The genus Sidera (Hymenochaetales, Basidiomycota) comprises white-rot, mono- or dimitic fungi with poroid or hydnoid hymenophore. It has a worldwide distribution albeit with fewer species present in the Southern Hemisphere. Although recent studies revealed the existence of several new Sidera species, there are still taxonomic inconsistencies and obscure phylogenetic relationships amongst certain taxa of the genus. In this work, a large number of Sidera collections were used to obtain an updated phylogeny, based on ITS and 28S rDNA sequences by including new material from Mediterranean Europe. The monophyly of the genus was strongly supported and all species with poroid hymenophore formed a highly-supported lineage with two major subclades. In total, 23 putative species were recognised. Amongst those, five are considered to possibly represent entities new to science, but further work is required since they are represented by single specimens or environmental sequences. Examined collections originally named S. lenis from southern Europe were grouped within S. vulgaris. Similarly, several collections under various names were hereby identified as S. vulgaris, including those of the recently described species S. tibetica. Furthermore, a critical discussion (based on morphoanatomical findings) is made on the key features that could be used to distinguish S. lenis from S. vulgaris.

Basidiomycetes, biodiversity, fungal phylogeny, Mediterranean Europe, mushroom, white-rot fungi

The genus Sidera Miettinen & K.H. Larss. (Sideraceae, Hymenochaetales, Basidiomycota) was established to harbour four resupinate, wood-inhabiting, white-rot species, in accordance with morphological and phylogenetic evidence: S. lenis (P. Karst.) Miettinen (type species), S. lowei (Rajchenb.) Miettinen, S. lunata (Romell ex Bourdot & Galzin) K.H. Larsson and S. vulgaris (Fr.) Miettinen (Miettinen and Larsson 2011). Amongst them, S. lowei was originally described from Brazil (Rajchenberg 1987), while all the others were described from material collected in Europe. Recently, the diversity of the genus was significantly expanded as a result of phylogenetic studies using nuclear ribosomal markers (ITS and LSU) to describe new taxa from boreal, temperate and tropical habitats (Du et al. 2019, 2020; Liu et al. 2021, 2022, 2023; Xu et al. 2023).

Morphologically, Sidera species are characterised by resupinate, whitish to cream-coloured, yellowish or buff, rarely pinkish or bluish basidiomata, poroid hymenophore with middle-sized to small pores (and, in only one case, hydnoid hymenophore; i.e. S. lunata), monomitic or dimitic hyphal system, generative hyphae with clamps, rather loosely arranged skeletal hyphae, presence of rosette-like crystals in subiculum and/or trama, hymenial cystidia as thin-walled hyphidia (cystidioles) and minute, allantoid to lunate, hyaline, thin-walled, negative in Melzer’s reagent and acyanophilous basidiospores (Miettinen and Larsson 2011).

After re-examining the lectotype of Physisporus lenis (designated by Lowe 1956), Niemelä and Dai (1997) recognised two morphologically distinct species: one that was more common in boreal, mostly pine-inhabiting old-growth forests, i.e. Skeletocutis lenis (P. Karst.) Niemelä and another species that came across more as generalist – in terms of substrate preference – and with a more southern distribution, i.e. Sk. vulgaris (Fr.) Niemelä & Y.C. Dai. In the original description of the genus Sidera, Miettinen and Larsson (2011) used two sequences which were then considered to represent S. vulgaris, both originating from Oceania (Tasmania, Gates FF257; New Zealand, Ryvarden 37198). However, no sequences from European collections were included in their phylogenetic analysis. The sequence from Tasmania nested close to S. lowei, while the sequence from New Zealand appeared as sister to S. lenis, indicating that they possibly correspond to two distinct species. Recent phylogenetic studies in the genus Sidera repeatedly used the aforementioned two specimens (Du et al. 2019, 2020) without including any S. vulgaris material from Europe. In fact, in one of these publications (Du et al. 2020), the specimen from Tasmania was identified as S. minutipora (Rodway & Cleland) Y.C. Dai, F. Wu, G.M. Gates & Rui Du. Moreover, Liu et al. (2021) used the term “Sidera vulgaris sensu lato” to refer to the New Zealand sequence and to some newly-generated sequences from Chinese collections, which were later linked to several new species, i.e. S. americana Z.B. Liu & Yuan Yuan, S. borealis Z.B. Liu & Yuan Yuan, and S. tibetica Z.B. Liu, J. Yu & F. Wu (Liu et al. 2022, 2023). Therefore, the phylogenetic position of S. vulgaris within the genus remains obscure and controversial.

