Growth, Differentiation and Sexuality

observed when the mammal LE component SCP3
is expressed in somatic cells (Yuan et al. 1998;
Zickler and Kleckner 1999). In Sordaria humana,
LEs are tubular and form numerous bulges that are
variable in size and location along chromosomes.
Bulgesaremorefrequentatthejunctionsbetween
synapsed and unsynapsed regions and are gone at
pachytene, but their role, if any, remains unknown
(Zickler and Sage 1981).
Five fungi (S. pombe, A. nidulans, Schizosaccharomyces
octosporus, Schizosaccharomyces
japanicus and Ustilago maydis) are among the
very few exceptions (with Drosophila male) that
do not form SCs (Olson et al. 1978; Fletcher
1981; Egel-Mitani et al. 1982; Bähler et al. 1993;
Kohli and Bähler 1994). Detailed analyzes by EM,
combined with FISH in time-course experiments
of synchronized cells, showed clearly that no
classical SC is formed in S. pombe (Bähler et al.
1993; Scherthan et al. 1994; Molnar et al. 2003).
Fission yeast, however, forms linear structures
that are likely functionally analogous to AEs
(review in Molnar et al. 2003). Differences with
standard continuous AEs/LEs are nevertheless
observed: although their total length per nucleus
varies with ploidy, both length and number are
highly variable from one nucleus to another.
FISH with telomeric, centromeric and interstitial
region probes reveal that homologues
occupy distinct territories, with maximum pairing
of probes during the stage when the linear
elements are longitudinally parallel in the horsetail
elongated nucleus (Scherthan et al. 1994).
Those examples clearly illustrate the importance
of studying meiosis in a variety of different
organisms.
C. Synaptonemal Complex
and the Recombination Process
Studies of synchronous budding yeast meiocytes
have allowed to draw a parallel between SC formation
and the recombination steps (Padmore et al.
1991; Hunter and Kleckner 2001). DSBs occur at
early leptotene – thus, before pairing and SC. The
next step, namely, the appearance of single-end
invasion intermediates (see Sect. III.), is concomitant
with the initiation of the central element and
is completed by the end of SC formation. Double
Holliday junctions formation occurs during
pachytene, and the resolution of DHJs to mature
crossovers occurs at the end of pachytene (Börner
Fungal Meiosis 427
et al. 2004 and references therein). Complete SC
along each pair of homologues likely both promotes
the maturation of recombination intermediates
and stabilizes homologous associations throughout
the period when crossovers are being formed.
Accordingly, budding yeast C. cinereus and S.
macrospora mutants that are impaired in recombination
steps show various SC formation defects.
For example, neither AEs nor SCs are complete
in a C. cinereus mutant defective for the nuclease
Mre11p involved in several steps of DNA repair
and homologous recombination (Gerecke and
Zolan 2000). No SCs are formed in the absence of
DSBs (Celerin et al. 2000; Storlazzi et al. 2003). Also,
SC appears to be nucleated at sites of recombination
interactions that eventually mature into crossovers.
In wild-type S. macrospora, numbers of interstitial
SC initiation sites correspond well to the number
of COs, chiasmata and recombination nodules, and
are decreased in two mutants with decreased COs
(Zickler et al. 1992). As CO interference precedes
initiation of SC, it is possible that the spreading of
interference may license SC polymerization.
D. Recombination Nodules, the Substructures
of the Synaptonemal Complex that Correlate
with Crossover and Noncrossover Exchanges
Recombination nodules are electron-dense structures
associated with forming or completed SC in
all investigated organisms (Fig. 20.5A). They were
termed recombination nodules (RNs) by Carpenter
(1975), on the basis of their correlation with
COs in Drosophila oocytes. Further investigations
identified two types of RNs, early nodules and
late nodules, which can be distinguished from
one another on the basis of stage of appearance,
frequency, shape, size, and staining properties.
Early nodules (ENs) are spherical or ellipsoidal
structures associated with axial elements and
the forming SCs from late leptotene until early
pachytene (Fig. 20.5C). Late nodules (LNs) are
denser, less variable in shape, and appear during
pachytene (Fig. 20.5A) and in lower numbers; they
sometimes persist through diplotene (review in
von Wettstein et al. 1984; Carpenter 1987; Zickler
and Kleckner 1999). Distributions of both types
of nodules were extensively investigated in four
mycelial fungi: N. crassa (Gillies 1972, 1979; Bojko
1988, 1989), S. macrospora (Zickler 1977; Zickler
et al. 1992), S. commune (Carmi et al. 1978) and C.
cinereus (Holm et al. 1981).