ToL-Reviewed

This tree represents the current knowledge on phylogenetic relationships of distinct members of the hymenochaetoid clade, and it is strongly based on some analyses of ribosomal DNA sequences (Wagner and Fischer, 2001; Hibbett and Binder 2002; Wagner and Fischer, 2002a; Wagner and Fischer, 2002b; Wagner and Ryvarden, 2002; Moncalvo et al., 2002 ).

Containing group: Agaricomycetes

Introduction

The hymenochaetoid clade is one of the eight major clades of Homobasidiomycetes suggested by Hibbett and Thorn (2001), and corroborated by Binder and Hibbett (2002) and Moncalvo et al. (2002). The hymenochaetoid clade probably comprises all known Hymenochaetaceae, as well as some members of at least nine different families of Homobasidiomycetes, as revealed by the aforementioned recent molecular phylogenetic studies.

Although the Hymenochaetaceae are typically represented by the yellowish to reddish brown lignicolous bracket fungi, the non-hymenochaetacean species of the hymenochaetoid clade exhibit a striking diversity of forms, colours and habits, which includes mushrooms (Fig. 1), lilac to violet bracket fungi (Fig. 2), some members of Corticiaceae s.l. that resemble a splash of paint on trunks and twigs of trees (Fig. 3), and even one species that was for a long time in the "Guinness Book of World Records" as possessing the largest known fruiting body of a fungus, Bridgeoporus nobilissimus (Fig. 4), a rare and endangered species, restricted to very large specimens of noble firs in Pacific Northwest old-growth forests in North America (Burdsall et al., 1996).

Many species of the hymenochaetoid clade, especially Phellinus s.l. are important phytopathogens, causing white rot in living trees in natural and cultivated forests of both conifers and flowering plants (Larsen and Cobb-Poule, 1990; Ryvarden, 1991, 1993): Phellinus pini is the etiological agent of red ring rot, the most economically important of the stem rotters of pines (Harrington and Wingfield, 1998); Phellinus weirii causes the laminated root rot in Douglas-fir and other commercially important species of conifers in northwestern North America (Thies and Rona, 1995); and Fomitiporia punctata and Fomitiporia mediterranea are associated with "esca" disease of grapevines (Fischer, 2002). On the other hand, some representatives of the hymenochaetoid clade have been used for medicinal purposes by traditional human societies in Asia (Lim et al., 2003.), and many metabolic compounds isolated from them have antineoplasic and antioxidant activities (Ajith and Janardhanan, 2003; Shon et al., 2003). Furthermore, because of their ability of degrading natural recalcitrant molecules (e. g. liginin, the main component of wood) they have been studied to be used for the biodegradation of xenobiotics (non-natural, man-made compounds, e. g. plastics, biocides etc.) (Wesenberg et al., 2003).

Characteristics

Except for only one common cytological character, this clade apparently has no morphological, physiological or ecological synapomorphy, and it is almost exclusively defined on molecular grounds. Although not confirmed for all the studied species regarded as members of the hymenochaetoid clade, they all appear to share at least one nonmolecular character in common, the dolipore septa with imperforate parenthesomes (Moore, 1985; Oberwinkler, 1985; Langer and Oberwinkler, 1993; Keller, 1997; Muller et al., 2000, see Characteristics of Hymenomycetes).

The Hymenochaetaceae are rather well-defined. They always possess clampless generative hyphae (Fig. 6) and brownish basidiomata (Fig. 5) that darken when moistened in alkali solutions (permanent positive xanthochroic reaction). Setoid structures, especially unbranched setae (Figs. 7-8), but also branched asterosetae occur frequently in several taxa (Oberwinkler, 1977). However, the hymenochaetoid species assigned to the other families not only lack this combination of characters but also have other markedly different nonmolecular characters.

Hymenochaetoid fungi exhibit almost all possible forms of basidiomata. Ressupinate (Fig. 9), effuse-reflex (Fig. 10) and pileate (Fig. 11) forms predominate, but stipitate (Fig. 12) forms also occur. The same is true for hymenophores, which can be smooth (Fig. 13), toothed (Fig. 14), poroid (Fig. 15), and even lamelate (Fig. 16) (Parmasto, 2001; Goes-Neto et al., 2001). They exhibit all the three main patterns of hyphal systems (mono, di and trimitic), and sterile elements (other than setoid structures) of the hymenium are often found in non-hymenochaetacean species (Hibbett and Thorne, 2001).

