Introduction to Fungi, Third Edition

424 HYMENOASCOMYCETES: PEZIZALES (OPERCULATE DISCOMYCETES)
molecular phylogenetic data (Spatafora, 1995;
Percudani et al., 1999; Hansen et al., 2001).
Although Tuber and other ascomycetes were formerly
classified in a separate order (Tuberales),
they are now placed in the Pezizales (Trappe,
1979) as a separate family, the Tuberaceae.
This family is closely related to the Helvellaceae
(see above).
14.7.1 The truffle ascocarp
The ascocarp is generally globose, varying in size
from about 1 to 8 cm in diameter and, exceptionally,
may weigh up to 1000 g. It is differentiated
into an outer, usually dark peridium
which in some species, e.g. T. melanosporum or
T. aestivum, may bear pyramidal scales, and an
inner, fertile gleba. The appearance of the gleba
is marbled because it is traversed by light- and
dark-coloured veins (Fig. 14.7a). The lightcoloured
veins are sterile, consisting of a loose
network of hyphae and air, whilst the darker
veins are fertile, made up of more closely packed
hyphae, paraphyses and asci (Parguey-Leduc
et al., 1991; Barry et al., 1995; Callot, 1999;
Janex-Favre & Parguey-Leduc, 2002).
The asci are unitunicate, subglobose and
contain 2 6 ascospores. They do not discharge
their spores violently, and lack a specialized
apical apparatus or operculum. The ascospores
are at first hyaline, but later develop yellow to
dark brown (melanized) thick walls which may
be spiny or thrown into reticulate, honeycomblike
folds (Figs. 14.7b, 14.8). Many of the fruit
bodies have a strong smell and flavour, and are
excavated and eaten by animals such as badgers,
wild boar, mice, moles, shrews, squirrels and
rabbits (Trappe & Maser, 1977). Hypogeous
ascocarps (and basidiocarps) form an important
component of their diet. Spore dispersal is
brought about in this way and ascospore germination
is probably enhanced by passage through
the gut of the mammal. Several different volatile
chemical substances have been detected from
truffle fruit bodies, but the most common
and abundant is dimethyl sulphide, attractive
to ‘truffle flies’ (Suillia spp.) which lay their eggs
on the ascocarps (Pacioni et al., 1990, 1991).
Similar substances are emitted by stinkhorns
(Phallus spp.), which likewise attract flies (see
p. 590). Claus et al. (1981) have shown that the
steroid hormone 5a-androst-16-en-3a-ol is also
produced by Tuber spp. Since this is the main
sex hormone produced by boars, its presence in
truffles may account for the enthusiasm and
efficiency with which sows locate and excavate
truffles. It is possible that some of the odoriferous
substances are produced by the activity of
microbes associated with ascocarp.
14.7.2 The life cycle of true truffles
Surprise discoveries may happen even in
seemingly well-studied life cycles such as those
of Tuber spp., in which a sympodulosporic conidial
state somewhat resembling Geniculosporium
(see Fig. 12.13) has been described recently (Urban
et al., 2004). It is as yet unclear how frequent
this state is in nature or among other Tuber
spp. and which role, if any, it might play in their
ecology.
The traditional life cycle of Tuber is based
solely on sexual reproduction (see Giovannetti
et al., 1994). The haploid ascospores germinate
to form hyphae with monokaryotic segments,
and the mycelium grows towards the roots of
potential mycorrhizal partners, usually trees,
but is unable to form mycorrhiza. Anastomosis
between monokaryotic mycelia derived from
different ascospores results in the formation
of a dikaryotic mycelium which forms sheathing
mycorrhiza with suitable hosts (Fasolo-
Bonfante & Brunel, 1972). Ascocarp development
is initiated by the aggregation and differentiation
of hyphae which at this stage remain
attached to long roots and obtain nutrients
from the host tree. According to Janex-Favre
and Parguey-Leduc (2002), in T. melanosporum
the primordium of the ascocarp consists of an
ascogonium with its trichogyne, surrounded at
the base by an envelope of sterile investing
hyphae. Within the glebal tissues to the inside
of the primordium, fertile cells develop. At the
tips of these fertile cells, the two nuclei of the
dikaryon fuse to give a diploid nucleus. This
is followed by meiosis and one or more mitoses
so that the ascospores may be uni- or multinucleate
(Delmas, 1978). Ultrastructural studies