Introduction to Fungi, Third Edition

26 INTRODUCTION
genera, e.g. Hypocrea (Fig. 12.15) or Cordyceps
(Fig. 12.33), the multicellular ascospores may
break up into part-spores within the ascus prior
to discharge. The ascospore wall may be thin or
thick, hyaline or coloured, smooth or rough,
sometimes cast into reticulate folds or ornamented
by ridges, and it may have a mucilaginous
outer layer which is sometimes extended to form
simple or branched appendages, especially in
marine ascomycetes where they aid buoyancy
and attachment. In many cases, ascospores are
resting structures which survive adverse conditions.
They may have extensive food reserves in
the form of lipids and sugars such as trehalose.
Because the formation of ascospores involves
meiosis, they are important not only as a means
of dispersal and survival but also in genetic
recombination.
It is obvious that there is no such thing as
a typical ascospore. Neurospora tetrasperma will
serve as an example of an ascospore whose
structure has been extensively studied (Lowry &
Sussman, 1958, 1968). This fungus is somewhat
unusual in that it has four-spored asci and the
ascospores are binucleate. The spores are black,
thick-walled and shaped rather like a rugby
football, but with flattened ends. The name
Neurospora refers to the ribbed spores, because
the dark outer wall is made up of longitudinal
raised ribs, separated by interrupted grooves. The
structure of a spore in section is shown in
Fig. 1.19. Within the cytoplasm of the spore
are the two nuclei, fragments of endoplasmic
reticulum (not illustrated), swollen mitochondria
and vacuoles, bounded by single unit membranes.
The wall surrounding the protoplast is
composed of several layers. The innermost layer
is the endospore, outside of which is the
epispore. The ribbed layer is termed the perispore.
Between the ribs are lighter intercostal
veins containing a material which is chemically
distinct from the ribs. This material is continuous
over the whole surface of the spore, giving
it a relatively smooth surface. The spore germinates
by the extrusion of germ tubes from a preexisting
germ pore, a thin area in the epispore at
either end of the spore. In many ascomycetes a
trigger is required for germination, e.g. heat
shock in Neurospora or a chemical stimulus, for
example in ascomycetes which grow and fruit on
the dung of herbivorous mammals and whose
spores are subjected to digestive treatment.
1.4.4 Basidiospores
Basidiospores are the sexual spores which
characterize a large group of fungi, the
Basidiomycota or basidiomycetes. In comparison
with the morphological diversity of ascospores,
basidiospores are more uniform. They also show
a smaller size range, from about 3 to 20 mm,
which is possibly related to their unique method
of discharge. They are normally found in
groups of four attached by tapering sterigmata
to the cell which bears them, the basidium. At
the time of their discharge all basidiospores
Fig1.19 Neurospora tetrasperma.T.S.
ascospore. Simplified diagram based on
an electron micrograph by Lowry in
Sussman and Halvorson (1966).