Introduction to Fungi, Third Edition

BOLETOID CLADE
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consumption. The toxic principle(s) appear to be
unknown as yet.
Fig19.22 Pulvinic acid-type pigments typical of members
of the boletoid clade. (a) Involutin, a brown pigment produced
by Paxillus involutus. (b) Variegatic acid, a yellow pigment.
(c) The dark blue oxidation product of variegatic acid upon
bruising the fruit bodies of Boletus erythropus and certain
other Boletus spp. (d) Grevillin B produced by a range of
Suillus spp.
seems to be responsible for gastric upsets within
a few hours of consuming raw or undercooked
specimens, whereas haemolysis due to an
allergic reaction can cause delayed liver
failure and kidney damage only after repeated
19.5.2 Boletaceae
Boletus (300 spp.)
Formerly comprising a broad range of porebearing
fungi, the genus Boletus has been divided
into a number of groups, often now recognized
as separate genera. Species of Boletus are ectomycorrhizal.
They have medium-sized, large or
very large fleshy basidiocarps with a tubular
hymenophore. The pores marking the openings
of the hymenial tubes may be of the same
yellowish-green colour as the tubes or may be
coloured orange to blood-red as in B. erythropus
(Plate 9f) and B. satanas. The pigments in Boletus
are of the pulvinic acid group (Gill & Steglich,
1987), with variegatic acid (Fig. 19.22b) being the
most common. Within seconds of bruising or
cutting the basidiocarps of certain species such
as B. erythropus, their flesh and pores become
discoloured blue or bluish-black due to the
enzyme-mediated oxidation of variegatic acid
(Fig. 19.22c) and xerocomic acid.
One of the most common species, B. chrysenteron
(Fig. 19.24b), sometimes classified in the
separate genus Xerocomus, is easily recognized by
the exposure of yellow or red flesh when the
cap surface skin cracks. It is associated with
broad-leaved trees. The stem of some species
may be punctate, i.e. dotted with tiny warts (e.g.
B. erythropus; Plate 9f), or veined (e.g. B. edulis).
There are several species with delicious edible
basidiocarps; amongst the best-known are B.
edulis (cep or penny bun; Fig. 19.21c), B. badius
(bay bolete; Plate 9h) and B. appendiculatus.
Although it is possible to grow mycelium of B.
edulis in pure culture, it is as yet impossible to
induce it to form basidiocarps, and therefore this
much-prized edible species continues to be
collected in forests (Wang & Hall, 2004). Some
Boletus spp. are safe to eat only after cooking,
e.g. B. luridus and B. erythropus, whereas others
have poisonous basidiocarps, notably B. satanas
(devil’s bolete) and B. satanoides.
Boletus parasiticus forms fruit bodies attached
to the basidiocarps of the earth ball Scleroderma.
Doubts have been expressed as to whether this