6 Wood Discoloration

30 2 Biology
a haploid mycelium with defined tester strains whose species affiliation is
known only results in a dikaryotic/diploid mycelium if the isolate belongs to
the same species. Mating is also possible between dikaryotic/diploid mycelium
and monokaryotic/haploid mycelium (Buller phenomenon) in this way that
one nucleus of the dikaryon enters a monokaryon of the same species. Complete
absence of interfertility between monokaryons of the True dry rot fungus, S.
lacrymans, and the Wild dry rot fungus, Serpula himantioides, (Harmsen
et al. 1958) showed that both fungi are independent species. That is the True
dry rot fungus should no more described as domestic variant of the wild
species adapted to buildings, which was later confirmed by DNA techniques
(Chap. 2.4.2.2).
Intersterility must be approached cautiously, however, because intersterile
populations (intersterility groups, ecotypes) occur that cannot be separated
morphologically. For example, in Heterobasidion annosum (Chase and Ullrich
1990), monokaryons isolated from fruit bodies sampled in pine forests
(P-isolates) did not pair with isolates from spruce trees (S-isolates) (Korhonen
1978a), and F-isolates were specialized for fir (Capretti et al. 1990). The different
groups have been recently attributed to three distinct species (Niemelä and
Korhonen 1998). Comparably, the five intersterility groups A, B, C, D, E within
the annulate Armillaria mellea complex (Korhonen 1978b) were assigned to
five biological species (Guillaumin et al. 1993). For edible mushrooms of the
Pleurotus species, three North American, eight European, and five Asian intersterility
groups have been found (Bao et al. 2004a).
Another genetic system referred to as somatic or vegetative incompatibility
restricts plasmogamy between genetically different heterokaryotic dikaryons.
In 1929, Fomitopsis pinicola was the first basidiomycete to be studied by means
of somatic incompatibility (cf. Högberg et al. 1999). The somatic incompatibility
system can be defined as the rejection of nonself mycelia following hyphal
anastomosis (Worrall 1997), thus assuring the isolation of unrelated individuals
in nature. Cultures of the same genotype form a common mycelium, while
cultures of different genotypes of a species or of different species separate themselves
by a demarcation zone. Two isolates are incompatible if they carry different
alleles at one or more vic loci. Self/nonself recognition is normally related
to genetic uniqueness (Hansen and Hamelin 1999). Thus, there is a correspondence
between the delimitation of genets by DNA fingerprints and vegetative
compatibility tests. In some Basidiomycetes, however, vegetatively compatible
isolates are not necessarily genetically identical or similar individuals, clones
or genets, but closely related genets by chance may share similar vegetative
compatibility alleles and do not recognize self from nonself. Kauserud (2004)
grouped the European isolates of S. lacrymans into five widespread vegetative
compatibility groups (VCGs). Due to low genetic variation of the fungus, the
VCGs may not represent clones or inbred lineages, but rather different genets
by chance share similar vic alleles (Kauserud et al. 2004a).
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