Growth, Differentiation and Sexuality

370 L.A. Casselton and M.P. Challen
Kües et al. 2002). Several interesting genes have
been characterised (see Chaps. 15 and 19, this
volume) but we are a long way from understanding
the downstream pathways in any of the fungi
that we have described above. Progress is likely
to change dramatically now that the genome
sequences of these fungi are becoming available
(http://www.broad.mit.edu/annotation/fungi/fgi/),
and microarray technology will aid identification
of major subsets of genes whose transcription
alters in response to mating.
As yet, few genes have been identified that
are direct targets of the pheromone response factor
(other than the mating type genes themselves)
or the homeodomain protein heterodimer. Fortuitously,
a direct target gene of the b heterodimer
of U. maydis resides in the a2 mating type locus,
lga2 (Romeis et al. 1997) but its function appears
to be irrelevant to mating (Bortfeld et al. 2004).
The lga2 promoter has a sequence motif that binds
the b proteins, and this is remarkably similar to
the binding site of the corresponding a1/a2 heterodimer
of S. cerevisiae.ThebproteinsofU. maydis
are much longer than the yeast proteins and,
unlike a1/α2, the b proteins are transcriptional activators.
Another b-target gene (frb52), encoding
a putative DNA polymerase, was identified in a major
screen to detect genes activated or repressed by
the b heterodimer (Brachmann et al. 2001). Several
interesting gene functions were identified that
act downstream of b, one being a MAPK (Kpp6)
required for fungal penetration of the host tissues
(Brachmann et al. 2003). In C. cinereus, a potential
target of the A protein heterodimer was identified
by the classical genetic approach of looking for
mutants blocked in clamp cell formation. The clp1
gene is induced by activating the A pathway, and
has an essential sequence motif in its promoter that
resembles the b protein binding sites in lga2 and
frb52 (Inada et al. 2001).
Expressed off a constitutive promoter, clp1
is sufficient to activate the entire clamp cell
programme, showing that although there may
be many promoters that bind the HD1/HD2
heterodimer, clp1 may be the only critical target
required to activate the pathway in C. cinereus.
Makino and Kamada (2004) have identified several
mutants that are defective in nuclear migration;
these could be potential targets of the predicted
pheromone response factor Hmg1. The Pcc1
protein has been implicated in regulating clamp
cell fusion (see above) but the pcc1 mutant has an
unusual phenotype, which is why it was identified;
it constitutively produces unfused clamp cells,
as demonstrated by Murata et al. (1998). These
authors suggested that Pcc1 might act as a repressor
of the clamp cell pathway. This may well be
an essential, if indirect function of an activated
pheromone response. The A and B regulated
pathways alternate during dikaryotic cell growth
and are mutually exclusive. The A pathway leads
to tip cell extension and nuclear division, whereas
the B pathway requires cell–cell recognition which,
as in mating S. cerevisiae cells, is likely to require
cell cycle arrest before cell fusion.
IV. Concluding Remarks
The mating type genes are not conserved between
basidiomycetes and ascomycetes (see Chap. 15, this
volume, and Hiscock and Kües 1999 for a detailed
account of both ascomycete and basidiomycete
mating types) but this does not mean that the
pathways they regulate are different. The mating
typelocusissimplyastrategyforpreventingconstitutive
sexual development by requiring mates to
bring together the full complement of necessary
genes (see Casselton 2002); the genes that are
sequestered there are the result of evolutionary
accident. Pheromone signalling is an essential
part of mating in both groups of fungi, and we
see a strong conservation in the response induced
by pheromone signalling, once we look beyond
the apparent differences in life style. There are
only two mating types in ascomycetes, and the
pheromones and receptors are not mating type
genes – they are regulated by the mating type genes
(see Coppin et al. 1997, and Chap. 16, this volume).
As in U. maydis and C. neoformans, pheromones
can act as chemoattractants, and are secreted into
the surrounding environment where binding to
a compatible receptor may induce the formation of
mating structures that aid cell fusion. S. cerevisiae,
U. maydis and C. neoformans all change shape in
response to pheromone stimulation, increasing
in size or forming pegs or filaments that orient
towards the source of pheromone and promote
cell fusion. In the filamentous ascomycetes such
as Neurospora crassa, both mating types may
differentiate male and female cells, the female cells
being the protoperithecia which, when fertilised,
develop into the fruiting bodies in which the
sexual cycle is completed. Emerging from the protoperithecia
is a filament known as the trichogyne