Introduction to Fungi, Third Edition

BOLETOID CLADE
555
fungus (P. nameko) is cultivated commercially on
sawdust-based substrate in Far Eastern countries
for its edible basidiocarps (Chang & Miles, 2004).
It is a moisture-loving fungus, growing in nature
on dead trunks and stumps of deciduous
trees at high altitudes in Japan and Taiwan. Its
mating system is bipolar. Arthroconida develop
on monokaryotic and on dikaryotic mycelia.
Similarly, basidiocarps develop on monokaryons
and on dikaryons.
19.4.14 Cortinariaceae
Cortinarius (c. 2000 spp.)
Because of its large number of species, this genus
provides one of the toughest challenges to fungal
taxonomists as well as field mycologists. In
Britain alone, 230 species have been listed. The
genus has been divided into several subgenera
(e.g. Cortinarius, Dermocybe, Leprocybe, Phlegmacium,
Myxacium, Telamonia). Species in the subgenus
Myxacium have slimy caps and stems derived
from a glutinous universal veil, whilst in the
subgenus Phlegmacium only the cap is sticky
whereas the stem is dry. The genus is distributed
in temperate regions of the Northern
Hemisphere. All species are mycorrhizal with
trees, so that they fruit in woodlands and
woodland margins. The fruit bodies are small
to large, buff, clay-coloured, orange-brown or
sometimes very colourful, e.g. violet in C. violaceus
or blood-red in C. sanguineus, C. purpureus and
related species. Young fruit bodies are enveloped
in a filamentous or glutinous veil and developing
gills are protected by a fibrillose cortina which
may be evanescent or may persist, attached to
the stem (Fig. 19.20e). The spores are mostly
warty, and the spore print is cinnamon to rustbrown.
It is unwise to attempt to eat Cortinarius
basidiocarps because the edibility of many
species is unknown and some are deadly poisonous.
The most notorious among them is
C. orellanus, which contains the bipyridyl toxin
orellanine (Fig. 19.15h), the cause of delayed
renal failure. Other toxins are cortinarins, which
are cyclic peptides. The long delay of 2 20 days
between ingestion of the mushroom and the
onset of symptoms (nausea, vomiting, diarrhoea,
gastric upset and abdominal pain) are
characteristic features of Cortinarius poisoning.
Death may ensue 2 6 months later (Bresinsky &
Besl, 1990; Michelot & Tebbett, 1990).
19.5 Boletoid clade
This clade includes not only the Boletales, but
some other groups with basidiocarps which are
dissimilar in appearance. Traditionally, the
Boletales, typified by the genus Boletus, have
included forms with fleshy, mushroom-like basidiocarps
with tubular hymenophores. Later,
based on morphological and chemical criteria,
the concept was expanded to include gill-bearing
forms such as Paxillus and Hygrophoropsis.
Relationships were also suggested between
poroid boletes and resupinate forms such as
Coniophora or Serpula, and gasteroid genera such
as Scleroderma or Rhizopogon. Molecular phylogenetic
techniques have confirmed these relationships
and a boletoid clade has been recognized to
embrace this wider concept (Bruns et al., 1998;
Hibbett & Thorn, 2001). We shall study representatives
of a gill-bearing group (Paxillaceae),
poroid groups (Boletaceae, Suillaceae) and resupinate
forms (Coniophoraceae).
19.5.1 Paxillaceae
Paxillus (15 spp.)
Most species of Paxillus are ectomycorrhizal
(Wallander & Söderström, 1999), but P. atrotomentosus
fruits on conifer stumps. The basidiocarps
are soft and fleshy. A characteristic feature
is that the hymenial tissue separates readily
from the flesh of the cap. The roll-rim Paxillus
involutus (Fig. 19.21a) has a brown, funnel-shaped
cap with an inrolled margin, decurrent gills and
brown spores. Upon bruising, an intense brown
colour develops due to the accumulation of
the pigment involutin (Fig. 19.22a), a member
of the shikimic acid-derived pulvinic acid
family typical of Boletales. Considering that it
is a mycorrhizal species, P. involutus has an
unusually wide host range, with 23 different
tree species listed by Wallander and Söderström
(1999). It is most commonly associated with birch
(Betula) and oak (Quercus) in acid woodlands.