Growth, Differentiation and Sexuality

302 R. Debuchy and B.G. Turgeon
main. The MAT1-2 idiomorph contains MAT1-2-1
and another gene, MAT1-2-2, which also extends
into the common flanking sequence that is shared
with MAT1-1-3, encoding the HMG domain. The
MAT1-2-2 ORFbeginsjustattheboundarybetween
MAT1-2 and the flanking sequence. The function
of MAT1-2-2 is as yet unknown.
b) Self-Compatible Sordariomycetes
The structure of the mating-type genes in selfcompatible
Sordariomycetes has been determined
for only three species: Gibberella zeae, Sordaria
macrospora, andChaetomium globosum (Fig. 15.2
and Table 15.2). G. zeae carries linked counterparts
of the self-incompatible Sordiariomycete MAT
genes, i.e., three genes structurally identical
to the MAT1-1 mating-type gene sequences in
self-incompatible G. fujikuroi (G. moniliformis),
separated from MAT1-2-1 by 611 bp (Yun et al.
2000). S. macrospora contains a homolog of
MAT1-1-1 (called Smt A-1) andMAT1-1-2 (called
Smt A-2) whereasthecounterpartofMAT1-1-3
(called Smt A-3) lacks the region encoding the
HMG domain specific to the MAT1-1-3 genes found
in self-incompatible Sordariomycetes (Pöggeler
et al. 1997). The Smt A-3 gene is located 813 bp
upstream of the MAT1-2-1 gene (called Smt a-1)
and is co-transcribed with Smt a-1 (Pöggeler and
Kück 2000). This structure was proposed, by the
authors, to derive from an unequal crossover
between the MAT1-1-3 gene and the MAT1-2
idiomorph in a putative self-incompatible ancestor
of S. macrospora. Itisnotknownifthetruncated
Smt A-3 gene still encodes a protein essential for
thesexualcycle,orifitprovidesapromoterand
a transcriptional start site for the downstream
Smt a-1 gene, or if it serves as a regulatory
miniORF. The recently sequenced self-compatible
C. globosum belongs to a group closely related to P.
anserina (Liu and Hall 2004). It contains a MAT1-1
mating type structurally identical to the MAT1-1
(mat-) idiomorph of P. anserina, andaMAT1-2-1
gene on a different supercontig. Surprisingly,
C. globosum contains two different copies of
the MAT1-1-2 gene, one at the MAT1-1 locus,
the other 1540 bp downstream of the MAT1-2-1
gene.
Hybridization of cloned portions of the
idiomorphs of N. crassa to the genome of related
self-compatible species has revealed three
classes of hybridization pattern correlated with
phylogenetic relationships (Dettman et al. 2001).
The hybridization patterns of N. terricola provide
evidence for the presence of MAT1-1 and MAT1-2
sequences (Glass et al. 1988), but subsequent
probing with portions of the idiomorphs reveals
that MAT1-1 has lost sequences in the region
corresponding to the MAT1-1-3 gene (Beatty
et al. 1994). Another distinct phylogenetic group
consists of N. africana, N. dodgei, andN. galapagosensis,
which may be individuals of the
same species. N. lineolata represents a distinct
lineage from the others, but still closely related.
These four self-compatible species or isolates all
contain the MAT1-1 sequence but none of them
contains a sequence that hybridizes to probes
encompassing the MAT1-2-1 (mat a-1) gene,
suggesting that it is absent in these fungi (Glass
et al. 1988, 1990). A third group corresponds to
a clade containing Gelasinospora calospora and
N. sublineolata (Anixiella sublineolata). These
two self-compatible species contain both MAT1-1
and MAT1-2 sequences (Beatty et al. 1994). These
analyses suggest that MAT1-1-1 and MAT1-1-2
are consistently present in all self-compatible
Sordariomycetes. Some of these species have lost
either MAT1-1-3 or MAT1-2-1, but no species has
lost both of these genes, which encode proteins
with a HMG domain. We propose that the essential
functions of the lost MAT1-1-3 or MAT1-2-1 are
taken on by the remaining HMG encoding gene.
The functions of the non-redundant MAT1-1-1
and MAT1-1-2 genes cannot be compensated
for by any other gene, precluding their loss in
self-compatible Sordariomycetes. Therefore, we
predict that the minimal mating-type gene structure
of self-compatible Sordariomycetes should
be MAT1-1-1, MAT1-2-1, and at least one HMG
encoding gene.
c) Asexual Sordariomycetes
The idiomorph structure of asexual Sordariomycetes
has been completely established for
two hypocreales: Fusarium oxysporum and Paecilomyces
tenuipes. Arie et al. (2000) demonstrated
that different forma specialis (f. sp.) of asexual
F. oxysporum contain either the MAT1-1 or the
MAT1-2 idiomorphs. Subsequent analysis of the
MAT1-1 and MAT1-2 idiomorphs of F. oxysporum
f. sp. lycopersici revealed that they contained
MAT1-1-1, -2, -3 and MAT1-2-1 genes, respectively
(Yun et al. 2000). These genes are structurally
indistinguishable from the functional MAT genes
present in sexual Sordariomycetes, notably the