadius).
The topology of the TEF1-α tree is shown in Figure 3. All species in the romellii species complex are recovered as separate from each other in strongly or well-supported clades. The relations between these species are slightly different from the ones recovered in the nrITS and combined datasets, but none of these relations receive any significant support. In the TEF1-α, tree Pluteus siccus appears as sister to P. vellingae, which is different from the topology recovered in the nrITS and combined datasets, where P. siccus is sister to P. parvicarpus. The position of P. siccus in the nrITS tree is strongly supported (96%/0.99) while in the TEF1-α tree it is not (87% and no support in Bayesian tree).

Taxonomy
Below, we present the detailed descriptions of Pluteus romellii and P. fulvibadius, as accepted and delimited here. We propose the following Holarctic species in the/romellii clade as new: P. austrofulvus (Eastern Nearctic), P. parvicarpus (Eastern Palearctic), P. parvisporus (Eastern Nearctic), P. siccus (Eastern Palearctic) and P. vellingae (Holarctic). We establish the synonymy of P. pallescens with P. romellii, accept the previously suggested synonymy of P. lutescens and P. romellii, and discuss the current status of several older names related to the taxa in the/romellii clade.  7: 19 (1943). Pileus 12-60(-80) mm broad, hemispherical or campanulate-convex, rarely conical, later plano-convex to applanate, without or with obtuse umbo; hygrophanous or not; translucently striate to sulcate at the margin or not, sometimes slightly eroded; usually yellow-brown, light brown to dark brown, rarely cinnamon brown, reddish brown or beige; with darker brown, rarely almost blackish center; surface smooth, rarely indistinctly velvety, matte or not, strongly to indistinctly radially venose from the center towards the margin. Lamellae L = 24-90(-110), l = 1-3, free, moderately crowded to crowded, slightly to distinctly ventricose, 2-10 mm broad, whitish or yellowish when young, later pink, often with yellow tinges, with white or concolorous, rarely yellowish flocculose even or eroded edge. Stipe (22-)25-91 × 1.5-7(-9) mm, cylindrical, sometimes with slightly broadened base, solid or fistulose; surface smooth or longitudinally innately fibrillose, covered with concolorous floccules especially at the lower part, whitish to yellow, rarely with very pale brownish tinge, with more distinct color at the base. Basal mycelium tomentose or absent, whitish to very pale yellowish when present. Context watery, white in the pileus, pale yellow, yellow to lemon-yellow in the stipe, sometimes whitish only with yellow tinge at the base or with very pale brownish tinge. Smell indistinct or slightly sweet, taste indistinct or mild.
Habit  Notes: The original description of P. romellii [10] is quite vague, and mainly characterizes this species as similar to P. nanus, with spores 6-7 µm, and growing on soil in Bavaria (Germany). Modern authors have interpreted P. romellii as a species in sect. Celluloderma with a brown pileus, a yellow stipe and no elongated elements in the pileipellis (e.g., [4,22]. This concept is accepted here, and the application of the name is stabilized by the selection of an epitype from central Europe (BRNM 761731) based on morphologically typical basidiomata fully corresponding with the description by Vellinga [4], and well-defined by nrITS + TEF1-α sequences, together with a lectotype based on original illustration [10]. Most authors have accepted the name Pluteus lutescens (Fr.) Bres. as a younger taxonomic synonym of P. romellii e.g., [4,22,23,26,54], and the same interpretation is accepted here.
The clade we accept as Pluteus romellii sensu stricto is recovered as well-supported in the analyses of the nrITS + TEF1-α dataset ( Figure 1) and also in the individual nrITS ( Figure 2) and TEF1-α ( Figure 3) phylogenies.
