Introduction to Fungi, Third Edition

DEVELOPMENT OF ASCI
243
Fig 8.14 Ascospore puffing in
Aleuria aurantia. Athickwhitecloud
of ascospores has been released by
a cluster of apothecia. Reprinted
from Fuhrer (2005), with
permission by Bloomings Books
Pty Ltd.Original image kindly
provided by B. Fuhrer.
general surface of the hymenium. Their tips may
be phototropic, as in the coprophilous fungus
Ascobolus, and this ensures that the ascospores
are directed upwards, towards the light. In some
discomycetes, especially those with operculate
asci (e.g. Ascobolus, Peziza), large numbers of
ripe asci may discharge their spores simultaneously,
a phenomenon known as puffing
(Fig. 8.14). This may also occur, but less obviously,
in forms with inoperculate asci, e.g. Sclerotinia
and Rhytisma. Puffing results in a cloud of ascospores
being discharged for greater distances than
with spores discharged from a single ascus
(Buller, 1934; Ingold, 1971).
In the flask fungi (pyrenomycetes), such as
Sordaria or Podospora, as an ascus ripens it
elongates and takes up a position inside the
ostiole, often gripped in position by a lining layer
of hairs, periphyses (Gr. prefix peri ¼ near,
around, roundabout). In this case the asci
discharge their spores in turn. The necks of the
perithecia in Sordaria and Podospora are phototropic
so that the ascospores are shot towards the
light. A variant of this method of discharge is
found in fungi whose perithecia have necks very
much longer than the length of the asci, such
as Ceratostomella and Gnomonia. Here, before
discharge, the asci break at their bases and
detached asci move up a canal inside the
perithecial neck and are held in place within
the ostiole by periphyses as they discharge their
spores (Ingold, 1971).
The behaviour of the bitunicate type of ascus
during discharge has been described as the
Jack-in-the-box mechanism (Ingold, 1971). The
outer wall is relatively rigid and inextensible.
As the ascus expands, the outer wall ruptures
laterally or apically (see Figs. 8.12h,i) and the
inner wall then stretches before the ascus
explodes. Ascus discharge is thus a two-stage
process. This type of mechanism is found in
Loculascomycetes such as Sporormiella (Fig. 17.18),
Leptosphaeria (Fig. 17.3) and Pleospora (Figs. 17.1,
17.9).
In Cochliobolus the ascus is bitunicate but the
endotunica is incomplete at its base, i.e. vestigially
or partially bitunicate. In C. cymbopogonis the
eight ascospores are spirally coiled around each
other in the ascus and each has a recurved tip
(Figs. 8.15a,b). The ectoascus bursts open near
its tip and the sheaf of ascospores is expelled,
the incomplete endoascus forming a thimblelike
cap over the tips of the ascospores as they
pass through the long pseudothecial neck. The
spores are not explosively discharged but
are extruded, en masse, from the neck of the
pseudothecium in a long tendril from which
they are dispersed by rain splash. In water the
spores separate from each other and push away
the endotunica which earlier capped their tips
together (see Fig. 8.15d; El-Shafie & Webster,
1980; Alcorn, 1981).
It is likely that in most cases the spores are
spatially separated from each other as they are