Introduction to Fungi, Third Edition

EVOLUTION AND PHYLOGENY OF GASTEROMYCETES
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70 years ago (Baura et al., 1992). In contrast, the
most ancient fossil gasteromycete found so far,
an earth star resembling Geastrum, dates back to
the Cretaceous period some 65 70 million years
ago (Krassilov & Makulbekov, 2003).
One selective environmental pressure towards
the secotioid and ultimately gasteromycete habit
might be drought, since the very nature of the
active basidiospore discharge mechanism by
drop fusion (see p. 493) precludes its function at
low humidity. It is perhaps no coincidence that
secotioid fungi are particularly common in arid
regions (Thiers, 1984). Secotioid fruit bodies are
generally assumed to be the first step towards
typical gasteromycete forms such as earth balls,
puffballs and false truffles (Reijnders, 2000).
However, mycologists are still at a loss to explain
how the fantastically complicated fruit bodies,
e.g. of the stinkhorns or bird’s nest fungi, could
have evolved from there.
Given the ease with which secotioid fruit
bodies can arise, it is hardly surprising that
gasteromycetes have evolved several times independently
from hymenomycete ancestors, as
indicated by numerous phylogenetic studies
(see Fig. 19.2; Hibbett et al., 1997b; Hibbett &
Thorn, 2001). In subsequent sections of this
chapter we shall consider the three most
important groupings which are as follows (see
Table 20.1):
1. Members of the euagarics clade (Section
19.4). The puffballs (Lycoperdaceae) and bird’s
nest fungi (Nidulariaceae) as well as a few
smaller groups of gasteromycetes belong to this
group. The Lycoperdaceae are close to
Macrolepiota (Krüger et al., 2001), whereas the
Nidulariaceae cannot be placed accurately as yet
but are likely to have arisen on a separate
occasion. A further independent evolutionary
event was that leading to the marine gasteromycete
Nia vibrissa (Binder et al., 2001).
2. Members of the boletoid clade (Section
19.5). Several gasteromycetes have their origin
in the boletoid clade (Binder & Bresinsky, 2002).
The most important group is the family
Sclerodermataceae, i.e. the earth balls and their
relatives (Scleroderma, Pisolithus, Astraeus) which
are closely related to Gyrodon. Another example
is Rhizopogon which is close to Suillus. Like their
actively spore-discharging relatives, these boletoid
gasteromycetes are important ectomycorrhizal
associates of trees.
3. The gomphoid phalloid clade. This
group contains the coral fungi and similar
basidiomycetes with exposed hymenia and
active basidiospore discharge (Ramaria,
Clavariadelphus, Gomphus; see p. 575), as well as
several important groups of gasteromycetes,
namely the earth stars (Geastrum spp.), the
cannonball fungus (Sphaerobolus), and the stinkhorns
and their allies. The phylogeny of this
grouping has been discussed by Humpert et al.
(2001).
Although the artificial nature of the gasteromycetes
as a taxonomic group has been known
or suspected for many decades, it still comes as a
shock to most mycologists to realize just how
strongly convergent the evolution of these fungi
has been. For instance, the implications from the
results of phylogenetic studies (see Table 20.1)
are that the earth stars (Geastrum spp.) have
arisen independently of the barometer earth star
(Astraeus), that the raindrop-mediated bellows
mechanism of basidiospore release through
an apical pore in puffballs and earth stars has
evolved at least three times, and that the
peridioles in the bird’s nest fungi (Cyathus,
Crucibulum), in Sphaerobolus and in Pisolithus are
analogous rather than homologous structures.
Referring to these and other findings made by
molecular phylogeneticists, Reijnders (2000) concluded
that ‘if this key denotes real affinities,
morphologists must be ashamed of their wrong
conclusions’.
Ingold (1971) regarded the gasteromycetes
as a biological group which, having lost the
active spore discharge mechanism of their
hymenomycete ancestors, have attempted a
remarkable series of experiments in spore
liberation. In order to explore this aspect of
gasteromycete biology, we shall consider these
fungi together in the present chapter, but
drawing on the taxonomic framework as set
out in Chapter 19.