Growth, Differentiation and Sexuality

Table. 15.2. Structure of the MAT locus in self-compatible Euascomycetes
Mating Types in Euascomycetes 299
Accession number Genes Features
Loculoascomycetes
Cochliobolus homomorphus AF129741 MAT1-2-1::MAT1-1-1 HMG+α domain
Cochliobolus luttrellii AF129740 MAT1-1-1::MAT1-2-1 α domain+HMG
Cochliobolus kusanoi AF129742 MAT1-1-1::MAT1-2-1 HMG
MAT1-1-1 α domain
Cochliobolus cymbopogonis AF129744 MAT1-1-1 α domain
AF129745 MAT1-2-1 HMG
Sordariomycetes
Gibberella zeae AF318048 MAT1-1-1 α domain
MAT1-1-2 HPG domain
MAT1-1-3 HMG
MAT1-2-1 HMG
Sordaria macrospora Y10616 Smt A-1 α domain
Smt A-2 HPG domain
Smt A-3 None identified
Sm a-1 HMG
Chaetomium globosum Broad Institute MAT1-1-1 α domain
MAT1-1-2 HPG domain
MAT1-1-3 HMG
MAT1-2-1 HMG
MAT1-1-2 like HPG domain
Eurotiomycetes
Emericella nidulans AY339600 MATB-1 (MAT-1, MATB) a α domain
AF508279 MAT1-2 (MAT-2, MATA) a HMG
a Names in parenthesis are different nomenclatures of the same gene
gene, and between it and ORF1 (Fig. 15.2). Notably,
thereisalsoagroupofself-compatibleisolates
carrying only MAT1-1-1, a situation analogous to
certain Neurospora self-compatible isolates (e.g., N.
africana) whereonlythemat A-1 gene is detected
(Glass et al. 1988, 1990). The remaining isolates of
Stemphylium examined have undetermined sexual
states and may be asexual, self-incompatible, or
possibly debilitated selfers.
These analyses indicate that mating-type
gene arrangements are diverse in self-compatible
Loculoascomycetes. A striking observation is that
MAT1-2-1 was not detected in some Stemphylium
isolates, which suggests that the HMG regulatory
domain might be dispensable for sexual cycle
control. Another possibility may be proposed,
based on the recent finding that each MAT gene
of Cochliobolus spp. appears to encode a protein
with both HMG and α1activities(LuandTurgeon,
unpublished data; see Sect. III.C.1); the regulatory
functions of the lost MAT1-2-1 gene may be taken
on by the remaining MAT1-1-1 gene. Evolution
of the target genes of MAT1-2-1 would then be
necessary to bring them under the control of the
HMG domain of MAT1-1-1. A first step for the
validation of this hypothesis will be to demonstrate
that Stemphylium MAT proteins have structures
similar to those of Cochliobolus spp.
c) Asexual Loculoascomycetes
The evolutionary conservation of MAT genes in
sexual Loculoascomycetes prompted Turgeon and
colleagues to ask if there were homologs of these
genes in related species that had never been demonstrated
to reproduce sexually (Sharon et al. 1996).
This investigation was first conducted with Bipolaris
sacchari, a pathogen of sugarcane. Among
21 field isolates, 19 were demonstrated to carry a
MAT1-2-1 gene and two carried a MAT1-1-1 gene.
Recently Schoch, Saenz, and Turgeon (unpublished
data) have updated this study, using more isolates
and several molecular characters to investigate
mating-type distribution and phylogenetic relationships
of asexual B. sacchari. Combined data
from both MAT genes, plus the 5 ′ MAT flank, the
3 ′ MAT flank, and several additional sequences,
confirm that B. sacchari isolates exist in the field
as both mating types. All datasets indicated that
the B. sacchari isolates fall into two well-supported
groups, with representatives of both mating types
in each group. The deduced MAT1-2-1 protein of