Introduction to Fungi, Third Edition

586 HOMOBASIDIOMYCETES: GASTEROMYCETES
the dye ball Pisolithus, and the barometer
earth star Astraeus. Binder and Bresinsky (2002)
have examined the phylogeny of the group
and have recommended its partitioning into
several families. All members seem to be ectomycorrhizal
with trees. Here we shall consider
the two most important genera, Pisolithus and
Scleroderma.
Pisolithus
The best-known species is P. tinctorius (¼
P. arhizus), which is so called because its
immature gasterocarps, when injured, produce
an intense black dye (see Plate 11b). Because of
the variability of gasterocarp appearance, the
taxonomy of Pisolithus has been problematic, and
initially P. tinctorius was thought to be of panglobal
distribution, capable of associating with
almost any ectomycorrhiza-forming tree species
(Marx, 1977). Pisolithus is now known to consist of
more than 10 species (Cairney, 2002; Martin et al.,
2002), with P. tinctorius distributed throughout
the Northern Hemisphere and associated mainly
with Pinus and Quercus. The centre of evolution of
the genus is probably Australia, and P. marmoratus
associated with Eucalyptus is regarded as the
Southern Hemisphere equivalent of P. tinctorius.
This and other species have been spread to South
America, South East Asia and Africa, together
with their host trees (Eucalyptus, Acacia) which are
used in intensive forestry and in reforestation
programmes (Dell et al., 2002; Martin et al.,
2002). In addition to this anthropogenic dispersal,
there is also evidence that Pisolithus can travel
long distances as air-borne basidiospores, e.g.
from Australia to New Zealand (Moyersoen et al.,
2003).
Pisolithus spp. may be displaced by other
ectomycorrhizal fungi in cool, wet situations
(McAfee & Fortin, 1986) but are prominent in
extreme environments, e.g. dry habitats with
sandy soil, or areas polluted with heavy metals
(Walker et al., 1989; Smith & Read, 1997). In such
situations, the growth of mycorrhizal trees can
be increased several-fold relative to uninoculated
controls. Benefits of Pisolithus infections to the
host tree include enhanced provision of nutrients
and water, detoxification of heavy
metals, and protection against soil-borne plant
pathogens. The considerable promise of Pisolithus
is reflected by an immense body of literature
which has been summarized admirably by
Cairney and Chambers (1997) and Chambers
and Cairney (1999).
Pisolithus has a tetrapolar mating system
(Kope & Fortin, 1990), and although monokaryons
can infect tree roots, a full-scale ectomycorrhizal
association requires a dikaryotic
mycelium. The establishment of a mycorrhiza
proceeds in several steps. Chemotropic growth of
Pisolithus hyphae towards host root tips is
followed by the secretion of glycoprotein fibrils
by the fungus during initial contact (Lei et al.,
1990). Dead or moribund cells in the root cap
region are infected first; the mantle is then
established within 48 h, and a Hartig net formed
subsequently (Horan et al., 1988; Lei et al., 1990).
Only root material grown after initial contact is
colonized. Rhizomorphs radiate outwards for
several metres, and these may partly account
for the success of the Pisolithus mycorrhiza,
especially in dry habitats. Another factor may
be the formation of sclerotia which enable the
fungus to survive adverse conditions in the soil
(Grenville et al., 1985). Eventually, fruit body
initials are formed in the soil, with the maturing
gasterocarps pushing through the surface.
Basidiospores are produced inside numerous
peridioles which disintegrate to release their
spores passively (Plate 11b). The formation and
maturation of peridioles proceeds from the tip
to the base of the gasterocarp which gradually
breaks up in the process. Gasterocarps can be
sizeable, up to 20 cm tall.
Studies on Pisolithus mycelia in Australian
eucalypt forests have revealed that genetically
distinguishable individuals (genets) may be
variable in size, ranging from less than 2 m 2 to
50 m 2 or more. Since these are interspersed,
the smaller genets are interpreted as the result
of recent re-colonization events from winddispersed
basidiospore inoculum (Anderson
et al., 1998, 2001).
Scleroderma
There are about 25 species of Scleroderma (Sims
et al., 1995), three common temperate examples
being S. bovista, S. citrinum (Fig. 20.5) and