Growth, Differentiation and Sexuality

264 R. Fischer and U. Kües
2. Chlamydospores typically arise as single units
endogenously within pre-existing vegetative
hyphal cells, by protoplast contraction and
formation of an inner, thickened secondary
cell wall.
3. Sporangiospores are formed, usually in
multiple numbers, by cytoplasmic cleavage
within specialized spore mother cells building
a sporangium.
4. Conidiospores, or conidia, are exospores which
develop externally through localized bulging
(budding) and subsequent constriction from
asporogenouscell.
There are many variations in conidia production
– Kirk et al. (2001), for example, present schemes
for 43 different conidial ontogonies. More generally,
holoblastic and enteroblastic conidia are distinguished
in terms of whether both cell wall layers
or only the inner cell wall layer of the sporogenous
cell contribute to the swelling. Enteroblastic
phiallidic conidia are produced from a specialized
cell called the phialide, by de novo production
of a cell wall which is not linked to that of the
spore-producing cell. Furthermore, spore production
might occur directly in, or on cells of the vegetative
mycelium or, alternatively, on sporophores
produced as specialized structures on the vegetative
mycelium. Based on the types of spores they
produce, conidiophores and oidiophores are distinguished.
Sporophores might occur individually or
in compact groups such as conidiomas (Kendrick
1979a,b; Esser 2001; Kirk et al. 2001).
Fungal spores from different species, or different
types of spores from one species usually vary in
size and morphology, and they may differ in their
nuclear load. Uninucleate spores may be haploid or
diploid, or spores may carry two or more genetically
identical or different nuclei. Moreover, spores
may consist of only one cell or they may be bi- or
multicellular (Kendrick 1979a,b; von Arx 1981; Ellis
and Ellis 1998; Kirk et al. 2001). Related to their
actual functions and to environmental conditions,
many spores have melanin or other pigments incorporated
in their thickened cell walls. Melanin
incorporation gives fungal cells a better rigidity,
resistance to lytic enzymes and oxidative stresses,
protection against physical stresses such as cold,
heat and UV, and protection against antagonistic
activities from other organisms (Frederick et al.
1999; Wheeler et al. 2000). In addition, the surface
of spores is often made hydrophobic by an outer
rodlet layer of hydrophobins, small cysteine-rich
hydrophilic secreted proteins which self-assemble
into amphipathic films (Kershaw and Talbot 1998;
Jeffs et al. 1999; see Chap. 19, this volume, and The
Mycota, Vol. 8, Chap. 7), and may also contribute
to thermotolerance (Ying and Feng 2004). Wettable
spores, by contrast, lack hydrophobins (Ásgeirsdóttir
et al. 1997). Spores of the Chytridiomycota
are motile by presence of a flagellum (zoospores),
whereas spores of all higher fungi lack flagellas and
are non-motile (Esser 2001; Kirk et al. 2001).
A single fungal species may form different
types of spores at the same time, or under different
environmental conditions and/or at different
places (von Arx 1981). For example, the cereal
rust fungus Puccinia graminis produces four different
types of asexual spores (distinctively called
pycnidiospores, aecidiospores, urediospores and
teliospores) which readily serve to demonstrate
the wide range of spore features and functions.
Inthecomplexlifecyclewithhostchanges,the
minute hyaline pycnidiospores are unicellular,
uninucleate haploid conidiospores with a simple
cell wall, acting as spermatia to fertilize hyphae
of opposite mating type on the alternate host.
From the resulting dikaryon with two haploid
nuclei in the cells, aecidiospores arise as a new
type of larger, thin-walled, unicellular dikaryotic
exospores needed for transfer to the main host.
Large, thick-walled, pigmented unicellular dikaryotic
uredospores are generated as exospores on
the main host for infection of other main hosts
and the large two-celled, thick-walled melanized
diploid teliospores for hibernation (Esser 2001,
Agrios 2005).
In view of the enormous variability of modes of
asexual spore production, spore morphologies and
other spore features, it is not surprising that these
represent commonly used criteria in fungal taxonomy,
particularly for those fungi lacking a sexual
state (Kendrick 1979a,b; von Arx 1981; Ellis and
Ellis 1998). The variability of spores and spore formation
within the fungal kingdom is far too broad
to possibly be present within a single book chapter.
Here, we therefore concentrate on a few model
species, preferentially those for which genetic data
are also available (Sect. IV.).
B. Ecological Aspects of Spore Production
Many fungal spores are airborne. It is mostly mixtures
of spores from saprophytic species and phytopathogens
which are detected in the air but spores