Introduction to Fungi, Third Edition

MAGNAPORTHACEAE
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cAMP levels were raised by chemical treatment,
much of the phenotype of hypovirus infection
including hypovirulence was induced. This
hints at an interplay between several signalling
pathways, as outlined for Magnaporthe grisea
(Fig. 12.48).
Experiments with CHV-1 have also provided
an insight into the sexual reproductive system
of C. parasitica because the virus suppresses the
genes encoding the mating pheromones which
could thus be identified (Zhang et al., 1998). Both
male and female pheromones were suppressed
by CHV-1 infections. The reason why infection
leads selectively to female sterility is unclear
but may be because a certain proportion of
conidia, which double up as spermatia, remains
uninfected by the virus during conidiogenesis.
Biological control of Cryphonectria parasitica
by Hypovirus
The transmission of Hypovirus from one strain of
C. parasitica to another is mediated by anastomosis,
which requires vegetative compatibility.
In C. parasitica as in Podospora anserina (p. 320),
there are several genetic loci controlling vegetative
compatibility, and anastomosis as well
as virus transmission occur readily between two
strains possessing identical alleles at all six vegetative
incompatibility (vic) loci. Heteroallelism
at one or more loci restricts anastomosis and
reduces the percentage of virus transmission,
whereby mismatches at certain loci have a more
restrictive effect than those at others (Cortesi
et al., 2001). Of course, virus transmission in the
field will be higher in populations containing
a low diversity of vic alleles. Such is the case
in Europe, where hypovirulent strains were
observed to spread rapidly after their discovery.
There are different strains of Hypovirus which
may be mildly hypovirulent, i.e. only moderately
restricting canker development and asexual
sporulation in Cryphonectria, or may be aggressive.
The latter permit Cryphonectria to form
only very small cankers, but also greatly reduce
asexual sporulation. Since the virus can be disseminated
at least to a certain extent in conidia
of C. parasitica, mild virus strains may spread
faster in nature than aggressive ones.
In addition to relying on the natural spread
of hypovirulent strains of C. parasitica in Europe,
it has proven possible to implement a biological
control strategy by inoculating active cankers
with a paste containing a mixture of hypovirulent
strains differing in their vic alleles.
Success has been obtained especially in chestnut
orchards or in regions where hypovirulent
strains were rare in the field (Heininger &
Rigling, 1994). Active cankers can be converted
into healing cankers if one of the inoculated
hypovirulent strains can anastomose with the
fully pathogenic strain. In addition, the natural
spread of hypovirulence was observed around
sites of release (Heininger & Rigling, 1994).
In contrast, in the eastern USA where a
great diversity of vic alleles exists in the wild,
biological control measures have not generally
been successful except in isolated forests.
Another reason for the difficulties may be that
an aggressive hypovirus strain was chosen for
initial release experiments, which strongly
reduced the ability of C. parasitica to produce
conidia (Nuss, 1992). Current strategies are using
the fact that cDNA of Hypovirus can be stably
integrated into the genome of C. parasitica,
and produces double-stranded viral RNA in the
fungus. Whereas the RNA of the virus is not
transmissible via sexual reproduction into ascospores,
the integrated genomic DNA, of course,
is transmitted so long as the viral RNA, which
causes female sterility, is absent from the fungal
cytoplasm. This strategy promises to be more
successful than previous release experiments
because the vic alleles are re-mixed during
sexual reproduction, thereby facilitating the
introduction of the virus into a population
with diverse vic alleles (Dawe & Nuss, 2001).
12.9 Magnaporthaceae
This small family (9 genera, 26 species) is
currently homeless, having been excluded from
the Diaporthales (see p. 373) with which it
was formerly thought to be associated (Berbee,
2001; Castlebury et al., 2002). We include it here