IntroductionIn the life cycle of sexually reproducing organisms meiosis halves the chromosome number in the germ line cells and produces haploid gametes. This halving is achieved by two consecutive divisions following a single round of DNA replication. The second (equational) division resembles mitotis: sister chromatids segregate into daughter nuclei. However, the first (reductional) division has unique features. During the first meiotic division homologous chromosomes pair, undergo high levels of recombination, and the resulting chiasma formation assists their segregation into daughter nuclei. In most organisms, pairing of homologous chromosomes in meiotic prophase is accompanied by the formation of synaptonemal complexes (SC). The synaptonemal complex is an evolutionary well-conserved, strictly meiosis specific, proteinaceous structure. In early prophase, after DNA replication axial elements (AE) start to connect the sister chromatids. By the pachytene stage of meiotic prophase, chromosome pairing and synaptonemal complex development culminate in the formation of a tripartite structure: the axial elements (now called lateral elements, LE) are connected by a central component (for reviews, see Zickler and Kleckner, 1999;Roeder, 1997;Kleckner, 1996).The fission yeast Schizosaccharomyces pombe is a haploid, unicellular eukaryote. Naturally, S. pombe cells undergo meiosis directly after mating of two cells of opposite matingtype (zygotic meiosis). However, diploid cells heterozygous for mating-type can be maintained, and synchronous meiosis can be induced by shifting the culture to nitrogen-free medium (azygotic meiosis) (Egel, 1973;Egel and Egel-Mitani, 1974). Meiosis in fission yeast has unusual features. In prophase, the meiotic nucleus oscillates between the cell poles (Chikashige et al., 1994). These movements confer an elongated shape to the nucleus [horse-tail nucleus (Robinow, 1977)], and are led by the SPB and the attached telomere cluster (Chikashige et al., 1994;Chikashige et al., 1997). Thus, the bouquet structure of chromosomes bundled at the telomeres is maintained during the whole meiotic prophase in fission yeast. Homologous chromosome pairing and recombination occur during horse-tail movements. Mutants impaired in telomere clustering or nuclear movement show decreased homologous pairing and recombination, indicating the importance of these events in homolog juxtaposition (Shimanuki et al., 1997;Cooper et al., 1998;Nimmo et al., 1998;Yamamoto et al., 1999). Fission yeast is highly proficient in meiotic recombination but shows no crossover interference (Munz, 1994).It has been long known that fission yeast does not form synaptonemal complexes. Instead, filamentous structures (linear elements) appear in meiotic prophase. They resemble the axial cores of other eukaryotes (Olson et al., 1978;Hirata and Tanaka, 1982). The adaptation of the nuclear spreading technique to fission yeast made possible a detailed analysis of linear element formation in meiotic time-course experiments (Bähler et al., 1993). ...

To determine whether recombination and/or sister-chromatid cohesion affect the timing of meiotic prophase events, the horsetail stage and S phase were analyzed in Schizosaccharomyces pombe strains carrying mutations in the cohesin genes rec8 or rec11, the linear element gene rec10, the pairing gene meu13, the double-strand-break formation genes rec6, rec7, rec12, rec14, rec15, and mde2, and the recombination gene dmc1. The double-mutant strains rec8 rec11 and rec8 rec12 were also assayed. Most of the single and both double mutants showed advancement of bulk DNA synthesis, start of nuclear movement (horsetail stage), and meiotic divisions by up to 2 hr. Only mde2 and dmc1 deletion strains showed wild-type timing. Contrasting behavior was observed for rec8 deletions (delayed by 1 hr) compared to a rec8 point mutation (advanced by 1 hr). An hypothesis for the role of cohesin and recombination proteins in the control of the G 1 -to-S transition is proposed. Finally, differences between azygotic meiosis and two other types of fission yeast meiosis (zygotic and pat1-114 meiosis) are discussed with respect to possible control steps in meiotic G 1 .

rec7 is involved in intra-and intergenic meiotic recombination in all tested regions of the genome of the fission yeast Schizosaccharomyces pombe. Segregational analysis in a rec7 gene disruption mutant revealed frequent occurrence of two-spored asci. Spores giving rise to diploid colonies were shown to derive from skipping of the second meiotic division. Nondisjunction of homologous chromosomes at the first meiotic division was also frequent. The cytological structures and processes, such as formation of linear elements, pairing of homologous chromosomes, and clustering of telomeres and centromeres, are regular in the mutant. Northern blot experiments revealed meiosis-specific expression of rec7. Screening of a meiotic cDNA library also identified transcripts from the opposite strand in the rec7 region. A Rec7-GFP fusion protein was localized in the nucleus of whole cells before karyogamy, during prophase, and after meiosis I. On spreads of prophase nuclei approximately 50 foci of Rec7-GFP were counted. Some of the observed phenotypes of the disruption mutant and the N-terminal sequence homology suggest that Rec7p is a functional homolog of Rec114p of Saccharomyces cerevisiae. The observed phenotypes of the disruption and the appearance of Rec7-GFP in mating haploid cells and after meiosis I are consistent with Rec7p functions before, during, and after meiotic prophase.