6 Wood Discoloration

2.3 Sexuality 27
the basidiospores have one allele and half the other. Compatible matings occur
between monokaryons with different mating type factors. The inbreeding level
is 50%. The outbreeding level in populations of bipolar polypores is over 90%.
In tetrapolar (bifactorial) species, incompatibility is controlled by two series
of multiple alleles at two loci on different chromosomes. The two pairs
segregate independently at meiosis. Four different mating types rise from one
dikaryon. In a fruit body of an isolate, basidiospores of the mating type AxBx,
AxBy,AyBx and AyBy develop. These spores germinate to monokaryons. Fully
compatible matings of monokaryons (+ in Table 2.5) occur when both factors
are heterozygous (A#B#): AxBx and AyBy as well as AxBy and AyBx. In addition,
there are hemicompatible matings, in which only one factor is different: AxBx
and AxBy as well as AyBy and AxBy.
The inbreeding level is 25%. The outbreeding level is very high. In Schizophyllum
commune 450 A factors and 90 B factors can combine to over 40,000
mating types (Raper and Miles 1958). Many Ascomycetes and about 25% of
the examined Basidiomycetes are bipolar heterothallic (e.g., Oligoporus placenta).
About 65% Basidiomycetes are tetrapolar (Raper 1966). Bipolar mating
predominates among brown-rot fungi and tetrapolar mating among white-rot
fungi (Rayner and Boddy 1988). Of 25 investigated brown-rot polypores, 17
were bipolar, three were tetrapolar, three were heterothallic with type of mating
system undetermined, one was homothallic, and one was reported by different
authors as bipolar and tetrapolar (Ryvarden and Gilbertson 1993). The biological
significance of heterothallism is that inbreeding is limited and outbreeding
is enhanced, promoting gene flow between populations and decreasing the rate
of speciation.
Combination and recombination of the genetic material with plasmogamy,
karyogamy, and haploidization, but without sexual organs, gamets and changes
of generations, can take place by parasexuality, particularly in Deuteromycetes
(Jennings and Lysek 1999). Nuclei of a hypha migrate by anastomosis into
another hypha and multiply and spread there. In the case of a heterokaryon,
Table 2.5. Mating scheme of tetrapolar heterothallic fungi
AxBx AxBy AyBx AyBy
AxBx − A B +
AxBy A − + B
AyBx B + − A
AyBy + B A −
− incompatible (A=B=), + compatible (A#B#)
A common-A heterokaryon (A=B#): conjugate nuclear division and clamp formation
blocked, variable nucleus content per hypha,
B common-B heterokaryon (A#B=): nuclear migration and clamp cell fusion blocked (“false
clamps”)
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