Introduction to Fungi, Third Edition

SORDARIALES
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degenerates. Cultures derived from single ascospores
form perithecia readily. Occasionally,
however, smaller uninucleate ascospores may
occur in some asci and when such spores are
germinated, the resulting mycelium does not
fruit. Instead, perithecia only develop when
certain strains derived from uninucleate ascospore
cultures are paired together. On each such
strain, spermatia and ascogonia bearing trichogynes
are formed, but these are self-incompatible;
perithecia only develop if trichogynes of
one strain are spermatized by spermatia of
a genetically distinct strain. Thus, although the
behaviour of the large ascospores suggests that
P. anserina is homothallic, it is clear that the
underlying mechanism controlling perithecial
development is a heterothallic one of the usual
bipolar type (i.e. with (þ) and ( ) strains). Most
of the large ascospores (about 97%) contain
nuclei of the two distinct mating types (Esser,
1974).
The fact that such a high proportion of the
binucleate ascospores in four-spored asci contain
nuclei of two distinct mating types implies some
regulated process of nuclear movement and
arrangement. The normal sequence of nuclear
divisions occurs during ascus development,
including the two nuclear divisions of meiosis
and a post-meiotic mitosis (PMM). The plane of
the two meiotic nuclear divisions lies parallel to
the long axis of the developing ascus, but the
spindles formed during PMM lie at a right angle
to it (Fig. 12.5). Delimitation of the ascospores
(closure) caused by invagination of the ascosporedelimiting
membrane is associated with a ‘cage’
of microfilaments surrounding each spore
initial. At the same time a ‘rope’ of microfilaments
running along the whole length of the
ascus develops, and the cage of each ascospore
initial becomes attached to it (Figs. 12.5a,b).
Pairing between nuclei of differing mating
types occurs during the cleavage of the ascospores
and this appears to be mediated by astral
microtubules radiating from their closely associated
spindle pole bodies (SPBs) and pulling the
two nuclei together (Thompson-Coffe & Zickler,
1994). This pairing between genetically different
nuclei during ascospore formation is similar
to the recognition mechanism leading to
Fig12.5 Nuclear alignment in the pseudohomothallic
four-spored fungi Podospora anserina and Neurospora
tetrasperma. Fine continuous lines represent the spore cell
membrane (omitted in c for clarity); dotted lines represent
microfibrils; thicker continuous lines represent microtubules.
Spindle pole bodies (SPBs) are shown as black bars and nuclei
as open circles or ovals. (a) Following post-meiotic mitosis
(PMM) the eight nuclei are seen in pairs linked to each other
by microtubules which radiate from the SPBs. A rope of
actin myosin is forming along the centre line of the ascus.
(b) The nuclei become rearranged and move to a staggered
formation along the microfibrillar rope as microfibrillar cages
form and microfibrils extend upwards towards the SPBs.The
spore membranes begin to invaginate. (c) Nuclei are re-aligned
in a row, presumably by the actin myosin rope and cage
assembly. Redrawn fromThompson-Coffe and Zickler (1994),
with permission from Elsevier.
karyogamy in the crozier prior to meiosis.
Where the SPBs are not closely associated,
uninucleate spores develop.
It is interesting to compare nuclear behaviour
within the asci of the two pseudohomothallic
four-spored ascomycetes Podospora anserina and
Neurospora tetrasperma (see Fig. 12.6). In N. tetrasperma,
the mating type idiomorphs A and a lie
close to the centromere so that A and a almost
invariably go with the centromeres to opposite
poles of the first meiotic division spindle (M I ),
i.e. there is first-division segregation of the
alleles for mating type. The second division
spindles (M II ) then overlap. The third nuclear