Introduction to Fungi, Third Edition

DEVELOPMENT OF ASCI
237
These nuclei then undergo a mitotic division
so that eight haploid nuclei result (Fig. 8.10f).
The eight nuclei may divide further mitotically
so that each ascospore is binucleate, or,
if still more mitoses follow, the ascospore
becomes multinucleate. For example, a single
mitosis occurs in the immature ascospores of
Neurospora crassa, and the spores remain binucleate
for 2 3 days after they have been delimited.
Later, a series of four or more synchronous
mitoses occur after the spores have become
pigmented so that they contain 32 or more
nuclei when they are mature (Raju, 1992a).
Where the ascospores are multicellular, there
are repeated nuclear divisions accompanied by
the formation of septa which divide up the spore.
In some ascomycetes more than eight ascospores
are formed, usually in numbers which are a
multiple of eight, e.g. in the coprophilous genera
Podospora and Thelebolus. In others the eight
multicellular ascospores break up into partspores,
e.g. in Hypocrea (Fig. 12.15c) and
Cordyceps spp. (Fig. 12.33b). In Taphrina ascospores
may bud mitotically within the ascus so that
the mature ascus contains numerous yeast cells
(Fig. 9.2c). Asci with fewer than eight spores
are also known, e.g. in Neurospora tetrasperma
where the four ascospores are binucleate, in
Phyllactinia guttata where there are two ascospores
(Fig. 13.14e), or in Monosporascus cannonballus
which has a single ascospore.
8.6.1 Cleavage of ascospores
In many ascomycetes, studies of the fine structure
of asci during cleavage of the ascospores
have shown that a system of double membranes
continuous with the endoplasmic reticulum
extends from the envelope of the diploid fusion
nucleus (Fig. 8.11). The double membrane develops
to form a cylindrical envelope lining the
young ascus. This peripheral membrane cylinder
or lining layer is termed the ascus vesicle or
ascospore-delimiting membrane. The ascospores
are cut out from the cytoplasm within the
ascus by infolding and fusion of the inner
edges of the double membrane around a portion
of cytoplasm and a nucleus (Fig. 8.11d). In some
ascomycetes, e.g. Taphrina, a peripheral
membrane cylinder has not been observed and
the nuclei within the ascus become enveloped by
ascospore-delimiting membranes formed by
direct invagination of discrete parts of the
ascus plasma membrane.
Between the two layers of the ascosporedelimiting
membrane enclosing the ascospores,
the primary spore wall is secreted. The inner
membrane forms the plasma membrane of the
ascospore and the outer membrane becomes
the spore-investing membrane. Secondary wall
material is secreted within the primary wall.
There may be several such layers. In Sordaria
humana a total of four spore wall layers have
been distinguished, a primary wall layer and
three secondary layers (Read & Beckett, 1996).
The secondary wall layers are often quite thick,
and in dark-walled ascospores the pigment is
usually laid down within the secondary wall
layers. The spore wall may be smooth or
extended to form a variety of ornamentations
such as spines, ridges or reticulations. The ascus
epiplasm, i.e. the residual cytoplasm remaining
outside the spores after these have become
cleaved out, may continue to play a part in the
formation of the ascospore wall. For example,
in Ascobolus immersus, the outer leaf of the sporedelimiting
membrane may extend irregularly
outwards into the surrounding epiplasm to form
a perisporic sac within which secondary wall
material is deposited, derived from the epiplasm,
and passing as globular bodies through the
membrane of the perisporic sac. This secondary
wall material ornaments the ascospore wall but
is not involved in the formation of the purple
pigment characteristic of Ascobolus ascospores
(Wu & Kimbrough, 1992).
In many ascomycetes the outermost layer of
the ascospore wall, the perispore, is mucilaginous,
as seen for example in Ascobolus immersus
(Fig. 14.5), Sordaria fimicola (Fig. 12.1c) and
Pleospora herbarum (Fig. 17.10a). The properties
of this outer wall layer may aid in the lubrication
of the spore and also enable it to be compressed
as it emerges from the ascus. Further, it may aid
in the attachment of ascospores to substrata.
It may also cause ascospores to stick together
to form multisporous projectiles, an adaptation
which results in an increased distance of