Introduction to Fungi, Third Edition

SPORES OF FUNGI
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Fig1.21 Sexual resting structures. (a) Zygospore of Rhizopus sexualis.The zygote has been produced by fusion of two gametangia
and has laid down a thick wall with warty ornamentations. (b) Oospore of Phytophthora erythroseptica.The oogonium (o) has grown
through the antheridium (a), and the oosphere has picked up a fertilization nucleus in the process. a kindly provided by H.-M. Ho;
reprinted from Ho and Chen (1998) with permission of Botanical Bulletin of Academia Sinica.
produced spore of the Oomycota (Chapter 5),
although oospores are also found in the
Monoblepharidales (Chytridiomycota; Fig. 6.25).
In the Oomycota, oospore development begins
with the formation of one or more oospheres
within the larger gametangium, the oogonium.
After fertilization, i.e. the receipt of an antheridial
nucleus by the oosphere, this lays down a
thick wall and becomes the oospore. The number
of oospores per oogonium may vary, and this is
an important taxonomic criterion. Meiotic
nuclear divisions precede oosphere and antheridial
maturation in the Oomycota and nuclear
fusion follows fertilization, so that the oospore is
diploid. The oospore develops a thick outer wall
and lays down food reserves, usually in the form
of lipids. In the Peronosporales the outer wall of
the oospore is surrounded by periplasm, the
residual cytoplasm left in the oogonium after
the oospheres have been cleaved out. Oospores
are sedentary (memnospores) and are important
in survival rather than dispersal. They
often require a period of maturation before
germination can occur and may remain dormant
for long periods.
1.4.7 Chlamydospores
In most groups of fungi, terminal or intercalary
segments of the mycelium may become packed
with lipid reserves and develop thick walls
within the original hyphal wall (Fig. 1.22).
The new walls may be colourless or pigmented,
and are often hydrophobic. Structures of
this type have been termed chlamydospores
(Gr. chlamydos ¼ a thick cloak). They are formed
asexually. Generally there is no mechanism for
detachment and dispersal of chlamydospores,
but they may become separated from each other
by the collapse of the hyphae producing them.
They are therefore typical memnospores, forming
important organs of asexual survival, especially
in soil fungi. Chlamydospores may develop
within the sporangiophores of some species
of the Mucorales, e.g. in Mucor racemosus (see
Fig 7.14). The Glomales, which are fungal
partners in symbiotic mycorrhizal associations
with many vascular plants, reproduce primarily
by large, thick-walled chlamydospores. These
develop singly or in clusters (sporocarps) on
coarse hyphae attached to their host plants.
They are sedentary in soil but may be dispersed