Growth, Differentiation and Sexuality

316 R. Debuchy and B.G. Turgeon
2. Target Genes Involved in Fertilization
ThewealthofdataavailableaboutmatinginS. cerevisae(Kurjan
1993) led to the quick identification of
the putative target genes required for fertilization
in filamentous fungi, notably the genes encoding
the pheromone precursors and the pheromone receptors.
Precursor pheromone genes similar to the
yeast MFa and MFα genes have been identified in
C. parasitica (Zhang et al. 1993, 1998; Turina et al.
2003), M. grisea (Shen et al. 1999), N. crassa (Bobrowicz
et al. 2002; Kim et al. 2002) and P. anserina
(Coppin et al. 2005b; see Chap. 16, this volume). In
all these self-incompatible fungi, transcripts of the
prepheromone genes are present only in a matingtype-specific
manner; MFα- andMFa-like genes
are expressed only in MAT1-1 or MAT1-2 strains,
respectively. Pheromone receptor genes pre-1 and
pre-2 have been identified in N. crassa (Pöggeler
and Kück 2001; Kim and Borkovich 2004). Kim
and Borkovich (2004) observed that RNA levels of
pre-1 in mat A strains were more than 100-fold
higher than those in mat a strains. Data obtained
by Pöggeler and Kück (2001) suggested that transcription
of the pheromone receptor genes is mating
type-independent. The use of a fluffy mutant
by this latter group, instead of a wild-type strain,
Fig. 15.9. Function of the mating-type
genes of P. anserina during the sexual cycle.
Arrows connote positive regulation and
lines ending in bars connote repression. In
the sexual organs, the mating-type genes
control the functions required for fertilization.
In the fruiting body, the mating-type
genes control the functions required for internuclear
recognition and the development
of the ascogenous hyphae. Genetic experiments
suggested that FPR1 and FMR1 have
a nucleus-limited expression during internuclear
recognition (reviewed in Coppin
et al. 1997). The nucleus-limited expression
of FPR1 in the mat+ nucleus controls the
expression of a specific set of proteins that
were assumed to remain in the vicinity of
this nucleus and to determine a mat+ identity.
A similar rationale applies to SMR2 for
the establishment of a mat– nuclear identity.
FMR1 does not display a nucleus-limited
expression in the genetic test (Arnaise et al.
1997). However, its interaction with SMR2,
demonstrated in the yeast two-hybrid
system, is assumed to prevent its diffusion
to adjacent nuclei during internuclear
recognition. Interactions between nuclei
with different nuclear identity was assumed
to trigger the internuclear recognition
process and the developmental arrest
may explain this discrepancy. The control of the
prepheromone genes and pre-1 by mat A-1 and
mat a-1 was substantiated by the loss of transcription
of these target genes in strains containing mat
A-1 and mat a-1 mutant alleles leading to sterility
(Bobrowicz et al. 2002; Kim et al. 2002; Kim and
Borkovich 2004).
Genome sequences offer the possibility to
search for all potential binding sites of the MAT
DNA-binding proteins. This type of search has
been undertaken to identify putative downstream
target genes of FMR1, SMR2 and FPR1 in P. anserina
(Hdidou, Coppin and Debuchy, unpublished
data). Prepheromone genes (mfm and mfp) and
pheromone receptor genes (pre-1 and pre-2) of
P. anserina were used for the identification of
putative target sites of the MAT transcription
factors. Comparison of the promoter sequences
of the mfm and pre-1 genes allows us to define
two common sequences of 12 bp. Screening of the
entire P. anserina genome with these sequences
reveals only one gene, KEX1, unambiguously
related to fertilization due to its role during α-like
prepheromone processing (see Chap. 10, this
volume). Screening of the P. anserina genome with
sequences common to the promoter region of the
mfp and pre-2 genes of P. anserina does not show