Growth, Differentiation and Sexuality

17 The Mating Type Genes of the Basidiomycetes
L.A. Casselton 1 ,M.P.Challen 2
CONTENTS
I. Introduction ......................... 357
A. BreedingSystems................... 358
B. DevelopmentalPathways ............. 358
II. Molecular Analysis of Mating Type Genes . . 362
A. TetrapolarSpecies .................. 362
1. The b and A Genes Encode
aTranscriptionFactor ............. 362
2. Homeodomain Protein Interactions . . 364
3. The a and B Genes Encode
PheromonesandReceptors ......... 365
4. The Pheromones and Receptors . . . . . . 366
B. BipolarSpecies..................... 366
C. HomothallicSpecies................. 367
D. Cryptococcus neoformans Mating Type
Locus ............................ 368
III. Downstream Regulation
of Development ....................... 369
A. PheromoneSignalling ............... 369
B. HeterodimerTargets ................ 369
IV. Concluding Remarks ................... 370
References ........................... 371
I. Introduction
The basidiomycete fungi are a diverse group that
includes the mushrooms, many of which are valuable
crop species, the plant pathogenic smuts and
rusts, and yeast-like species such as the saprophytic
Rhodosporidium toruloides and Cryptococcus neoformans,
an opportunist pathogen in humans.
Understanding the mating systems of these fungi
hasapplicationsinthedesignofbreedingprogrammes
for crop improvement and in the elucidation
of pathogenicity determinants. Mating generally
leads to a dramatic change in cellular morphology,
and the mating type genes themselves are
directly involved in bringing about these changes.
Significantly, the genes encode members of protein
families and signalling molecules that are ubiq-
1 Department of Plant Sciences, University of Oxford, South Parks
Road, Oxford OX1 3RB, UK and Warwick HRI, University of Warwick,
Wellesbourne, Warwick CV35 9EF, UK
2 Warwick HRI, University of Warwick, Wellesbourne, Warwick
CV35 9EF, UK
uitous in eukaryotic cells. Mating pathways thus
present unique opportunities tostudy the functions
of these proteins and to gain a general insight into
how they regulate eukaryotic cellular development.
The function of the mating type genes is
to impose barriers on self-mating and thereby
promote outbreeding, which maintains variability
within the population. In ascomycete fungi this is
achieved by having two mating types determined
by alternative forms of a mating type locus
that have no DNA homology and contain genes
encoding functionally different proteins (see
Chap. 15, this volume). In the basidiomycetes,
typically each mating type locus contains similar
genes, and compatible mates are those that
have different alleles of the same set of genes
(reviewed by Casselton and Olesnicky 1998). The
genes may be multiallelic, so that there can be
many more than two versions of the mating type
locus. There may be two mating type loci, each
containing a different set of multiallelic genes.
This enormous diversity, found particularly in
mushroom species, can generate several thousands
of different mating types. Although superficially
complex, the underlying mechanisms that permit
the recognition of compatible mating partners
are highly conserved between ascomycetes and
basidiomycetes, as we attempt to show here. The
best described basidiomycete models are the homobasidiomycete
mushrooms Coprinus cinereus
(Coprinopsis cinerea, Redhead et al. 2001) and
Schizophyllum commune, the hemibasidiomycete
corn smut Ustilago maydis and C. neoformans.
Conservation in genome organisation in homobasidiomycetes
(Kües et al. 2001) and degenerate
PCR cloning strategies now make it possible to
isolate the genes from other species, and it is of
considerable interest to evolutionary biologists to
compare mating type gene sequences because they
are considered to have evolved more rapidly than
genes encoding conserved metabolic functions
(May et al. 1999; James et al. 2004a).
The Mycota I
Growth, Differentation and Sexuality
Kües/Fischer (Eds.)
© Springer-Verlag Berlin Heidelberg 2006