Introduction to Fungi, Third Edition

EUAGARICS CLADE
545
flesh of the fruit body. The cytology of basidial
development shows no unusual features.
Surface properties, especially the wettability
of hyphal surfaces, play an important role in
mycelial growth and fruit body development in
Schizophyllum. Aerial hyphae, including those
making up the basidiocarp, are strongly hydrophobic
(i.e. non-wettable) because they are
covered by parallel rodlets of special proteins,
hydrophobins, which were first discovered
during research with S. commune. Hydrophobins
are amphipathic, i.e. they arrange themselves
into sheets with one wettable (hydrophilic)
face and a non-wettable (hydrophobic) face.
At an air water interface or over the surface
of a hypha, the hydrophilic face attaches to the
meniscus or to the hyphal surface whilst the
hydrophobic layer faces outwards, giving it its
non-wettable properties (Wessels, 1997, 2000).
Schizophyllum commune has at least four hydrophobin
genes. The gene SC3 is active in both
monokaryons and dikaryons, whereas SC1, SC4
and SC6 are active in dikaryons only. When
grown experimentally in liquid culture, the
hyphae of S. commune secrete the monomers of
SC3p which accumulate at the air liquid interface
and also lower the surface tension of
the culture liquid. This enables individual
hyphae to penetrate the interface, and to grow
into the air, simultaneously becoming coated
with hydrophobin. The function of the product
of the SC4 gene, the hydrophobin SC4p, has been
clarified. It covers the surface of the hyphae
lining the numerous air channels which traverse
the basidiocarp, preventing them from being
clogged by water and thus permitting gas
exchange to continue during respiration.
Volvariella
There are about 50 species of Volvariella, and their
taxonomic position is still unsettled but may
be close to Schizophyllum (Moncalvo et al., 2002).
The genus includes several species cultivated
for their edible basidiocarps. The best known is
V. volvacea, the paddy straw mushroom, widely
grown in Asia on rice straw, cotton waste and
other cellulose-rich agricultural waste products.
In terms of world production it is one of
the most important edible mushrooms. It is a
warm-temperature fungus which can grow vegetatively
at 32 34°C, with an optimum temperature
for fruiting of 28 30°C. Under favourable
conditions, the period between inoculation and
harvest of fruit bodies is 8 10 d, the shortest in
any cultivated fungus (Chang & Miles, 2004).
The basidiocarps of Volvariella spp. are enclosed
by a universal veil which persists as a prominent
cup-like volva (Fig. 19.16c). There is no ring.
The gills are free and the spore print is pink.
Volvariella volvacea has an unusual life cycle
(Chang & Yau, 1971; Chiu, 1993). It is homothallic
and its mycelium is haploid, being made up of
multinucleate segments. There are no clamp
connections and it is reported that nuclei can
pass through the transverse septa which separate
adjacent hyphal segments. Brown, thick-walled,
multinucleate chlamydospores are borne on
specialized branches of the aerial mycelium.
They serve as asexual propagules under adverse
conditions, germinating by hyphal growth.
Young basidia contain two haploid nuclei
which fuse to form a diploid nucleus. Meiosis
gives rise to four haploid nuclei, one entering
each of the four basidiospores. Basidiospores are
therefore normally uninucleate (occasionally
binucleate).
Volvariella bombycina, the silver silk straw
mushroom, has a similar life cycle. It fruits
readily in laboratory culture and its basidiocarp
development has been studied by Chiu and
Moore (1990b). Development is normally hemiangiocarpic,
but angiocarpic (i.e. with the
hymenium enclosed until a late stage) and
gymnocarpic (hymenium exposed) development
may also occur. Hemi-angiocarpic development
has been arbitrarily divided into five
phases: the bulb, button, egg, elongation and
mature stages. During the button stage, a
schizogenous cavity is formed, enclosed by the
universal veil and basal bulb. The hymenophore
develops within this cavity. Elongated cylindrical
hyphae of uniform size develop on ridges,
initiating the formation of the gills. These
ridges do not extend to the future stipe, so the
gills remain free. During the late egg stage, i.e.
before rupture of the universal veil, local
proliferation of the gills occurs, and at the
site of proliferation the hymenophore is folded