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Sexuality


The wood-inhabiting Ascomycetes and Basidiomycetes are either homothallic or heterothallic (Ryvarden and Gilbertson 1993). Homothallic fungi are self-fertile, that is no second mating type is required for sexual reproduction. Fertilization takes place at the same mycelium.

Many Ascomycetes and about 10% of the Basidiomycetes belong to this type. Heterothallism includes both bipolar and tetrapolar fungi. In bipolar (uni-factorial) species, incompatibility is controlled by a series of multiple alleles at one locus. Any dikaryon has two alleles that segregate at the 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 inpopulations of bipolar polypores is over 90%.

In tetrapolar (bifactorial) species, incompatibility is controlled by two se-ries  of multiple alleles at two loci on different chromosomes. The twopairs segregate independently at meiosis. Four different mating types rise from one dikaryon. In a fruit body of an isolate, basidiospores of the mating type A,13x, 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#): Ax13„ and AyBy as well as AxBy and AyBx. In addition, there are hemicompatible matings, in which only one factor is different: AO, and A,By as well as AyBy and AxBy.

The inbreeding level is 25%. The outbreeding level is very high. In Schizo-phyllum  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 pla-centa). 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 biolog-ical 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 ofgenerations, 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,

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