Introduction to Fungi, Third Edition

508 BASIDIOMYCOTA
A 3 B 3 , A 3 B 4 , A 4 B 3 and A 4 B 4 would be compatible
with all the spores of the original fruit body, on
the assumption that the essential requirement
for fertility is that in any cross both alleles
should differ at both loci. This high value for
outbreeding success implies the existence of a
large number of different mating type factors,
and estimates based on isolates from worldwide
collections of fruit bodies indicate that the
number of mating type factors of certain species
may be many thousands (Raper, 1966).
Our understanding of the structure and
function of the mating type factors is derived
from studies of three species, namely the two
Homobasidiomycetes S. commune and C. cinereus,
and the maize smut fungus Ustilago maydis
(Ustilaginomycetes). Extensive literature is available
on general aspects of this topic (see Kües &
Casselton, 1992; Kämper et al., 1994; Kothe, 1996;
Kronstad & Staben, 1997; Brown & Casselton,
2001; Casselton, 2002) and on the individual
species C. cinereus (Kües, 2000), S. commune
(Stankis et al., 1990; Ullrich et al., 1991; Kothe,
1999) and U. maydis (Banuett, 1995).
18.9.4 Functions of the A and B loci
There is a close similarity between the functions
and structure of the A and B mating type loci
in S. commune and C. cinereus as described in
Table 18.2. The fact that they influence
many different functions indicates that their
gene products are active as regulatory proteins.
The A locus encodes two peptides which together
make up a heterodimer transcription factor,
i.e. the molecule is active only if its two halves
are different from each other (see below and
Fig. 23.8). The B locus of S. commune and C. cinereus
directly encodes the peptide pheromone and a
transmembrane receptor for pheromones of
compatible strains. In this way it differs from
the mating system of the ascomycete yeast
S. cerevisiae in which the mating type loci
encode regulatory genes whose products stimulate
the transcription of pheromone and pheromone
receptor genes located elsewhere in the
genome (see Fig. 10.5).
18.9.5 Structure of themating type factors
The results of crossing experiments between
compatible monokaryons indicate that recombination
may occur within the A and B loci
to give novel mating types. This implies that
both loci are complex. For S. commune it has
been proposed that the A locus contains two subloci,
Aa and Ab. Similarly the B locus contains two
sub-loci, Ba and Bb. For each sub-locus, pairing
tests revealed a number of ‘alleles’: 9 Aa, 32Ab,
9 Ba and 9Bb (Ullrich et al., 1991). Recombination
between the Aa and Ab ‘alleles’ gives rise to 288
(i.e. 9 32) different A specificities. When these
are multiplied by the 81 B specificities, over
20 000 possible mating types are generated. In
C. cinereus there are an estimated 160 A specificities
and 79 B specificities which together
generate over 12 000 mating types (Casselton,
2002). The actual number of Aa and Ab alleles is
not known (Casselton & Olesnicky, 1998).
Table18.2. Functions of the A and B loci in Schizophyllum commune and Coprinus cinereus. The functions operate
only if there are different specificities at the A and B loci.
Locus
A-regulated
B-regulated
Function
Pairing of nucleiin dikaryon
Initiation of clamp cell formation
Synchronized nuclear division
Septation
Nuclear exchange between monokaryons
Septal dissolution and nuclear migration
Peg formation and clamp cell fusion
Pheromone production