Introduction to Fungi, Third Edition

EUAGARICS CLADE
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(see Figs. 19.15b,c). Of these, the more damaging
is a-amanitin which binds to hepatocytes (liver
cells). It inhibits a nuclear polymerase responsible
for transcribing DNA into mRNA, resulting
in reduced protein synthesis at the ribosomes
and ultimately the death of cells. Phalloidin acts
by binding to G-actin in liver cells and brings
about an efflux of potassium ions and lysosomal
enzymes, which leads to cell destruction
(Chilton, 1994).
The treatment of patients suffering from
A. phalloides poisoning includes forced vomiting
and other means of evacuating the stomach,
orally applied activated charcoal to adsorb the
toxins, external dialysis, infusion with silybinin,
an extract from the milkthistle Silybum marianum
which competes with a-amanitin for adsorption
on to hepatocytes, and liver transplantation
(Faulstich & Zilker, 1994).
The effects of ingesting basidiocarps of
A. muscaria are less severe, being mainly hallucinogenic.
Characteristic symptoms are drowsiness,
followed by deep sleep in which vivid
dreams occur. Recovery is generally complete,
with no permanent or prolonged ill after-effects.
The two toxins chiefly involved are the alkaloids
ibotenic acid and muscimol (Figs. 19.15d,e;
Michelot & Melendez-Howell, 2003), and the
former is readily converted to the latter in the
gut. The molecular structures of ibotenic acid
and muscimol closely resemble those of two
neurotransmitters, glutamic acid and g-aminobutyric
acid (GABA), respectively. Certain ethnic
groups (e.g. in Siberia) have taken advantage of
the hallucinogenic properties of A. muscaria to
experience euphoria. Its use has extended to
semi-religious practices in which shamans have
induced themselves into trances in which they
claim to have powers of revelation. Within 1 h of
ingestion, ibotenic acid and muscimol are
detectable in the urine of humans and also
reindeer, and this may account for the tradition
of drinking such urine in order to obtain a
‘second-hand kick’. Such practices have now been
discontinued in favour of alcoholic beverages,
although A. muscaria is still occasionally taken as
a recreational drug.
The name ‘fly agaric’ refers to the supposed
insecticidal properties of A. muscaria caps. When
soaked in milk the caps attract flies, possibly
brought there in response to an attractant,
diolein. The flies ingest the Amanita flesh, but it
is likely that they are intoxicated rather than
killed.
Amanita muscaria, easily recognized by its red
cap adorned by white volva scales, is the bestknown
of all macro-fungi, featuring in countless
illustrations, folk tales, religious and semireligious
ceremonies. It has been considered as
the ‘tree of life’ or ‘tree of knowledge’ (Wasson,
1968). Michelot and Melendez-Howell (2003) have
given an account of its chemistry, biology,
toxicology and ethnomycology. It is widely
distributed in Europe, North America and Asia,
and has probably been introduced into other
parts of the world as a mycorrhizal partner on
the roots of imported trees. Molecular phylogeny
studies suggest that it should be separated into
at least three groups, corresponding to Eurasian,
Eurasian subalpine and North American regions
(Oda et al., 2004).
19.4.4 Pluteaceae
Pluteus (c. 300 spp.)
Species of Pluteus are saprotrophs, growing
mainly on rotting wood. Pluteus cervinus is
common, fruiting on deciduous tree stumps,
logs, fallen branches and on sawdust heaps.
Characteristic features are the free gills and
pink spore print. On the surface of the hymenium,
tapering, thick-walled cystidia crowned by
pointed prongs protrude (Fig. 19.9d). Their function
is unknown.
The Pluteaceae are related to the
Amanitaceae, and these two families appear as
sister groups in phylogenetic analyses (Moncalvo
et al., 2002).
19.4.5 Pleurotaceae
This family includes only two genera, Pleurotus
and Hohenbuehelia (Thorn et al., 2000; Moncalvo
et al., 2002).
Pleurotus (20 spp.)
Pleurotus spp. (oyster mushrooms) have laterally
attached, lamellate basidiocarps. Most cause
white rot decay of wood. The basidiocarps are