Introduction to Fungi, Third Edition

SORDARIALES
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heterokaryotic (A þ a) cultures have the potential
advantage of hybrid vigour. They can also enter the
sexual cycle and produce ascospores without waiting
for a sexual partner. This is very useful in situations
where rapid completion of the life cycle is
advantageous.
Neurospora and the biological clock
Many organisms show a daily (circadian) rhythm
in their activities, entrained by a combination
of light and temperature (Dunlap, 1999). When
transferred to a uniform environment, they may
continue to display the same rhythm, suggesting
that it is controlled by some internal clock.
Important research helping to interpret the
molecular basis of circadian rhythmicity in
fungi as well as other eukaryotes is being
performed on N. crassa (Davis, 1995; Liu, 2003;
Dunlap & Loros, 2004). A band mutant (bd) was
discovered which, when grown in culture in long
tubes, formed alternating bands of macroconidia
interspersed by non-sporulating bands. In continuous
darkness, bands continued to form, with a
periodicity of 21.5 h, little affected by temperature.
From the bd mutant, further mutants were
developed by mutagenesis in which the frequency
of the circadian rhythm was affected. The frq
locus was identified as a key control element
of the frequency of sporulation, with partial
loss-of-function mutations capable of shortening
the frequency to as little as 16 h or extending
it to 29 h. The effect of temperature on the
circadian rhythm seems to be controlled by the
temperature-dependent alternative splicing of
the frq mRNA, giving either of two major FRQ
proteins (Colot et al., 2005). Several other loci are
also involved in the integration of the rhythm
with temperature and light. The white collar wc-1
and wc-2 gene products are especially important,
forming a heterodimer (WCC ¼ white collar
complex) which stimulates frq transcription. The
FRQ dimer, in turn, inhibits existing WCC and
lowers the continued expression of WC-1, thus
introducing a circadian pattern into the cycle
such that the levels of FRQ proteins hit their
lowest point late at night, and WC-1 (and WCC
activity) late in the day (Dunlap & Loros, 2004).
It should be noted that regulation is partly at
the level of gene expression, partly by
translation of existing mRNA molecules, and
partly by protein phosphorylation/dephosphorylation.
Entrainment of the rhythm, i.e. the
switching on of the clock, is sensitive to blue
light and the photo-receptor has been identified
as the WC-1 component of WCC coupled with the
chromophore FAD (flavin-adenine dinucleotide).
12.2.4 Chaetomium (Chaetomiaceae)
There are over 80 species of Chaetomium (von Arx
et al., 1986), many of which are cosmopolitan,
growing in soil and fruiting on cellulose-rich
substrata such as seeds, textiles in contact with
soil, straw, sacking and dung. Wood infected by
Chaetomium spp. may undergo a superficial decay
known as soft rot. Wood inside buildings
damaged by flooding or water used in fire
control is particularly susceptible. Most species
are saprotrophic and cellulolytic, but some have
been isolated from human lesions. Some produce
mycotoxins (Ugadawa, 1984) and others have
been used in biological control because of their
competitive ability to colonize cereal stubble,
thus displacing plant-pathogenic fungi (Dhingra
et al., 2003). Many potentially valuable chemical
compounds such as enzymes (cellulases, xylanases),
pharmaceutical products and antifungal
substances have been extracted from Chaetomium
in culture. Chaetomium thermophile is thermophilic
and has potential for use in composting palm
oil fibre for recycling biomass (Suyanto et al.,
2003).
The perithecia of Chaetomium are superficial,
barrel-shaped, thin-walled and, in most species,
clothed with projecting, dark, stiff hairs. In
C. elatum, one of the commonest species, the
hairs are dichotomously branched. In others,
e.g. C. cochliodes, the body of the perithecium
bears straight or slightly wavy, unbranched
hairs, whilst the apex bears a group of spirally
coiled hairs. The hairs are roughened or ornamented,
and the type of ornamentation is an aid
to identification (Guarro & Figueras, 1989). It is
likely that the perithecial hairs have special
functions. Jerking caused by the movement of
the hairs over each other on drying may help in
ascospore dispersal. Another possible function is
to deter grazing of the perithecia by insects and