6 Wood Discoloration

28 2 Biology
some nuclear fusions and after haploidization new combinations occur. Usually,
the diploid nuclei are unstable, and their ploidy number is regulated to the
haploid stage by elimination of chromosomes or discharge of sections (Müller
and Loeffler 1992).
Illustrated by the tetrapolar heterothallic Serpula lacrymans,interstockmating
of ten isolates is demonstrated in Table 2.6 (Schmidt and Moreth-Kebernik
1991c). First, the four different mating type monokaryons of each isolate were
obtained after fruit body stimulation (Schmidt and Moreth-Kebernik 1991b)
and subsequent inbreeding according to Table 2.5. Then the 10×4 monokaryons
were paired one with another in all possible combinations on agar. As in S.
lacrymans, like in many Basidiomycetes, only the dikaryon forms clamps, it
can be detected in the light microscope. In contrast, only the monokaryons of
the fungus show abundant arthrospores. The heterokaryons of the type A=B#
and the “false clamps” mycelia (A# B=) can be recognized from the mating
diagram by calculation or by further pairings. The mating types of the isolate
monokaryonsareshownintheuppertablepart.
The mycelium of the F1-dikaryons of S. lacrymans grew faster at about 20 ◦ C
than that of the two parental monokaryons (Schmidt and Moreth-Kebernik
1991a), like this applies also to Lentinula edodes (Schmidt and Kebernik 1987)
and Stereum hirsutum (Rayner and Boddy 1988). Thus, dikaryotic mycelium,
which grows out from compatible monokaryons, looks like a bow tie (Fig. 2.18),
that is, dikaryons can usually be detected macroscopically.
In a sample of ten isolates, theoretically 20 different A and B factors can
occur. In the S. lacrymans sample, there were however only four A and five
B factors. This limited number of mating alleles contrasts with the regular
observation of a high number of mating alleles in other Basidiomycetes (May
et al. 1999) and indicated that S. lacrymans has a narrow genetic base.
Fig.2.18. Bow tie-like outgrowth of the
faster growing dikaryon (D) ofSerpula
lacrymans from the slowly growing
monokaryons (M)
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