Growth, Differentiation and Sexuality

2. Sordariomycetes
a) Gibberella
All self-compatible, Gibberella zeae (nine species)
isolates investigated by O’Donnell et al. (2004)
carry contiguous MAT1-1 and MAT1-2 matingtype
regions configured as initially described
by Yun et al. (2000; Fig. 15.2). Five additional,
related species examined by O’Donnell and
coworkers, and the G. moniliformis/fujikuroi
and Fusarium oxysporum MAT genes (Yun et al.
2000) carry only one of the two possible MAT
idiomorphs. Phylogenetic analyses by O’Donnell
et al. (2004) support a monophyletic origin of
the linked MAT genes, and also suggest that the
self-compatible lifestyle is derived from a selfincompatible
ancestor within this group. This
finding is in contrast to proposed polyphyletic
evolutionary origins of self-compatibility in
Cochliobolus (Yun et al. 1999), but in agreement
with the Stemphylium findings (Inderbitzin et al.
2005).
There are no published data regarding an evolutionary
mechanism, but the observation that the
genes that flank MAT are faithfully conserved in G.
zeae and most of the fungi described in Fig. 15.3
supports the notion that self-compatibility in G.
zeae may have arisen by a recombination event
that brought the MAT1-1 and MAT1-2 idiomorphs
together in a single isolate, as proposed earlier by
Yun et al. (1999). The most parsimonious event
would be recombination between homologous sequences
in the 3 ′ flanks of MAT1-2-1 and MAT1-1-1
of ancestral self-incompatible isolates of opposite
mating type. With currently available information,
it is not possible to find evidence for a candidate
recombination point, since the G. zeae MAT1-1-1
3 ′ flank is unlike the MAT1-1-1 3 ′ flanks of selfincompatible
isolates in the databases. The history
of this stretch of sequence in G. zeae is unknown.
b) Neurospora and Sordaria
Pöggeler (1999) subjected MAT and GPD sequences
of nine self-compatible and eight self-incompatible
taxa of Neurospora and Sordaria to phylogenetic
analyses, and concluded that:
1. Neurospora and Sordaria are monophyletic
units, based on both GPD and MAT.
2. Self-compatible and self-incompatible species
within both genera are distinct.
Mating Types in Euascomycetes 313
3. There was a single evolutionary origin of selfcompatibility.
4. Neurospora self-compatible strains of A/a-type
group separately from A-type.
5. Overall the data suggest early divergence of
self-compatibility and self-incompatibility,
before Neurospora and Sordaria speciated
(Pöggeler 1999).
As with the Loculoascomycetes, self-compatibility
in Neurospora and Sordaria appears to be
derived from self-incompatible ancestors. Unlike
Cochlibolus, but like Stemphylium and G.
zeae, self-compatibility likely arose once; all
extant self-compatible isolates within Neurospora
or Sordaria are monophyletic. The molecular
path of conversion from one lifestyle to
the other is undescribed, but likely occurred
by a recombination mechanism similar to
that described for Cochliobolus (Yun et al.
1999).
3. Leotiomycetes: Aspergillus spp.
Geiser et al. (1996) suggested that heterothallism
isthederivedstateinAspergillus species, based
on phylogenetic analyses using β-tubulin and hydrophobin
sequences, and that heterothallism may
have arisen once. Comparisons of three genomes,
self-compatible A. nidulans, asexual A. fumigatus,
and asexual A. oryzae, haveprovidedinsightinto
this question (Galagan 2005; Rydholm et al. 2005).
A. nidulans has MAT genes at two unlinked loci in
the genome (Dyer et al. 2003). A. fumigatus and
A. oryzae each carry a single MAT gene – A. fumigatus
has a MAT1-1-1 ortholog whereas A. oryzae
has a MAT1-2-1 ortholog.Astudyoflargenumbers
of field isolates of both indicate that both
mating types are found, as with all asexual fungal
populations studied to date (Sect. III.), a finding
that hints, once again, to the possibility that
both species may be capable of sexual reproduction.
Although the genomic data provide a glimpse
of mating lifestyle evolution in Aspergillus, this
view is based on comparison of species that
are quite distantly related, unlike evaluation
of closely related Cochliobolus taxa differing
in reproductive lifestyle. Nevertheless, MAT
structure in these three Aspergillus taxa supports
the hypothesis of Geiser et al. (1996), and
suggests that self-incompatibility is the derived
state.