Reappraisal of the Genus Exsudoporus (Boletaceae) Worldwide Based on Multi-Gene Phylogeny, Morphology and Biogeography, and Insights on Amoenoboletus

Abstract

The boletoid genera Butyriboletus and Exsudoporus have recently been suggested by some researchers to constitute a single genus, and Exsudoporus was merged into Butyriboletus as a later synonym. However, no convincing arguments have yet provided significant evidence for this congeneric placement. In this study, we analyze material from Exsudoporus species and closely related taxa to assess taxonomic and phylogenetic boundaries between these genera and to clarify species delimitation within Exsudoporus. Outcomes from a multilocus phylogenetic analysis (ITS, nrLSU, tef1-α and rpb2) clearly resolve Exsudoporus as a monophyletic, homogenous and independent genus that is sister to Butyriboletus. An accurate morphological description, comprehensive sampling, type studies, line drawings and a historical overview on the nomenclatural issues of the type species E. permagnificus are provided. Furthermore, this species is documented for the first time from Israel in association with Quercus calliprinos. The previously described North American species Exsudoporus frostii and E. floridanus are molecularly confirmed as representatives of Exsudoporus, and E. floridanus is epitypified. The eastern Asian species Leccinum rubrum is assigned here to Exsudoporus based on molecular evidence, and a new combination is proposed. Sequence data from the original material of the Japanese Boletus kermesinus were generated, and its conspecificity with L. rubrum is inferred as formerly presumed based on morphology. Four additional cryptic species from North and Central America previously misdetermined as either B. frostii or B. floridanus are phylogenetically placed but remain undescribed due to the paucity of available material. Boletus weberi (syn. B. pseudofrostii) and Xerocomus cf. mcrobbii cluster outside of Exsudoporus and are herein assigned to the recently described genus Amoenoboletus. Biogeographic distribution patterns are elucidated, and a dichotomous key to all known species of Exsudoporus worldwide is presented.

Keywords: Boletales; biogeography; bolete diversity; molecular phylogeny; taxonomy.

Figure 1

ML phylogenetic tree of Exsudoporus, Amoenoboletus and allied genera generated from a multilocus (ITS + nrLSU + tef1-α + rpb2) dataset. PP values ≥ 0.7 and BS support values ≥ 50% are shown at the nodes. Thickened branches indicate PP ≥ 0.95 and BS support ≥ 70%. Newly sequenced collections are indicated in bold; type specimens are indicated with an asterisk (*). Two-letter country codes (ISO 3166-1 alpha-2) reflecting origin of specimens are given.

Figure 2

Basidiomes of E. permagnificus: (A) IB 19800750, holotype collection; (B) GS1275; (C) MG558; (D) MG662; (E) MG662; (F) MG829; (G) AB B11-03. Photos by: (A) R. Kuhnert; (B) G. Simonini; (C–F) M. Gelardi; (G) R. Kuznetsov.

Figure 3

Microscopic features of E. permagnificus: (A) basidiospores; (B) basidia; (C) cheilocystidia and pleurocystidia; (D) caulocystidia; (E) pileipellis. Bars: (A–D) = 10 µm; (E) = 20 µm. Drawings by M. Gelardi.

Figure 4

Distribution of spore size of E. permagnificus (23 collections) using “isoprobability ellipse”. Shown is the distribution of the average values of spore size of any of the collections, at the confidence of 68% (corresponding to one standard deviation).

Figure 5

Stipe base hyphae of E. permagnificus in Melzer’s reagent. (A) GS1717, (B) GS1717, (C) GS336, (D) GS784, (E) GS1001, (F) GS 1275. Bars: 20 µm. Photos by G. Simonini.

Figure 6

Basidiomes of E. floridanus: (A) FLAS-F-61008; (B) unnumbered collection. Basidiomes of E. frostii s. l.: (C) TO AVBB11; (D) TO AVBB10; (E) iNat 35326745 (TO AVBB12); (F) iNat 30897161 (TO AVBB13); (G) FLAS-F-60742. Basidiome of E. ruber: (H) TNS-F-37407, holotype collection of Boletus kermesinus. Photos by: (A,G) L. Kaminsky; (B) A. Farid; (C) L. Craig; (D) R. Abbott; (E,F) R. K. Antibus; (H) Y. Taneyama.