Growth, Differentiation and Sexuality

etween dispersed DNA repeats. In N. crassa,
duplications of chromosome segments of the order
of megabases are subject to RIP, implying that RIP
mayalsoplayanevolutionaryrolein“weeding
out” spontaneous chromosome rearrangements
(Perkins et al. 1997).
C. A “Checking” Mechanism that Operates
After Karyogamy
In addition to prekaryogamy mechanisms, N.
crassa has developed ways of controlling the
integrity of its genome during meiotic prophase
(Fig. 20.1E). When during pairing an asymmetrical
situation is detected, such as a deletion or an
insertion in one of the homologous chromosomes,
the presence of this unpaired sequence of DNA
activates a process called meiotic silencing by
unpaired DNA (MSUD). This, in turn, silences all
paired and unpaired copies of the sequence/gene in
the genome; silencing holds also for even numbers
of copies, but inserted at different positions along
the chromosomes (Shiu et al. 2001; Shiu and
Metzenberg 2002). The mechanism appears to be
post-transcriptional, based on the observation that
mutations in the suppressor of ascus dominance-1
(sad-1), coding for an RNA-dependent RNA polymerase
(RdRP), suppress the ascus dominance
exerted by unpaired copies of reporter genes
(Shiu et al. 2001; Shiu and Metzenberg 2002).
The post-transcriptional gene-silencing nature of
meiotic silencing may involve at least two more
genes in the pathway: the suppressor of meiotic
silencing sms-2 coding for an Argonaute-like
protein, and the suppressor of meiotic silencing
sms-3 coding for a Dicer-like protein (Kutil et al.
2003; Lee et al. 2003). Production of the silencing
signal (e.g., small interfering RNAs) does not
affect the expression of adjacent genes, as the aberrant
transcript seems restricted to the unpaired
sequence (Kutil et al. 2003). Also, both size and
degree of homology directly correlate with the
silencing efficiency but the presence of promoter
elements in the unpaired DNA is not required to
induce silencing (Lee et al. 2004).
MSUD potentially plays a role in development:
if the unpaired gene encodes a protein required
for the completion of meiosis and/or sporulation,
MSUD will arrest development at that particular
stage when the product of the gene is required
(Shiu et al. 2001). Interestingly, meiotic silencing
is also important from an evolutionary point of
Fungal Meiosis 419
view. Shiu et al. (2001) showed that sad-1 mutants
could suppress the interbreeding inability between
N. crassa and three related species: N. sitophila, N.
tetrasperma, and N. intermedia. A significant increase
in fertility was observed in crosses between
the tested species, suggesting that an important
barrier between two closely related species is, in
fact, the existence of numerous small rearrangements
in the genome.
III. Meiotic Recombination
Theuniquefeatureofmeiosistogenerateoffspring
that are genetically different from their parents
is important to maintain genetic diversity, but is
actually an indirect consequence of the fact that
homologous recombination is mechanically indispensable
for meiosis. During mitosis, the two sister
chromatids separate from one another by releasing
cohesion along their entire length. During meiosis,
by contrast, homologous chromosomes separate
at division I: they must, therefore, be connected
to one another to avoid that the pulling
force of the spindle will segregate them randomly.
This is achieved by programmed high levels of
DNA double-strand breaks (DSBs), which give rise
to recombinant non-sister chromatids that ultimately
will join homologues together via chiasmata.
The need for inter-homologue links is illustrated
by phenotypes of recombination-deficient
mutants: non-recombinant chromosomes are single
when they are captured by microtubules, and
thus undergo missegregation leading to aneuploid,
sterile gametes (e.g., review in Lichten 2001; Bishop
and Zickler 2004).
A. Initiation
Recombination occurs during the long prophase
of the first meiotic division, at a much higher frequency
than during vegetative/somatic growth (review
in Pâques and Haber 1999). It is initiated by
the programmed formation of a large number of
DSBs – thus, in a risky way. Repair of these DSBs
differs from that of DSBs occurring in mitotic cells
because meiotic recombination uses a homologue
non-sister chromatid for repair, whereas mitotic
recombinational repair uses the sister chromatid
or repeated sequences on the same chromosome
(e.g., review in Käfer 1977; Pâques and Haber 1999;
van Heemst and Heyting 2000).