Arrhenia schistidicola (Hygrophoraceae, Agaricales) – a new species on the moss Schistidium crassipilum from western Norway

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INTRODUCTION
During a study of the botany of mortar walls in the town Bergen in 2019, a small Arrhenia Fr. (Fries 1849) was found growing on the moss Schistidium crassipilum H.H. Blom, and a closer study was initiated to reveal its identity.
The genus Arrhenia comprises small-sized brown-grey species with or without stipe and with or without lamellae, and many grow in association with photosynthetic cryptogams such as mosses (e.g., Lawrey et al. 2009, Voitk et al. 2022).The North American species have been treated by Redhead (1984), the Fennoscandian by Elborne (2008) and the Norwegian by Høiland (1976).

MATERIAL AND METHODS
The material is deposited in BG.Fresh material of caps was examined using a Zeiss Stemi 2000-C stereo microscope, and microscopically on razor-blade-cut sections mounted in 10 % KOH, using a Zeiss Axiolab compound microscope.The spores were measured in water.
The Geneious multiple alignment algorithm uses progressive pairwise alignment and the alignment was subsequently manually checked by LL.Sequences generated for this study are deposited in GenBank under accession numbers OP584486 and OP584487.
Initial blast searches in GenBank revealed that the most similar sequences are found in the Arrhenia spathulata group, which corresponds to the second well-supported clade of Voitk et al. (2020).We therefore downloaded sequences of 11 representatives of the A. spathulata group from GenBank to investigate the nrITS variation in relation to the segregation of species and the relations of our samples within this clade (Table 1).One sequence of A. obscurata (D.A. Reid) Redhead, Lutzoni, Moncalvo and Vilgalys was downloaded and used as the outgroup (Voitk et al. 2020).Calculations of genetic distances (observed p-distance) were done in Geneious and phylo-

RESULTS AND DISCUSSION
The aligned complete ITS sequence data set consisted of 14 sequences with 618 nucleotides.The topology of the phylogenetic trees was the same in all phylogenetic analyses (NJ, ML, MP).The sequences from our samples consistently form a monophyletic clade with A. spathulata Redhead as a monophyletic sister group with high branch support (Figure 1).The samples of A. schistidicola that we sequenced differ from each other in two nucleotides, which in our alignment corresponds to an observed within-species p-distance of 0.4 % (Table 2).Generally, genetic distances between species in the A. spathulata group vary as much as between 7 and 27 %, except for A. elegans (Pers.)Redhead, Lutzoni, Moncalvo and Vilgalys and A. subglobispora (G.Moreno, Heycoop and E. Horak) Redhead, Lutzoni, Moncalvo and Vilgalys, which are separated by a distance of 1.4 % (10 nucleotide differences).The genetic distances between A. schistidicola and the two closest related species in the phylogenetic tree are ca 4.6 % (A.spathulata) and ca 9 % (A.retiruga (Bull.)Redhead) respectively.Hence, barcode gaps support the new species.However, the great variation in amount of genetic variation within species in the A. spathulata group indicates the need of a taxonomic revision of the group.
The new species keys out in Nordic literature (Høiland 1976, Elborne 2008) as A. retiruga, with no stipe and similar anatomy of the pileus.However, molecular results show that it is closer related to A. spathulata.Arrhenia spathulata was described on a specimen collected in Western France (Fries 1828), and Fries distinguished it from A. retiruga based on stipe formation (Redhead 1984).Some modern descriptions of A. spathulata state that it has a stipe, while others say that it may or may not have one (Høiland 1976, Redhead 1984, Barrasa and Rico 2003, Elborne 2008).This variability probably reflects the presence of more than one taxon Etymology: Growing on species of the moss genus Schistidium Holotype: Norway: Hordaland (Vestland), Bergen, Breistølen 5, Lat./Long.60.2333 N, ⁰ 5.1924 E, altitude ca 120 m a.s.l., on ⁰ Schistidium crassipilum on vertical W-oriented mortar wall, 2019-11-16, leg. D.O. Øvstedal (BG F-16674, GenBank OP584486).
Distribution: Norway.Ecology: Arrhenia schistidicola has only been found growing on the acrocarpous moss Schistidium crassipilum, which is distributed in most of Europe and found north to Finnmark in Norway (Blom 1996).The fungus, being heterotrophic, needs a carbon source, which in this case must be products from the dead or living moss.Since the attach point between the moss and the fungus is not necrotic, A. schistidicola is not an obvious parasite.The hyphae most probably live inside the moss cells (endophyte).

Figure 1 .
Figure 1.Phylogenetic tree generated from Maximum Likelihood analysis based on nrITS sequence data.ML values > 70 are indicated above branches and Bootstrap values (MP) > 70 are indicated below.Scale bar represents the estimated number of nucleotide substitutions from the most recent node (ML).

Table 1 .
Data of Arrhenia sequences (nrITS) used in this study.