In order to resolve this issue and provide an updated phylogeny of the genus Sidera, several collections from Mediterranean Europe, initially identified as Skeletocutis sp., Sk. lenis and Sk. vulgaris from Mediterranean Europe, were included in this study, together with a large number of publicly available sequences. In addition, answers were sought to the following key questions: (a) Could available specimens confirm the presence of S. lenis in the Mediterranean Region? (b) Is there adequate evidence that S. vulgaris indeed has a cosmopolitan distribution? (c) Which Sidera taxa are accommodated within S. vulgaris sensu lato and are they related to new species correctly introduced in this genus? (d) What are the key morphological features to distinguish S. vulgaris from S. lenis?

This study mainly deals with the taxonomic uncertainty associated with collections under the names S. vulgaris and S. lenis. A great deal of confusion stems from the erroneous initial identifications of such specimens and, as explained below, further obstacles were raised by the description of the (allegedly) new species S. tibetica (Liu et al. 2022).

Miettinen and Larsson (2011), who introduced Sidera as a new genus to accommodate – amongst others – the dimitic polypores Sk. vulgaris and Sk. lenis, studied the former by examining three collections: one European (Poland, Niemelä 5981) and two from the Southern Hemisphere. However, sequence data were available only from the latter two. As our results show, the collection from New Zealand (Ryvarden 37198), that was considered to represent S. vulgaris and was included as such in several phylogenetic studies, was assigned to clade B in the present study (as ‘Sidera sp. 1’). Furthermore, the collection from Tasmania (G. Gates FF257) was grouped in clade C and was identified as S. minutipora (Du et al. 2020). Therefore, it is clear that these two collections are not related to S. vulgaris. Moreover, several specimens included either as S. vulgaris or S. vulgaris sensu lato in the previous publications (Liu et al. 2021, 2022) were later linked to other species of this genus by the same group of authors (Liu et al. 2023), for example, to S. americana (Dai 12730 from USA and Dai 19173 from Canada) and S. borealis (Cui 11216 and Dai 22822 from China).

Most importantly, another new Sidera species was recently introduced under the name S. tibetica (Liu et al. 2022). It was described on the basis of two sequenced specimens from Tibet, but without being examined versus any real/authentic S. vulgaris collections. In fact, the three “S. vulgaris” collections included in the aforementioned study were Ryvarden 37198 (New Zealand), Dai 19173 (Canada) and Dai 22822 (China). However, the first of these corresponds to Sidera sp. 1 (as the present work demonstrated), the second to S. americana and the third to S. borealis. In addition, all S. tibetica sequences from the work of Liu et al. (2022) [and from other recent publications where this name was also erroneously used (e.g. Volobuev (2023))] were grouped (100%, 1.00) together with a large number of S. vulgaris specimens originating from Europe (#15), Asia (#3) and USA (#4). It should additionally be noted that Liu et al. (2021) had indications that the allegedly new species occurred also in Europe (since they have examined the specimen Dai 21057, initially identified as S. vulgaris sensu lato from Belarus). However, they did not include it in the description of S. tibetica (Liu et al. 2022), whereas it was used in their more recent study under this name (Liu et al. 2023). Therefore, it is evident that S. tibetica was erroneously introduced as a new species since the respective examined material corresponds to S. vulgaris, i.e. the already existing species, previously described from specimens originating from Sweden (Niemelä and Dai 1997). Apparently, the lack of sequence data from the type material of S. vulgaris and not including correctly identified collections of the appropriate geographic origin (e.g. Europe) were the main reasons for this major issue detected in the publication of Liu et al. (2022). Hence, S. tibetica should be considered as a synonym of S. vulgaris.