Pluteus pallescens P.D. Orton was originally described as a species in sect. Celluloderma with a brown-colored, markedly hygrophanous pileus and a stipe "white or whitish then discoloured dirty cream or pale dirty yellowish from base" [20]. Orton emphasized the hygrophanous pileus and the broader pleurocystidia to separate P. pallescens from his concept of P. nanus (Pers.) P. Kumm., and the colors of the stipe (lacking grey colors) to separate P. pallescens from P. griseopus P.D. Orton. Later, after reexamining the original description and the type material of P. satur Kühner & Romagnesi, Orton concluded that both taxa were the same, and synonymized P. pallescens with P. satur [22]. It should be noted that, in the description of P. satur given by Orton [22], the stipe is described as "white or whitish then pale creamy grey" with no specific mention of the yellow base that was indicated in the original description of P. pallescens. Citérin & Eyssartier [26] reduced P. pallescens to a form of P. satur, characterized by a strongly striate and markedly hygrophanous pileus. We were able to obtain a nrITS sequence from the isotype collection of P. pallescens, and this sequence falls within the molecular variation of P. romellii s. stricto as accepted here (Figures 1 and 2). We consider P. pallescens to be a younger synonym of P. romellii. Despite the suggestions of Orton [22] and Citérin & Eyssartier [26] of a close relation between P. pallescens and P. satur, our ongoing revision of sect. Celluloderma points in a different direction. The original material of Pluteus satur at the Conservatoire et Jardin botaniques de la Ville de Genève (G00052523 and G00052524) has differently shaped pleurocystidia, and moreover the nrITS sequences generated from this material indicates that P. satur belongs in the/cinereofuscus clade as defined by Menolli et al. [39] (Data not shown). la Ville de Genève (G00052523 and G00052524) has differently shaped pleurocystidia, and moreover the nrITS sequences generated from this material indicates that P. satur belongs in the /cinereofuscus clade as defined by Menolli et al. [39] (Data not shown).    Notes: This morphological variant of P. romellii is characterized by the white basidiomata [78]. Phylogenetic analyses of the type collection confirm that f. albidus is just an albino variant of the normally brightly colored P. romellii ( Figure 4).  Notes: This morphological variant of P. romellii is characterized by the white basidiomata [78]. Phylogenetic analyses of the type collection confirm that f. albidus is just an albino variant of the normally brightly colored P. romellii (Figure 4).
The existence of albino forms in several species of Pluteus sect. Celluloderma makes morphological identification challenging, as many species are defined by the colors of pileus and stipe. The relatively broad pleurocystidia of P. romellii f. albidus help in identifying this taxon in the absence of molecular data.
Pluteus aff. Pileus 14-35 mm in diam., initially convex, then plano-convex to applanate; slightly hygrophanous or not, usually striate at the margin when mature; surface smooth, entirely wrinkled or wrinkled only at the center, sometimes cracked, pure yellow, yellowbrown, greyish brown or brown with yellow areas. Lamellae free, crowded to moderately crowded, ventricose, whitish when very young, later pink, with concolorous edges. Stipe 24-50 × 1.8-3.5 mm, cylindrical, solid, yellow when young or whitish at upper part and yellowish or pale yellow-brown beneath, longitudinally fibrillose. Smell indistinct, taste mild, indistinct.
Habit, habitat, phenology and distribution: Often solitary, on decayed wood of deciduous trees (logs, fallen branches) including Betula, Carpinus, Quercus. May-January. Widely distributed in Eurasia, from Spain to the Russian Far-East, and possibly Japan.

Notes:
The description above is based on the collections identified in Figure 1 as Pluteus aff. romellii. The holotype of Pluteus romellii var. luteoalbus falls within this group of sequences, but not all collections of P. aff. romellii fit the morphological concept of P. romellii var. luteoalbus. By its original description, Pluteus romellii var. luteoalbus differs from Pluteus romellii var. romellii by its completely different colors: a pileus that is yellow, darker at umbo; lamellae that are whitish to pale yellow (later pinkish by pink spore print), and a whitish stipe; context whitish in pileus, whitish to pale yellow in the stipe [79]. This variety was originally described and was only known from the Czech Republic. Lécuru [80] then recombined this taxon to form rank, as Pluteus romellii f. luteoalbus (Ševčíková & Borov.) Lécuru, but without any further taxonomic or molecular discussion.
Pluteus splendidus A. Pearson was originally described from England as a species with a bicolored pileus (chrome yellow at the center and yellow brown color towards the margin) and a chrome yellow or a lemon yellow stipe [61]. Vellinga & Schreurs [23] reexamined the original collections of P. splendidus, designated one of them as a lectotype, and concluded that P. splendidus was a synonym of P. romellii. Lécuru [80] recently recombined Pluteus splendidus A. Pearson as P. romellii f. splendidus (A. Pearson) Lécuru. We cannot

Notes:
The description above is based on the collections identified in Figure 1 as Pluteus aff. romellii. The holotype of Pluteus romellii var. luteoalbus falls within this group of sequences, but not all collections of P. aff. romellii fit the morphological concept of P. romellii var. luteoalbus. By its original description, Pluteus romellii var. luteoalbus differs from Pluteus romellii var. romellii by its completely different colors: a pileus that is yellow, darker at umbo; lamellae that are whitish to pale yellow (later pinkish by pink spore print), and a whitish stipe; context whitish in pileus, whitish to pale yellow in the stipe [79]. This variety was originally described and was only known from the Czech Republic. Lécuru [80] then recombined this taxon to form rank, as Pluteus romellii f. luteoalbus (Ševčíková & Borov.) Lécuru, but without any further taxonomic or molecular discussion.
