Introduction to Fungi, Third Edition

SORDARIALES
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kilns and bakeries where they cause serious
trouble because of their rapid growth and
sporulation. For this reason N. sitophila is sometimes
called the red bread mould. Strains of
N. intermedia are used in the preparation of the
fermented food ‘omchom’ (ontjom) in which
conidia are used to inoculate soybean or
peanut solids from which oil and protein have
been extracted by pressing.
Neurospora as a genetic tool
Neurospora has been widely used in genetic and
biochemical studies (Perkins, 1992; Davis, 1995).
The development of auxotrophic mutants
deficient in successive steps of arginine
biosynthesis led to the proposition of the ‘onegene
one-enzyme’ hypothesis by Beadle and
Tatum (1941) who were awarded the Nobel
Prize in 1958. There is a very extensive literature
relating to the genus. The loci of over 1000 genes
have been mapped (Perkins et al., 2000), and the
complete genome of N. crassa has now been
sequenced. It is haploid and has seven chromosomes.
The best-known species are N. crassa and
N. sitophila, both of which are eight-spored and
heterothallic. Neurospora tetrasperma is fourspored
and pseudohomothallic. Some other
species, e.g. N. africana, N. dodgei and N. terricola,
are homothallic (Coppin et al., 1997). The homothallic
species do not form conidia.
The reasons why Neurospora has proven so
useful as a tool in biochemical and genetic
research are: (1) that it is haploid; (2) that wildtype
strains have simple nutritional requirements,
namely a carbon source, simple mineral
salts and one vitamin, biotin; (3) that mutations
can be induced readily by the use of chemical
mutagens or UV irradiation of conidia; (4) that
growth and sexual reproduction is rapid; and
(5) that tetrad analysis by dissecting asci is
straightforward. By the use of marked strains it
is now not even necessary to dissect asci because
tetrad analysis can be performed on octets of
projected ascospores. Another advantage of
Neurospora is that cultures can be preserved for
long periods in suspended animation as spores
stored over silica gel, or following lyophilization
or freezing.
The life cycle of Neurospora
The life cycle of N. crassa is illustrated diagrammatically
in Fig. 12.7. The name Neurospora is
derived from the characteristically ribbed ascospores.
The ascospore walls bear dark, raised,
thicker ribs separated by thinner, paler,
branched or unbranched inter-costal veins
(Figs. 1.19 and 12.8). The spores are multinucleate
and have reserves of lipids and the carbohydrate
trehalose. The dark ascospores of N. crassa are
viable for many years and do not germinate
readily unless treated chemically (e.g. by furfural)
or by heat shock (e.g. 60°C for 20 40 min).
In contrast, the conidia are killed by such heat
treatment. Following treatment, the ascospores
germinate through a germ pore at either or both
ends, forming at first a globose, inflated vesicle
and then a coarse, incompletely septate, rapidly
growing mycelium, each segment of which is
multinucleate. Cytoplasm and organelles including
nuclei pass freely through the septa.
Within 24 h, the mycelium can begin asexual
reproduction. The cues inducing conidial development
include light, desiccation, and nutrient
deprivation. Upright branches develop which,
instead of continuing to grow by hyphal tip
elongation, undergo repeated apical budding.
The resulting cells are separated from each other
by incomplete septa with a wide central pore.
These cells have been termed proconidia
(Springer & Yanofsky, 1989). The proconidia
continue to bud apically, forming multinucleate
macroconidia (blastoconidia) separated from
each other by septa with narrower pores. The
septa thicken and develop a centripetal furrow
which widens, leaving a central strand of
material, the connective, by which the spores
remain attached to each other. Further conidia
develop by budding of the terminal conidium
on a chain and, when the terminal conidium
gives rise to two buds, the chain branches
(Figs. 12.8d,e; Hashmi et al., 1972). Conidia of
this type belong to the form genus Chrysonilia
(formerly Monilia). The individual segments of the
spore chain break apart and are readily dispersed
by wind. The surface of macroconidia in wildtype
strains is made up of hydrophobin rodlets
which impregnate it, but the conidia of the
easily wettable mutant eas are devoid of rodlets