Introduction to Fungi, Third Edition

DEVELOPMENT OF ASCI
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appendages (see Figs. 12.3, 12.4; Beckett et al.,
1968). The appendages of adjacent spores intertwine
so that the spores are discharged strung
together in the manner of a slingshot (Ingold,
1971). In some aquatic ascomycetes the ascospores
have extensions of the spore wall which aid
in attachment. Pleospora scirpicola, which forms
ascocarps on the submerged parts of culms
of Schoenoplectus lacustris, an inhabitant of the
shoreline of freshwater lakes, canals and slowmoving
rivers, has long, mucilaginous, tapering
extensions from each end of the ascospore
(Fig. 17.1d). Appendaged ascospores are especially
common in marine ascomycetes. The appendages
develop in a variety of ways and unfurl in sea
water, slowing down their rate of sedimentation
and increasing the likelihood of their attachment
to underwater substrata such as wood
(Hyde & Jones, 1989; Hyde et al., 1989; Jones,
1994).
Germ pores or germ slits, through which
germ tubes emerge on spore germination, are
found in many ascomycetes, especially those
with thick dark-pigmented walls. Germ pores,
representing thin areas in the spore wall, occur
at each end of the spore in Neurospora and
germination may occur at either or at both
ends. In Sordaria humana there is a single germ
pore at the lower end of the ascospore plugged
by a pore plug (Read & Beckett, 1996). The
ascospores of Xylariaceae, e.g. Xylaria, Hypoxylon
and Daldinia, have black walls with a hyaline
germ slit running along the length of the spore
(Figs. 12.10 and 12.14).
Because the division which follows the fournucleate
stage is mitotic and because the division
plane is usually parallel to the length of the
ascus, adjacent pairs of spores starting from the
tip of an ascus are normally sister spores and
are thus genetically identical. Rare exceptions to
this situation are occasionally found where the
division planes are oblique, or for other reasons
(see Raju, 1992a).
8.6.2 The ascus wall
The wall of the ascus consists of several
distinguishable layers. The outer layer is laid
down first and inside it is a succession of laterformed
layers so that the mature wall may
consist of four or more layers (Bellemère, 1994;
Read & Beckett, 1996). The wall material
includes chitin, polysaccharides and proteins,
but there is no evidence of lipid. The ascus
wall is elastic. All or parts of it may stretch
considerably during ascospore liberation, and
contraction of the elastic wall provides the force
for ascospore discharge. During discharge all
the layers of the ascus wall may remain
attached to each other, thus appearing as a
single layer. Such asci are termed unitunicate
(Lat. tunica ¼ a garment). Despite the term
unitunicate which refers to the behaviour
(i.e. function) of the ascus wall during ascus
dehiscence, the wall of unitunicate asci is often
composed of two superposed tunicae, a thin,
single-layered or double-layered exoascus and
a thicker endoascus. The endoascus may be
fibrillar, or at first granular and then with
parallel or reticulate fibrils (Parguey-Leduc
& Janex-Favre, 1984). During ascospore discharge
the two layers of the ascus wall
remain attached, i.e. they do not glide over
each other.
A variant of the unitunicate type of ascus
dehiscence is found in the lichenized ascomycetes
Lecanora and Physcia (the Lecanora or rostrate
type of dehiscence). In Physcia stellaris the ascus
has a prominent amyloid dome. The ripening
ascospores push against this dome and on
ascospore discharge it is extruded to form
a rostrum (Lat. rostrum ¼ beak) which extends
upwards to the surface, whilst its base remains
attached to the upper part of the wall of the
ascus (see Figs. 8.12e,f; Honegger, 1978).
In other ascomycetes, the ascus wall appears
distinctly two-layered (bitunicate) when viewed
with the light microscope (Luttrell, 1951;
Reynolds, 1971, 1989). The layers of many (but
not all) bitunicate asci separate at ascospore
discharge into two functionally distinct layers
(see Fig. 8.12), and such asci are termed fissitunicate
(Dughi, 1956). Fissitunicate asci are particularly
common in the Loculoascomycetes.
Development of the wall of a bitunicate ascus
takes place in two stages prior to ascospore
formation. The first stage involves the growth
of the ascus initial and the expansion of the