Growth, Differentiation and Sexuality

368 L.A. Casselton and M.P. Challen
(see Raper 1966). In C. cinereus,insomebutnotall
backgrounds, such phenotype is induced in transformants
carrying extra B genes (Kües et al. 2002).
Strains with mutations at both the A and B loci are
constitutive for the entire mating pathway; they
resemble in most respects true dikaryons, and produce
fertile fruiting bodies in which meiosis is normal
(Swamy et al. 1984). These strains have proved
invaluable in many genetic selection programmes
because they permit the selection of recessive mutations
that affect all stages of development, from
dikaryosis to sporulation, and which would not be
detectable in normal matings with two genetically
different nuclei (Granado et al. 1997; Cummings
et al. 1999; Inada et al. 2001; Kamada 2002).
Molecular analysis of two A mutants of C.
cinereus revealed that there had been a major
deletion within the A locus, such that only a single
gene remained, and this was a chimeric gene
consistingofthemajorpartofanHD2 gene fused
to the 3 ′ end of an HD1 gene (Kües et al. 1994;
Pardo et al. 1996). Such fusion genes are sufficient
to constitutively activate the clamp cell pathway
in the mutant and in any other background into
which the gene is introduced (Kües et al. 1994).
As mentioned above, heterodimerisation brings
together different functional domains of an active
transcription factor, the chimeric A gene is predicted
to encode a minimal heterodimer that has
the essential HD2 homeodomain for DNA binding
and sufficient of the HD1 protein to provide the
nuclear targeting sequences and the activation domain
(Asante-Owusu et al. 1996). Self-compatible
mutations in the B genes may occur in either
the receptor genes or the pheromone genes and,
in either case, lead to constitutive activation
of the pheromone-dependent pathway, nuclear
migration and clamp cell fusion. Receptor gene
mutations may result in constitutive activation
of the receptor or altered specificity towards
a normally incompatible pheromone whereas
pheromone mutations alter receptor specificity
(Olesnicky et al. 1999, 2000; Fowler et al. 2001).
D. Cryptococcus neoformans Mating Type
Locus
The two versions of the C. neoformans mating type
locus are totally dissimilar to the basidiomycete
loci described so far. Each locus extends over
some 100 kb (Lengeler et al. 2002; Hull et al. 2005).
The alternative loci of serotype D strains are
illustrated diagrammatically in Fig. 17.6. As can
be seen, within these sequences there are mating
type-specific genes encoding the pheromone receptors
(STE3a and STE3α) and the corresponding
pheromones (MFα1, MFα2 and MFα3 in the
MATα locus and MFa1, MFa2 and MFa3 in the
MATa locus). Pheromone signalling is important
in mating and, as shown in Fig. 17.3, it induces
mating structures in mating cells in the same way
as it does in U. maydis.Oncethematingcellshave
fused, and by analogy with homobasidiomycetes,
pheromone signalling is expected to be important
in maintaining the dikaryophase, particularly
clamp cell fusion. Based on our understanding
of mating in homobasidiomycetes, one would
expect that heterodimerisation between an HD1
and an HD2 protein would be required for mate
recognition and sexual development in C. neoformans.
This is indeed so. A gene encoding an HD1
protein (SXI1α)residesattheborderoftheMATα
locus (Hull et al. 2002), and a gene encoding an
HD2 protein (SXI2a) resides at the corresponding
position of the MATa locus (Hull et al. 2005). As
in S. cerevisiae, mating is necessary to bring this
compatible pair of proteins together. Sxi1α has
been implicated in a/α-cell identity (Hull et al.
2002), and a direct interaction between the proteins
(as determined by a two-hybrid analysis) is
required for sexual development (Hull et al. 2005).
In addition to the pheromone and receptor
genes and the HD1 and HD2 genes, theMAT loci
contain mating type-specific genes encoding elements
of the MAPK phosphorylation pathway,
Ste20p, the first kinase in the pathway, Ste11p, the
first kinase in the MAPK module (MAPKKK) and
the homologue of the S. cerevisiae pheromone response
pathway target transcription factor, Ste12p.
There are many other genes within these extended
loci that have no obvious role in mating; the different
alleles of these genes are variable in position, sequence
and orientation, variability which, together
with transposons and remnants of transposons,
acts to generate extended regions of homologous
chromosomes where recombination is suppressed.
Fraser and Heitman (2004) have pointed out that
the unusual organisation of genes around the extended
MAT loci in both C. neoformans and U.
hordei, and the fact that there are just two mating
types, rather than the many seen in other basidiomycetes,
have remarkable parallel with sex chromosomes
in other organisms, and they present
models to explain how these complex loci may have
evolved.