Growth, Differentiation and Sexuality

any gene with an obvious relationship to fertilization.
Although this study has allowed us to identify
several candidate genes whose role in the sexual
cycle remains to be established, it is surprising
that no STE gene identified in yeast has been
found in this search. Coppin et al. (2005b) have
demonstrated that the MAT regulatory proteins
control genes required for post-transcriptional
modification of pheromone precursors. None of
these genes, which are well defined from yeast
studies, have been found, except KEX1. This
suggests that either MAT transcription factors are
not the direct activators/repressors of these genes,
or that each MAT transcription factor binds different
cofactors and/or has different binding sites
in different target genes. This possibility would
preclude the identification of common binding
sites from the different target genes expressed
in female organs and male cells. Biochemical
evidence for the binding of MAT transcription
factors to the available target genes should be the
first step to resolve this question.
3. A Possible Common Scheme
for Fertilization Control in Euascomycetes
The dual activator/repressor functions found for
the MAT transcription factors in P. anserina may
be extended to N. crassa. The prepheromone gene
ccg-4 is transcribed specifically in mat A strains,
as demonstrated by Bobrowicz et al. (2002). This
gene was also found to be transcribed in the a m33
mutant strain, which contains a mutant mat a-1 allele
but has wild-type mating behavior. Bobrowicz
and coworkers proposed that the mat a-1 m33 allele
retains its ability to activate the a-like pheromone
gene, but has lost a repressing function for ccg-4.
This situation is reminiscent of the fertilization
control evidenced in P. anserina, although the a m33
strain does not display the self-fertile phenotype
observed in P. anserina mat mutants. It must be
noted that the self-fertile phenotype is hardly detectable
in the absence of mutations that increase
theproductionofmalecells,andhadescapedour
initial examination of mat mutants.
The deletion of the mating-type locus of the
self-compatible G. zeae (Fig. 15.2) resulted in one
half of the strains being completely sterile but the
other half produced perithecium-like structures
(Desjardins et al. 2004). These structures were
smaller in size and more variable in shape than
perithecia produced by wild-type strains, and did
not contain any ascospores. We would suggest that
Mating Types in Euascomycetes 317
these perithecium-like structures result from the
basal expression of the mating-type target genes
in a context where they are neither activated nor
repressed. This basal expression of the target genes
may be sufficient for fertilization, but appropriate
regulatory proteins would be required for further
development such as internuclear recognition. The
perithecia-like structures were not produced in
strains carrying a deletion of either of the MAT1-1
or MAT1-2 mating-type sequences. This observation
suggests that MAT1-2-1 represses the MAT1-1
target genes required for fertilization while the
MAT1-1 products have a similar effect on MAT1-2
target genes, according to the regulatory pathway
evidenced in P. anserina. Therefore, even in selfcompatible
species, the mating-type transcription
factors may have a repressor activity on the target
genes required for fertilization. The compatibility
between the MAT1-1-1 and MAT1-2-1 activator
and repressor functions in the same genome could
be obtained by an epigenetic event, silencing either
the one or the other mating-type information,
thus resulting in a functionally self-incompatible
mechanism. A similar hypothesis was initially
proposed by Metzenberg and Glass (1990).
It appears that in self-incompatible Cochliobolus
species, MAT1-1-1 encodes a protein that contains
an alpha box domain and a partial HMG box
domain, and MAT1-2-1 encodes protein containing
a HMG box domain plus a partial alpha box
domain (Lu and Turgeon, unpublished data; see
Sect. III.C.1 and Fig. 15.5). Both MAT1-1-1 and
MAT1-2-1 proteins alone are capable of promoting
sexual development in a heterologous genetic
background. However, both proteins may be inactive
in their respective self-incompatible genetic
backgroundduetothepresenceofacommonrepressor.
A mutation that blocks the repressor activity
would allow MAT to initiate the early stages
of sexual development (e.g., fruiting-body formation)
in the absence of the opposite mating type.
A C. heterostrophus REMI mutant, generated in
a MAT1-2 strain, is available that produces abundant,
but barren pseudothecia when selfed. This
type of mutant, although self-compatible, is able to
form fertile pseudothecia when mated to a MAT1-1
but not a MAT1-2 albino tester, indicating the selfincompatible
mating specificity is maintained. The
native MAT1-2-1 gene is intact in the REMI mutant,
and unlinked to the mutation site (Turgeon
and Lu, unpublished data). These data suggest that
both MAT1-1 and MAT1-2 haploid strains could
initiate sexual development if a protein is blocked