Semicentenialea rex Rosling, Aime & Kluting 2026, sp. nov.

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Semicentenialea rex Rosling, Aime & Kluting sp. nov. Figs 2, 3, 4 Diagnosis. All cultured strains are separated from other taxa, currently known only from eDNA, in the class by ribosomal sequences possessing the following characters in ITS 2: CCTGTTTGAGTGTCATAATAC at position corresponding to bases 181–201 in MH 843997. Holotype. SWEDEN • Jädraås, Ivantjärnsheden Field Station (60.8145, 16.5070, altitude 185 m), from surface-sterilized root tips of Pinus sylvestris (Pinaceae), August 2013, H. Urbina HU 4064 (Holotype: UPS collection F-1085350, as dry metabolically inactive culture; ITS = GenBank MH 843997. Ex-type cultures in culture collections. CBS 150710 (Netherlands), HU 4064 (PUL, USA), DSM 120069 (DSMZ, Germany), HU 4064 (Rosling lab, Sweden). Etymology. Latin, rex, for king, referring to King Carl XVI Gustaf, King of Sweden. Description. Colonies extending at 0.3–0.4 mm per day at room temperature on modified MMN media. Colonies pale ochre towards the center, becoming lighter and more diffuse near the margin with no reverse color when grown for eight weeks at ambient room temperature (Fig. 2 A). Towards the center, colonies appearing opaque, damp, slightly glistening, leathery to slightly wrinkly, and slightly raised from the agar; producing aerial mycelium occasionally. Margin somewhat irregular, appearing more filamentous, more effuse, translucent, and slightly submerged. Hyphae hyaline, septate, producing clamp connections (Fig. 2 B – D, Suppl. materials 2, 3: movies S 1, S 2); primarily binucleate, but occasionally multi-nucleate (Fig. 2 C, D, Suppl. material 1: fig. S 3, Suppl. materials 2, 3: movies S 1, S 2), 1–1.5 mm in diameter; producing no detectable odors. Hyphae producing three types of swellings: basidium-like cells (hereinafter BLC), intercalary inflated cells, and probasidium / teliospore-like cells (hereinafter TLC) (Fig. 2 D, Suppl. material 1: fig. S 5), all often occurring simultaneously on a small area of the culture. BLCs obovoid, thin-walled, emerging directly or on a short stalk from generative hyphae, subtended by a clamp connection (Fig. 3 A – L, Suppl. material 1: fig. S 4 A – D, F – H), with two nuclei in the obovoid cell, giving rise to a lateral cell with one nucleus (Fig. 3 G – K). The obovoid BLC sometimes with a ring with a strong calcofluor white staining, indicating a possible separation of the cell to two in later developmental phases (Fig. 3 G – H). Four nuclei occurring in a putatively initial phase of the BLC formation (Fig. 3 L). The obovoid BLCs occasionally arising from TLCs (Fig. 3 H, Suppl. material 4: movie S 3). Intercalary inflated cells typically separated by a septum on either side, sometimes with budding structures near the septa (Fig. 4 A – D, Suppl. material 1: fig. S 5). TLCs globose, thick-walled (Fig. 4 E – S, Suppl. material 1: figs S 5, S 6, Suppl. material 5: movie S 4), especially frequent in the older parts of the culture, but also present intermixed with the BLC cells close to the growth edge. A spore-like cell developing inside TLCs with two or three nuclei (Fig. 4 F – K), being released when the TLC cell walls degrade (Fig. 4 K, N). Inside the TLCs, spot-like structures stain strongly with calcofluor white, which we interpret as being related to the early stage of the cell wall development of the spore-like cells (Fig. 4 M). TLCs frequently giving rise to a lateral cell with one nucleus (Fig. 4 I, J, Suppl. material 1: fig. S 3 B). TLCs detaching from the hyphae and germinating (Fig. 4 N – U), producing knobby promycelial-like hyphae (Fig. 4 U). Habitat and distribution. Associated with roots of Pinus sylvestris, but environmental sequences captured by SH 1268087.09 FU indicate that the species also occurs in soils where other potential host plants dominate. Near global distribution based on eDNA sequences captured by SH 1268087.09 FU with records from North America, South America, Australia, Asia and Europe. Additional material examined. SWEDEN • Jädraås, Ivantjärnsheden Field Station (60.8145, 16.5070, altitude 185 m), surface sterilized root tips of Pinus sylvestris (Pinaceae), August 2013, H. Urbina HU 4068 (PUL culture collection; ITS = GenBank PQ 270019), H. Urbina HU 4069 (PUL culture collection; ITS = GenBank MH 844000); HU 4107 (PUL culture collection; ITS = GenBank MH 844024); HU 4147 (PUL culture collection; ITS = GenBank MH 844045). FINLAND. Hyytiälä Forest station, Mämmilampi site (61.8400, 24.2600, altitude 180 m), surface sterilized root tips of Pinus sylvestris (Pinaceae), summer 2002 J. Heinonsalo JH 144 (HAMBI Culture Collection, ITS = GenBank LK 052815), Scots pine forest near Pieksamäki (62.3700, 27.0700, altitude 120 m) surface sterilized root tips of Pinus sylvestris (Pinaceae), autumn 2008 J. Heinonsalo JH 169 (HAMBI Culture Collection, ITS = GenBank LK 052826). Notes. Semicentenialea rex sp. nov. represents the first named species of what has previously been known as ‘ Clade GS 25 ’ (Tedersoo et al. 2017). There are currently no formally described species closely related to S. rex sp. nov. The closest described relatives to S. rex sp. nov. belong to other classes within the Pucciniomycotina (Basidiomycota), most likely within the Classiculomycetes. Several cultures representing the species were captured from root tips at the sampled location at Ivantjärnsheden Field Station, Jädraås, Sweden. Two additional cultures were obtained from ectomycorrhizal roots of P. sylvestris in central Finland. The species corresponds to the UNITE SH 1268087.09 FU (https://doi.org/10.15156/BIO/SH1268087.09FU) with over 2600 environmental ITS sequences. The species is by far the most frequently recovered USH of the lineage as captured by TH 069974.

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