Introduction to Fungi, Third Edition

420 HYMENOASCOMYCETES: PEZIZALES (OPERCULATE DISCOMYCETES)
Fig14.4 Oedocephalum conidial state of Peziza subviolacea
(¼ P. praetervisa). (a) Conidiophores terminating in a
club-shaped vesicle bearing numerous dry blastoconidia.
(b) Details of developing conidia (top) and a vesicle from which
the conidia have been detached (bottom). (c) Conidia, two of
which are germinating. Scale bar ¼ 40mm (a,b) and 20 mm(c).
From Webster et al. (1964), with permission from Elsevier.
coprophilous, growing on the dung of herbivorous
animals, but A. carbonarius grows on old
bonfire sites. Common coprophilous species
are A. furfuraceus (¼ A. stercorarius) which is very
commonly found on old cattle dung, often
along with A. immersus (Figs. 14.5 and 14.6,
respectively). Whilst these species are heterothallic,
some others, e.g. A. crenulatus (¼ A. viridulus),
are homothallic. Characteristic features of all
species are the purple colour of the ascospores
and the protruding, operculate asci. Ascobolus
furfuraceus forms yellowish saucer-shaped apothecia
up to 5 mm in diameter, and when mature
the surface of the apothecium is studded with
purple dots which mark the ripe asci (Plate 6d).
As the asci mature they elongate above the
general level of the hymenium. The ascus tips
are phototropic and this ensures that when they
explode the spores are thrown away from
the dung. The ascospores have a mucilaginous
perispore which aids attachment. Ascobolus
immersus has yellow globose apothecia about
1 2 mm in diameter, with very large ascospores
(about 70 30 mm). The perispores cause all
the eight ascospores to adhere to form a single
projectile about 250 mm long, capable of being
discharged for up to 30 cm horizontally. In
general, multi-spored projectiles have a lower
surface-to-volume ratio and are projected further
than single spores (see p. 317 for Sordaria).
There is a general trend among coprophilous
fungi towards multi-spored projectiles. In the
genus Saccobolus, which also belongs to the
Ascobolaceae, all eight spores are firmly cemented
together by their perispores.
The spores of Ascobolus become attached to
herbage and, when eaten by a herbivore, germinate
in the faeces. It is likely that digestion
stimulates spore germination. Most spores fail
to germinate on nutrient media but can be
triggered to do so by treatment with 0.4% NaOH
or bile salts, and incubation at 37°C. The purple
pigment in the spore wall develops late and is
deposited within the perispore from the ascus
epiplasm. Immature spores are colourless. The
spore wall bears longitudinal colourless striations
in some species, e.g. A. immersus (Fig. 14.5)
and A. furfuraceus (Fig. 14.6). Both species can be
grown and induced to form apothecia in culture
(see Webster & Weber, 2001).
14.5.1 Mating behaviour
There is variation in the mating behaviour of
different species of Ascobolus. A single ascospore
culture of A. scatigenus (¼ A. magnificus) does not
produce apothecia. Sex organs (coiled ascogonia
and antheridia) are formed only when mycelia
of different mating types are grown together.
Each strain is hermaphroditic, i.e. is capable
of developing both ascogonia and antheridia.
However, A. scatigenus is self-incompatible,
i.e. the antheridia of one strain do not fertilize
the ascogonia borne on the same mycelium.
The ascospores of this fungus are of two types, A