Growth, Differentiation and Sexuality

318 R. Debuchy and B.G. Turgeon
in the self-incompatible genetic background. The
REMI mutated gene may correspond to the putative
repressor hypothesized above.
B. Functions of Mating-Type Genes During
Fruiting-Body Development
Mating-type genes are required for the postfertilization
development of fruiting bodies, but
the specific stages controlled by these genes remain
elusive, although in most cases a common feature
seems to be the control of internuclear recognition.
1. C. heterostrophus
Complementation of ΔMAT strains with various
fragments of each idiomorph leads to the identification
of a sequence that is not necessary for pseudothecia
formation, but is required for the completion
of the sexual cycle after mating (Wirsel
et al. 1998). This 160-bp sequence is localized in
the 3 ′ UTR of MAT1-1-1 and MAT1-2-1 transcripts,
outside the idiomorph sequence. This sequence
is therefore identical in MAT1-1-1 and MAT1-2-1
transcripts, but must be present in each transcript
for ascospore production. The authors have proposed
that this sequence is required for proper
localization of MAT mRNAs, leading to recognition
between nuclei of opposite mating type and
to normal meiosis. No cytological observations are
available to allow specification of the defect resulting
from the absence of the 160-bp region in the
MAT loci.
2. G. zeae
Deletion of either MAT1-1 or MAT1-2 mating-type
sequences from self-compatible G. zeae (Fig. 15.2)
resulted in two strains that have a self-incompatible
mating-type structure and behavior (Lee et al.
2003). In crosses of the mat1-1; MAT1-2 strain with
a wild-type strain, the perithecia contain mat1-1;
MAT1-2 nuclei and nuclei of the self-compatible
parent. Lee et al. (2003) analyzed the progeny to
establish what the proportion is of the two possible
nuclear recognition events: self-recognition of
wild-type nuclei, and internuclear recognition
between a wild-type nucleus and a mat1-1; MAT1-2
nucleus. Surprisingly, they found only asci contributed
by both parents, suggesting that wild-type
and mat1-1; MAT1-2 nuclei recognize each other
preferentially. This preferential recognition might
be attributed to an undefined mechanism that
favors recruitment of nuclei with different genetic
backgrounds, a phenomenon known as “relative
heterothallism” in E. nidulans (Hoffmann et al.
2001). This explanation is excluded here, however,
as the two strains used for crossing differ only by
the deletion of the MAT1-1 mating-type genes.
Similar results were obtained in crosses of MAT1-1;
mat1-2 strains with wild-type strains. The finding
of Lee et al. (2003) clearly demonstrates that
the partnering of nuclei is not random during
perithecium development, but it remains to be
established why wild-type nuclei do not contribute
to the progeny while they do so in a wild-type
cross. A possible explanation is that their crossing
partner induces the wild-type nuclei to switch to
theoppositematingtypebyanepigeneticcontrol
of mating-type gene expression (Metzenberg and
Glass 1990). It would be interesting to test if
wild-type progeny issued from a first cross with
MAT1-1; mat1-2 or mat1-1; MAT1-2 strains of
G. zeae are still able to cross with both MAT1-1;
mat1-2 and mat1-1; MAT1-2 partners, or if they
have conserved the putative epigenetic switching
of their first cross, as proposed by Metzenberg and
Glass (1990).
3. N. crassa
In N. crassa, several mutations in mat A-1 or mat
a-1 prevent mating but none was found to impede
fruiting-body development after mating, precluding
the investigation of the role of these genes after
fertilization. Surprisingly, mutations in either mat
A-2 or mat A-3 do not affect fruiting-body development,
although these mutations result in stop
codons before the HPG and HMG domains of mat
A-2 or mat A-3, respectively (Ferreira et al. 1998).
However, Glass and Lee (1992) have isolated a strain
called A IIRIP that displayed very low fertility. Subsequent
RT-PCR experiments show that in A IIRIP , mat
A-2 and mat A-3 are not transcribed, probably as
a result of RIP mutations and methylation (Ferreira
et al. 1998). This leads to the intriguing conclusion
thatmatA-2andmatA-3are redundantgenes,since
inactivation of both is required to observe a defect
during the sexual cycle. Examination of developing
A IIRIP × a perithecia showed that formation
of ascogenous hyphae is infrequent, although occasional
croziers can be observed (Glass and Lee
1992). Glass’ group proposed that these genes are
involved in orchestrating the multiple mitotic divisions
of opposite mating-type nuclei before cellularization,
or in the control of the internuclear