Introduction to Fungi, Third Edition

DEVELOPMENT OF ASCI
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may converge and appear as a kind of apical
basket which has been termed by Chadefaud
(1942) the nasse apicale (Fr. nasse ¼ keep net,
eel trap).
In some asci the ascus wall does not function
in ascospore discharge, but dissolves or
disintegrates at maturity, the spores being
released passively. Such asci are termed prototunicate
(see Fig. 8.12a). This type of ascus is
characteristic of certain groups of ascomycetes
such as the Eurotiales and Onygenales but they
are also found in unrelated groups (Currah, 1994).
Examples are Eurotium (Fig. 11.16) and Gymnoascus
(Fig. 11.9).
8.6.3 The apical apparatus of asci
The apical dome of the ascus may be modified in
various ways. In certain types of discomycete
with an open saucer-like fruit body or apothecium,
the ascus is capped by a wall which has
an annulus of thinner wall material forming a
lid or operculum (Lat. operculum ¼ a cover, lid)
(van Brummelen, 1981). When the ascus explodes
to discharge its ascospores, the operculum may
be lifted off completely or may hinge to one side
(see Figs. 14.5, 14.6). Such asci are operculate
(Figs. 8.12b,c). However, the majority of ascomycetes
have no ascus lid; they are inoperculate
and when ascospore discharge occurs, the tip
of the ascus opens by a pore (Fig. 8.12d).
The presence or absence of an operculum is
a character used in the classification of
discomycetes. Operculate asci are characteristic
of Pezizales including genera such as Aleuria,
Ascobolus and Pyronema (see Chapter 14).
Inoperculate discomycetes include Helotiales
such as Sclerotinia (Figs. 15.1, 15.2) and many
other orders. In a few cases, e.g the lichenized
ascomycete Pertusaria and the coprophilous
fungus Ascozonus, the ascus may burst by one
or two longitudinal slits at the apex (see
Fig. 8.12g). Such asci are described as bilabiate
(i.e. two-lipped).
Other kinds of specialized structures found in
ascus tips are generally referred to as the apical
apparatus. Their functions relate to the mechanism
of discharge (see below). In many perithecial
fungi the tip of the ascus contains an apical
ring or annulus. This is a specially thickened
inward extension of the apical wall of the ascus,
arranged in the form of a cylindrical flange
(Fig. 8.13). In some fungi (e.g. Xylaria) the annulus
is amyloid, i.e. it stains blue with Melzer’s iodine,
an aqueous solution of I 2 in KI (Beckett &
Crawford, 1973). In other ascomycetes, reddishbrown
(dextrinoid) staining may be observed,
whereas in yet others (e.g. Sordaria) the annulus
does not stain with iodine (Read & Beckett, 1996).
When ascospores are discharged the annulus
is everted, i.e. turned inside out like a sleeve
(see Fig. 8.13b). The cylindrical opening of the
annulus is considerably less than the diameter of
the ascospores which pass through it as shown
in Fig. 8.13a for Xylaria longipes and Fig. 12.1 for
Sordaria fimicola, so that the annulus must be
sufficiently elastic to expand and contract as an
ascospore passes through it. The function of the
annulus is to act as a sphincter, minimizing the
decrease in hydrostatic pressure inside the ascus
as spore discharge proceeds. It may also separate
the spores from each other as they pass through,
and by gripping the tapering rear portion of an
ascospore impart some force which helps to
expel it (Ingold, 1954a). Filiform (i.e. needleshaped)
ascospores are discharged singly and
not in groups. This is well shown in the ergot
fungus Claviceps purpurea and its allies such as
Cordyceps (Figs. 12.27, 12.33). Their ascus
apices are capped by a swollen plug of wall
material pierced by a narrow pore. Ascospores
are squeezed out through the pore, sometimes
with an interval of several seconds between
successive discharge events (Ingold, 1971).
Other elaborations of the upper part of the
ascus have been reported (Beckett, 1981;
Bellemère, 1994).
The actual form of the mature ascus is very
variable. In prototunicate forms, i.e. with nonexplosive
ascospore release, the ascus is often
a globose sac, but in the majority of ascomycetes
the ascus is cylindrical, and the spores are
expelled from the ascus explosively.
8.6.4 Hamathecium
In many cases the asci are surrounded by
packing tissue in the form of paraphyses