Pluteus splendidus A. Pearson was originally described from England as a species with a bicolored pileus (chrome yellow at the center and yellow brown color towards the margin) and a chrome yellow or a lemon yellow stipe [61]. Vellinga & Schreurs [23] reexamined the original collections of P. splendidus, designated one of them as a lectotype, and concluded that P. splendidus was a synonym of P. romellii. Lécuru [80] recently recombined Pluteus splendidus A. Pearson as P. romellii f. splendidus (A. Pearson) Lécuru. We cannot currently define the identity of P. splendidus. Some collections of P. aff. romellii have a bicolored pileus, indicating that the name P. splendidus could be considered for this lineage. Unfortunately, no molecular data is available that reliably represents P. splendidus. Until molecular data become available from either the original collections of P. splendidus or modern collections from the UK, preferably from England, that fit the original description, the status of P. splendidus remains uncertain. currently define the identity of P. splendidus. Some collections of P. aff. romellii have a bicolored pileus, indicating that the name P. splendidus could be considered for this lineage. Unfortunately, no molecular data is available that reliably represents P. splendidus. Until molecular data become available from either the original collections of P. splendidus or modern collections from the UK, preferably from England, that fit the original description, the status of P. splendidus remains uncertain. This clade is phylogenetically well-supported in the TEF1-α analysis ( Figure 3), but receives no support in the nrITS ( Figure 2) or combined analyses ( Figure 1).
All available evidence points to Pluteus aff. romellii being a separate lineage from P. romellii s. stricto as accepted here, even if the nrITS phylogeny fails to recover P. aff. romellii as a distinct, well-supported clade. We refrain from formally describing this taxon as new, or from raising var. luteoalbus to species rank, as the name P. splendidus might be the correct name for this lineage. Should it prove in the future that the name P. splendidus is not applicable for this lineage, we recommend elevating taxon Pluteus romellii var.  This clade is phylogenetically well-supported in the TEF1-α analysis (Figure 3), but receives no support in the nrITS (Figure 2) or combined analyses ( Figure 1).
All available evidence points to Pluteus aff. romellii being a separate lineage from P. romellii s. stricto as accepted here, even if the nrITS phylogeny fails to recover P. aff. romellii as a distinct, well-supported clade. We refrain from formally describing this taxon as new, or from raising var. luteoalbus to species rank, as the name P. splendidus might be the correct name for this lineage. Should it prove in the future that the name P. splendidus is not applicable for this lineage, we recommend elevating taxon Pluteus romellii var. luteoalbus to species level as Pluteus luteoalbus. Pileus 15-50 mm in diameter, hemispherical or campanulate when young, expanding to convex or plano-convex, often with a low, broad umbo; surface smooth, slightly to strongly rugose around the center, remaining intact or with small cracks in older specimens; with predominantly brown or yellow-brown colors (Munsell: 7.5YR 6/6-6/8, 5/4-5/8, 4/4-4/6; 10YR 6/6-6/8, 5/4-5/8, 4/4-4/6), sometimes with an olive-brown tint, young specimens might have predominantly green colors (5GY 3/4-5/4) that fade and become brown to olive-brown as they mature, darker at the center; dry, hygrophanous, slightly paler on drying; margin translucently striate. Lamellae crowded, free, ventricose, up to 8 mm broad, white or yellow when young, remaining yellow before turning pink as spores mature, with even or slightly flocculose edges, white or concolorous. Stipe 20-70 × 2-6 mm, cylindrical, with slightly broadened base (up to 8 mm wide); surface pale to bright yellow all over (2.5Y 8/6-8/8 or brighter), smooth or slightly fibrillose, sometimes with abundant white mycelium at base. Context in stipe and pileus white to pale yellow near the surfaces of pileus and stipe. Smell and taste indistinct.
Habit, habitat, phenology and distribution: often gregarious, sometimes solitary, commonly growing on well-decayed wood of angiosperms, also recorded on mulch, on Picea litter, or apparently terrestrial. In temperate forests or transitional temperate/boreal forests.

Notes:
We accept Pluteus fulvibadius, originally described from Oregon (USA), as the correct name for one of the North American lineages in the romellii species complex. Based on the collections examined and publicly available records, this species is widely distributed in Western North America, and it is also present in Eastern North America, but it has only been recorded so far in the Northern states of the USA (Michigan, Minnesota) and in Canada (Québec). The morphological characteristics of our studied collections fit well the type collection revision by Minnis & Sundberg [27].
The nrITS sequences for the North American P. fulvivadius have 4-7 evolutionary events separating them from the Eurasian P. romellii and P. aff. romellii. Their TEF1-α sequences differ in 12-18 evolutionary events from P. romellii and P. aff. romellii (see Table S2 for a complete comparative overview). In all of the analyses, P. fulvibadius is recovered as a clade separate from the Eurasian collections, but with varying levels of support (Figures 1-3).
