Growth, Differentiation and Sexuality

appear very dissimilar (see consensus in Figs. 15.7
and 15.8). Phylogenetic analysis of the HMG domains
from the MATA family shows that MAT1-2-1
proteins form a distinct subfamily that does not
contain any other HMG proteins, except MAT1-1-3
from P. brassicae and R. secalis (data not shown).
Moreover, SMR2 from P. anserina was shown to interact
with FMR1 (Arnaise et al. 1995), and similar
phenotypes conferred by FMR1 and SMR2 mutations
support the idea that these two proteins cooperate
in the regulation of their target genes (Zickler
et al. 1995; Arnaise et al. 1997, 2001). These data
support the idea that MAT1-1-3 proteins bind to
DNA in a very different way from that of MAT1-
2-1/SOX-TCF proteins. Further investigations on
DNA-binding modalities of fungal HMG transcription
factors would be required to substantiate classification
of these proteins, by function.
IV. Evolution of Mating Types
Two fascinating questions attend any discussion of
mating-type gene evolution. The first is how dimorphic
sex chromosomes or dissimilar regions
(idiomorphs) of a chromosome evolved, and the
second is how different reproductive lifestyles, i.e.,
self- or non-self-compatibility, evolve. The evolutionary
origins of the dissimilar MAT idiomorphs
of self-incompatible Ascomycetes are unknown at
this point, but data are accumulating on the matter.
One hypothesis is that the small pockets of identity
in the otherwise unlike MAT idiomorphs reflect
common ancestry (Coppin et al. 1997). Conceivably,
these pockets are remnants of a series of mutagenic
events in a single ancestral gene(s). The mutations,
coupled with recombination suppression,
might have led to the highly divergent extant MAT
genes that now encode different products (Turgeon
et al. 1993). A similar scenario has been proposed
for the evolution of the Y chromosome. Indeed,
acquisition of sex-determining genes, recombination
isolation, and addition of genes not involved in
mating are shared between fungal mating-type loci
and mammalian X and Y chromosomes; it has been
suggested that these elements may be early steps
linking the evolution of sex chromosomes in diverse
organisms (Lahn and Page 1999; Fraser et al.
2004). This section focuses on how Ascomycetes
change from one reproductive lifestyle to another
(self-incompatibility to self-compatibility, and vice
versa).
Mating Types in Euascomycetes 311
A. Phylogenetic Analyses of Mating Type
1. Loculoascomycetes
a) Cochliobolus
Because MAT genes control the reproductive process,
comparison of their sequences reflects life
history and should reveal mechanisms underlying
changes in reproductive mode. A combination of
molecular and phylogenetic data have been used
to determine the direction of evolution of reproductive
lifestyle in Cochliobolus (Yun et al. 1999)
and in Stemphylium (Inderbitzin et al. 2005). In
both cases, examination of MAT structure in many
species from diverse geographical locations, plus
phylogenetic treatments, support the hypothesis
that the direction is likely from self-incompatible
to self-compatible.
To address the issue of which mode of fungal
sexual reproduction is ancestral in Cochliobolus,
Yun et al. (1999) compared extant MAT sequences
from self-incompatible and self-compatible
species. Structural organization of MAT loci of
self-incompatible C. heterostrophus, C. carbonum,
C. victoriae, C. ellisii, C. sativus and C. intermedius
species is highly conserved; each strain carries
asingleMAT gene, either MAT1-1-1 or MAT1-
2-1 (Fig. 15.2). By contrast, all self-compatible
Cochliobolus species carry both MAT genes in
one genome, but as described in Sect. III. and
Fig. 15.2, the structural organization of each
locus is unique. In two cases (C. luttrellii and C.
homomorphus), the genes are fused into a single
ORF; the gene order in C. luttrellii is reversed in
C. homomorphus. In the remaining two cases, the
genes are not fused. In C. kusanoi, the organization
is 5 ′ MAT1-2-13 ′ –3 ′ MAT1-1-15 ′ , and part of the
sequence between the genes is similar to a portion
of the β-glucosidasegenenormallyfound3 ′ of both
MAT genes in self-incompatible C. heterostrophus
(Fig. 15.3). To the 5 ′ of MAT1-2-1 is a perfect
inverted repeat of a 561-bp region containing
123 bp of the 5 ′ end of the MAT1-1-1 ORF fused to
the 5 ′ end of MAT1-2-1, and 145 bp of a different
fragment of the β-glucosidase gene, separated
from each other by 293 bp. C. cymbopogonis
carries both homologs of the self-incompatible
idiomorphs, but these are not closely linked.
It is not known if they reside on the same or
different chromosomes. Thus, the MAT genes have
close physical association in three Cochliobolus
self-compatible species, but not in the fourth.
The fused MAT genes in self-compatible species