Introduction to Fungi, Third Edition

TUBER (TUBERACEAE)
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of neighbouring ascospores, the perisporic sac
may be lined by vesicles. Secondary wall formation
results from deposition of material derived
from the epiplasm onto the epispore. Dense,
vesicle-bound bodies originally present in the
epiplasm are responsible for the purple coloration
of the secondary spore wall. The pigmented
secondary wall layer is not uniformly thick and
the hyaline striations in the spore wall represent
crevices in the material deposited.
14.5.4 Studies on gene recombination
Ascobolus immersus has proved a useful tool
for interpreting the mechanism of gene
recombination through crossing-over during
meiosis using ascospore colour as a marker.
Recombination is detected by the simple technique
of scoring the ratio of the spores of different
colour in octads of spores shot away from hybrid
apothecia. Although the wild-type strains have
purple ascospores, several series of mutants
with pale spores have been found. When crosses
are made using certain non-allelic spore colour
mutants, recombinants develop resulting from
two types of event: (1) crossing-over, giving
reciprocal recombinants and (2) gene conversion,
yielding non-reciprocal recombinants corresponding
to only one of the four products
of meiosis. Gene conversion is a process where
one allele of a gene converts another allele at the
same locus to its own type. Crossing-over results
in a 4 : 4 ratio of coloured to colourless spores
whilst a conversion is detected by the presence
of coloured and colourless spores in different
ratios e.g. 7 : 1, 6 : 2 or 5 : 3. Conversions have
been interpreted as implying a double-strand
replication of one part of a chromatid whilst
the corresponding part of the other is not
replicated (Lamb, 1996).
14.6 Helvella (Helvellaceae)
Helvella, a genus containing about 40 species, is
mainly distributed in northern temperate areas
(Dissing, 1986; Abbott & Currah, 1997). Helvella
spp. are known as saddle fungi because the
ascocarp is differentiated into a hollow, nonfertile
stipe and a curved, saddle-shaped fertile
part folded over it. They are also called false
morels. Common species are H. crispa (Plate 6f)
and H. lacunosa. Some species form ectomycorrhiza,
and e.g. H. crispa is a mycorrhizal
partner with beech, Fagus sylvatica. A characteristic
feature of the Helvellaceae is that their
ascospores are quadrinucleate. This feature,
also found in certain truffle-like fungi with
hypogeous ascocarps, has been used as evidence
placing genera such as Hydnotrya, species of
which also form sheathing mycorrhizae, into
the Pezizales instead of the Tuberales where
they were previously classified (Trappe, 1979).
More recent molecular phylogenetic studies
have confirmed a close relationship between
the Helvellaceae and the Tuberaceae, the main
truffle-containing family (Percudani et al., 1999).
14.7 Tuber (Tuberaceae)
About 100 species of Tuber are known. They
are called the ‘true truffles’, ascomycetes which
form subterranean fruit bodies in which the
hymenium is not open to the exterior. It has
been suggested that hypogeous fruiting is an
adaptation for surviving drought and that the
genera of fungi which have adopted this strategy
(including ascomycetes and basidiomycetes)
have evolved from ancestors with epigeous fruit
bodies. According to this view the subterranean
ascocarps of Tuber and other ascomycetous
truffle genera are modified apothecia. The term
stereothecium, defined as a more or less solid
fleshy ascoma with asci which are either solitary,
i.e. scattered relatively evenly throughout
the medullary excipulum, or grouped in dispersed
pockets, has been proposed for this
type of ascoma (O’Donnell et al., 1997; Hansen
et al., 2001). Supporting evidence that stereothecia
are modified apothecia is that, during
their ontogeny, ascocarps are at first open to
the exterior and close over later as the hymenium
develops (Barry et al., 1993; Callot, 1999;
Janex-Favre & Parguey-Leduc, 2002). This morphological
evidence has been corroborated by