Introduction to Fungi, Third Edition

30 INTRODUCTION
Fig1.22 Chlamydospores formed by soil-borne fungi. (a) Intercalary hyphal chlamydospores in Mucor plumbeus (Zygomycota).
(b) Terminal chlamydospore in Pythium undulatum (Oomycota). Both images to same scale.
by wind or by burrowing rodents which eat the
spores. Chlamydospores may also develop within
the multicellular macroconidia of Fusarium spp.
and may survive when other, thin-walled cells
making up the spore are degraded by soil microorganisms.
Similar structures are found in old
hyphae of the aquatic fungus Saprolegnia (see
Fig. 5.6g), either singly or in chains. In this genus,
the chlamydospores may break free from the
mycelium and be dispersed in water currents.
Chlamydospores which are dispersed in this way
are termed gemmae (Lat. gemma ¼ a jewel).
The term chlamydospore is also sometimes
used to describe the thick-walled dikaryotic spore
characteristic of smut fungi (Ustilaginales;
Chapter 23) but the term teliospore is preferable
in this context. Hughes (1985) has discussed the
use of the term chlamydospore.
1.4.8 Conidia (conidiospores)
Conidiospores, commonly known as conidia, are
asexual reproductive structures. The word is
derived from the Greek konidion, a diminutive
of konis, meaning dust (Sutton, 1986). Conidia
are found in many different groups of
fungi, but especially within Ascomycota and
Basidiomycota. The term conidium has, unfortunately,
been used in a number of different ways,
so that it no longer has any precise meaning.
It has been defined by Kirk et al. (2001) as
‘a specialized non-motile (cf. zoospore) asexual
spore, usually caducous (i.e. detached), not
developed by cytoplasmic cleavage (cf.
sporangiospore) or free cell formation (cf. ascospore);
in certain Oomycota produced through the
incomplete development of zoosporangia which
fall off and germinate to produce a germination
tube’. In many fungi conidia represent a means
of rapid spread and colonization from an initial
focus of infection.
In general, conidia are dispersed passively, but
in a few cases discharge is violent. For instance, in
Nigrospora the conidia are discharged by a squirt
mechanism (Webster, 1952), and in Epicoccum
(Fig. 17.8) discharge is brought about by the
rounding-off of a two-ply septum separating the
conidium from its conidiogenous cell (Webster,
1966; Meredith, 1966). In the Helminthosporium
conidial state of Trichometasphaeria turcica, drying
and shrinkage of the conidiophore is associated
with the sudden development of a gas phase,
causing a jolt sufficient to project the conidium
(Meredith, 1965; Leach, 1976).
There is great variation in conidial ontogeny.
This topic will be dealt with more fully
later when considering the conidial states of
Ascomycota, and at this stage it is sufficient to
distinguish between the major types of conidial
development, which may be either thallic or
blastic. Cells which produce conidia are conidiogenous
cells. The term thallic is used to describe
development where there is no enlargement of
the conidium initial (Fig. 1.23a), i.e. the conidium
arises by conversion of a pre-existing segment of
the fungal thallus. An example of this kind is
Galactomyces candidus, in which the conidia are