Leiostroma Fr.
Abstract
Leiostroma Fr., Novitiae Florae Suecicae 5: 80, 1819 Description. Basidiomata annual, effused, arid, soft and easily crumbling, nearly white to gray or brown. Hymenial surface smooth or spinulose. Hyphal structure monomitic; hyphae clamped, hyaline or grayish-brownish, normally spaced, basidia-bearing hyphae well-differentiated. Crystals abundant, angular or sand-like. Cystidia absent. Hyphidia present, with a well-differentiated, often widened stem and richly branched, densely encrusted apical outgrowths. Basidia four-celled, ellipsoid-ovoid to obconical, sessile, thin-walled or with a distinct wall, exposed or embedded, occasionally with a reduced stalk-like base; some basidia with a grayish-brownish cytoplasm. Basidiospores thin-walled, narrowly to broadly cylindrical, curved. On dead wood of angiosperms and conifers. Generic type. Thelephora acerina var. abietis Fr. (= Corticium abietis (Fr.) Romell) (see comments below). As reintroduced here, Leiostroma encompasses four species having soft, easily crumbling, effused basidiomata with more or less pronounced tints of gray or brown. The same colouration is also characteristic for the contents of (at least some) hyphae and hymenial cells. All included species lack cystidia and possess very characteristic, abundant, hyaline, sand-like crystalline encrustation. These encrusting bodies are often very densely arranged, which makes microscopic structures difficult to observe. They form continuous shields on subicular hyphae and, in mature basidiomata, also on subhymenial hyphae and occasionally on hymenial cells (Fig. 28). These features, as well as the lack of cystidia, separate Leiostroma spp. from the similar-looking smooth Adustochaete species. Moreover, basidia-bearing hyphae are well differentiated in Leiostroma and usually clearly wider than subhymenial hyphae; in Adustochaete spp., no perceptible difference between these hyphal types was observed. Some Proterochaete spp. (P. furfurosa, P. letendreana, and P. mucedinea) macroscopically similar to Leiostroma can be differentiated by their more densely arranged, partly glued hyphae (vs. spaced in the latter genus), as well as the presence of cystidia (except for P. letendreana) and granular, brownish, cyanophilous deposits on hyphae and hymenial cells. Differences between the two genera are discussed in more detail under Proterochaete. Mature, faded basidiomata of Tegmenticium peritrichum can be mistaken for Leiostroma spp.; however, it has cystidia and is devoid of incrustation on hyphae and hymenial cells. Here we reinstate the generic name Leiostroma to encompass, to some extent, a species, formerly known as Exidiopsis calcea in North Europe, as well as three closely related species from the southern part of Europe, Macaronesia and Africa. Liu et al. (2022) uncritically took E. calcea and, without clarifying its identity, combined it in their newly described genus, Alloexidiopsis. The genus was typified with the simultaneously introduced A. schistacea, an effused, Heterochaete - like species from the southern part of China. The latter species is probably identical with Heterochaete cretacea Pat. from Vietnam (Patouillard 1924). Although it was shown by Liu et al. (2022) as forming a strongly supported clade with the so-called E. calcea and several other species, the question about their relationships, in our opinion, cannot be solved solely on the basis of the ribosomal gene-based phylogenies. Available ITS sequences of A. schistacea reveal a relatively low similarity (less than 94 %) with E. calcea s. auct. and three other species dealt with below. LSU sequences of this species provide no additional information because they show A. schistacea to be very close to and, at the same time, equally distant from E. calcea s. auct. with its closest relatives on one hand and Heteroradulum spp. on another (98.5–98.7 % similarity). Due to the lack of the specimens suitable for obtaining additional genetic markers, we leave the question about the phylogenetic placement of A. schistacea open. In the following paragraphs, we argue for resurrecting the genus name Leiostroma for E. calcea s. auct. and its closest relatives; our arguments in favor of discarding the species name Thelephora calcea Pers. (basionym of E. calcea) are provided under Leiostroma farinellum. The genus Leiostroma was introduced by Fries (1819) without any species assigned to it (“ Species plures ”). Two years later, Fries (1821) lowered the rank of Leiostroma to the section in the genus Thelephora and described six species therein, i. e. Thelephora acerina, T. cinerea, T. fallax, T. maculaeformis, T. amphibolia, and T. sterilis. It seems this list reflects his original views on the scope of Leiostroma because the description of the section in Systema expands the earlier generic description of Leiostroma and does not contradict it. Donk (1957) treated Leiostroma sensu Fries 1821 as a subgenus of Thelephora and typified it with T. maculaeformis. He argued that T. maculaeformis was a ‘ constant member’ of Leiostroma, while two other candidates for typification, T. cinerea and T. acerina, had been subsequently removed by Fries himself and placed in other sections or genera. Later Donk (1963) detected that Leiostroma was published as a genus already in 1819; he, however, maintained his earlier typification of it with T. maculaeformis, providing no new arguments. The type material of T. maculaeformis was studied by Eriksson (1950) who found it to be conspecific with Peniophora polygonia (Pers.) Bourdot & Galzin. The latter species is a member of Peniophora in the current sense (Hallenberg et al. 1996). Before 1982, Donk’s typification looked harmless because Leiostroma was considered a devalidated name, i. e. that published prior to Fries’ Systema Mycologicum and therefore having no priority over generic names introduced afterwards. However, after changing the starting point for validly published fungal genera in 1982, Leiostroma becomes an older name for Peniophora, a large and widely accepted genus of crust fungi. This name change looks very unfortunate, and we therefore argue in favor of abandoning Donk’s typification and conserving Leiostroma with another generic type. Under Code Art. 10.2, any of the six mentioned species mentioned by Fries (1821) is suitable for lectotypification of the