Growth, Differentiation and Sexuality

364 L.A. Casselton and M.P. Challen
only between proteins encoded by different alleles
of the genes. In any one mating in U. maydis (e.g.
bW1-bE1×bW2-bE2), two functionally equivalent
heterodimers form (bW1/bE2 and bW2/bE1;
Kämper et al. 1995). The 5 ′ regions of the genes
are highly variable, as is the promoter region,
so a compatible gene combination cannot be
generated by recombination. Such combinations
can be made experimentally, and are sufficient
without further mating to activate b-regulated
development (Gillissen et al. 1992).
In C. cinereus, we see the same pairs of
HD1/HD2 genes, but the locus now extends over
some 25 kb and contains representative members
of three different pairs of genes (designated a, b
and d), all of which are multiallelic (Kües et al.
1992; Pardo et al. 1996). These three groups
of genes have clearly arisen by duplication but
are now functionally independent (i.e. they are
paralogous). Mating partners are compatible
provided they have different alleles of just one pair
of genes. Because of the redundancy in function,
it is common to find that few A loci contain all
six genes – only one such locus was identified
amongst nine investigated (Pardo et al. 1996). The
A5 and A3 loci, for example, have only one gene of
the d pair, A3 has the HD1 gene, and A5 has the
HD2 gene; these encode compatible proteins that
can heterodimerise in A3 × A5matings.Thereare
inactive pseudogenes in some loci; the first locus
sequenced, A42, contained an additional HD1
gene which was thought to represent a c gene pair
(Kües et al. 1992) – hence, the designation d for
the third gene pair! If mating partners contained
different alleles of all three gene pairs, six different
but functionally equivalent heterodimers would be
formed. Locus organisation is maintained because
the DNA sequences that comprise allelic versions
of the genes (both coding and flanking sequence)
are sufficiently different to prevent homologous
recombination between alleles and, thus, bring
compatible combinations of genes together.
During evolution of this complex, there has been
random mixing of the three groups of genes
to generate many different allele combinations
(May and Matzke 1995; Pardo et al. 1996). Large
numbers of A mating specificities can be generated
by having relatively few alleles of the three groups
of genes. A population analysis of the C. cinereus
A locus identified four alleles of the a genes, ten
of the b genes and three of the d genes, sufficient
to generate 120 genetically different A loci (May
and Matzke 1995). Recombination between the
different groups of genes at the C. cinereus A locus
can still occur with a frequency of 0.07% (Day 1960,
1963b), which is due to a short 7.0-kb sequence
of homology that exists between the a and the b
gene pairs in all versions of the A locus (Kües et al.
1992; May and Matzke 1995). Recombination has
the beneficial effect of generating non-parental
A mating specificities but the disadvantage that
these are fully compatible with all other sibs
from the mating. Increasing sib-compatibility
reduces the efficiency of an outbreeding system, so
evolutionary pressure will have acted to maintain
close linkage of the gene pairs. The a gene pair
in the C. cinereus locus corresponds to the Aα
locus (hence, α-complex in Fig. 17.4), and the b
and d genes to the Aβ locus (hence, β-complex
in Fig. 17.4) described by Day (1960, 1963b).
In S. commune, where recombination between
A genes was first described (see Raper 1966),
the two groups of genes are separated at much
greater distance (1-17cM), depending on the
strains (Raper et al. 1960), and the Aα and Aβ
genes clearly reside at two distinct loci. The Aα
locus contains a single pair of HD1/HD2 genes for
which there are nine alleles (Stankis et al. 1992).
Asyet,onlyasinglegeneoftheAβ complex has
been identified (Shen et al. 1996) but there are
apredicted32Aβ specificities (Raper 1966), and it
is likely that at least two pairs of genes would be
needed to generate this number.
2. Homeodomain Protein Interactions
An important property of the homeodomain proteins
encoded by the HD1 and HD2 genes is their
ability to discriminate between large numbers of
potential dimerisation partners. With 25 alleles of
the b genes of U. maydis, we can calculate that there
are potentially 625 heterodimer interactions; 25
are self-interactions that are incompatible whereas
the remaining 600 are predicted to be possible
and all equally capable of activating b-regulated
development. In C. cinereus, there are many
more incompatible interactions because proteins
encoded by paralogous genes are also unable to
heterodimerise (Pardo et al. 1996). Studies with U.
maydis bproteinsandC. cinereus and S. commune
A proteins have shown that specificity resides in
the N-terminal domains; exchanges between these
domains generated altered specificities, and these
domains are sufficient to mediate protein–protein
dimerisation in vitro (Banham et al. 1995; Kämper
et al. 1995; Magae et al. 1995). The N-terminal