Introduction to Fungi, Third Edition

MATING SYSTEMS IN BASIDIOMYCETES
509
In both S. commune and C. cinereus it has been
possible to obtain details of the structure of the
sub-loci at a finer level of resolution. For
example, the Aa locus of C. cinereus contains one
gene pair (designated a1 and a2) and the Ab locus
two gene pairs (b1 and b2, d1 and d2), each with a
number of alleles. In the field, many strains have
lost one or more of their maximum complement
of six A genes (Fig. 18.19). The gene products
are the subunits of the heterodimer transcription
factor. The two different subunits (1 and 2)
are encoded by compatible alleles. For example,
a functional heterodimer can be formed from
the product of an a allele at the a1 position
(e.g. a1 1) and a different one at the a2 position
(e.g. a2 2, a2 3, etc., but not a2 1). Likewise,
heterodimers can be formed between the
products of two compatible b or two d alleles,
but there is a great deal of functional redundancy
in the system in the sense that it is
sufficient if only one of six possible heterodimers
is formed. In other words, compatibility between
two strains at the A locus is ensured if compatible
alleles are present at the a or b or d genes.
Casselton and Olesnicky (1998) have calculated
that only 5 6 alleles would be required at each
gene pair (e.g. 5 6 6) to account for the
estimated 160 unique A gene combinations in
C. cinereus.
The genetics of mating type behaviour in the
maize smut fungus Ustilago maydis is, in many
respects, similar to that described for C. cinereus
and S. commune. This fungus is dimorphic, with
a monokaryotic, saprotrophic yeast-like state
which can be readily cultured, and a dikaryotic
mycelial state which is parasitic on maize
and requires living host cells for growth (see
Fig. 23.1). Ustilago maydis is tetrapolar, unlike
most smut fungi which are bipolar. Incompatibility
is governed by two loci, designated
a and b. Unfortunately the functions of the
b mating type factor in U. maydis correspond to
those of the A factor in C. cinereus and S. commune,
and vice versa. There are two alleles at the a locus
(a 1 and a 2 ) and about 25, possibly more, at the b
locus. The a locus encodes a pheromone and a
pheromone receptor; it controls the switch to
filamentous growth but not pathogenicity. The b
locus encodes the production of a heterodimeric
DNA-binding protein involved in self/non-self
recognition and also in pathogenicity; two
different b alleles are required for pathogenic
growth (see p. 643).
Much lower numbers of alleles have been
estimated for the bird’s nest fungi (Gasteromycetes;
see p. 581). For instance, Cyathus striatus
has 4 A and 5 B alleles, and Crucibulum vulgare 3 A
and about 16 B alleles. It has been suggested that
Fig18.19 The A loci of two different field strains of Coprinus cinereus. Neither strain has the complete set of six genes, but none the
less four different functional heterodimers canbe formed from gene products of different (compatible) alleles at the a, b and d genes.
Only one would be sufficient for overall compatibility at the A locus. Note that the position of the d genes is reversed relative to
those of the a and b genes.Genetic recombination events are possible in homologous DNA regions (thick lines) but not within the
regions covered by the gene pairs a, b or d. Redrawn and modified from Casselton and Olesnicky (1998).