Growth, Differentiation and Sexuality

420 D. Zickler
In a wide range of organisms including the
fungi budding and fission yeastsC. cinereus and S.
macrospora, DSBs are induced by the evolutionarily
conserved and meiosis-specific topoisomerase
II-related Spo11 protein (Celerin et al. 2000; Keeney
2001; Sharif et al. 2002; Storlazzi et al. 2003). Spo11p
is transiently covalently attached to the 5 ′ ends of
the DNA fragments and after removal, DSBs are
rapidly resected on their 5 ′ -strand termini to produce
3 ′ -single-stranded tails, which bind to a protein
complex that includes the RecA analogs Rad51
and Dmc1 (Keeney 2001). This strand transfer complex
promotes later recombination steps that occur
during zygotene and early pachytene (see below).
DSBs are formed in haploid meiosis of budding
yeast, and thus independently of homologous interactions
(de Massy et al. 1994).
All known spo11 mutants strongly reduce CO
and chiasma formation, but exhibit also interesting
differences in their pairing and progression phenotypes.
In the fungal, plant and mammal species
studied so far, spo11 mutants display severe pairing
defects (Celerin et al. 2000; Keeney 2001; Storlazzi
et al. 2003 and references therein). Irradiation of
budding yeast, C. cinereus and S. macrospora,mutant
meiocytes partially corrects these defects, consistent
with a requirement for break-induced events
to ensure both recombination and pairing (Celerin
et al. 2000; Storlazzi et al. 2003; Tesse et al. 2003).
By contrast, pairing is normal in spo11 mutants of
Drosophila melanogaster and Caenorhabditis elegans
(review in McKee 2004). The most striking result
to emerge from these studies concerns the role
of Spo11p in meiotic progression. Chromosomes
segregate randomly at both divisions in spo11 mutants
of plants, yeasts and S. macrospora, leading
to reduced fertility (Sharif et al. 2002; Storlazzi
et al. 2003). By contrast, in C. cinereus and mice,
spo11 mutants undergo programmed cell death after
prophase (Celerin et al. 2000; Romanienko and
Camerini-Otero 2000).
Spo11p does not act alone: in budding yeast,
there are at least nine other proteins required for
DSBs (Arora et al. 2004 and references therein) and
one of these, called Rec103/Ski8, is also found to
be a direct partner of Spo11p in S. macrospora
(Tesse et al. 2003). SKI8 was originally identified
in budding yeast on the basis of its superkiller (of
RNA viruses) mutant phenotype, and has likely
roles in RNA metabolism during the vegetative cycle
in yeasts and S. macrospora (Tesse et al. 2003;
Arora et al. 2004 and references therein). Interestingly,
Ski8p redistributes from cytoplasm to nu-
clei only at meiosis, and localizes to chromosomes
specifically at early meiotic prophase. The localizations
of Ski8p and Spo11p are mutually interdependent
(Tesse et al. 2003; Arora et al. 2004). Four
of the other proteins (Rec102, Rec104, Mer2 and
Mei4) have so far no obvious homologues in other
organisms, including fission yeast and N. crassa
(Borkovich et al. 2004). This may reflect a species
difference in some aspects of recombination initiation
or a lack of sequence conservation for proteinswithconservedfunctions.Thesameistrue
for the meiosis-specific RecA homologue Dmc1,
absent in N. crassa, but present in C. cinereus and
Pleurotus ostreatus (Nara et al. 2000; Mikosch et al.
2001; Borkovich et al. 2004). Also, in contrast to
yeasts and N. crassa, Ustilago maydis has an orthologue
(Brh2) of BRCA2, the tumor suppressor
essential for the error-free repair of DSBs. In plants
and Ustilago, BRCA2 is required for meiotic DSB
repair (Kojic et al. 2002).
B. From Initiation to Recombination Products
Some DSBs mature into inter-homologue crossingovers
(COs), also called reciprocal exchanges
(CRs), in which the arrangement of flanking
regions is modified because the recombination
intermediates resolve in such a way that each of
the two interacting chromatids is cleaved and
religated to its homologue (Fig. 20.2, right).
The remaining DSBs do also interact with their
homologous partner but the two molecules will be
resolved back without accompanying exchange of
their flanking regions (Fig. 20.2, left). This type of
recombination is called noncrossovers (NCOs) or
non-reciprocal recombinations (NCRs), and will
result in non-Mendelian segregation of the studied
marker, also defined as “gene conversion”. In
heterozygous crosses of eight-spored ascomycetes,
gene conversions are seen as two wild-type and
six mutant ascospores or six wild-type and two
mutant ascospores (Fig. 20.2, left), in contrast to
the 4:4 parental marker segregation issued from
crossovers (Fig. 20.2, right). Gene conversion
was first described in 1934 by H. Zickler in the
ascomycete Bombardia lunata, “rediscovered”in
budding yeast by Lindegren in 1953 and definitively
demonstrated by Mitchell in 1955 with the
use of linked markers on either side of the studied
locus. When 6:2 segregation was observed at the
pyridoxine locus of N. crassa, the linked markers
segregated with normal 4:4 ratios, indicating that