Growth, Differentiation and Sexuality

296 R. Debuchy and B.G. Turgeon
locus has been found. This locus will be described
below in self-incompatible, self-compatible, and
asexual species, respectively.
1. Loculoascomycetes
a) Self-Incompatible Loculoascomycetes
The simplest idiomorphs are those of selfincompatible
Loculoascomycetes (Fig. 15.2).
For all taxa in this group, MAT1-1 and MAT1-2
idiomorphs consist of one ORF corresponding
to genes encoding the α1 and HMG proteins,
respectively (Table 15.1). Following standard
nomenclature, described above, the genes have
been designated MAT1-1-1 and MAT1-2-1. The
size of the idiomorphs varies with species, but
both encoded MAT genes have the same orientation
with respect to their chromosome. For C.
heterostrophus, the first described for this class,
each ORF occupies almost all of the idiomorph
sequence (Turgeon et al. 1993). Expression analysis
revealed that both MAT1-1-1 and MAT1-2-1
transcripts start and stop in common sequences
flanking the idiomorphs, resulting in transcripts
that are almost twice the size of the corresponding
ORF (Leubner-Metzger et al. 1997). Multiple
transcripts, resulting from the use of at least two
transcription start sites and alternative splicing of
5 ′ UTR introns, are produced. The 5 ′ UTR of the
MAT messages contain μORFs that could control
translation of the downstream coding sequence.
However, the transcription initiation sites and
the 5 ′ UTR region of the MAT genes have been
deleted in both parents without penalty to fertility,
indicating that these regions do not contain
information essential for mating (Wirsel et al.
1998). By contrast, the 3 ′ UTR of the idiomorphic
genes contains structural information that is
necessary for completion of the sexual cycle. The
molecular function of this sequence present in
the 3 ′ common flanking sequence of MAT1-1 and
MAT1-2 idiomorphs is as yet unknown. This one
gene/one idiomorph structure has also been found
in C. ellisii, C. carbonum, C. victoriae,andC. intermedius
(Yun et al. 1999). The MAT idiomorphs of
Didymella rabiei (sexual state of Ascochyta rabiei;
Barve et al. 2003), Mycosphaerella graminicola
(sexual form of Septoria tritici; Waalwijk et al.
2002) and Phaeosphaeria nodorum (sexual state of
Stagonospora nodorum; Bennett et al. 2003) also
contain a single ORF that starts and terminates in
the idiomorph, but the sizes of the idiomorphic se-
quences are larger than those of C. heterostrophus
(Table 15.1). Transcription initiation sites have not
been established for these fungi. It is possible that,
in contrast to C. heterostrophus, the promoter is
localized in the MAT-specific sequences, raising
the possibility that, unlike C. heterostrophus,
each MAT gene responds to specific transcription
regulatory factors.
b) Self-Compatible Loculoascomycetes
Self-compatible Loculoascomycetes display various
arrangements of MAT1-1-1 and MAT1-2-1
genes in their haploid genome (Fig. 15.2 and
Table 15.2). C. cymbopogonis carries complete
MAT1-1-1 and MAT1-2-1 counterparts, in contrast
to C. luttrellii and C. homomorphus that contain
MAT gene fusions (Yun et al. 1999). These fusions
consist of either MAT1-1-1::MAT1-2-1 (C. luttrellii)
or MAT1-2-1::MAT1-1-1 (C. homomorphus)
recombinant genes. Possible evolution of these
self-compatible species from self-incompatible
ancestors is detailed in Sect. IV. A more complex
structure has been found in C. kusanoi (Yun et al.
1999). This fungus contains a complete MAT1-1-1
homologlinkedtoaMAT1-1-1::MAT1-2-1 chimeric
gene that is devoid of the α1 encoding region but
contains the HMG encoding sequence. The MAT
locus from a self-compatible species outside of the
genus Cochliobolus, Mycosphaerella zeae-maydis
(Didymella zeae-maydis), has been characterized
(Yun 1998). In this case, M. zeae-maydis MAT has
complete versions of the C. heterostrophus MAT
counterparts tandemly arranged, head to tail,
separated by about one kilobase of non-coding
sequence.
A total of 106 diverse isolates of Stemphylium
have been examined to determine the structure
of the mating-type loci (Inderbitzin et al. 2005).
Isolates were found to contain both separate and
linked MAT genes. Where separate, MAT loci
contain either a MAT1-1-1 or MAT1-2-1 gene in
the same 5 ′ ->3 ′ orientation as in Cochliobolus,
between upstream ORF1 and downstream BGL1
genes (Fig. 15.3). Self-compatible isolates were
isolated from field ascomata or conidia and
demonstrated to undergo self-mating in the laboratory.
These isolates had two types of MAT locus
configuration. In the first type, the two MAT genes
are linked. The MAT1-1-1 gene and both flanks are
inverted and connected to MAT1-2-1 oriented in
the original MAT gene direction (as in Cochliobolus),
approx. 200 bp upstream of the MAT1-2-1