Growth, Differentiation and Sexuality

B. sacchari is 97% identical to its counterpart in C.
heterostrophus (Sharon et al. 1996). Another asexual
species, Alternaria alternata,wasfoundtocontain
MAT1-1 or MAT1-2 idiomorphs that are structurally
similar to those of C. heterostrophus (Arie
et al. 2000). For both asexual species, the function
of the cloned asexual genes was tested in C. heterostrophus
and shown to induce the formation of
fertile pseudothecia. This test demonstrated that
asexual B. sacchari and A. alternata possess functional
orthologs of the mating-type genes of sexual
species. The lack of sexual cycle in at least one
B. sacchari isolate may result from a MAT transcriptional
defect. Northern blot analyses failed
to detect any B. sacchari MAT1-2-1 transcript in
isolate 764.1 of B. sacchari, although the heterologous
C. heterostrophus MAT1-1-1 and MAT1-2-1
geneswereexpressed.Aconclusivestatementregarding
expression of the B. sacchari MAT genes
awaits examination of expression in additional isolates
by RT-PCR, informed by the updated phylogenetic
analysis described above. Expression of both
C. heterostrophus MAT genes in B. sacchari does not
induce the formation of fertile pseudothecia, suggesting
that the sexual defect cannot be attributed
only to the transcription defect of the endogenous
MAT1-2-1 gene. A transcription defect hypothesis
seemstobeexcludedfortheA. alternata case, since
RT-PCRanalyses indicatedthat both MAT genes are
transcribed. These data support the argument that
asexual Loculoascomycetes arise from sexual progenitors,
and indicate that the cause of asexuality
maybedefectsinanygeneinthemating-typepathway,
such as the target genes of the MAT regulatory
factors.
2. Sordariomycetes
a) Self-Incompatible Sordariomycetes
Two self-incompatible Sordariomycetes, N. crassa
and P. anserina, have served as model systems to
investigate the structure of the idiomorphs, which
were found to be similar in other self-incompatible
speciesfromthisgroup.TheMAT1-1 idiomorph
contains three genes: MAT1-1-1, which is the hallmark
of the idiomorph, MAT1-1-2 and MAT1-1-3
(Fig. 15.2). These genes are designated mat A-1, A-2
and A-3 in N. crassa, FMR1, SMR1 and SMR2 in P.
anserina, respectively (Table 15.1). The MAT1-1-2
gene is located upstream of MAT1-1-1 (Fig. 15.2).
The MAT1-1-2 proteins are characterized by a HPG
domain (see Sect. C) but their molecular function
Mating Types in Euascomycetes 301
is not yet known. Investigation of SMR1 indicated
that this is not a bona fide mating-type gene, since
mating does not display any defect whatever the
location of SMR1, in either the one or the other
parent, and even when present in both parents (Arnaise
et al. 1997). MAT1-1-2 is consistently present
in all Sordariomycetes, and no homolog has been
identified in species outside this taxon. These data
strongly suggest that MAT1-1-2 is a characteristic of
the Sordariomycete mating-type system, and that
its molecular function is linked to a specific feature
of Sordariomycete mating type. A third gene,
MAT1-1-3, is present in the MAT1-1 idiomorph
of self-incompatible Sordariomycetes. The MAT1-
1-3 gene is located at the end of the idiomorph
distal to MAT1-1-1 (Fig. 15.2). MAT1-1-3 proteins
are characterized by the presence of a HMG domain.
The MAT1-2 idiomorph contains one ORF encoding
a protein with a HMG domain (Fig. 15.2),
corresponding to the MAT1-2-1 gene (mat a-1 in
N. crassa and FPR1 in P. anserina, Table 15.1).
Pöggeler and Kück performed a detailed analysis
of the idiomorphic region upstream of the mat a-1
ORF (Pöggeler and Kück 2000) in N. crassa, and
demonstrated that the mat a-1 transcript extended
at least 1252 nucleotides upstream of the putative
translational start. This transcript contained several
μORFs and one miniORF of 79 residues with
a 147-bp intron. The authors proposed that this
miniORF corresponds to a new gene called mat
a-2. However, mat a-2 coding sequence does not
contain any motif indicative of its molecular function,andithasnohomologinMAT1-2
idiomorphs
of other Sordariomycetes. Moreover, Chang and
Staben have tested the mating behavior of N. crassa
strains containing truncated versions of the mat
a-1 gene lacking most of the 5 ′ untranslated region,
and notably the mat a-2 sequence (Chang
and Staben 1994). These strains mated as an a
wild-type strain, indicating that the idiomorphic
region upstream of the mat a-1 ORF does not contain
any gene or structure essential for mating in
N. crassa.
The idiomorphs of Magnaporthe grisea display
a structure differing from the consensus observed
in other self-incompatible Sordariomycetes
(Fig. 15.2 and Table 15.2; Kanamori and Arie, personal
communication). The MAT1-1-1 gene has an
orientation opposite to its orientation in other Sordariomycetes.
Surprisingly, the MAT1-1-3 ORF extends
from the idiomorph into the flanking sequence,
and this region includes the HMG do-