Book of Abstracts (PDF) - International Mycological Association

IMC7 Tuesday August 13th Lectures
order to illustrate of common and different evolutionary
themes.
193 - Pheromone receptors of the basidiomycete
Schizophyllum commune distinguish single amino acid
differences among active mating pheromone ligands
T.J. Fowler * , M.F. Mitton, E.I. Rees & C.A. Raper
University of Vermont, Microbiology and Molecular
Genetics Dept., 208 Stafford Hall, 95 Carrigan Drive,
Burlington, VT 05405, U.S.A. - E-mail:
tfowler@zoo.uvm.edu
The linked, multigenic loci B-alpha and B-beta of the
homobasidiomycete Schizophyllum commune contribute to
the specification of more than 20,000 mating types. Nine
versions each of B-alpha and B-beta have been described
by genetic methods; only portions of a few of these
versions have been described by molecular techniques.
These versions carry different but related sets of genes
encoding lipopeptide pheromones and 7-transmembranedomain
receptors. We sequenced the linked pair B-alpha3-
B-beta2 and we can account for all known mating activities
of a B-alpha-B-beta complex for the first time. There are
eleven pheromone genes and two receptor genes within the
B-alpha3-B-beta2 complex. The amino acid sequences of
predicted pheromones and receptors from B-alpha3-Bbeta2
were compared with other S. commune mating
pheromones and receptors to decipher how specificity is
achieved in their interactions. These pheromones and
receptors can be functionally expressed in Saccharomyces
cerevisiae and show the same specificity as seen in S.
commune. Using a combination of heterologous and
homologous expression systems, we can show that single
amino acids in some pheromones control which receptors
(i.e. mates) are activated for pheromone response. The
receptors likewise can be manipulated to have altered
pheromone recognition or to be active in the absence of
pheromone. Thus, a finely tuned set of molecules maintains
outbreeding while prohibiting self-activation of the mating
process.
194 - Mating recognition in ascomycetes
R. Debuchy
Institut de Génétique et Microbiologie, Bâtiment 400,
Université Paris-Sud, F-91405 Orsay, France. - E-mail:
debuchy@igmors.u-psud.fr
In heterothallic Euascomycetes, mating is controled by the
mating-type alleles (idiomorphs) which allow compatible
strains to recognize each other and to proceed to
fertilization. Fertilization is not immediately followed by
karyogamy. Instead, the parental nuclei divide mitotically
several times inside the female organ and are intermingled
in plurinucleate cells. Then nuclei from male and female
origin, i. e. nuclei with opposite idiomorphs, recognize
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Book of Abstracts
each other and form dikaryotic hyphae. This step involves
at least two key events: internuclear recognition (IR) and
cellularization. The dikaryotic hyphae divide mitotically,
maintaining a strict ratio of 1:1 of each parental nucleus.
Eventually pairs of nuclei fuse and meiosis and ascospore
formation ensue immediately. A major challenge is to
understand the relations between the idiomorphs and the
developmental events of the sexual cycle. Podospora
anserina is used as a model system for investigating the
functions of its mat+ and mat- idiomorphs and the
molecular mechanism of IR. The mat+ sequence contains
one gene, FPR1 and the mat- sequence contains three
genes: FMR1, SMR1 and SMR2. FPR1, FMR1 and SMR2
encode transcriptional factors which control fertilization
and IR. A model, based on genetics, has been proposed for
the control of IR by the mat genes and will be presented. IR
is followed by a developmental arrest which is overcome
by SMR1. The current hypotheses about the molecular
mechanism of IR will be discussed.
195 - Gene deletion and unidirectional mating type
switching in Ceratocystis
T.C. Harrington 1* , J. Steimel 1 , D.L. McNew 1 & S.
Kanematsu 2
1 Iowa State University, Department of Plant Pathology,
Ames, Iowa 50011, U.S.A. - 2 National Institute of Fruit
Tree Science, Department of Apple Research, Morioka
020-0123, Japan. - E-mail: tcharrin@iastate.edu
Four of the five major clades within Ceratocystis have
homothallic species. Within the C. coerulescens clade there
are two closely related species with contrasting mating
systems: C. eucalypti is heterothallic, while MAT-2 strains
of C. virescens are self-fertile due to unidirectional mating
type switching. Most strains of C. virescens recovered from
nature are self-fertile and behave as MAT-2 in crosses,
progeny from selfings or from crosses segregate in a 1:1
ratio for mating type, and MAT-1 progeny are self-sterile.
Mating type idiomorphs and flanking regions were
sequenced for both mating types of these two species. In C.
eucalypti, the idiomorph has either the MAT1-1-2 and the
MAT1-1-1 genes or only the MAT1-2-1 gene, depending on
mating type, typical of a heterothallic ascomycete. All
strains of C. virescens have the MAT1-1-2 and MAT1-1-1
genes (as in MAT-1 strains of C. eucalypti), but self-fertile
strains also have the MAT1-2-1 gene and another copy of
the MAT1-1-1 gene. Half of the progeny recovered from
selfings retain this arrangement of four mating type genes
in the idiomorph and are self-fertile, but the other half
(MAT-1, self-sterile) are missing a 4.7 kb fragment that
includes the MAT1-2-1 gene and one copy of the MAT1-1-1
gene. The inherited deletion irreversibly changes the
mating type to MAT-1 and apparently allows mating with
MAT-2 nuclei. This is the first sequencing of the DNA
fragment deleted in unidirectional mating type switching.