Introduction to Fungi, Third Edition

350 HYMENOASCOMYCETES: PYRENOMYCETES
Fig12.24 Important metabolites from
Clavicipitales. (a) Cyclosporin A, produced by
Tolypocladium inflatum.This is used extensively as
an immunosuppressive drug, e.g. after organ
transplantations. (b) Cephalosporin C,
a b-lactam antibiotic with activity against
Gram-positive and Gram-negative bacteria.
and sits like a cap over it. The ovary, which would
normally develop into a caryopsis filled with
grain, becomes replaced by fungal tissue. For this
reason the disease caused by C. purpurea has been
termed a replacement disease (Luttrell, 1980).
The fungal structure which develops is considerably
longer than the ovary which it replaces and
it differentiates into a sclerotium up to 3 cm in
length, the foot of which continues to obtain
nutrients via the vascular connection in the host
rachilla. The sclerotium is made up of three
distinct layers: a thin purplish-brown rind, a
discontinuous layer of mealy white tissue, and
a central layer of translucent gelatinous tissue
(Luttrell, 1980).
Ergot is the French name for a cock’s spur,
referring to the curved, banana-shaped sclerotia
which project from the inflorescence of infected
grasses and cereals in late summer (Fig. 12.26a).
The sclerotia fall to the ground and overwinter
near the surface of the soil. They need a period of
low temperature before they can develop further.
A chilling period of 0°C for at least 25 days
is optimal for further development. The main
reserve substance of the sclerotium is lipid,
which may account for 50% of the dry weight.
It is likely that the chilling period is necessary
before enzymes capable of mobilizing the lipid
reserves develop (Cooke & Mitchell, 1970).
Sclerotia do not remain viable in soil in the
field for more than a few months, often being
invaded by fungi, bacteria, mites and insects
(Cunfer & Seckinger, 1977). The following
summer, sclerotia develop one or more perithecial
stromata (clavae) about 1 2 cm high, shaped
like miniature drumsticks (Fig. 12.26c). The
perithecial stromata are positively phototropic
(Hadley, 1968).
The enlarged spherical head or capitulum
contains a number of perithecia which are embedded
in the stroma, each surrounded by a
distinct perithecial wall (Fig. 12.27a). The cytological
details of perithecial development have
been studied in C. purpurea by Killian (1919) and
in C. microcephala (regarded by some as a form of
C. purpurea) by Kulkarni (1963). In the outer layers
of the head of the perithecial stroma, clubshaped
multinucleate antheridia and ascogonia
undergo plasmogamy. Ascogenous hyphae made
up of predominantly binucleate segments
develop from the base of the ascogonium, and
the tips of the ascogenous hyphae form croziers