Skeletocutis nivea

Korhonen, Aku, Seelan, Jaya Seelan Sathiya & Miettinen, Otto, 2018, Cryptic species diversity in polypores: the Skeletocutisnivea species complex, MycoKeys 36, pp. 45-82 : 45

Skeletocutis nivea complex

Description.

Basidiocarps (Fig. 4) annual to sometimes perennial; half-resupinate (resupinate with a pileate edge) to resupinate; hard when dry; surface of pileus white to ochraceous, sometimes turning black when old (Fig. 4C); pore surface cream coloured with ochraceous tints, bluish or greenish colour sometimes develops in the tubes (Fig. 4E); context and subiculum with coriaceous consistency and whitish colour; pores 6-10 per mm; tube layer darker than context.

Hyphal structure: context and subiculum seemingly trimitic (Fig. 5C); hyphae are parallel near cap surface, forming a homogenous, coriaceous texture; skeletal hyphae prevailing, unbranched, thick-walled and often solid, refractive; generative hyphae relatively scarce, clamped, sometimes with (unevenly and irregularly) thickened walls and rarely with sandy encrustation, rarely producing generocystidia (encrusted tips of generative hyphae) with thorny encrustation; 'binding hyphae’ (Fig. 5 C–D) 1 –2– 4 µm wide, arbuscule-like, simple-septate side-branches of generative hyphae, thin-walled to solid and refractive, developing later than skeletal hyphae and sometimes missing in young parts of context/subiculum but becoming dominant in older parts, sometimes filling up the old tube layer.

Trama (Fig. 5G) monomitic to dimitic; hyphae interwoven, tightly subparallel; generative hyphae 1-3 µm wide, usually prevailing, clamped, thin-walled or sometimes with slightly thickened walls; skeletal hyphae (Fig. 6 A–C) looking different from those in context and subiculum, sparse, sometimes apparently missing, originating from tramal generative hyphae, winding and irregularly wide (up to 5+ µm) with spacious lumen, walls usually only slightly thickened, slightly refractive; generative hyphae in dissepiment edges (Fig. 5E and G) ca. 2 µm wide, thin-walled, slightly undulating, often somewhat irregularly shaped towards the tips, bare to richly encrusted with sandy crystals.

Hymenium with fusiform cystidiols (Fig. 5F), often weakly differentiated and inconspicuous but sometimes with strongly elongated apices; hyphal pegs (Fig. 5G) common; heavily encrusted, thorny generocystidia (Fig. 6D) originating from subhymenial hyphae and emerging through hymenium, common especially in older parts of hymenium but sometimes forming amongst dissepiment edge hyphae; basidia (Fig. 6 E–G) (5 –)6–9(– 10) ×(2.2–)2.7–3.7(– 4) µm wide, tetrasterigmatic.

Basidiospores (Fig. 7) narrowly allantoid, 2.5 –4.0×0.4– 0.9 µm, Q’=3.4– 7.0, IKI-, CB- (contents CB+).

Discussion.

The tramal hyphal structure in S. nivea and S. ochroalba has traditionally been described as monomitic. However, our microscopic study revealed two distinct hyphal types existing in the trama of all species in the S. nivea complex. Amongst the normal clamped and thin-walled generative hyphae, there are usually at least some notably wider and slightly thick-walled hyphae which seem to lack clamps. We call these special hyphae tramal skeletal hyphae. They appear to originate from the generative hyphae in the trama and reach down almost to the pore mouths. Usually the lack of clamps, greater width and thicker walls help to tell them apart from generative hyphae in the trama. Although the tramal skeletal hyphae are usually wide and only slightly thick-walled, some specimens of S. nivea had narrower and solid skeletal hyphae in the trama.

Sometimes the tramal hyphal structure is dominated by the skeletal hyphae but sometimes they seem to be missing completely or occur only sporadically in otherwise monomitic tramal structure (at least in S. nemoralis and S. semipileata ). They can also be difficult to detect when the whole tramal structure becomes sclerified and generative hyphae also develop thickened walls, which was observed in some specimens of S. nivea . In general, clear detection of tramal skeletal hyphae is easiest in a squash mount from very thin longitudinal slices of the tube layer which have been properly thinned to an almost disintegrated state.

The nature of the arbuscule-like 'binding hyphae’ has been discussed by David (1982) and Niemelä (1985) and both express some reservations about using the term ‘trimitic’ to describe the S. nivea complex. They point out that the 'binding hyphae’ in the morphospecies S. nivea and S. ochroalba originate as clampless side-branches of the generative hyphae and, hence, they are not binding hyphae proper, such as those of Trametes . David (1982) studied the staining reactions of the hyphal walls and noted that the walls of the 'binding hyphae’ are congophilic and non-metachromatic whereas the walls of the skeletal hyphae are non-congophilic and metachromatic. Our observations confirm that all species in the S. nivea complex appear to be similar in this respect.