We accept "fulvibadius" as the correct orthography for this species, despite Index Fungorum, MycoBank, MyCoPortal and other online sources accepting "fulvobadius" (at the time of writing the first version of this manuscript). Murrill uses "fulvibadius" in the original description, and consistently elsewhere in the original publication [13]. The reasoning behind the correction to "fulvobadius" is not clear to us. If Article 60.10 of the ICN [81] is followed, fulvibadius is the correct orthography, and no other grounds for orthographic correction listed under Article 60.1 apply here. After an informal discussion by one of the authors (A. Minnis) with the Nomenclature Committee for Fungi, some of these databases have corrected their spelling to "fulvibadius".
Among the North American species in the P. romellii species complex, P. fulvibadius can be characterized by the relatively larger basidiomata, the lamellae often with bright yellow colors before the spores mature and the relatively larger basidiospores (see comparisons under P. austrofulvus and P. parvisporus).
Pluteus vellingae (see description below), while not part of the romellii species complex, is quite similar in morphology, and its geographical range overlaps with that of P. fulvibadius. Pluteus vellingae has generally smaller basidiomata, the lamellae are usually white or very pale yellow before sporulation and the basidiospores are slightly smaller (avl × avw = 5.7-6.2 × 5.1-5.6 µm), but molecular data might be necessary to confidently identify collections of both taxa.
Minnis & Sundberg [27] considered Pluteus melleipes Murrill a synonym of P. fulvibadius. Revision of the type material of P. melleipes revealed no significant differences with P. fulvibadius as described in Minnis & Sundberg [27], and as accepted here. Both Homola [21] and Minnis & Sundberg [27] suggested that there might be Eastern and Western North American variants under a broader concept of P. fulvibadius (as "P. lutescens" in Homola's work) that accepted here. Minnis & Sundberg [27] suggested that if those variants were confirmed by future work the name P. melleipes (originally described from New York) could be adopted for the Eastern variant. No consistent differences, morphological or molecular, were found among the collections of P. fulvibadius from Western and Eastern North America studied here. The name P. melleipes was evaluated as a candidate for the species described here as P. austrofulvus and P. parvisporus, but morphology (including basidiospore size) and geographical distribution of P. melleipes fit P. fulvibadius better than any of the other North American taxa in the romellii species complex. One of the Californian collections studied here (UCSC-F-00818) has marked green colors in the pileus of young basidiomata (Figure 8), then fades to the usual brown colors. This characteristic led us to consider the name Pluteus californicus McClatchie [82]. This species was originally described from California as having a pileus with "surface rugosevenose, hygrophanous, greenish drab, becoming cinnamon drab". It is unclear, however, whether P. californicus belongs to the /romellii clade. Murrill [13] and Smith and Stuntz [83] described this species as having yellow colors on the stipe, but that is not mentioned in the original description by McClatchie [82], who described the stipe as "pale drab"; moreover, neither of those authors studied any additional collections of P. californicus. No fresh material attributable to P. californicus has been collected since its original description. Green tones of the pileus are not uncommon in some of the taxa in the /cinereofuscus clade of sect. Celluloderma, and P. californicus could be considered for some the Western North American taxa in that group, but it might be best considered a doubtful name, since no modern collections examined by us can be confidently considered to correspond to P. californicus. One of the Californian collections studied here (UCSC-F-00818) has marked green colors in the pileus of young basidiomata (Figure 8), then fades to the usual brown colors. This characteristic led us to consider the name Pluteus californicus McClatchie [82]. This species was originally described from California as having a pileus with "surface rugosevenose, hygrophanous, greenish drab, becoming cinnamon drab". It is unclear, however, whether P. californicus belongs to the/romellii clade. Murrill [13] and Smith and Stuntz [83] described this species as having yellow colors on the stipe, but that is not mentioned in the original description by McClatchie [82], who described the stipe as "pale drab"; moreover, neither of those authors studied any additional collections of P. californicus. No fresh material attributable to P. californicus has been collected since its original description. Green tones of the pileus are not uncommon in some of the taxa in the/cinereofuscus clade of sect. Celluloderma, and P. californicus could be considered for some the Western North American taxa in that group, but it might be best considered a doubtful name, since no modern collections examined by us can be confidently considered to correspond to P. californicus.
