Growth, Differentiation and Sexuality

312 R. Debuchy and B.G. Turgeon
provide a snapshot of the genetic link between
self-incompatibility and self-compatibility, and
thus clues to the genetic mechanism mediating the
change from one lifestyle to the other.
Inspection of the sequence at the MAT fusion
junction in C. luttrellii revealed that 345
nucleotides from the 3 ′ end of the MAT1-1-1
ORF and 147 nucleotides from the 5 ′ end of the
MAT1-2-1 ORF are missing, compared to the C.
heterostrophus self-incompatible homologs. The
deletions are consistent with the hypothesis that
a crossover event occurred within the dissimilar
C. heterostrophus genes at positions corresponding
to the fusion junction. Inspection of the C.
heterostrophus genes reveals 8 bp of sequence
identity precisely at the proposed crossover site,
which would explain this arrangement. A single
crossover within this region would yield two
chimeric products, one of which is identical to
the fused MAT gene actually found in C. luttrellii
(Fig. 15.2). A similar scenario can be proposed
for C. homomorphus; in this case, the fused gene
is missing 27 nucleotides from the 3 ′ end of
MAT1-2-1 and 21 nucleotides from the 5 ′ end of
MAT1-1-1, compared with the C. heterostrophus
MAT genes. Examination of the C. heterostrophus
MAT sequences at positions corresponding to the
C. homomorphus fusion junction reveals 9 bp of
identity (with one mismatch), and thus a putative
recombination point. The mechanism of conversion
from self-incompatibility to self-compatibility
is likely a recombination event between small
islands of identity in the otherwise dissimilar MAT
sequences.
To determine whether phylogenetic analyses
support a convergent origin for self-compatibility,
Yun et al. (1999) used maximum likelihood and
parsimony trees inferred from the ribosomal
gene internal transcribed spacer (ITS) and from
glyceraldehyde phosphate dehydrogenase (GPD)
datasets. All of the resulting trees show that
self-compatibility is polyphyletic. None of the selfcompatible
species on the tree clustered together
in any of the 15 most parsimonious trees or in
maximum likelihood trees. Thus, phylogenetic
evidence clearly supports independent evolution
of self-compatibility in the four Cochliobolus
species, and underpins the structural evidence.
b) Stemphylium
To investigate the origin of self-fertility in Stemphylium,
Inderbitzin et al. (2005) compared the
position of self-compatible species in an organismal
phylogeny constructed using sequence data
from four loci, unrelated to mating type, and in
a phylogeny of the mating-type genes. The MAT
datasets were analyzed using likelihood, parsimony,
Bayesian and neighbor-joining methods.
Several conclusions were reached:
1. Linked MAT regions are derived from ancestral,
separate MAT regions, as suggested by the
basal position of isolates with separate MAT
regions in phylogenetic analyses.
2. There was 100% support from all analyses for
the monophyly of MAT1-1 from fused regions,
and for the monophyly of MAT1-2 from fused
regions.
3. Organismal phylogenies support polyphyly of
the isolates with fused MAT regions.
4. In contrast to the Cochliobolus history, where
self-fertility appears to have originated in
different species by independent mating-type
gene fusions, Inderbitzin et al. (2005) found
evidence for a single fusion, present in 76
self-fertile isolates, which appears to have been
laterally transferred across lineages.
5. The isolates that harbor only MAT1-1-1 may
represent a third lineage that evolved selfing
ability independently, with unknown genetic
changes responsible for its evolution.
Evolution of fused MAT regions from separate selfincompatible
progenitors was supported by DNA
sequence comparison, showing that selfers with
both MAT genes could have originated from a selfsterile
ancestor by the inversion of an ancestral
MAT1-1-1 gene and its flanking regions, creating
a crossover site and allowing the fusion of the inverted
MAT1-1-1 region to MAT1-2-1 (Fig. 15.2; Inderbitzin
et al. 2005). If the sequence of a MAT1-
1-1 locusinanincompatiblestrainisalignedto
thesameregioninaself-compatiblestrainwith
both MAT regions, it is clear that the flanking sequence
carrying ORF1 is identical, until approx.
300 bp 3 ′ of ORF1, at which point the sequence
loses identity but, in fact, is the reverse complement
of the MAT1-1-1 region for about 1740 bp.At
this point, the sequence returns to identity with the
5 ′ flank of a MAT1-2-1 gene, in the native orientation.
Thus, homothallism in Stemphylium appears to
have evolved from outcrossing ancestors, by fusion
of MAT1-1 and MAT1-2, by lateral transfer of this
fused region across lineages, and in certain cases,
where MAT1-1 is present only, by unknown means.