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The Chaotic Taxonomy of Ganoderma sessile

The word Ganoderma is Greek from “Gano” meaning shiny and “derma” meaning skin. Ganoderma was erected as a genus in 1881 by Karsten and included only one species, G. lucidum (Curtis) Karst. Previously, this taxon was characterized as Boletus lucidus Curtis (1781) and then Polyporus lucidus (Curtis) Fr. (1821) (Karsten 1881). The species P. lucidus was characterized by having a laccate pileus and stipe, and this is a character that Murrill suspects was the reason for Karsten’s division because only one species was included, G. lucidum (Murrill 1902). Patouillard revised Karsten’s genus Ganoderma to include all species with pigmented spores, adhering tubes and laccate crusted pilei, which resulted with a total of 48 species classified under the genus Ganoderma in his 1889 monograph (Murrill 1902; Patouillard 1889). Until Murrill investigated Ganoderma in North America in 1902, previous work had focused solely on European species including, for example, G. lucidum, G. resinaceum Boud. (1890) and G. valesiacum Boud. (1895) (Adaskaveg 1986; Murrill 1902, 1908).

Murrill described 17 new Ganoderma species in his treatises of North American polypores (Murrill 1902, 1908), including for example, G. oregonense, G. sessile, G. tsugae, G. tuberculosum and G. zonatum. Most notably and controversial was the typification of Ganoderma sessile, which was described from various hardwoods only in the United States (Murrill 1902, 1908). Ganoderma sessile was distinguished based on a sessile fruiting habit, common on hardwood substrates and occasionally having a reduced, eccentric or “wanting” stipe (Murrill 1902, 1908). In 1908, Atkinson considered G. tsugae and G. sessile as synonyms of G. lucidum, but erected the species G. subperforatum from a single collection in Ohio on the basis of having “smooth” spores (Atkinson 1908). Although he did not recognize the genus Ganoderma, but rather kept taxa in the genus Polyporus, Overholts agreed with Atkinson, and considered G. sessile as a synonym of the European G. lucidum (Overholts 1915).

Although no data was reported, in 1920 in an update on Polyporaceae of North America, Murrill conceded that G. sessile was closely related to the European G. lucidum (Murrill 1920). It is difficult to determine if this concession was the result of political scrutiny or from a scientific foundation, due to the retention of the name by Murrill in a later publication (Murrill 1922) and a single herbarium collection in 1926 (www.mycoportal.org).

Approximately a decade later, Haddow considered G. sessile a unique taxon, but suggested Atkinson’s G. subperforatum was a synonym of G. sessile, on the basis of the “smooth” spores, which was the original basis of G. subperforatum when earlier erected by Atkinson in 1931 (Haddow 1931). Until this point, all identifications of Ganoderma taxa were based on fruiting body morphology, geography, host, and spore characters.

In 1948 and then amended in 1965, Nobles characterized the cultural characteristics of numerous wood-inhabiting hymenomycetes including Ganoderma taxa (Nobles 1948, 1965). Her work laid the foundation for culture-based identifications in this group of fungi (Nobles 1965). Nobles recognized that there were differences in cultural characteristics between G. oregonense, G. sessile, and G. tsugae (Nobles 1948, 1965). Although Nobles recognized G. lucidum in her 1948 publication as a correct name for the taxon from North American isolates that produce numerous broadly ovoid to elongate chlamydospores (12.0-21.0 x 7.5-10.5 μm), she corrected this misnomer in 1968 by amending the name to G. sessile (Nobles 1965). Clarifying further, Bazallo & Wright and Steyaert agree with Haddow’s distinction between G. lucidum and G. sessile on the basis of “smooth” spores, but they synonymize G. sessile with G. resinaceum, a previously described European taxon (Bazzalo and Wright 1982; Steyaert 1980). Later Adaskaveg and Gilbertson demonstrated the similarity in culture morphology and that vegetative compatibility was successful between the North American taxon recognized as ‘G. lucidum’ and the European G. resinaceum (Adaskaveg and Gilbertson 1986).

In the monograph of North American Polypores written in 1986, which is still the only comprehensive treatise on this group of fungi unique for North America, the authors Gilbertson and Ryvarden do not recognize G. sessile, but rather recognize the five following laccate species being present in the U.S.: G. colossum (Fr.) C.F. Baker (current name: Tomophagus colossus (Fr.) Murrill), G. curtisii, G. lucidum, G. oregonense, and G. tsugae (Gilbertson and Ryvarden 1986). They considered G. sessile along with other laccate species synonyms with G. lucidum, and in the comments under G. lucidum they discuss the taxonomic chaos of this particular species complex, and leave it somewhat unresolved.

With the rise of molecular phylogenies in the late 20^th century, species concept hypotheses were tested to determine the relatedness amongst the nuanced morphological variabilities of the laccate Ganoderma taxa. In 1995, Moncalvo et al constructed a phylogeny of the rDNA, which was the universally accepted locus at that time, and found five major clades amongst the 29 isolates tested (Moncalvo et al. 1995). It turned out that G. lucidum was not a monophyletic species, and further work needed to be done to clarify this taxonomic problem. Interestingly, they also found that G. resinaceum from Europe, and the North American ‘G. lucdium’, which Adaskaveg and Gilbertson found to be biologically compatible in vitro, did not cluster together (Adaskaveg and Gilbertson 1986;  Moncalvo et al. 1995). Moncalvo et al. reject biological species complexes as a sole tool to distinguish a taxon, and suggested using a combination between biological and phylogenetic species concepts to define unique Ganoderma taxa (Adaskaveg 1986; Moncalvo et al. 1995).