Pluteus rugosidiscus Murrill is macroscopically similar to P. fulvibadius, especially to the specimens with green or olive tones in the pileus, but differs in the predominantly lageniform to fusiform pleurocystidia, with a long-to-short pedicel and/or long neck, and cheilocystidia predominantly lageniform with short neck or (broadly) utriform [2,29,53]. Phylogenetically, P. rugosidiscus is not closely related to any of the species in the/romellii clade [2]. Pluteus rugosidiscus Murrill is macroscopically similar to P. fulvibadius, especially to the specimens with green or olive tones in the pileus, but differs in the predominantly lageniform to fusiform pleurocystidia, with a long-to-short pedicel and/or long neck, and cheilocystidia predominantly lageniform with short neck or (broadly) utriform [2,29,53]. Phylogenetically, P. rugosidiscus is not closely related to any of the species in the /romellii clade [2].  MB 844669. Etymology: Makes reference to the yellow colors of the stipe, that characterize this species as part of the/romellii clade, and its more southern distribution relative to P. fulvibadius.
Diagnosis: Differs from P. fulvibadius by the smaller basidiospores and geographical distribution; differs from P. parvisporus by the bigger basidiospores, cracking pileus surface and gregarious habit.
Pluteus fulvibadius Murrill has a more northern distribution in Eastern North America (Minnesota, Michigan, and Québec). Macroscopically it has slightly bigger basidiomata, and the lamellae are bright yellow before turning pinkish as the spores mature. The basidiospores of P. fulvibadius are, on average, larger than those of P. austrofulvus (avl × avw = 6.4-7.2 × 5.6-6.1 µm in P. fulvibadius). Pluteus fulvibadius has predominantly broadly clavate or utriform pleurocystidia. Notes: Pluteus austrofulvus is recovered in the nrITS and TEF1-α phylogenies as a distinct lineage, separate from the other two North America taxa in the romellii species complex: P. fulvibadius and P. parvisporus.
Pluteus vellingae, while morphologically similar, is phylogenetically distant from all taxa in the romellii species complex. Morphologically, it can appear similar to P. austrofulvus, but the pileus is not hygrophanous and it lacks an externally cracking surface. The pleurocystidia in P. vellingae are mostly clavate or broadly clavate, while in P. austrofulvus they are mostly ovoid or broadly fusiform. Pileus 10-15 mm in diameter, hemispherical or campanulate when young, expanding to convex or plano-convex, with or without a low, broad umbo; surface smooth, slightly to strongly rugose around center or all over, remaining entire, not cracking; with predominantly brown or yellow-brown colors (Munsell: 10YR 5/6-5/8, 6/6-6/8); dry, not markedly hygrophanous; margin slightly translucently striate. Lamellae crowded, free, ventricose, up to 3 mm broad, white when young, remaining white before turning pink as spores mature, with even or slightly flocculose edges, white or concolorous. Notes: Pluteus parvisporus occurs in the same geographic area as P. austrofulvus, and both have been collected in the same or nearby localities, either at the same time or at different times in the season. Pluteus parvisporus tends to fruit solitary, and externally it lacks the cracking pileipellis that appears in many collections of P. austrofulvus. Basidiospore length and Q values are the best morphological characters to separate both taxa (see comments under P. austrofulvus).
The small basidiospores (with average length below 6 µm) separate Pluteus parvisporus from Pluteus fulvibadius, which, in addition, has a more northern distribution in Eastern North America (Minnesota, Michigan, and Québec). Macroscopically, it has bigger basidiomata, and the gills are bright yellow before turning pinkish as the spores mature.

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lacks the cracking pileipellis that appears in many collections of P. austrofulvus. Basidiospore length and Q values are the best morphological characters to separate both taxa (see comments under P. austrofulvus).
The small basidiospores (with average length below 6 µm) separate Pluteus parvisporus from Pluteus fulvibadius, which, in addition, has a more northern distribution in Eastern North America (Minnesota, Michigan, and Québec). Macroscopically, it has bigger basidiomata, and the gills are bright yellow before turning pinkish as the spores mature.
Basidiospores [340,12,9]  Notes: Pluteus vellingae is characterized by a yellow-brown to brown, not or only slightly hygrophanous, mostly rugose pileus; scarce to numerous, broadly clavate to clavate or ovoid pleurocystidia; narrowly to broadly clavate or narrowly utriform cheilocystidia, globose to broadly ellipsoid basidiospores and growth on coniferous or deciduous wood. Molecularly, P. vellingae is quite different from all taxa in the P. romellii species Notes: Pluteus vellingae is characterized by a yellow-brown to brown, not or only slightly hygrophanous, mostly rugose pileus; scarce to numerous, broadly clavate to clavate or ovoid pleurocystidia; narrowly to broadly clavate or narrowly utriform cheilocystidia, globose to broadly ellipsoid basidiospores and growth on coniferous or deciduous wood. Molecularly, P. vellingae is quite different from all taxa in the P. romellii species complex and also from its two closest relatives in the/romellii clade, P. parvicarpus and P. siccus (Table S2). The separation of all these taxa based on morphological features could be challenging. complex and also from its two closest relatives in the /romellii clade, P. parvicarpus and P. siccus (Table S2). The separation of all these taxa based on morphological features could be challenging.   Pluteus vellingae has often (but not exclusively) been collected on coniferous wood, which is not a common habitat for species in the /romellii clade. Of the species described here, only P. fulvibadius has been confirmed to occur on coniferous wood or duff. Pluteus fulvibadius has generally larger basidiomata, with bright yellow lamellae that remain that color until sporulation, and larger basidiospores (on average 6.4-7.2 × 5.6-6.1 µm). Pluteus austrofulvus has a hygrophanous pileus, with an often-cracked surface, and predominantly ovoid to broadly fusiform pleurocystidia. P. parvisporus has smaller basidiospores (on average 5.2-5.6 × 4.4-5.1 µm).
