Introduction to Fungi, Third Edition

570 HOMOBASIDIOMYCETES
non-outcrossing strains may have the selective
advantage in being able to exploit a particular
habitat. They are also resistant to potential
takeover or conversion to unfit or unstable
genomic combinations.
Like many wood-rotting basidiomycetes,
Stereum spp. show antagonistic reactions in the
form of discoloured barrage zones where different
strains confront each other, as seen in
transverse sections of tree branches which
contain more than one heterokaryon (Rayner &
Boddy, 1988). Similar zones also develop on
culture plates when dissimilar heterokaryons
meet. Confrontations between heterokaryotic
(secondary) and homokaryotic (primary) mycelia,
equivalent to ‘di mon’ mating in Coprinus
cinereus and Schizophyllum commune (see p. 508),
have been studied in S. hirsutum in search of
evidence of the ‘Buller phenomenon’ (Coates &
Rayner, 1985a). Such confrontations may be fully
compatible, where both types of nuclei from the
heterokaryon are potentially competent in bringing
about dikaryotization of the homokaryon, or
hemi-compatible, where only one kind of
nucleus is competent. The Buller phenomenon
does occur in S. hirsutum, with either one or two
new heterokaryons formed per interaction. The
genotypes of the new heterokaryons can be
classified as composite (acceptor homokaryon
plus heterokaryon component), parental (identical
with the donor heterokaryon) and novel
(different from all three parental and composite
combinations). There is preferential selection of
a non-sib-related component of the donor heterokaryon,
i.e. a nucleus from an unrelated strain,
not derived from the same fruit body as the
recipient homokaryon.
The bow-tie reaction
Because S. hirsutum is bipolar, 50% of sib-matings
(i.e. matings between primary mycelia from
basidiospores of the same basidiocarp) are
compatible, whilst non-sib matings are uniformly
compatible because of multiple alleles
at the mating type locus. In about one-third of
incompatible sib-related matings a distinctive
pattern of mycelial interaction occurs in agar
cultures termed the ‘bow-tie’ reaction (see
Fig. 19.28a; Coates et al., 1981). A band of
appressed sparse mycelium, shaped like a bowtie
in being widest at the edges, develops
between the two homokaryons, and is bounded
by narrow regions with exude watery droplets.
Hyphae within the bow-tie band often burst,
have granular contents and produce abundant
irregular lateral branches (Figs. 19.28b,c). It is
believed that the bow-tie region of a culture
is occupied by a weakly growing heterokaryon.
The bow-tie region often expands and may
replace the mycelium of one of the component
monokaryons. In some cases, a darkened zone
of mutual antagonism develops between them.
The development of bow-ties is controlled by
a multi-allelic genetic factor, the B-factor,
unlinked to the mating type C-factor.
Heterozygosity of the B-factor results in bowtie
formation which is only visible between
incompatible homokaryons. The bow-tie
phenomenon, although discovered in S. hirsutum,
is not a feature peculiar to this fungus. Similar
behaviour has been found in mating type
compatible pairings of some other basidiomycetes,
e.g. S. gausapatum, Phanerochaete velutina,
Mycena galopus and Coniophora puteana (Coates &
Rayner, 1985b).
Mating in Stereum sanguinolentum
The mating behaviour of S. sanguinolentum is also
unusual (Calderoni et al., 2003). Karyogamy,
meiosis and post-meiotic mitosis may occur in
the four-spored basidia or spore primordia.
Most basidiospores contain two nuclei and are
heterokaryotic, producing a mycelium capable
of fruiting. Stereum sanguinolentum therefore
shows secondary homothallism or pseudohomothallism.
Pairings between single-basidiospore
isolates from the same basidiocarp (intrabasidiome
pairings) are somatically compatible
but virtually all pairings between isolates from
different basidiocarps (inter-basidiome pairings)
are somatically incompatible, with demarcation
lines developing between them. This indicates
that in S. sanguinolentum there are numerous
vegetative incompatibility groups (Calderoni
et al. 2003).