Introduction to Fungi, Third Edition

510 BASIDIOMYCOTA
the small number of incompatibility factors may
be related to the specialized method of dispersal
in which numerous basidiospores are packaged
into a peridiole.
The molecular basis of bipolar (unifactorial)
mating systems may be quite similar if one
considers that here the A and B loci are simply
linked to each other in close proximity on the
same chromosome (see p. 637).
18.9.6 The Buller phenomenon
Buller (1931) discovered that a monokaryon of
C. cinereus paired with a dikaryon of the same
fungus could be converted to the dikaryotic
state. The same phenomenon has been reported
in some other bipolar and tetrapolar fungi.
Conversion, i.e. dikaryotization, is brought
about by nuclear migration from the dikaryon
into the monokaryon. Different kinds of combination
(di mon matings) are possible (Raper,
1966).
There are two kinds of legitimate combinations.
(1) In fully compatible combinations, a
monokaryon is compatible with both nuclear
components of the dikaryon, e.g. bipolar (A 1 þ
A 2 ) A 3 or tetrapolar (A 1 B 1 þ A 2 B 2 ) A 3 B 3 . (2) In
hemicompatible combinations, a monokaryon is
compatible with only one of the nuclear components
of the dikaryon, e.g. bipolar (A 1 þ A 2 ) A 2
or tetrapolar (A 1 B 1 þ A 2 B 2 ) A 1 B 1 .Inillegitimate
(incompatible) combinations, a monokaryon is
compatible with neither nuclear component of
the dikaryon, e.g. tetrapolar (A 1 B 1 þ A 2 B 2 ) A 1 B 2
or A 2 B 1 .
Surprising features were discovered in some
such pairings. In compatible pairings using
Schizophyllum it was found that the selection of
a compatible nucleus from the dikaryon to
dikaryotize the monokaryon was not a matter
of chance. Consider the fully compatible di mon
mating (A 1 B 1 þ A 2 B 2 ) A 3 B 3 . If conversion of
the monokaryon by one of the nuclei from
the dikaryon were entirely random, dikaryons
(A 1 B 1 þ A 3 B 3 ) and (A 2 B 2 þ A 3 B 3 ) would be equally
frequent. However, this is not the case; there is
evidence of preferential selection of one mating
type over the other, but the reasons for this
selection are obscure. A second unexpected
feature is the discovery that dikaryotization can
occur in incompatible pairings. A possible reason
for this phenomenon is that somatic recombination
between the nuclei in the original dikaryon
can occur to give rise to a nucleus compatible
with that of the monokaryon (Raper, 1966).
An unusal mating phenomenon which is the
equivalent of the Buller phenomenon has been
discovered in Armillaria mellea. The vegetative
phase of this fungus is mainly diploid, not
dikaryotic. Its mating system is bifactorial, i.e.
tetrapolar, controlled by A and B loci. When
diploid and haploid mycelia are paired in certain
combinations, mating occurs, i.e. the diploid
mycelium is capable of dikaryotizing the haploid
monokaryon (Anderson & Ullrich, 1982).
18.10 Fungal individualism:
vegetative incompatibility
between dikaryons
When genetically distinct dikaryons belonging to
the same species are paired together, they do not
coalesce. Although their hyphae may fuse
together, the cells of the resulting heteroplasmon
die and often become darkly pigmented.
This is the result of vegetative incompatibility. In
contrast, when genetically identical dikaryons
are paired, their mycelia intermingle. Vegetative
incompatibility is readily demonstrated in
culture (Fig. 18.20c) and is recognizable in the
field as black bands of fungal cells at the interface
between adjacent dikaryotic colonies in
decaying tree stumps (Fig. 18.20a). The phenomenon
was first discovered in the bracket fungus
Trametes versicolor, the cause of white-rot in
deciduous trees, but has since been found to be
widespread amongst different ecological groups
of basidiomycetes, including wood rotting, coprophilous,
ectomycorrhizal and plant pathogenic
species, and has led to the concept of fungal
individualism (see Todd & Rayner, 1980; Rayner,
1991a,b). Pairing tests between dikaryotic
isolates facilitate the determination of the
limits and extent of an individual mycelium
of a basidiomycete. In T. versicolor, tree trunks