Introduction to Fungi, Third Edition

SORDARIALES
325
Mating type factors in Podospora anserina
As explained above, wild-type P. anserina is
pseudohomothallic, each binucleate ascospore
normally containing both distinct mating type
idiomorphs matþ and mat , whilst uninucleate
ascospores contain either matþ or mat . The
genes which control mating type specificity have
been labelled FPR1 and FPR2 (fertilization plus
and minus regulators). The molecular structure
of both genes has been determined (Debuchy &
Coppin, 1992; Coppin et al., 1997). The matþ locus
contains 3800 + 200 base pairs (bp) with the FPR1
gene within it, whilst the mat locus is larger
and contains 4700 + 200 bp, enclosing FPR2 and
three regulatory genes SMR2, SMR2 and FMR1.
There is a close similarity between the structure
of the mating type genes in P. anserina and
Neurospora crassa.
Studies on incompatibility in
Podospora anserina
Incompatibility is usually defined as the genetic
control of mating competence, but this concept
extends beyond the sexual phase to the vegetative
phase. Two different systems provide genetic
control, namely homogenic and heterogenic
incompatibility (Esser & Blaich, 1994).
Homogenic incompatibility is caused by the
sexual incompatibility of nuclei carrying identical
idiomorphs, and it thus favours outbreeding.
In contrast, in heterogenic incompatibility (also
known as heterokaryon, somatic or vegetative
incompatibility), the coexistence of nuclei in a
common cytoplasm is inhibited by the genetic
difference in one or more genes. Thus heterogenic
incompatibility restricts outbreeding. It
may also play a role in speciation. Another
consequence is the reduced risk of transmission,
following hyphal anastomosis, of the spread of
infectious cytoplasmic elements such as mycoviruses
or transposons (e.g. in Cryphonectria,
p. 375).
Heterogenic incompatibility was discovered
when attempts were made to cross strains of
P. anserina of different geographic origin. When
two mycelia grow towards each other and intermingle,
hyphal anastomosis occurs. Nuclear
exchange is not inhibited but is followed by an
antagonistic reaction, sometimes accompanied
by death of the fusing cells and by profuse
branching of adjacent cells. This barrage
phenomenon, observed as a white or colourless
zone between two mycelia, occurs irrespective of
mating type. Perithecium formation may occur
in inter-racial crosses of differing mating types
but the number of perithecia is much reduced. In
some pairings, one or both of the reciprocal
crosses between the different mating partners
are unsuccessful.
Nine unlinked loci are now known to be
involved in the control of heterokaryon incompatibility,
and these are termed het loci. A het
locus can be defined as a locus in which
heteroallelism cannot be tolerated in a heterokaryon.
The nine het loci comprise five allelic
systems (in which different alleles of the same
gene provoke vegetative incompatibility) and
three non-allelic systems (involving the interactions
of two specific alleles from different loci).
One locus (het-V) is simultaneously involved in an
allelic and a non-allelic interaction (Saupe, 2000).
The molecular structures of the genes at some of
these loci have been characterized. The different
genes encode very different products in the form
of HET polypeptides. Complexes between the
different HET polypeptides may function in the
recognition process between self and non-self
and may act as the trigger to mediate biochemical
events causing vegetative incompatibility.
Alternatively, HET heterocomplexes may function
to poison the cell and thus may directly
mediate growth inhibition and death (Glass et al.,
2000).
The het-s/het-S allelic system of P. anserina is of
particular interest. Both alleles encode polypeptides
differing only in a few amino acids,
and incompatibility results if a heterodimer is
formed. However, whilst the HET-S protein is
immediately active, HET-s is initially translated
as an inactive form, HET-s , which is present in
a soluble form in the cytoplasm. Biologically
active HET-s molecules may arise by a rare
spontaneous rearrangement to another tertiary
conformation, and HET-s molecules have the
ability to convert HET-s to their own state by
catalysing this conformational change. This
interaction may lead to the formation of aggregates.
Once initiated, the conversion of HET-s to