Introduction to Fungi, Third Edition

578 HOMOBASIDIOMYCETES: GASTEROMYCETES
Ellis and Ellis (1990) and Pegler et al. (1995).
Species with hypogeous gasterocarps (‘false
truffles’) have been described by Pegler et al.
(1993).
20.2 Evolution and phylogeny of
gasteromycetes
In theory, the evolution of a gasteromycete
from a hymenomycete ancestor requires only
two morphogenetic changes, i.e. the production
of a closed fruit body accompanied by the loss
of the active spore discharge mechanism.
The coincidence of these two features is shown
by several examples of secotioid fruit bodies,
i.e. basidiocarps in which the margin of the
pileus fails to become detached from the stipe
(Thiers, 1984). Watling (1971) described such
aberrant development in the agaric Psilocybe
merdaria growing in culture, where the failure
of the fruit body to expose its hymenium
coincided with morphological changes to the
gills and basidia. Chiu et al. (1989) reported a
similar case in Volvariella bombycina, and Hibbett
et al. (1994) demonstrated that a naturally
occurring recessive allele in a single gene is
responsible for converting the lamellate fruit
body of Lentinus tigrinus into a secotioid one.
Except for the L. tigrinus mutant in which an
existing hymenium is belatedly overgrown by a
veil (see Fig. 19.6b), most secotioid forms seem to
arise as a developmental defect causing incomplete
differentiation of an agaric- or bolete-type
basidiocarp (Thiers, 1984; Hibbett et al., 1997b).
Several species pairs are known in nature in
which a secotioid form is closely related to
a mushroom-type species, e.g. Montagnea and
Podaxis (Fig. 20.1) related to Coprinus comatus
(Fig. 19.14c), Gastroboletus related to Boletus,
Gastrosuillus related to Suillus, Hydnangium related
to Laccaria, orThaxterogaster related to Cortinarius
(Thiers, 1984; Mueller & Pine, 1994; Hopple &
Vilgalys, 1999). Such evolutionary trends
towards ‘gasteromycetation’ may be ongoing,
and e.g. Gastrosuillus laricinus is thought to
have arisen from Suillus grevillei as recently as
Fig 20.1 Fruit bodies of the secotioid
mushroom Podaxis pistillaris, a close relative of
the ink cap Coprinus comatus (see Fig.19.14c).
(a) Young fruit body. (b) Mature disintegrating
fruit body.Original photographs kindly
provided by A.E. Ashford.