Introduction to Fungi, Third Edition

330 HYMENOASCOMYCETES: PYRENOMYCETES
strain can be transferred to trichogynes of the
opposite strain by flooding with sterile water. In
nature it is possible that mites or insects are
involved in transfer. There is evidence that a
microconidium produces a pheromone which
induces directional growth (positive chemotropism)
of a trichogyne of opposite mating type
towards it before plasmogamy occurs (Bistis,
1983). The transfer of macroconidia or hyphae
of the opposite strain to a trichogyne can also
effect fertilization. Fusion between the trichogyne
and the fertilizing cell is followed by
the migration of one or more nuclei from the
fertilizing cell down the trichogyne into the
ascogonium. The development of ripe perithecia
occurs within 7 10 days and follows the typical
general ascomycete pattern. This has been
described by Nelson and Backus (1968) in two
homothallic species. Much is now known of the
genetic control of sexual development in
Neurospora and several genes have been identified
which control steps in the process. The cytological
details of ascus development have also
been worked out. After the eight-nucleate
stage in the developing ascus of N. crassa, several
mitoses ensue so that a fully developed
ascospore may contain as many as 32 nuclei
(Raju, 1992a).
Mating type genes in Neurospora
In N. crassa, heterokaryons are not normally
formed between mycelia of opposite mating
types, and this implies that plasmogamy usually
occurs only between a trichogyne of one strain
and a fertilizing agent (e.g. a microconidium,
macroconidium or hypha) of the opposite strain.
This condition is termed restricted heterokaryosis.
Even within one mating type, the ability
to form heterokaryons is under genetic control,
i.e. there is heterokaryon incompatibility as in
Podospora anserina and heterokaryons generally
develop only if the het genes which control
compatibility are homoallelic (see p. 325).
Several het genes have been identified (Mylyk,
1976; Perkins, 1992), and Micali and Smith (2003)
have provided evidence of a yet more complex
regulation of heterokaryon incompatibility in
the shape of suppressor genes which modify the
effect of het and mating type genes. When the
mycelia of unlike genotype anastomose, cytoplasmic
incompatibility results in vacuolation
and disorganization of cell contents in the
region of the anastomosis. Similar cytoplasmic
reactions are visible when anastomosis occurs
between the hyphae of wild-type strains differing
in mating types. In contrast to N. crassa, heterokaryons
are readily formed between different
mating type strains of N. tetrasperma, which thus
exhibits unrestricted heterokaryosis.
The molecular structure of the A and a
idiomorphs has been elucidated in N. crassa.
They are strikingly dissimilar (Glass et al., 1988).
The A idiomorph is composed of a region of
5301 bp bearing little similarity to the a idiomorph
comprising 3235 bp (Glass et al., 1990;
Staben & Yanofsky, 1990). The A idiomorph of
N. crassa is also involved in heterokaryon incompatibility.
There are similarities in the structure
and functions of the mating type idiomorphs
between N. crassa, Podospora anserina and the
yeasts Saccharomyces cerevisiae and
Schizosaccharomyces pombe, but there are also
differences (p. 266; Glass & Lorimer, 1991). The
idiomorphs of N. crassa are larger than those of
the yeasts. Mating type idiomorphs are present
in several homothallic species of Neurospora
which hybridize with the A DNA probe of N.
crassa (Coppin et al., 1997). This and other lines of
evidence suggest that the heterothallic condition
was primitive and that the homothallic condition
has probably arisen several times in the
course of evolution and may have a selective
advantage (Metzenberg & Glass, 1990).
The pseudohomothallic condition, represented
by N. tetrasperma, is of interest. This
species is functionally homothallic in that the
mycelium from a single heterokaryotic ascospore
containing nuclei of two distinct mating types
can develop perithecia directly. However, homokaryotic
mycelia can develop from uninucleate
ascospores and also from about 20% of macroconidia.
Such mycelia are capable of outcrossing.
Raju (1992b) has summarized,
Thus N. tetrasperma appears to have the best of
both worlds. On one hand the single-mating-type
homokaryotic cultures offer N. tetrasperma the
advantages of outbreeding. On the other hand the