Introduction to Fungi, Third Edition

506 BASIDIOMYCOTA
fungus Bulbillomyces farinosus (anamorph Aegerita
candida). The anamorphic state (Fig. 18.18)
appears on the surface of wet wood from freshwater
streams as white clusters of clamped cells
between which air is trapped, giving the propagule
buoyancy. This is a typical feature of aeroaquatic
fungi which form asexual propagules
in air on the surface of leaves and branches of
trees previously submerged in fresh water (see
Section 25.3).
18.9 Mating systems in
basidiomycetes
18.9.1 Homothallic systems
About 10% of the Basidiomycota which have been
tested are homothallic (Raper, 1966). Three types
of homothallic behaviour may be distinguished,
namely primary, secondary and unclassified
homothallism.
Primary homothallism
In Coprinus sterquilinus a single basidiospore
germinates to form a mycelium, which soon
becomes organized into binucleate segments
bearing clamp connections at the septa. There
is no genetic distinction between the two nuclei
in each cell, and this mycelium is capable of
forming fruit bodies.
Secondary homothallism
(pseudohomothallism)
In Coprinus ephemerus f. bisporus the basidia
bear only two spores, but the spores are heterokaryotic.
After meiosis two nuclei enter each
spore and a mitotic division may follow. On
germination, a single spore forms a dikaryotic
mycelium with clamp connections, capable of
fruiting. Occasional spores, on germination, give
rise to non-clamped mycelia, and fruiting occurs
only if these are paired in certain combinations,
showing that the fungus is basically heterothallic.
The cultivated mushroom, Agaricus bisporus,
also has a mating system of this type. Although
most basidia bear two spores, four-spored basidia
do occur, and when monosporous cultures
derived from basidiospores from four-spored
basidia are crossed, they produce fruiting mycelia
in certain combinations. It has been suggested
that a simple bipolar system (see below) is
operating (Miller, 1971; Raper et al., 1972).
This situation occurs in a number of other twospored
basidiomycetes and is closely paralleled
by that found in certain four-spored ascomycetes
such as Neurospora tetrasperma (Raper, 1966).
Fungi showing both secondary homothallism
and heterothallic behaviour are said to be
amphithallic.
Unclassified homothallism
The four-spored wild mushroom, Agaricus campestris,
is homothallic in the sense that a mycelium
derived from a single spore is capable of fruiting.
There is nuclear fusion in the basidium, followed
by two nuclear divisions, presumably meiotic.
However, paired nuclei, conjugate nuclear divisions
and clamp connections have not been
observed.
18.9.2 Heterothallic systems
Amongst the remaining 90% of the
Basidiomycota reported to be heterothallic, we
can distinguish bipolar and tetrapolar
conditions.
Bipolar
In species such as Coprinus comatus (the shaggy
ink-cap) and Piptoporus betulinus (the birch polypore),
when mycelia obtained from single spores
from any one fruit body are mated together,
dikaryons are formed in half the crosses. This can
be explained on the basis of a single gene
(or factor) with two alleles. Because only a
single factor is involved, the genetic basis for
the bipolar condition is described as unifactorial.
Segregation of the two alleles at meiosis ensures
that a single spore carries only one allele.
Dikaryons are only formed between monokaryons
carrying different alleles at the mating
type locus. In fact, it is known that there may
be numerous mating type alleles in a population
of fruit bodies collected over a wide area (see
below). About 25% of Basidiomycota examined
have been shown to be bipolar. Most members of
the Uredinales and Ustilaginales have mating