Introduction to Fungi, Third Edition

318 HYMENOASCOMYCETES: PYRENOMYCETES
ascus is only about 4 mm, the latter acts as a
sphincter, gripping the spores as they leave the
ascus. Projectiles which vary in size from one to
eight spores may be formed. The larger the
number of spores in the projectile, the greater
the distance of discharge due to the fact that the
surface-to-volume ratio of single spores is greater
than that of multiple-spored projectiles, so that
the effects of wind resistance are disproportionately
high (Ingold & Hadland, 1959). Thus,
single-spored projectiles have a mean discharge
distance of about 1.5 cm whilst for eight-spored
projectiles the distance is about 6 cm. The necks
of the perithecia are phototropic and, as in
many other coprophilous fungi, this adaptation
ensures that the spores are projected away
from the dung substratum. The ascospores of
S. fimicola have a distinct mucilage envelope
which enables them to adhere to herbage, but in
S. humana the sheath is much reduced or absent.
The ascospores of S. fimicola can survive for long
periods. On drying, gas vacuoles (de Bary bubbles)
may appear in the spore cytoplasm, but despite
this the spores remain viable, and the bubbles
disappear upon rehydration (Ingold, 1956;
Milburn, 1970). Ascospore germination is
enhanced by digestive treatment in the herbivore
gut and this effect can be simulated in the
laboratory by treatment with pancreatin or
sodium acetate.
Perithecium development
There have been numerous studies on perithecial
development and structure in Sordaria, e.g. in
S. fimicola (Mai, 1977), S. humana (Uecker, 1976;
Read & Beckett, 1985) and S. macrospora (Hock
et al., 1978). Intercalary multinucleate cells of
vegetative hyphae give rise to multinucleate,
spirally coiled, septate ascogonia which are not
associated with antheridia. In these species,
there is no trichogyne and there are no microconidia
(spermatia). The ascogonium is enveloped
by branched investing hyphae which
originate from the ascogonial stalk or from
adjacent vegetative hyphae to form a spherical
protoperithecium, i.e. an immature ascoma
which does not, at this stage, show differentiation
into a neck or ostiole. The outer region of
the protoperithecium is made up of about five
layers of rounded cells which have thick,
pigmented (melanized) walls. Lying inside them
are several layers of flattened, thinner, nonpigmented
cells (see Fig. 12.1). Differentiation of
the innermost cells of the protoperithecium
gives rise to the centrum consisting of elongate,
free-ended, thin-walled, septate paraphyses and
ascogenous cells. As the ascoma matures, it
changes in outline from spherical to pearshaped
with an elongate neck, thus developing
into a perithecium. Neck development is associated
with meristematic activity of the cells at
the base of the neck and by the appearance of
short, tapering periphyses which line it. By
pushing against each other the periphyses
create the ostiole, the opening to the outside
through which the asci will discharge their
spores. Thus, the cells are pushed apart rather
than lysed, and this process of ostiole development
is called schizogenous. Phototropism of the
perithecial neck is associated with differential
enlargement of the periphyses.
The ascogenous hyphae arise on a placentalike
mound at the base of the centrum and
elongate upwards. They show the usual type of
crozier found in many ascomycetes with a
binucleate penultimate cell (the mother cell of
an ascus), a terminal cell and a stalk cell (see
Fig. 8.10). Proliferation of the ascogenous hypha
occurs by fusion of the recurved terminal cell
with the stalk cell. Nuclear fusion in the ascus
mother cell is followed by meiosis, yielding four
haploid nuclei. Two further mitotic divisions
produce 16 nuclei. Cleavage of the cytoplasm
accompanied by wall formation results in eight
binucleate ascospores. The fine structure of
ascospore development has been studied by
Mainwaring (1972).
Perithecium development is affected by environmental
factors such as temperature and
nutrient supply. For S. macrospora, Hock et al.
(1978) have shown that in pure culture on a
defined nutrient medium, perithecium development
requires a simultaneous supply of biotin
and arginine. In the presence of certain other
fungi such as Armillaria spp. and Mortierella spp.,
S. fimicola is stimulated to increased production
of perithecia and ascospores and it is likely that
this effect is related to the production of