Introduction to Fungi, Third Edition

GASTEROMYCETES IN THE GOMPHOID PHALLOID CLADE
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of a cadaverous smell or by colour, with gasterocarps
of species like Clathrus ruber (Plate 11e) and
C. archeri (Plate 11f) appearing dark red due to the
accumulation of carotenoids, chiefly lycopene
(Fiasson & Petersen, 1973; Gill & Steglich, 1987).
There may be a synergism of attractions because
species with brightly coloured gasterocarps tend
to emit less evil smells than dull-coloured ones.
Common temperate examples of Phallaceae
are the graphically named stinkhorn Phallus
impudicus (Lat. impudicus ¼ shameless; Plate 11d)
and the dog’s stinkhorn Mutinus caninus
(Fig. 20.7d).
Phallus
In late summer and autumn, stinkhorns can
be detected readily by their smell. They can be a
common or even dominant component of the
population of basidiocarps on the forest floor, as
shown by Shorrocks and Charlesworth (1982) who
estimated some 50 000 70 000 gasterocarps km 2
per season in a woodland. In such situations,
stinkhorns and their primordia, the ‘eggs’,
provide a major breeding ground for mycophagous
flies. Eggs of P. impudicus are about 5 cm in
diameter and develop from an extensive system of
white mycelial cords which can be traced underground
to a buried tree stump (see Fig. 1.12b;
Grainger, 1962). A longitudinal section of an egg
(Fig. 20.9a) shows a thin papery outer and inner
peridium and a wider mass of jelly making up the
middle peridium. The central part of the gasterocarp
is differentiated into a cylindrical hollow
stipe and a folded honeycomb-like receptacle
which bears the fertile part of the gleba. Within
the young gleba are cavities lined by basidia
bearing up to nine spores (Fig. 20.9b), but as the
glebal mass ripens the basidia disintegrate.
Gasterocarps expand very rapidly: within a few
hours the stipe may elongate from about 5 cm to a
length of 15 cm or more, leaving behind the
peridial remains as a volva at its base.
A demonstration of gasterocarp erection in the
laboratory by incubation of freshly collected ripe
eggs in a moist chamber rarely fails to impress.
The sudden expansion is probably at the expense
of water stored within the jelly of the middle
peridium. The mean weight of expanded stipes is
more than twice that of unexpanded ones (Ingold,
1959). Expansion of the stipe of P. impudicus is
accompanied by breakdown of glycogen and its
conversion to sugar (Buller, 1933). A similar
conversion has been reported in P. indusiatus
(Fig. 20.7c) in which cells of the unexpanded
stipe are folded but expand to almost 12 times
their original volume during stipe elongation
(Kinugawa, 1965).
At about the same time as the stipe of
P. impudicus is elongating, the fertile glebal mass
begins to release strong-smelling volatile
substances which are attractive to flies, especially
bluebottles (Plate 11d). Depending on the analytical
methods used, the smell has been attributed
to a range of substances including methylmercaptan
and hydrogen sulphide (List & Freund,
1968) or dimethyl disulphide (see Fig. 15.4) and
dimethyl trisulphide (Borg-Karlson et al., 1994).
Once a fly has located a gasterocarp, it is
presented with the dark green glebal
mass of basidiospores embedded in a liquid
which contains sugars and also sweet-smelling
substances such as phenylacetaldehyde and
phenylethanol. Flies feed on the spore mass
which is removed within a few hours, leaving
behind the pale receptacle. The ingested basidiospores
are defaecated, apparently unharmed,
onto surrounding vegetation and the soil, often
within a short time of ingestion. Tuno (1998)
found that the gut of fruitflies (Drosophila spp.)
feeding on P. indusiatus and P. duplicatus contained
up to 240 000 basidiospores, and that of the larger
muscid flies up to 1.7 million. Basidiospore
germination was unaffected by passage through
the gut. However, it is unknown how the
mycelium from germinating basidiospores
succeeds in reaching fresh tree stumps, and
clonal spread by mycelial cords may be an
important additional mode of reproduction.
There have been few studies of the nutrition
and physiology of Phallus, but P. ravenelii has
been shown to make good vegetative growth on
a wide range of carbohydrates, and to require
thiamine (Howard & Bigelow, 1969). The veiled
stinkhorns P. indusiatus (Fig. 20.7c) and P. duplicatus
are grown in China as a culinary
speciality. Expanded fruit bodies are produced
commercially from inoculated wood both in
woodlands and indoors, and are marketed in a