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