Fomitiporia exigua Alves-Silva, Góes-Neto & DrechslerSantos, sp. nov.

Index Fungorum number:  IF 900086; Mycobank number: MB 900086; Facesoffungi number: FoF 13980; Fig. 1

Etymology – exigua, from exiguus (Latin)=little, small in all parts; it refers to the small basidiomata.

Holotype – FLOR 58558

White-rot fungus, found growing on living Protium heptaphyllum (Aubl.) Marchand, (Burseraceae). It is currently known in Brazil, in the Imerí Province, Amazonas state. Basidiomata perennial, pileate, sessile, and mostly broadly attached, subdimidiate, semicircular symmetric to asymmetric, spatulate, applanate to convex (in older basidiomata), obtriquetrous to rarely triquetrous, projecting 2–18.5 mm, 4–13 mm wide and 2–8 mm thick, with woody consistency when dried. Pileus glabrous, concentrically zonate with multiple narrow bands, moderately sulcate, brown, dark brown [6EF (6–8)] to black; margin acute, folded, well delimited around pore surface, sterile, light brown, yellowish brown to brown [5DE (5–8)]. Pore surface light greyish brown (5D8) to greyish brown (6F3), (8F3); pores round to angular, 8–12/mm, (60–)70–120 µm diam. (x̅=84 µm); dissepiments entire, 20–90 µm (x̅=51.4 µm) thick. Tubes distinct stratified, up to 6 layers interleaved with context, individual tube layers thin, up to 0.5 mm thick, brown [5EF (3–4)] to grayish brown (5F3), the older layers filled with whitish mycelium. Context simple, thin, concentrically zonate, with dense texture and woody consistency, brown to dark brown [6EF (6–8)], with a distinct dark line near to surface. Hyphal system dimitic in all parts; generative hyphae simple septate, hyaline to pale yellow, sparingly branched, 2–2.5 μm diam.; skeletal hyphae golden brown to reddish brown, unbranched, thick-walled, in the context and hymenophoral trama 3–4 μm diam., the lumen 1–2 μm wide. Hymenial setae absent. Cystidioles rare, fusoid, lanceolate, hyaline, and thin walled. Basidia subglobose to globose, hyaline, tetrasporic, 7–9×6–8 µm (ave=7.7×6.9 µm), Q=1–1.3 µm (x̅= 1.12 µm); basidioles identical in shape but slightly smaller than basidia. Basidiospores subglobose to globose, 4–4.5×4–4.5 µm (x̅=4.2×4.1 µm), Q=1.0–1.3 µm (aveQ =1.02 µm), hyaline, undextrinoid to slightly dextrinoid, cyanophilous, thick-walled, smooth. Crystals rhomboid, of variable size.

Material examined – Brazil, Amazonas: Novo Airão, Parque Nacional Anavilhanas, Igarapé Santo Antônio, 2° 24′ 42.5″ S 60° 58′ 08.9″ W, on a living Protium heptaphyllum (Aubl.) Marchand (Burseraceae), 6 Dec 2013, E.R. Drechsler-Santos 1256 (FLOR 58558 holotype).

GenBank numbers – LSU: KU663296, ITS: KU663323.

Notes – Fomitiporia exigua is mainly characterized by having perennial, small (less than 20 mm long and wide) and thin (less than 10 mm thick) basidiomata; convex pileus in the older basidiomata; and small pores (8–12/mm). Microscopically, it has small basidiospores, 4.2×4.1 µm on average and variable reactions in Melzer, from undextrinoid to slightly dextrinoid (Fig. 2f–g). Phylogenetically, it was recovered in F. langloisii–F. castilloi clade, sister to F. rondonii. As F. rondonii, it has pileate basidiomata, which is different from most of the species of this clade. Fomitiporia rondonii differs by having bigger basidiomata and basidiospores (5×4.8). In addition, F. exigua differs from all pileate species by having the smallest basidiomata in the genus and undextrinoid to slightly dextrinoid basidiospores.

Figure 1 – Fomitiporia exigua (FLOR58558, holotype) a–e Basidiomata ex-situ. f–g Slightly dextrinoid basidiospores. Scale bars: a–c, e=5 mm, d=1 mm, f–g=5 µm

Figure 2 – Phylogenetic tree of Fomitiporia based on ML analysis of combined ITS, 28S, tef1-α, and rpb2 sequences. The matrix comprises 3816 characters with gaps. The tree is rooted with Phellinus uncisetus (MUCL47061 and MUCL46231). The best-scoring RAxML tree with a final likelihood value of−lnL 28,484.23 is presented. The matrix had 1948 distinct alignment patterns, with 9.55% of undetermined characters or gaps. The best partition scheme and evolution models were estimated in PartitionFinder 2 (Guindon et al. 2010; Lanfear et al. 2017) under the linked model of branch lengths, greedy search algorithm (Lanfear et al. 2012), and Akaike information criterion for model selection and. The best-fit models, matrices and trees are available at Alves-Silva et al. (2023). In the BI analysis, runs converged to stable likelihood values (−lnL=28,067.41 and 28,075.73), and after the 50% burn-in, 7502 trees were used to compute a 50% majority-rule consensus tree and to estimate BPPs. ML bootstrap support (first set)≥70% and Bayesian posterior probabilities≥0.95 are given near each branch. The new species proposed are in blue