Introduction to Fungi, Third Edition

532 HOMOBASIDIOMYCETES
biotechnological application is the use of oxidative
enzymes for the bleaching of wood pulp for
paper production, or of textile dyes.
There is a further twist to the tale. Many
fungal metabolites, including veratryl alcohol
and 3-chloro-anisylalcohol (see Fig. 19.13), contain
methoxy groups or methyl esters, whose
methyl groups are added late during biosynthesis.
Various molecules may act as donors
of methyl groups, including chloromethane
(CH 3 Cl), a known greenhouse gas. A few woodrotting
fungi, notably Phellinus spp., have
gained notoriety because they produce vastly
more chloromethane than they require for
their biosynthetic machinery. Watling and
Harper (1998) estimated that atmospheric chloromethane,
which is mainly of natural origin,
accounts for 15 20% of the chlorine-mediated
ozone destruction in the stratosphere, and that
86% of the total fungal chloromethane contribution
can be attributed to the genus Phellinus.
19.4 Euagarics clade
As shown in Table 19.1 and Fig. 19.2, the
euagarics clade includes not only forms with
lamellate (i.e. agaricoid) basidiocarps formerly
classified in the Agaricales, but also forms with
other hymenial configurations. This means that
there are few, if any, reliable gross morphological
characters by which members of the euagarics
clade can be recognized. It is probable that
agaricoid basidiocarp types have evolved repeatedly
(Hibbett et al., 1997b). Kirk et al. (2001) have
recognized 26 families, 347 genera and about
10 000 species, but cautioned that there are
difficulties in defining these families at present.
In the account of some common genera which
follows, the estimated numbers of species have
been taken from Kirk et al. (2001) unless otherwise
indicated.
19.4.1 Agaricaceae
The Agaricaceae are one of the most diverse
families of the Agaricales, estimated to contain
over 50 genera and some 900 species. The spore
print (i.e. the accumulation of spores projected
from a basidiocarp) may be white or coloured.
There are also variations in the structure of the
hymenophoral trama and the surface of the
pileus. Partial and/or universal veils are usually
present. Despite these variations, molecular
evidence supports the view that the core genera
of this family, including Agaricus and Lepiota, are
monophyletic (Moncalvo et al., 2000, 2002;
Vellinga, 2004).
Agaricus (c. 200 spp.)
This is a large genus of fungi often synonymized
with the term ‘mushrooms’, distributed mainly
in temperate regions. Agaricus spp. are saprotrophs,
growing in pastures and woodland litter.
The mycelium is perennial and some species, e.g.
A. arvensis (horse mushroom), A. xanthodermus
(yellow stainer) and A. tabularis may fruit in rings.
From measurements of the annual rate of
increase in diameter, a ring of A. tabularis 60 m
in diameter was estimated to be 250 years old.
Agaricus basidiocarps are moderate to large in
size, generally firm and white but sometimes
changing colour upon bruising (e.g. in A. xanthodermus).
There is a ring on the stem (two rings in
A. bitorquis), but no volva. The gills are at first
pink due to the colour of the cytoplasm of the
spores, but, as the spores mature, their walls
darken to a purple-brown, the colour of the spore
print. Many species have edible basidiocarps
and some are prized as food, e.g. A. campestris
(field mushroom), A. arvensis, A. macrosporus and
A. silvaticus. Agaricus bisporus (Fig. 19.14a), now
sometimes called A. brunnescens, is the cultivated
white button mushroom (see below) and is
occasionally found in nature on manure heaps,
garden waste and roadsides, and under Cupressus
in coastal areas of California and France. The
basidiocarps of A. xanthodermus may cause gastrointestinal
upsets in some people, probably due
to the presence of phenol which also gives the
fruit bodies an unpleasant smell of carbolic acid
(Gill & Strauch, 1984).
Cultivation of Agaricus bisporus
The white button mushroom has been cultivated
for its edible fruit bodies for almost four
centuries since collectors discovered that its
spawn could be used to inoculate compost. The