Growth, Differentiation and Sexuality

424 D. Zickler
defects and pairing defects: S. macrospora and
budding yeast null and partial loss of function
alleles of spo11 and ski8 mutants show different
levels of pairing and synapsis (Tesse et al. 2003;
Henderson and Keeney 2004). Also, modulated
doses of irradiation of spo11 and ski8 mutants of S.
macrospora showed that the three steps of pairing
(described above) are distinguishable by their
differential dependence on DSBs (Tesse et al. 2003).
Therefore, recombination need not be initiated
by Spo11p to promote pairing: DSBs induced by
ionizing radiation can rescue both recombination
and pairing defects in spo11 mutants (Celerin
et al. 2000; Storlazzi et al. 2003). However, this
coupling is not universal. It is required in fungi,
plants and mammals, but DSB formation is not
required for efficient pairing in C. elegans and in D.
melanogaster female. Rather, pairing and synapsis
are likely established through chromosomal
pairing centers, even in mutant situations where
SPO11/DSBs are absent (review in McKee 2004).
C. The Bouquet Stage: a Specific Configuration
of Meiosis
From end of leptotene through early pachytene,
chromosome ends are attached to the inner sur-
Fig. 20.4. A–I Pairing, bouquet formation
and synapsis in wild-type S. macrospora.
Chromosome axes are visualized by Spo76-
GFP (see Sect. VI.), and nuclei are staged
by progression of ascus growth. A Early
leptotene. B,C Mid-leptotene nuclei showing
progressive recognition of homologues,
initially at telomere regions in B (arrows)
and then along most chromosomes in C
(arrow). D Late leptotene: all homologues
are aligned (arrows). E Early zygotene, with
partially unsynapsed regions (arrow) and
synapsed telomere regions that are grouped
into a loose bouquet formation (arrowhead).
F,G Zygotene nuclei (arrow points
to a non-synapsed region) with a loose
(F) andatight(G) bouquet conformation
(arrowheads). H Early pachytene, with
seven synapsed bivalents and releasing
bouquet (arrowheads point to the bouquet
area). I Telomere clustering is completely
released at mid-pachytene. Bar = 5 μm
face of the nuclear envelope and are grouped together
within a limited area, generally facing the
microtubule-organizing center (MTOC) in organisms
with a clearly defined MTOC. This telomere
polarization takes the form of a “bouquet”, so
named for its resemblance to the stems from a bouquet
of cut flowers (Fig. 20.4E–G). In most species,
the bouquet formation is rapid and transient (e.g.,
nuclei with a bouquet never exceed 5%–10% of
prophase nuclei in S. macrospora; Storlazzi et al.
2003). The bouquet formation is unique to meiosis
and highly conserved (review in Bass et al. 1997;
Zickler and Kleckner 1998; Scherthan 2001). Fission
yeast provides the most extreme example of
the bouquet arrangement: during telomere clustering,
the nucleus moves back and forth between
the cell poles for the entirety of meiotic prophase,
with the spindle pole body (SPB) located at the
leading edge of the moving nucleus (called “the
horsetail stage”). Telomere clustering and the subsequent
nuclear oscillation likely facilitate alignment
and pairing of homologues (Chikashige et al.
1994; Kohli and Bähler 1994). When telomere clustering
is impaired by depleting the dynein heavy
chainDhc1orintheabsenceofproteincomponentsofeitherthetelomere(Taz1orRap1)orthe
SPB (Kms1), both homologous pairing and recombination
are reduced (review in Ding et al. 2004).