Introduction to Fungi, Third Edition

SPORES OF FUNGI
25
Fig1.18 Sporangia in Mortierella (Umbelopsis)
vinacea. (a) Maturing sporangium in which
the cytoplasm is being cleaved into
numerous sporangiospores. (b) Release of
sporangiospores by breakdown of the
sporangial wall. Unusually, in M. vinacea the
sporangiospores are angular in shape.
The spores may be uni- or multinucleate and are
unicellular. They generally have thin, smooth
walls and are almost always globose or ellipsoid
in shape. They are formed by cleavage of the
sporangial cytoplasm. They vary in colour from
hyaline (colourless) to yellow, due to carotenoid
pigments in the cytoplasm. When mature, they
may be surrounded by mucilage, in which case
they are usually dispersed by rain splash or
insects, or they may be dry and dispersed by wind
currents. In some genera, e.g. Pilobolus, entire
sporangia become detached. The number of
sporangiospores per sporangium may vary from
several thousand to only one. The detachment
and dispersal of intact sporangia containing a
few sporangiospores or a single one is indicative
of the way in which conidia may have evolved
from one-spored sporangia.
1.4.3 Ascospores
Ascospores are the characteristic spores of the
largest group of fungi, the Ascomycota or
ascomycetes. They are meiospores and are
formed in the developing ascus as a result of
nuclear fusion immediately followed by meiosis.
The four haploid daughter nuclei then divide
mitotically to give eight haploid nuclei around
which the ascospores are cut out. Details of
ascospore development are described in Fig. 8.11.
In most ascomycetes, the eight ascospores are
contained within a cylindrical ascus, from which
they are squirted out together with the ascus
sap when the tip of the turgid ascus breaks down
and the elastic ascus walls contract. The distance
of discharge may be 1 cm or more. In some cases,
for example, the Plectomycetes (Chapter 11)
and in ascomycetes with subterranean fruit
bodies, such as the false truffles (Elaphomyces
spp.; Fig. 11.21) and truffles (Tuber spp. and their
allies; p. 423), ascospore release is non-violent
and their asci are not cylindrical but globose.
Ascospores vary greatly in size, shape and colour.
In size, the range is from about 4 5 1 mm in
small-spored forms such as the minute cup
fungus Dasyscyphus, to 130 45 mm in the lichen
Pertusaria pertusa. The shape of ascospores varies
from globose to oval, elliptical, lemon-shaped,
sausage-shaped, cylindrical, or needle-shaped.
Ascospores are often asymmetric in form with
a wider, blunter, anterior part and a narrower,
more tapering posterior. This shape increases
their acceleration as they are squeezed out
through the opening of the ascus. Ascospores
may be uninucleate or multinucleate, unicellular
or multicellular, divided up by transverse or
by transverse and longitudinal septa. In some