Introduction to Fungi, Third Edition

320 HYMENOASCOMYCETES: PYRENOMYCETES
Sordaria brevicollis and S. heterothallis are
heterothallic. Both species form minute spermatia
which are involved in the fertilization of
mycelia of the opposite mating type. Sordaria
brevicollis is a heterothallic relative of S. fimicola
according to Guarro and von Arx (1987). In
S. brevicollis it has been discovered that perithecium
development can occur in unmated
cultures of mating type A. In about 30% of such
perithecia one or two asci with viable ascospores
may form, with all spores being mating type A
(Robertson et al., 1998).
The molecular basis of homothallism has
been elucidated in S. macrospora by Pöggeler
et al. (1997). The molecular configuration of the
genes conferring the ability to mate is similar
to that in two heterothallic members of the
Sordariaceae, Podospora anserina and Neurospora
crassa. In Neurospora crassa (see Fig. 12.7) the
haploid mating types are designated A and a. The
‘alleles’ which confer mating competence in
P. anserina and N. crassa consist of dissimilar
DNA sequences termed idiomorphs which are
present at the homologous loci in the mating
partners. This term has been introduced to
denote sequences like those of mating types A
and a, which occupy the same locus in different
strains but are related neither in sequence nor
(probably) by common descent (Metzenberg &
Glass, 1990). In S. macrospora, the two idiomorphs
are contiguous (i.e. adjoin each other), and they
have been used in experiments to transform (þ)
and ( ) strains of P. anserina in order to induce
them to form perithecia.
12.2.2 Podospora and Schizothecium
(Sordariaceae)
The perithecia of Podospora and Schizothecium
develop on herbivore dung. In Podospora, the
upper part of the perithecium wall is often
ornamented by various kinds of ‘vestiture’
(Lundqvist, 1972) such as short or long single
hairs or long, pointed setae which may be
aggregated into a tuft to one side of the
perithecial neck as seen in P. anserina and
P. curvicolla. In Schizothecium, the hairs are
composed of swollen cells and agglutinate
together to form short scale-like tufts (Bell &
Mahoney, 1995). The separation between these
two genera has been confirmed by phylogenetic
analyses (Cai et al., 2005). Taking Schizothecium
and Podospora together, about 80 species are
known (Mirza & Cain, 1969; Lundqvist, 1972).
Different species show a degree of substrate
specificity. For instance, perithecia of P. curvicolla,
P. pleiospora and S. vesticola are especially common
on the dung of lagomorphs (rabbits and hares)
whilst P. curvula fruits commonly on horse dung
(Lundqvist, 1972; M. Richardson, 1972, 2001). The
reasons for these preferences are not known.
Some species have semi-transparent perithecia
within which the outline of the club-shaped asci
can be seen and the sequential development
and discharge of individual asci can be followed
(Fig. 12.3a). The number of spores in the ascus
varies from 4 to 512. Spore number has been
used as a taxonomic criterion in the past,
although the species concept has been widened
to include forms with a range of spore numbers.
For example, 8-, 16-, 32- and 64-spored forms of
Podospora decipiens are recognized. The name
Podospora (Gr. podos ¼ foot, spora ¼ seed) refers
to the mucilaginous appendage attached to one
or both ends of the black ascospore (Fig. 12.3b).
This character is also found in Schizothecium. In
some of the commonest species, P. curvula and
S. tetrasporum, the upper spore appendages are
attached to the cap of the ascus, and when the
ascus explodes, the spores, roped together by
their appendages, are propelled as a single slingshot
projectile (Fig. 12.3c). As in Sordaria, it has
been shown that multi-spored projectiles are
discharged further than single spores (Walkey &
Harvey, 1966a). The ascus wall breaks across, just
beneath the cap, and in contrast to Sordaria there
is usually no distinctive apical apparatus.
Developmental aspects
Many morphological and experimental studies
have been made on P. anserina, which Lundqvist
(1972) treated as a synonym of P. pauciseta. This
fungus was originally described from goose dung
but also fruits on the dung of sheep, horse,
cattle, mice, grouse and zoo animals.
Perithecium development has been described
for P. anserina (Beckett & Wilson, 1968; Mai,
1976) and Schizothecium spp. (Bell & Mahoney,