Introduction to Fungi, Third Edition

ASCOBOLUS (ASCOBOLACEAE)
421
Fig14.5 Ascobolus immersus.(a)Apothecium
showing two projecting asci. Immature asci can
be seen below the general level of the surface.
A single projectile consisting of eight adhering
ascospores is shown above the apothecium.
Note the operculum which has also been
projected. (b) Tip of ripe ascus showing the
operculum. (c) Tip of discharged ascus. In this
case the operculum has remained attached to
the ascus tip.
and a, and fertilization can only occur between
an A ascogonium and an a antheridium, or
vice versa. There is thus a gene for mating
type represented in two idiomorphs A and a,
and incompatibility is controlled by this gene
irrespective of the presence of both types of
sex organ on each strain. There is no morphological
difference between the two different
mating type strains.
A similar situation occurs in A. furfuraceus,
but here there are no antheridia. Instead, each
strain at first produces chains of arthrospores
or oidia (see Fig. 14.6c). The oidia can germinate
to form a fresh mycelium, i.e. they can function
asexually as conidia, but they also play a part
in sexual reproduction. Mites and flies may
transport oidia of one strain to the mycelium
of the opposite strain, and following this,
apothecia develop. The process of fertilization
has been studied by Bistis (1956, 1957) and Bistis
and Raper (1963). If an A oidium is transferred
to an a mycelium, the oidium fails to germinate
and within 10 h an ascogonial primordium
appears on the a mycelium (Fig. 14.6d). The
ascogonium consists of a broad coiled base and
a narrow apical trichogyne which shows chemotropic
growth towards the oidium and eventually
fuses with it. There is evidence that this sequence
of events is under hormonal control, and it
has been suggested that a fresh A oidium is not
immediately capable of inducing development
of ascogonial primordia, but must itself at first
be sexually activated by a messenger secreted
by the a mycelium. Following activation, the
oidium can induce ascogonial development.
By substitution experiments, it has been shown
that an A ascogonium can be induced to fuse
with an A oidium, i.e. an oidium of the same
mating type, but apothecia fail to develop
from such fusions. In compatible crosses, fertile
apothecia develop within about 10 days of fertilization,
each ascus producing four A and four
a spores.
In A. immersus there are no morphologically
distinguishable antheridia and there are
no oidia. When A and a mating type mycelia
are grown together in culture, multinucleate
ascogonia develop which are fertilized by fusion
with slender multinucleate hyphae of the
opposite strain. Experimentally, fertilization
can also be achieved using homogenized mycelial
fragments of one strain to spermatize a
strain of opposite mating type (Lewis & Decaris,
1974).