Introduction to Fungi, Third Edition

MATING SYSTEMS IN BASIDIOMYCETES
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systems of this type, although a few have more
complex systems.
Tetrapolar
In the coprophilous ink-cap Coprinus cinereus or
the wood-rotting Schizophyllum commune, fertile
dikaryons result in one-quarter of the matings
when primary mycelia derived from basidiospores
from a single fruit body are intercrossed.
The explanation originally proposed for this
situation was that incompatibility is controlled
by two genes (factors), with two alleles at each
locus. Because two separate factors are involved,
the genetic basis is termed bifactorial. Thus we
can denote the two genes as A and B and their
two alleles as A 1 , A 2 and B 1 , B 2 , respectively.
Consider the cross of a monokaryon bearing A 1 B 1
with another bearing A 2 B 2 . This would result in
a fertile dikaryon (A 1 B 1 þ A 2 B 2 ). Such a dikaryon
would form spores following meiosis and the
spores would be of four kinds: A 1 B 1 , A 2 B 2
(parentals), A 2 B 1 and A 1 B 2 (recombinants). In
most cases studied, the proportions of the four
kinds of spore are equal, showing that the A
and B loci are unlinked, i.e. borne on different
chromosomes.
Fertile dikaryons are only formed when the
alleles present at each locus in the opposing
monokaryons differ e.g. in crosses of the type
A 1 B 1 A 2 B 2 or A 2 B 1 A 1 B 2 . Where there is an
identical allele at either or both loci the cross
is unsuccessful. Thus the success of inbreeding
within the spores of any one fruit body is only
25% in tetrapolar species as compared with 50%
in bipolar forms.
A species with tetrapolar heterothallism
whose life cycle is unusual and difficult to
interpret is Armillaria mellea. Most of the cells of
the mycelium are monokaryotic, and there is no
evidence of clamp connections in the mycelium
or the rhizomorphs. Fruit body primordia arise
from the monokaryotic rhizomorphs, and the
cells of the young primordia are also monokaryotic.
However, cells making up the gill tissue
are dikaryotic, and these dikaryotic hyphae are
associated with clamp connections, whilst the
monokaryotic cells formed in the remaining
tissue of the stem and cap have no clamps.
Estimations of nuclear volume in monokaryotic
and dikaryotic cells suggest that the nuclei of
monokaryotic cells are diploid, whilst those of
dikaryotic cells are haploid. It is presumed that
the diploid nuclei undergo haploidization by an
unknown mechanism during the formation of
gill initials. Within the basidia, nuclear fusion
and meiosis occur, and a single meiotic product
enters each basidiospore. In the spore, the
nucleus divides mitotically, and one daughter
nucleus from each spore migrates back into the
body of the basidium and degenerates (Korhonen
& Hintikka, 1974; Tommerup & Broadbent, 1974;
Ullrich & Anderson, 1978; Anderson & Ullrich,
1982).
Variations in the life cycle of A. mellea have
been reported. In a form designated as ‘Japanese
A. mellea’, Ota et al. (1998) presented evidence that
four haploid nuclei appear after meiotic division
of the diploid nucleus in the young basidium.
These haploid nuclei fuse in pairs, resulting in
two diploid nuclei which migrate into two of
the developing basidiospores where they divide
mitotically. One nucleus from each basidiospore
returns to the basidium, leaving the spore
containing one diploid nucleus. Occasionally
nuclear migration fails to occur and the
spore remains binucleate. Spores with diploid
nuclei can complete the life-cycle by forming
a mycelium competent to fruit. Ota et al. (1998)
therefore concluded that the ‘Japanese
A. mellea’ illustrates a kind of secondary
homothallism.
18.9.3 Multiple alleles, complex loci
Although a single spore from one fruit body of
S. commune or C. cinereus is compatible with only
one-quarter of its fellow spores, crosses between
spores from fruit bodies of different origin often
result in 100% mating success, i.e. a spore from
one fruit body can mate successfully with 100%
of the spores from a different fruit body. The
explanation of this phenomenon is that a large
number of alleles is present in a population
representing the species as a whole, instead of
the single pair of alleles at each locus present
in any one dikaryotic mycelium. Suppose that a
second fruit body had the composition (A 3 B 3 þ
A 4 B 4 ), then all the four kinds of spore it produced,