Introduction to Fungi, Third Edition

SORDARIALES
329
Fig12.8 Neurospora crassa. (a) Ascus.
(b) Ascospores showing ribbed surface.
(c) Protoperithecium showing projecting
trichogyne. (d) Macroconidia from one-day-old
culture. (e) Enlarged view of developing
macroconidia. (f) Microconidia forming sticky
clusters. (g) Enlarged view showing origin of
microconidia. (a d,f) to same scale;
(e,g) to same scale.
and cohere in sticky masses (Beever et al., 1978).
The pink colour of the conidia is due to the
presence of a carotenoid pigment, neurosporoxanthin,
which is stimulated to develop by
light. The spores are formed in vast numbers.
In laboratories they can cause serious contamination
of other cultures, partly because of the
rapid growth of the mycelium (up to 5 mm h 1 ),
and partly because macroconidia can develop
in profusion beyond the rims of closed Petri
dishes. In humid environments, the mycelium
may grow through the cotton wool plugs of testtube
cultures and sporulate on the plugs. Shaw
(1993) has reported that macroconidia formed on
steamed pine logs in Queensland, Australia, are
collected by honeybees in their pollen baskets.
Cultures derived from a single ascospore also
develop two other types of reproductive structure.
In contrast to the large, dry, wind-dispersed
macroconidia which are formed within 1 2 days,
clumps of smaller, oval, uninucleate, sticky
microconidia develop after about 12 15 days
(Figs. 12.8f,g; Maheshwari, 1999). The conidiogenous
cells from which the microconidia develop
have been interpreted as reduced phialides. The
microconidia are capable of slow and variable
germination but function primarily as spermatia.
Coiled ascogonia, terminated by long tapering
trichogynes and surrounded at the base by
hyphae, also develop (Fig. 12.8c). Such structures
are termed protoperithecia or bulbils and each
often forms several trichogynes. In N. crassa and
N. sitophila, no further development occurs in
single ascospore cultures, i.e. each strain is selfincompatible.
Incompatibility is controlled by a
pair of mating type idiomorphs, A and a, and
if two compatible strains are grown together in
a Petri dish for a few days, microconidia of one