Growth, Differentiation and Sexuality

Mating Types in Euascomycetes 295
Fig. 15.1. Life cycle of Euascomycetes. Outer circle: self-compatible life cycle. Inner circle: self-incompatible life cycle
(adapted from a drawing by S.-W. Lu)
III. Description
of Mating-Type Structure
A. General Organization of Mating Types
In all self-incompatible filamentous Ascomycetes
examined to date, the mating-type locus contains
one of two different sequences occupying the same
chromosomal locus in their genome. The extent of
dissimilarity ranges from 1 to almost 6 kb. Metzenberg
and Glass used the word “idiomorph” to
denote these large sequences not obviously related
by structure or common descent (Metzenberg and
Glass 1990). The analysis of self-compatible species
indicates that, in most cases, both mating types are
contained in the haploid genome, usually tightly
linked, but sometimes not, and thus these loci cannot
be called idiomorphs, since there is no second
strain of opposite mating type. We retain the use
of the term “mating type”, instead of “idiomorph”,
for self-compatible species. A standardized nomenclature
for idiomorphs and mating-type genes has
been proposed, and, in most cases adopted, to
cope with the rapidly growing number of described
mating-type loci from various species (Turgeon
and Yoder 2000). This standardized nomenclature
is based on the constant presence in one idiomorph
of an ORF encoding a protein with a motif called
the α1 domain, found initially in the MATα1 transcription
factor of Saccharomyces cerevisiae. This
idiomorph is called MAT1-1, andthecorresponding
gene MAT1-1-1. The other idiomorph is characterized
by the constant presence of an ORF encoding
a protein with a DNA-binding domain of
the high-mobility group (HMG) type and is called
MAT1-2. The corresponding gene is called MAT1-
2-1. The number assigned to any particular gene
within an idiomorph corresponds to that of its homolog
in other fungi that have already characterized
MAT genes. If there is no apparent homolog,
the next sequential number is assigned. If a second
mating-type locus is discovered in the same fungus,
it would be MAT2.Thissystemisadheredtobythe
fungal community, with the exception of members
of the Neurospora crassa and P. anserina communities,
since designation of genes from these genera
was established before standard terminology
was proposed. To date, for self-incompatible Ascomycetes,
including the models Cochliobolus heterostrophus,
P. anserina,andN. crassa,onlyasingle