Growth, Differentiation and Sexuality

Fig. 20.1. A–E Diagrammatic representation of ascus formation,
with indication of the sites for RIP/MIP and MSUD.
The stages cartooned have all been demonstrated cytologically
(e.g., Raju 1980; Zickler et al. 1995). The two nuclei
of opposite mating types are shown as white and red nuclei.
A Mating between polynucleated ascogonium (left)
and uninucleated conidium through the trichogyne. B After
fertilization, the haploid nuclei proliferate in the ascogonium.
This heterokaryotic ascogonim then forms binucleated
dikaryotic cells containing nuclei of opposite mating
type, in which will occur RIP or MIP. In homothallic
species, this results in genetically identical binucleate
daughter cells. The tip cell bends to form a hook-shaped
cell called a crozier. C Different steps of the crozier development,
from one-celled to a three-celled structure (see
text). D The two upper nuclei of the three-celled crozier
fuse, giving rise to the only diploid nucleus of the fungal life
cycle. The two lower nuclei divide again in the basal cell,
and give rise to a second crozier. E Astheuppercroziercell
elongates into an ascus, karyogamy is immediately followed
by meiosis. MSUD occurs during the early steps of meiotic
prophase
ascomycetes) prolonged dikaryotic phase between
fertilization and karyogamy. The mechanism by
which formation of ascomycetal dikaryotic cells is
regulated remains unknown (e.g., Hoffmann et al.
2001).
A. Karyogamy and Premeiotic Replication
The two nuclei issued from a unique nucleus
(homothallic species) or the two nuclei of opposite
mating type (heterothallic species) divide
synchronously several times before fusing (karyogamy)
and entering meiosis (Fig. 20.1B–E).
Karyogamy of most ascomycetes is preceded by the
formation of a hook-shaped crozier cell containing
two haploid nuclei (Fig. 20.1B). These undergo
a simultaneous mitosis, with spindles positioned
such that one daughter nucleus from each parent is
Fungal Meiosis 417
present in the crook portion of the cell (Fig. 20.1C).
Septa form on each side of the crook, resulting in
a basal and a lateral cell flanking the binucleate
ascus-mother cell (Fig. 20.1C). Karyogamy takes
place as the ascus-mother cell begins to elongate
(Fig. 20.1D), followed immediately by the long
prophase of the first meiotic division (Fig. 20.1E;
review in Read and Beckett 1996). In P. anserina,
elongation of this upper cell, and therefore karyogamy,
requires wild-type levels of peroxisomes
(Berteaux-Lecellier et al. 1995). Meiosis can be
induced in the absence of karyogamy: haploid
meiosis proceeds up to ascospore formation
in monokaryotic asci of P. anserina (Zickler
et al. 1995). Diploidy per se is also not required:
tetraploid nuclei issued from diploid crosses of A.
nidulans as well as the highly polyploid (over 8n)
nuclei formed after karyogamy in the cro1-1/she4
mutants of P. anserina go through both meiotic
divisions (Elliot 1960; Berteaux-Lecellier et al.
1995). The rosette of over 100 asci formed in
a wild-type fruiting body results usually from
the establishment of one dikaryon made by
a single “male” and a single “female” nucleus (e.g.,
Johnson 1976), but exceptions are also observed
(e.g., Hoffmann et al. 2001).
Premeiotic replication is closely analogous to
its mitotic counterpart. In budding yeast, replication
utilizes the same specific origins that fire, with
the same relative frequencies and the same general
order, in both meiosis and mitosis (Collins and
Newlon 1994). However, a common feature of meiotic
S-phase is its extended duration, compared
to mitotic S-phases in the same organism (e.g.,
Cha et al. 2000). The mechanism of this prolongation
remains unknown. Premeiotic replication
is also a critical step for the meiotic recombination
process. DNA double-strand breaks (DSBs),
which initiate meiotic recombination and premeiotic
replication, are tightly coupled, at least in budding
yeast: DSBs do not form when replication
is blocked, and delaying replication in a region
causes a corresponding delay in DSB formation in
this region (Borde et al. 2000). Despite the general
realization of the importance of premeiotic
S-phase, timing of S-phase remains questionable
in mycelial ascomycetes (e.g., Farman 2002). Based
on microspectrophotometric quantitation of DNA
(a technique potentially subject to artifacts), replicationwasfoundtoprecedekaryogamyinNeottiella
rutilans, N. crassa and S. fimicola (Rossen
and Westergaard 1966; Iyengar et al. 1976; Bell and
Therrien 1977). By contrast, timing of S-phase is