Introduction to Fungi, Third Edition

EUAGARICS CLADE
535
primordium has grown to a height of 10 mm, the
orientation of the upper stalk region is complete.
Subsequent increase in height of the stipe is
almost entirely due to cell expansion, although
some nuclear and cell division also occur,
especially in the upper region of the stipe
where cell elongation is most marked
(Craig et al., 1977). The gills develop in radially
arranged ridges made up of downward-growing
hyphae terminating in a tightly packed palisade
of cells which are the basidia. Although the
hyphal segments making up the vegetative
mycelium and the stipe are multinucleate,
the number of nuclei in the basidia is reduced
to two. As the pileus continues to expand, the
partial veil is broken and persists as a ring
attached to the stem (Fig. 19.14a). There is
evidence that stipe elongation is promoted by
a factor of unknown chemical identity, which is
mainly produced by the ripening gills (Frazer,
1996). Konishi (1967) partially purified a
substance which enhanced stipe elongation.
The promoting substance included a mixture of
amino acids, and it is not known if they function
as nutrients or growth factors. It has also been
claimed that a compound found chiefly in gill
tissue of mushrooms, 10-oxo-trans-8-decenoic acid
(ODA), stimulates mycelial growth and enhances
elongation of the upper stipe (Mau et al., 1992).
Unfortunately, attempts to confirm these findings
have been disappointing (Champavier et al.,
2000).
Biochemical changes occur during basidiocarp
development (Hammond, 1985; de Groot
et al., 1998). Mannitol, glycogen and trehalose
accumulate in basidiocarp primordia, lowering
the water potential and thereby possibly causing
uptake of water from the mycelium. This would
lead to an increase in turgor pressure, causing
cell enlargement and fruit body expansion
(Hammond & Nichols, 1979). Rapid synthesis
of chitin is correlated with stipe elongation
(Craig et al., 1979).
A hydrophobin-like protein is secreted by the
vegetative mycelium of A. bisporus and an
abundant hydrophobin (ABH1) forms hydrophobic
rodlet layers on the surfaces of the cells in
certain regions of the basidiocarp, especially
those where there are air spaces, such as in the
outer regions of the pileus and stipe, in the veil
and in the core of the stipe, but not in the gills.
The hydrophobic layer may be responsible for the
non-wettability of the surface of the basidiocarp,
preventing the inflow of water from the outside
and possibly protecting against bacterial and
fungal parasites (Lugones et al., 1996).
Life cycle of Agaricus bisporus
The life cycle of A. bisporus is unusual (Miller,
1971). The majority of the spores formed on its
two-spored basidia are at first binucleate but
post-meiotic mitosis increases the number of
nuclei to four. The spores are heterokaryotic for
mating type, i.e. they contain non-sister nuclei
with dissimilar A mating type idiomorphs
(Evans, 1959; Elliott, 1985). The products of
meiosis in the basidium are four haploid
nuclei, two with mating type idiomorph A 1 and
two with A 2 . Since two nuclei enter each
basidiospore, the spores may be homokaryotic
(A 1 þ A 1 or A 2 þ A 2 ) or heterokaryotic (A 1 þ A 2 ).
Of the 12 possible pairings of the 4 haploid
meiotic products, 4 are homokaryons and 8 are
heterokaryons, i.e. in a ratio of 1 : 2. Evans (1959)
has claimed that the disproportionately high
ratio of heterokaryotic spores results from the
alignment of the nuclear spindles during meiosis
in the basidia.
On germination the heterokaryotic spores
give rise to a mycelium with multinucleate
hyphal segments capable of forming basidiocarps.
Mycelia from homokaryotic spores
can only fruit following anastomosis with mycelia
of opposite mating type. Basidia of normal
cultivated A. bisporus may rarely bear three
or four basidiospores, and most of these are
initially uninucleate. Mycelium from such an
aberrant spore will only fruit when mated with
a mycelium of opposite mating type. Thus
A. bisporus has two alternative kinds of mating
behaviour, secondary homothallism or bipolar
heterothallism. Such ambivalent behaviour is
sometimes termed amphithallic (Lange, 1952)
and is not confined to A. bisporus, being
also found in some other basidiomycetes with
two-spored basidia, e.g. certain species of Coprinus
(euagarics clade) and the four-spored Stereum
sanguinolentum (russuloid clade).