Habit, habitat, phenology and distribution: solitary, on fallen branches of deciduous trees. With the current knowledge P. parvicarpus fruits in August and is known only from the Russian Far East.
Notes: Pluteus parvicarpus is characterized by rather small or very small basidiomata with a yellowish brown pileus with olive-brown center, very rare pleurocystidia, cheilocystidia predominantly clavate, and globose or subglobose, relatively small basidiospores.
In the combined nrITS + TEF1-α phylogeny, P. parvicarpus occupies a sister position to P. siccus, from which it differs in 19 evolutionary events in its nrITS sequences and 29 in its TEF1-α sequences (Table S2). Morphologically, P. parvicarpus differs from P. siccus in the smaller basidiomata with brownish (not yellow) pileus, distinctly colored bright yellow stipe and geographical distribution in the Russian Far East.    P. parvicarpus differs from P. vellingae in the smaller basidiomata, less common and utriform pleurocystidia, smaller basidiospores and predominantly clavate/spheropedunculate cheilocystidia.
Among the species in the P. romellii species complex, only P. parvisporus has a similar basidiospore size. That species differs from P. parvisporus in the (narrowly) utriform or clavate cheilocystidia, and its distribution in Eastern North America. Molecularly, the nrITS and TEF1-α sequences of P. parvisporus and P. parvicarpus are very different from each other, with more than 50 individual differences in both cases (Table S2).
Pluteus siccus E.F. Malysheva, sp. nov. Figures 18 and 19. MycoBank number: MB 844712. Etymology: The epithet refers to the peculiarities of the ecology of the species, its discovery in an arid area.
Diagnosis: Differs from Pluteus romellii by its yellow pileus with distinct green hue, smaller basidiospores and polymorphic cheilocystidia, as well as distinct nrITS and TEF1-α sequences.
Among the species in the P. romellii species complex, only P. parvisporus has a similar basidiospore size. That species differs from P. parvisporus in the (narrowly) utriform or clavate cheilocystidia, and its distribution in Eastern North America. Molecularly, the nrITS and TEF1-α sequences of P. parvisporus and P. parvicarpus are very different from each other, with more than 50 individual differences in both cases (Table S2).
Pluteus siccus E.F. Malysheva, sp. nov. Figures 18 and 19. MycoBank number: MB 844712. Etymology: The epithet refers to the peculiarities of the ecology of the species, its discovery in an arid area.
Diagnosis: Differs from Pluteus romellii by its yellow pileus with distinct green hue, smaller basidiospores and polymorphic cheilocystidia, as well as distinct nrITS and TEF1α sequences.
Habit, habitat, phenology and distribution: in a small group, on decayed wood of deciduous tree. Known only from the holotype material collected in the Russian Far East.
Habit, habitat, phenology and distribution: in a small group, on decayed wood of deciduous tree. Known only from the holotype material collected in the Russian Far East.
Notes: Pluteus siccus is characterized by rather small basidiomata, a velvety pileus with greenish hue, utriform pleurocystidia often with slimy cap at apex, polymorphic cheilocystidia and small globose or subglobose basidiospores. Pluteus siccus is morphologically similar to P. romellii but differs from the latter in its velvety yellow pileus with a distinct green hue, smaller basidiospores and polymorphic cheilocystidia.
Based on molecular data, P. siccus occupies a very distant position apart from the romellii species complex, and phylogenetically it is more closely related to P. parvicarpus and P. vellingae. Pluteus parvicarpus significantly differs in smaller basidiomata with a brownish pileus and a bright yellow stipe, and geographical distribution in the Russian Far East. Pluteus vellingae differs by a brown rugose pileus, predominantly clavate pleurocystidia and larger basidiospores.
