Taxonomic and ecological significance of synnema-like structures/acanthophyses produced by Physisporinus (Meripilaceae, Polyporales) species

Abstract

Physisporinus, a genus in Polyporales, Basidiomycota, is a versatile fungus that lives as a wood decomposer, a potential pathogen of standing trees, and an orchid mycobiont. We previously reported that some Physisporinus species inhabiting wet wood in aquatic environments such as streams and waterfalls form synnema-like structures (SSs) bearing acanthophyses at their apices, and that they produce acanthophyses on vegetative hyphae when cultured on agar media. In this study, we investigated the acanthophysis-forming ability in Physisporinus and allied genera, and experimentally demonstrated the function of SSs. Phylogenetic analyses and observations of Meripilus, Physisporinus and Rigidoporus cultures showed that all of the strains forming acanthophyses belonged to Physisporinus, whereas strains of Meripilus and Rigidoporus did not produce acanthophyses. These findings suggest that SS/acanthophysis formation is a useful taxonomic character for members of Physisporinus. When Physisporinus strains were cultured under oxygen (O₂) concentrations of 5, 10, 20 and 40%, most of those cultured under 20% O₂ formed the most acanthophyses. According to these experimental data, the SSs/acanthophyses in Physisporinus were considered to have a respiratory function. Physisporinus probably acquired the SS/acanthophysis-forming ability to adapt to moist and/or aquatic habitats and to decay wet wood in which the O₂ concentration is often low.

Keywords: Meripilus; Rigidoporus; freshwater; oxygen concentration; phylogenetic analysis.

Fig. 1 - Basidiocarps (A-C), SSs (D) bearing acanthophyses (E), and rhizomorphs (F) of Physisporinus in nature. A: Whitish basidiocarps produced on wet wood nearby streams (P. cf. 2 eminens TUMH 65445). B: Basidiocarps (P. cf. 1 eminens TUMH 65440). C: Pore surface of basidiocarps (P. cf. 1 eminens TUMH 65442). D: SSs on the water-boundary part of wood in streams (the source for P. pouzarii TUFC 101965). Arrowheads show parts forming acanthophyses. E: Acanthophyses on the apex of SS (P. microacanthophysis TUMH 64311). F: Rhizomorphs on the submerged part of wood in streams (P. rhizomorphae TUMH 64298). Bars: C, D 1 mm; E 30 µm.

Fig. 2 - The outline of experiment for acanthophysis production under different O₂ concentrations (For details, see section 2.4. in materials and methods).

Fig. 3 - Phylogenetic tree of Meripilaceae in Polyporales inferred from connected sequences of the ITS and LSU regions of nrDNA by ML method. A total of 1,171 sites in the final data set were used for this analysis. The values at nodes indicate BS in ML method (≥ 70%), and bold branches mean BS ≥ 90% in the above method. The species names and numbers of strains used in this study are shown in bold, and the strain number followed by “S” or “R” indicates the isolate from a SS or rhizomorph. ^T on the sample number means the sequence obtained from the type specimen or ex-type culture. Filled circles show the strains forming acanthophyses in culture. Open squares show the strains having clamp connections at the septa of vegetative hyphae on agar media (white arrow indicates a monophyletic clade characterized by this feature). The strains without sufficient cultural investigations in the present study are unmarked.

Fig. 4 - The relationship between molecular phylogeny and acanthophysis formation in Physisporinus. A figure at the upper left is the reduced Fig. 3, and a box in it shows a magnified part for this figure. As with Fig. 3, filled circles show the strains forming acanthophyses in culture, and open squares show the strains having clamp connections at the septa of vegetative hyphae on agar media (white arrow indicates a monophyletic clade characterized by this feature). The strains without sufficient cultural investigations in the present study are unmarked. The sizes of acanthophyses of Physisporinus species investigated in this study are described under the species name. The appearance of a typical acanthophysis of each species is exhibited by photographs using SEM at the right of this figure. Bars: 10 µm.

Fig. 5 - The relationship between molecular phylogeny and acanthophysis formation in Physisporinus. A figure at the upper left is the reduced Fig. 3, and a box in it shows a magnified part for this figure. As with Fig. 3, filled circles show the strains forming acanthophyses in culture. The strains without sufficient cultural investigations in the present study are unmarked. The sizes of acanthophyses of Physisporinus species investigated in this study are described under the species name. The appearance of a typical acanthophysis of each species is exhibited by photographs using SEM at the right of this figure. Bars: 10 µm.

Fig. 6 - Cultural characteristics of the strains of Meripilus giganteus (A-C) and Physisporinus species (D-F) in Meripilaceae. A: Colony on 1.5% MA (TUFC 100564). B: Plectenchymata in mycelia (CBS 421.48). C: Vegetative hyphae (CBS 421.48). Arrow heads show clampless septa. D: Colony on 1.5% MA (P. pouzarii TUFC 101965). E: Clamp connection on a septum of vegetative hyphae (P. pouzarii TUFC 101965). F: Plectenchymata and vegetative hyphae (P. lineatus CBS 167.65). Arrow heads show clampless septa. Bars: B, C, E, F 10 µm.

Fig. 7 - Differences in the number of acanthophyses produced on agar discs under four different O₂ concentrations (5, 10, 20, and 40%) by six Physisporinus strains. The vertical axis of the bar graph shows the number of acanthophyses. “T” and “B” under the horizontal axis mean the agar discs inoculated on a CMA plate as top face up and back face up, respectively. “T” is shown by a blue bar and “B” by a white bar. “U” and “S” indicate the upper surface and side face of the agar discs.