UNUSUAL NUCLEAR-STRUCTURES IN MEIOTIC PROPHASE OF FISSION YEAST - A CYTOLOGICAL ANALYSIS

Earlier results from sectioned nuclei indicating that Schizosaccharomy ces pombe does not develop a classical tripartite synaptonemal complex (SC) during meiotic prophase are confirmed by spreading of whole nucl ei. The linear elements appearing during prophase I resemble the axial cores (SC precursors) of other organisms. The number of linear elemen ts in haploid, diploid, and tetraploid strains is always higher than t he chromosome number, implying that they are not formed continuously a long the chromosomes. Time course experiments reveal that the elements appear after DNA replication and form networks and bundles. Later the y separate and approximately 24 individual elements with a total lengt h of 34 mum are observed before degradation and meiotic divisions. Par allel staining of DNA reveals changes in nuclear shape during meiotic prophase. Strains with a mei4 mutation are blocked at a late prophase stage. In serial sections we additionally observed a constant arrangem ent of the spindle pole body, the nucleolus, and the presumptive centr omere cluster. Thus, S. pombe manages to recombine and segregate its c hromosomes without SC. This might correlate with the absence of crosso ver interference. We propose a mechanism for chromosome pairing with i nitial recognition of the homologs at the centromeres and suggest func tions of the linear elements in preparation of the chromosomes for mei osis I disjunction. With the spreading technique combined genetic, mol ecular, and cytological approaches become feasible in S. pombe. This p rovides an opportunity to study essential meiotic functions in the abs ence of SCs which may help to clarify the significance of the SC and i ts components for meiotic chromosome structure and function.