Introduction to Fungi, Third Edition

326 HYMENOASCOMYCETES: PYRENOMYCETES
HET-s spreads like an infection throughout a
mycelium at a rate of several mm h 1 . Further,
transmission can occur from a HET-s containing
hypha to a HET-s mycelium by anastomosis.
The ability of a protein to convert others to
its own state in an infectious transcriptionindependent
manner, accompanied by the
formation of cytoplasmic aggregates, has the
hallmarks of a prion disease (Coustou et al., 1997;
Coustou-Linares et al., 2001).
Senescence in Podospora anserina
Podospora anserina has been the subject of
research into senescence and has been treated
as a model of the ageing phenomenon in more
complex organisms (Griffiths, 1992; Bertrand,
2000; Silar et al., 2001). In P. anserina senescence is
defined as a diminution in the ability of cells to
proliferate and/or differentiate. This may or may
not culminate in cell death. In pure cultures of
P. anserina, senescence is marked by a progressive
reduction in growth rate and loss of ability to
form perithecia. Eventually it proves impossible
to transfer viable sub-cultures so that a given
isolate has a limited lifespan. Different isolates of
P. anserina have characteristic lengths of growth
before growth ceases, and the mean lengths of
growth can be used as a convenient indicator of
lifespan. For example, two races A and S grown
on cornmeal agar in glass tubes (20 150 mm) at
26°C in the dark had mean lengths of 15 and
170 cm, respectively (Smith & Rubenstein, 1973).
A hypothesis to explain the phenomenon of
senescence in P. anserina is that, after a period of
growth characteristic of a given race of the
fungus, a senescence factor appears in a culture
and is presumed to be produced, or to reproduce
itself, more rapidly than other cellular components.
The factor is transmissible through hyphal
anastomosis, i.e. fusion between a senescent
hypha and a non-senescent hypha results in the
non-senescent hypha acquiring the factor
controlling senescence. Senescence is inherited
maternally; it can be transmitted to 90% of the
progeny of a senescent protoperithecial strain
but to none of the progeny of a spermatial
parent. No nuclear mixing is involved.
A large number of genes (between 600 and
3000) can modulate lifespan; 50% increase it
and 50% diminish it (Rossignol & Silar, 1996).
Senescence is a complex process affected by
environmental factors and is also controlled
by the interactions between nuclear and mitochondrial
DNA (Osiewacz & Kimpel, 1999;
Osiewacz, 2002). The onset of senescence is
marked by the appearance of dysfunctional
mitochondria and of circular plasmid-like senility
DNAs derived from the mitochondria.
During the respiratory activities of mitochondria,
reactive oxygen species (ROS) are generated
as by-products and these molecules are able to
damage all cellular components, leading to
cellular dysfunctions such as the cytochrome
oxidase pathway (Osiewacz, 2002). To compensate
for these dysfunctions, ROS scavengers can
be produced which reduce the level of ROS.
Alternative oxidative pathways may also be
induced which may help in reducing the adverse
effects of ROS (Dufour et al., 2000; Lorin et al.,
2001). It is interesting that different mutants
with defective mitochondrial DNA associated
with growth arrest can be restored to wild-type
mitochondrial DNA by crossing, indicating
an important role of sexual reproduction in
this pseudohomothallic fungus (Silliker et al.,
1997).
12.2.3 Neurospora (Sordariaceae)
There are about 12 species of Neurospora, mostly
growing on soil. A key has been provided by
Frederick et al. (1969). Many species grow in
humid tropical and subtropical countries but
others have been reported from temperate areas
(Perkins & Turner, 1988; Turner et al., 2001). In
nature, the most conspicuous species colonize
burnt ground and charred vegetation following
fire caused by volcanic eruptions or deliberate
burning to clear vegetation (slash and burn) or
crop residues such as sugar cane. Within a few
days the burnt areas are covered by an orange or
pink powdery mass of macroconidia. The
commonest species here is N. intermedia (Perkins
& Turner, 1988). The association with burnt
ground is related to the fact that dormant
ascospores in the soil are stimulated to germinate
by heat. Neurospora species also grow in
warm humid environments such as wood-drying