Among the species in the romellii species complex, only P. parvisporus has a similar basidiospore size, but that species has green hues on the pileus, fruits solitarily and occurs in Eastern North America. Type study: There are two basidiomata stored in one bottle with Velenovský's solution [61]. One basidioma represents the holotype of P. sternbergii, while the second represents 'P. dominii', an unpublished name unknown to us. Both basidiomata have a lot of similar features described below, but different pleurocystidia (described separately).
Habit, habitat and phenology (from the protologue): stump of Populus in valley. In temperate area of Central Europe, Czech Republic. May-July.
Notes: Velenovský characterized Pluteus sternbergii as having a brown, smooth and rugulose pileus, a smooth yellow stipe and whitish to pink lamellae [15] Based on the original description by Velenovský, Vellinga & Schreurs [23] placed P. sternbergii in synonymy with P. romellii.
The molecular analysis of the holotype deposited in PRC preserved in Velenovský's solution repeatedly failed. Velenovský [15] mentioned globose spores, 7-8 µm, absence of pleurocystidia and obtusely rounded, vesiculose cheilocystidia. The holotype was stored together with another basidioma marked as "P. dominii". Both basidiomata have rare pleurocystidia and similar pileipellis, cheilocystidia and spores, the latter probably mixed due to long joint storage. Pleurocystidia of one basidioma are bigger and mostly (broadly) fusiform (A), while smaller pleurocystidia of second basidioma are mostly narrowly utriform to clavate (B).
We were able to obtain a nrITS sequence from another collection (PRM 154258) identified by Velenovský as P. sternbergii. We include this sequence in our analyses (Figures 1 and 2), but it is not closely related to any of the taxa in the/romellii clade. Broader analyses of sect. Celluloderma place P. sternbergii in the/cinereofuscus clade as defined by Menolli et al. [38] (data not shown). In view of the problematic status of the holotype of P. sternbergii, and to stabilize the usage of this name, we have selected collection PRM 154258 as the epitype of P. sternbergii.
Pluteus splendidus A. Pearson [13]. Different authors have interpreted this species differently in North American literature [21,83], but here we agree with Minnis & Sundberg [27] and consider this species a likely synonym of Pluteus fulvibadius. Before describing as new P. austrofulvus, P. parvisporus and P. vellingae we considered the possibility of using the name P. melleipes for each of these taxa. The final conclusion in all cases was that, of all the taxa in the romellii clade present in Eastern North America, (P. fulvibadius, P. austrofulvus, P. parvisporus and P. vellingae) it is actually P. fulvibadius the one that most closely matches the characteristics of P. melleipes, especially in basidiospore size and geographic distribution.
Pluteus sulphureus Velen.České Houby (Praze) 3: 608. 1921. Velenovský [15] characterized this species by a pale pileus covered with dark, appressed thin scales and a black-brown center, a smooth yellow stipe, lemon-yellow adnate lamellae, ovoid-globose spores measuring 4-5 µm and an absence of cystidia. Velenovský found it only once in a Quercus forest. Under the name P. sulphureus, there are three basidiomata preserved in the herbarium PRC (Velenovský no. 95!) in Velenovský's solution, but none of them represent a Pluteus species. The adnate lamellae described in the protologue and our own observations of the type collection confirm that P. sulphureus does not represent a Pluteus species. In the protologue of Pluteus chrysophaeus, the pileus was described as "saturate aureo". However, the lectotype (Tom. III., Table CCLIII) established by Justo et al. [2] includes drawing of very young basidiomata with brown pilei which may also resemble basidiomata of the P. romellii group. Pluteus chrysophaeus has been interpreted in different ways by different authors [2,19,20,22,23,53] and is best considered a doubtful name without modern application.

Discussion
In the present paper, we circumscribe the limits of the commonly used name Pluteus romellii, based on morphological and molecular data, and formally lecotypify and epitypify that species in order to stabilize the usage of the name. We also provide a detailed description, and a phylogenetic delimitation, of P. fulvibadius, one of the North American species in the romellii species complex. Two additional North American species in the romellii species complex are described as new to science (P. austrofulvus and P. parvisporus). An additional Eurasian lineage in this species complex (P. aff. romellii) was detected in the phylogenetic analyses, but it is not formally described here, as more collections and molecular data are needed to further clarify its taxonomic and nomenclatural situation. Three other species which are morphologically similar to P. romellii were recovered in the phylogenies as part of the/romellii clade, but these taxa are not part of the romellii species complex as accepted here. They are newly described here as P. vellingae, P. parvicarpus and P. siccus.
Geographically, there are two groups of taxa: one Eurasian (P. romellii, P. aff. romellii, P. siccus, P. parvicarpus) and one North American (P. fulvibadius, P. austrofulvus, P. parvisporus). Only P. vellingae does not conform to this pattern and has a confirmed distribution on both sides of the Atlantic.