Most of the members of the hymenochaetoid clade predominantly consist of saprotrophic, lignicolous, white-rot species, although mycorrhizal, forming both ectomycorrhiza (Danielson, 1984), and orchid mycorrhyza (Umata, 1995), and even agaricoid bryophyte-associated species (Moncalvo et al., 2002; Redhead et al., 2002) are also found in this clade.

Although they are not restricted to the hymenochaetoid clade, styrylpyrones are found in all studied hymenochatacean members of the clade (Fiasson, 1982), which, together with other phenolic compounds, confer the typical xanthocroic reaction.

Phylogenetic Position of the Hymenochaetoid Clade

Contrary to most of the Homobasidiomycetes, the hymenochaetoid clade, as well as the cantharelloid and gomphoid-phalloid clades, include species with imperforate parenthesomes (Hibbett and Thorn 2001, Binder and Hibbett 2002). As this character is also encountered in non-homobasidiomycetous Hymenomycetes (e. g. Auriculariales and Dacrymycetales), one can argue that the presence of imperforate parenthesomes is the plesiomorphic condition in the Homobasidiomycetes, and this point of view would be in accordance with the basal position of the hymenochaetoid clade in homobasidiomycete phylogeny. Hibbett and Thorn (2001), however, suggested that this character could be potentially homoplasic, because of the apparent co-occurrence of imperforate and perforate parenthosomes in the polyporoid clade (Keller, 1997), which would be in conformity with a non-basal position of this clade. Therefore, the position of the hymenochaetoid clade in homomobasidiomycete phylogeny still is controversial (Binder and Hibbett, 2002).

Discussion of Phylogenetic Relationships

Relationships among putative hymenochaetoid clades are not well resolved. According to Wagner and Fisher (2002a), there is at least some support for one less inclusive clade comprising most of the "typical hymenochaetaceae" (Inonotus s.s., Phylloporia, Fulvifomes, Inocutis, Fomitiporella, Aurificaria, Phellinus s.s., Pseudoinonotus, Fomitiporia, Porodaedalea, Onnia, Mensularia, and Pseudochaete). This group is in accordance with the classic concept of the Hymenochaetales sensu Oberwinkler (1977), and the taxa exhibit a holocoenocytic behavior of the somatic mycelium (Wagner and Fischer, 2001, 2002a), but its monophyly is still uncertain.

The remaining hymenochaetacean taxa (Coltricia, Coltriciella, Pyrrhoderma, Fuscoporia, Phellinidium, Asterodon, Phellopilus, Hymenochaete) appear intermingled with corticiaceous and polyporoid species of the genera Hyphodontia, Schizopora, Basidioradulum, Oxyporus, Bridgeoporus and Trichaptum, and, most surprisingly, also with gilled omphalinoid fungi, such as Cotylidia, Rickenella, Loreleia, Contumyces and Sphagnomphalia, as was revealed by previous studies (Hibbett and Donoghue, 1995, Hibbett et al. 1997, Ko et al., 1997, Hibbett et al., 2000; Hibbett and Donoghue, 2001; Moncalvo et al., 2002; Redberg et al., 2003; Redhead et al., 2002).

In summary, the position of the hymenochaetoid clade and the relationships among the putative more inclusive clades are definetely not well resolved. Thus, the great challenging tasks concerning this clade are (i) to discover which non-hymenochaetacean taxa are in this group and (ii) to characterize this clade more properly.

Other Names for Hymenochaetoid clade

• Hymenochaetaceae
• Hymenochaetales

References

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Information on the Internet

• The WWW Virtual Library: mycology. An index to Mycological Resources in the internet.
• Deep Hypha Research Coordination Network. Deep Hypha is a project to coordinate and provide resources for research in fungal systematics.
• AFTOL: Assembling the Fungal Tree of Life. Collaborative research in fungal phylogenetics.