genus. However, the first five species are currently classified among well-established genera of corticioid fungi, i. e. Dendrothele Höhn. & Litsch. (T. acerina), Peniophora Cooke (T. cinerea, T. fallax, and the already mentioned T. maculaeformis), and Phanerochaete P. Karst. (T. amphibolia). Selecting any of these species as a lectotype of Leiostroma would result in discarding one of these currently accepted corticioid genera (Dendrothele, Peniophora, or Phanerochaete) from use because it is older than any of them. As far as we know, no authentic specimens of the sixth species, Thelephora sterilis, exist. In the subsequent reintroduction of the section Leiostroma, Fries (1828: 224) retained four species from the previous treatise (T. acerina, T. maculaeformis, T. amphibolia, and T. sterilis) and added seven more species, i. e. T. corrugata, T. laevigata, T. episphaeria, T. bufonia, T. circinata, T. typhae, and T. epiphylla. Again, choosing any of these species as the generic type of Leiostroma is problematic: T. corrugata and T. episphaeria are currently treated under Hydnoporia, T. laevigata is a species of Amylostereum, T. circinata is the generic type of Sarcopodium (Ascomycota), T. typhae is the generic type of Epithele, and T. epiphylla is that of Athelia. All these currently accepted generic names (except Athelia and Sarcopodium) are younger than Leiostroma. As for T. bufonia, its identity remains uncertain. There are five specimens in Persoon’s herbarium in Leiden identified as T. bufonia, representing various bark-inhabiting fungi. It seems that Fries was uncertain about identity of this species, and it was certainly not the source of his concept of Leiostroma. This leads us to a conclusion that there are no species associated with Leiostroma which could be suitable for the typification of this genus without causing problems for the current classification of fungi. Therefore, we have to return to the original description of the genus by Fries in 1819. The Code Art. 10.2 provides us with a choice: either to select a lectotype of Leiostroma among the species associated with its later reintroduction as a section in the genus Thelephora, or to choose the type otherwise. The first option looks inconvenient because – as we demonstrated above – it would lead to a disposal of the currently accepted and well-defined genera. So, we follow the second option aligning it, as much as possible, with Fries’ concept of Leiostroma as he developed between 1819 and 1828. In both Systema and Elenchus, Fries listed Thelephora acerina as the first species in the section Leiostroma. He most likely viewed it as the most “ typical ” representative of the section because the most important features of its introduction (basidiome shape and consistency, hymenium configuration) in Systema repeat those in the section’s description. The species was introduced with four varieties, of which the first, T. acerina var. abietis, is of special interest for the present study. It certainly refers to a taxon macroscopically similar to Dendrothele acerina but occurring on Picea and clearly implies a widely distributed conifer-dwelling species which was treated as Sebacina or Exidiopsis calcea after Bresadola (1898) and Wells (1961). In our opinion, the latter name is inapplicable to T. acerina var. abietis (see remarks under Leiostroma farinellum) and must be interpreted otherwise. Romell (1895) was the first who clarified the identity of var. abietis, and we see no reasons to reject his interpretation. He raised it to the species level; however, his Corticium abietis is preceded by Xerocarpus far
Authors 13
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University of Helsinki · Finnish Museum of Natural History
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Finnish Museum of Natural History, University of Helsinki, PO Box 7, 00014 Helsinki, Finland
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Komarov Botanical Institute of the Russian Academy of Sciences
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Komarov Botanical Institute, Prof. Popova str. 2, 197022 St. Petersburg, Russia
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Organismic Botany and Mycology, Institute of Plant Science and Microbiology, Ohnhorststraße 18, 22609 Hamburg, Germany
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Naturalis Biodiversity Center, Darwinweg 2, 2333 CR Leiden, Netherlands
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Société Linnéenne de Lyon, 33 rue Bossuet, 69006 Lyon, France
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Österplana Skattegården 2, 53384 Hällekis, Sweden
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Globe Institute, Department of Biology, University of Copenhagen, Universitetsparken 15, 2100 Copenhagen Ø, Denmark
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Institute of Biological Sciences, University of Oslo, P. O. Box 1045, Blindern, N- 0316 Oslo, Norway
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University of Tennessee at Knoxville
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Department of Ecology and Evolutionary Biology, University of Tennessee, 1406 Circle Drive, Knoxville, Tennessee 37996, USA
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Ecology and Genetics Research Unit, University of Oulu, P. O. Box 3000, FI- 90014 Oulu, Finland
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Czech Academy of Sciences, Biology Centre
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Biology Centre, Academy of Sciences of the Czech Republic, Branišovská 31, CZ 37005, České Budějovice, Czech Republic
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National University of Misiones
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Instituto de Biotecnología Misiones (InBioMis), Facultad de Ciencias Exactas, Químicas y Naturales, Universidad Nacional de Misiones, Ruta Nacional N 12, km 7.5, Posadas, Misiones, CP 3304, Argentina & Instituto Misionero de Biodiversidad (IMiBio), Ruta Nacional N 12, km 5, Puerto Iguazú, Misiones, CP 3370, Argentina
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University of Helsinki · Finnish Museum of Natural History
Affiliation as printed
Finnish Museum of Natural History, University of Helsinki, PO Box 7, 00014 Helsinki, Finland
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