Generally, there are few unique morphological or ecological differences between the taxa described here. Nevertheless, we do interpret the phylogenetic lineages as separate species and therefore have to describe and name them. Without a correct understanding of the natural history of species in the Pluteus romellii complex, and a transparent taxonomy and nomenclature, it will be impossible to obtain more accurate data about the distribution, ecology, morphology and conservation status of these taxa. Regional endemics in this group need further studies to establish their possible conservation status. Morphological features are discussed below.
All Holarctic species with a brown pileus and yellow stipe in the/romellii clade have many common features and can be easily confused. Most species are characterized by a relatively small pileus in various shades of brown with a striate margin, yellow stipe, hyaline pleurocystidia, frequent thin-walled cheilocystidia and an euhymeniderm pileipellis.
Basidioma size varies from relatively small (pileus ≤ 15 mm) in P. parvisporus, P. parvicarpus, and P. siccus to medium-large (pileus ≥ 20 mm and up to 50 mm) in the other species described here. Extremely large basidiomata of P. romellii, with a pileus up to 80 mm, were collected from wood chips, which is a rich substrate often producing bigger basidiomata. On the other hand, basidiomata growing from small twigs tend to be very small. Pileus color is mostly brown to yellowish brown, but distinctly yellow pilei have been recorded in P. siccus and P. aff. romellii (incl. P. romellii var. luteoalbus); green or olive tones have been recorded in P. fulvibadius, P. siccus and P. parvicarpus.
Pluteus austrofulvus often has a pileus with cracks, revealing the white context underneath, a feature also observed in P. parvicarpus, while most other species usually have a non-cracked pileus. However, some collections can get this cracked surface in response to lack of humidity (e.g., P. romellii BRNM 825845). Lamellae of P. romellii, P. vellingae, and P. fulvibadius are often yellow, which might help for the first orientation in the field. While there is some intraspecific variability to this character, the North American collections of P. fulvibadius and P. vellingae seem to have specific patterns of lamellar coloration. In P. fulvibadius, the lamellae are often bright yellow, and the color remains intensely yellow until they turn pink as the spore mature. In P. vellingae, the lamellae start pale yellow, and their color often fades to almost white before the spores mature. The stipe of all species has some distinct yellow colors, either throughout the stipe length or just in its lower part. Some collections of P. romellii and P. aff. romellii have a white stipe, without distinct yellow colors.
Basidiospores of the species described here vary from globose to ellipsoid, but in most species, they are predominantly subglobose to broadly ellipsoid. Basidiospore size values have some overlap when comparing species, but the average values of length and width are useful for separating species. Pluteus parvisporus, P. siccus and P. parvicarpus have the smallest basidiospores (<6 µm long on average), while P. romellii, P. fulvibadius and P. austrofulvus have the larger ones (>6 µm long on average), while. P. vellingae and P. aff. romellii are somewhere in between. Pleurocystidia are very rare or scarce in P. siccus and P. parvicarpus, whereas they are scattered to common in the rest of the species described here. While there is a good degree of overlap in terms of morphology and size, some species tend to have a predominantly morphological characteristics that might be useful to identify collections; e.g., the pleurocystidia of P. fulvibadius are predominantly broadly clavate or utriform, while in P. austrofulvus they are mostly ovoid or broadly fusiform. A mucous cap was observed covering the apex of the pleurocystidia in P. fulvibadius (collection HRL3636) and P. siccus and the apex of the cheilocystidia in P. parvicarpus. All species have a welldeveloped strip of cheilocystidia covering the lamellar edge, often quite variable in shape and size, mostly hyaline but rarely with pale brown intracellular pigment. Caulocystidia are sometimes present in some taxa, but they are not a constant enough character to allow for species differentiation.
All species grow on deciduous wood, but Pluteus fulvibadius and P. vellingae are also known from coniferous wood. These species, as well as P. romellii, can apparently be terrestrial, but the presence of small pieces of decomposed wood in the soil can never be ruled out.

Conclusions
In this article, eight molecular lineages in the/romellii clade of the genus Pluteus are detected in the phylogenetic analyses, and their taxonomic status is discussed in detail. Pluteus fulvibadius and P. sternbergii are confirmed as separate species from P. romellii, while P. pallescens is synonymized with P. romellii. Pluteus sulphureus is rejected as not belonging to Pluteus. Pluteus austrofulvus, P. parvicarpus, P. parvisporus, P. siccus and P. vellingae are described as new species to science. Pluteus aff. romellii is recognized as a separate lineage from P. romellii sensu stricto, but further studies are necessary to clarify its taxonomic and nomenclatural status. A lectotype with an epitype collection for P. romellii and an epitype for P. sternbergii are designated here.