Introduction to Fungi, Third Edition

CLAVICIPITALES
349
thick apical cap perforated by a narrow pore
through which the ascospores are discharged,
singly and successively. The ascospores are
long, narrow and often multi-septate, breaking
up into part-spores. Most members (e.g. Claviceps,
Balansia, Epichloe) are pathogens or endophytes
of grasses, whereas Cordyceps parasitizes insects
or fruit bodies of the hypogeous ascomycete
Elaphomyces. Claviceps sclerotia are the source of
toxic alkaloids, and grasses infected with endophytic
Balansia may also be toxic to herbivorous
insects and mammals. The grass host may thus
be at least partially protected against insect
herbivory, and the effects of alkaloids on
grazing mammals can also be severe. The
conidia of certain species of Cordyceps show
promise as agents of biological control of insect
pests (Evans, 2003). Some members of the
Clavicipitales attack nematodes (Gams & Zare,
2003). For example, Atricordyceps (now called
Podocrella) is the teleomorph of Harposporium
anguillulae (Samuels, 1983) (see Fig. 25.8), and
phylogenetic analyses of nuclear ribosomal
DNA have also placed the nematophagous
fungus Drechmeria coniospora (see Fig. 25.8) in
the Clavicipitaceae (Gernandt & Stone, 1999).
The immunosuppressant drug cyclosporin A
(Fig. 12.24a) is produced by Tolypocladium
inflatum, anamorph of Cordyceps subsessilis
(Hodge et al., 1996). A group of b-lactam antibiotics,
the cephalosporins (Fig. 12.24b; see
p. 302), are derived from the anamorphic
Acremonium chrysogenum and A. salmosynnematum.
Many other valuable secondary metabolites have
been isolated from members of the Clavicipitales
(Isaka et al., 2003).
12.5.1 Claviceps
There are over 40 species of Claviceps, all of which
are parasitic on grasses, rushes and, occasionally,
sedges (Alderman, 2003). The best known species
is C. purpurea, the cause of ergot of grasses and
cereals. Other economically important species
are C. sorghi and C. africana which cause ergot
of sorghum (Frederickson et al., 1991), C. paspali
on Paspalum and C. fusiformis on pearl millet
(Pennisetum typhoides). Claviceps purpurea occurs in
temperate regions and has an exceptionally wide
host range for a biotrophic pathogen, infecting
over 400 grass species. The course of infection
has been described by Luttrell (1980), Tenberge
(1999) and Oeser et al. (2002).
Life cycle
The life cycle of C. purpurea is summarized in
Fig. 12.25. The primary inoculum is an ascospore
shot away from a perithecium which has developed
from an overwintered sclerotium. The time
of ascospore release coincides with anthesis in
a susceptible host. Ascospores germinate on
a grass stigma to form an intercellular mycelium
which grows down to the base of the ovary
towards the vascular bundle of the floret stalk
(rachilla), thus gaining access to the photosynthetic
products of the host. Subsequent growth is
upwards and within a few days a conidial stroma
develops beneath the ovary. A palisade of phialides
lining labyrinthine chambers is formed
from which a succession of unicellular, uninucleate
conidia develops in a sugary syrup. This
becomes visible on the grass florets as beads of
liquid termed honeydew (Fig. 12.26b). The conidial
stage was given the separate name Sphacelia
segetum before its connection with ergot was
understood. Honeydew contains glucose, fructose,
sucrose and other sugars (Mower &
Hancock, 1975a), and is attractive to insects,
which feed on it and in so doing disperse conidia
to healthy grass flowers, thus causing secondary
infection. Infection of a grass flower by Claviceps
results in increased translocation of water
and sucrose towards the diseased flower, and
infected flowers are more effective at acquiring
photosynthetic products from the host than
uninfected flowers (Parbery, 1996b). Within the
infected host tissue, conversion of host-derived
sucrose to mono-, di- and oligo-saccharides by the
fungus creates a continuing sink for sucrose
translocation, and evaporation at the surface of
the diseased grain results in increased osmotic
concentration of the sugars, possibly accelerating
the rate of translocation (Mower & Hancock,
1975b). The high osmotic concentration prevents
conidial germination until the honeydew has
been diluted.
As infection proceeds, the entire ovary is
pushed upwards by the developing fungal tissue