The molecular evidence provided by the phylogenetic analyses (Fig. 3, Fig. 4 and Suppl. material 2) shows that S. vulgaris is well discriminated from S. lenis since the respective terminal clades are properly defined and clearly separated. On the other hand, distinguishing S. vulgaris from S. lenis morphologically remains a difficult task; this resulted in incorrect identifications of the two species in Europe and elsewhere. Until very recently, the distribution of S. vulgaris in south Europe was considered to be unknown because, as stated, “… it was easily confused with S. lenis” (Bernicchia et al. 2020). It is now clear that S. vulgaris is the only species of the genus which is also present in Mediterranean Europe.

Our morphological studies, in conjunction with the verified identity of specimens from DNA sequencing, revealed that the most stable and reliable character to distinguish these two species is the pore size, which is clearly smaller in S. vulgaris, on average, more than six pores per mm, as opposed to less than six pores per mm in S. lenis (Table 2). The length of the basidia, furthermore, seems to be important in this regard, as they hardly exceed 10 μm in S. vulgaris, while they are always longer in S. lenis. Regarding the size and shape of basidiospores, our measurements indicate a much wider deviation, particularly with regards to the width, which largely affects the quotient (Q, length/width). Our work also suggests that S. vulgaris basidiospores may exceed 1.5 μm in width; this contradicts pertinent generic keys which placed a clear-cut value of 1.5 μm between these species (i.e. Ryvarden and Melo (2017); Bernicchia et al. (2020)). Apparently, this tiny value – and variations thereof – are very difficult to detect with accuracy. In contrast, the average spore length seems to be a more reliable character, since it does not exceed 4 μm in S. vulgaris, in contrast to S. lenis whose spores are usually longer. In our opinion, the presence of the stellate crystal agglomerations and the mucous deposit on the capitate generative hyphal tips are unstable characters, most likely affected by the age of basidiomata and the microscopy techniques used; thus, they are of questionable taxonomic value. In addition, the most important taxonomic features mentioned in the description of S. tibetica (Liu et al. 2022) are similar to (or not differing considerably from) those of S. vulgaris (Table 2). The deviations observed in spore size (especially) and pore density are small and they cannot be considered as significant since only three specimens of S. tibetica were used for its original description.

In this work, a large number of Sidera sequences (ITS and LSU rDNA) were analysed which included new material from Mediterranean Europe, as well as publicly available sequences. The monophyletic nature of the genus was strongly supported in the generated trees. Sidera lunata (characterised by a hydnoid hymenophore) was identified as the sister group to the remainder of the genus in the derived phylogeny, while species with poroid hymenophore formed a robustly supported lineage that was subdivided into two major clades. Amongst 23 species in total, five are possibly new to science, but since they are mostly represented by single collections, further work is needed before any definite conclusions could be drawn. The presence of S. lenis was assessed in north Europe, Russia and China, while examined collections from south Europe under this name were recovered within S. vulgaris. The latter species exhibits a Holarctic distribution. It occurs on dead wood of angiosperms and gymnosperms, including the regions of Eurasia where it was erroneously reported as S. tibetica. As we demonstrate, the description of this allegedly new species was based on collections that are hereby identified as S. vulgaris. This observation also emphasises the need to proceed with the epitypification of S. vulgaris since the type material maintained in Herbarium UPS may be too old for successful sequencing.

Curators of O (University of Oslo, Norway) and SALA (University of Salamanca, Spain) are thanked for loans of specimens. Our gratitude is also extended to A. Bernicchia and S.P. Gorjon for kindly providing access to herbarium collections.