In a recent multilocus phylogeny, the authors revealed that the global diversity of the laccate Ganoderma species included three highly supported major lineages that separated G. oregonense/G. tsugae from G. zonatum and from G. curtisii/G. sessile, and these lineages were not correlated to geographical separation (Zhou et al. 2015). These results agree with several of the earlier works focusing mostly on morphology, geography and host preference showing genetic affinity of G. resinaceum and G. sessile, but with statistical support separating the European and North American taxa (Zhou et al. 2015). Also, Ganoderma curtisii and G. sessile were separated with high levels of statistical support, although there was not enough information to say they were from distinct lineages. Lastly, G. sessile was not sister to G. lucidum. The phylogeny supported G. tsugae and G. oregonense as sister taxa to the European taxon G. lucdium sensu stricto (Zhou et al. 2015).

Description of Ganoderma sessile

Fruiting bodies annual and sessile (without a stipe) or pseudostipitate (very small stipe). Fruiting bodies found growing on trunks or root flares of living or dead hardwood trees. Very common taxon, being found in practically every state East of the Rocky Mountains. Mature fruiting bodies are laccate and reddish-brown, often with a wrinkled margin if dry. Fruiting bodies are shelf-like if on stumps or overlapping clusters of fan-shaped (flabelliform) fruiting bodies if growing from underground roots, and range in size of 3-20 cm in diameter. Hymenium white, bruising brown, and poroid with irregular pores that can range in shape from circular to angular. The context tissue is cream colored and can be thin to thick and on average the same length as the tubes. Black resinous deposits are never found embedded in the context tissue, but concentric zones are often found. Spores appear “smooth”, or nearly so, due to the fine (thin) echinulations from the endosporium. Elliptical to obovate to obpyriform chlamydospores formed in vegetative mycelium, and abundant in cultures (Murrill 1902; Nobles 1965; Zhou et al. 2015).

References

Adaskaveg, J. E. 1986. Studies of Ganoderma lucidum and Ganoderma tsugae (Delignification, Mating Systems, Root Rot, Cultural Morphology, Taxonomy). Dissertation. The University of Arizona.

Adaskaveg, J. E., and Gilbertson, R. L. 1986. Cultural studies and genetics of sexuality of Ganoderma lucidum and G. tsugae in relation to the taxonomy of the G. lucidum complex. Mycologia:694-705.

Atkinson, G. F. 1908. Observations on Polyporus lucidus Leys and some of its Allies from Europe and North America. Botanical Gazette 46:321-338.

Bazzalo, M. E., and Wright, J. E. 1982. Survey of the Argentine Species of the Ganoderma lucidum Complex. Mycotaxon 16:293-325.

Gilbertson, R., and Ryvarden, L. 1986. North American Polypores, vol. 1. Abortiporus to Lindteria, Fungiflora, Oslo. 443 pp., 1987. North American Polypores 2.

Haddow, W. R. 1931. Studies in Ganoderma. Journal of the Arnold Arboretum 12:25-46.

Karsten, P. 1881. Enumeratio Boletinarum et Polyporarum Fennicarum systemate novo dispositorum. Rev. Mycol. 3:16-18.

Moncalvo, J.-M., Wang, H.-F., and Hseu, R.-S. 1995. Gene phylogeny of the Ganoderma lucidum complex based on ribosomal DNA sequences. Comparison with traditional taxonomic characters. Mycological Research 99:1489-1499.

Murrill, W. A. 1902. The Polyporaceae of North America, genus I Ganoderma. Bull. Torrey Bot. Club 29:599-608.

Murrill, W. A. 1908. Agaricales (Polyporaceae). North Amer. Flora 9:73-131.

Murrill, W. A. 1920. Corrections and additions to the Polypores of temperate North America. Mycologia 12:6-24.

Murrill, W. A. 1922. Index to Illustrations of Fungi, XXIII-XXXIII. Mycologia 14:332-334.

Nobles, M. K. 1948. Studies in Forest Pathology. IV. Identification of Cultures of Wood-Rotting Fungi. Can. J. Res. 26:281-431.

Nobles, M. K. 1965. Identification of cultures of wood-inhabiting Hymenomycetes. Canadian journal of Botany 43:1097-1139.

Overholts, L. O. 1915. Comparative Studies in the Polyporaceae. Annals of the Missouri Botanical Garden 2:667-730.

Patouillard, N. 1889. Le genre Ganoderma. Bull. Soc. mycol. Fr. 5:64-80.

Steyaert, R. L. 1980. Study of Some Ganoderma Species. Bull. du Jardin Bot. Nat. de Belgique/ Bull. van de Nat. Plantentuin van Blegie 50:135-186.

Zhou, L. W., Cao, Y., Wu, S. H., Vlasak, J., Li, D. W., Li, M. J., and Dai, Y. C. 2015. Global diversity of the Ganoderma lucidum complex (Ganodermataceae, Polyporales) inferred from morphology and multilocus phylogeny. Phytochemistry 114:7-15.