Chromosome synapsis and recombination in yeast meiosis

Sammanfattning: Meiosis is a cell division process that produces haploid gametes from diploid cells. Several important meiotic events take place during prophase of meiosis I, most important being homologous chromosome pairing, meiotic recombination and formation of the synaptonemal complex (SC). These processes assure proper segregation of the homologous chromosomes into the haploid germ cells. Improper segregation of the homologos can cause chromosomal abnormality (aneuploidy), which causes various human disorders, notably mental retardation and pregnancy loss.This thesis focuses on the relationship between recombination and the formation of SCs, aggregates of SC-related materials (polycomplexes) and recombination enzymes during meiosis. We have investigated SC formation in the absence of recombination, nature of polycomplexes and recombination enzymes in relation to the SCs structures and recombination nodules (RNs) in yeast Saccharomyces cerevisiae.Studies on yeast mutants suggest that the formation of SCs can take place only in the presence of the initiation of meiotic recombination through the action of the Spo11 enzyme. We have investigated the spo11 mutant of yeast that lacks initiation of meiotic recombination and observed that a fraction of this mutant cells (1.0%) can form complete SCs with wild-type appearance. We have further analyzed a spo11 mutant strain that accumalates at pachytene (spo11?ndt80?), and found that the frequency of cells with mature SC formation was 10%. The SC structures were detected in both immuno-fluorescence and electron microscopy (EM). Other phenotypic criteria such as spore viability and homologous chromosome pairing measured by FISH with chromosome specific probes, agrees with several previous reports of the spo11 mutant. Our results suggest that although synapsis is strongly promoted by Spo11 induced DSBs in yeast, it can take place in the absence initiation of meiotic recombination by Spo11 enzyme.In different organisms, the SCs are found to aggregate as stacks to form polycomplexes (PCs) that commonly occur before or after SC formation. It has generally been believed that the PCs are not attached to the chromosomes. We have investigated the ndt80 mutant of yeast that arrest at pachytene and found that although the SCs in spread chromosome preparations appear as wild type SCs, they appear as PCs in the intact nuclei in EM. In fluorescence in situ hybridization (FISH) with chromosome specific probes, we have shown that the homologous chromosomes in this mutant undergo high level of pairing and are attached to the SCs. In immuno-electron microscopy, two independent anti-DNA antibodies preferentially labeled the lateral element of the PCs. Our data suggests that the SCs in these polycomplexes can be involved in binding of chromosomes and are functional in pairing. The recombination enzymes, which are involved in the meiotic recombination process are believed to be components of recombination nodules. In some organisms, both early and late recombination enzymes have been shown to be located on RNs, but not in yeast before. We have studied immuno-localization and co-localization of Msh4, Msh5 and Sgs1 recombination enzymes on RNs in pachytene yeast nuclei with a newly developed EM method and found their co-localization on RNs along SCs. Msh4 and Msh5 are found to be located at the edges of RNs on the SCs. We have further analyzed the temporal appearance/disappearance and co-localization pattern of early and late meiotic recombination enzymes in relation to the SC development by immuno-fluorescence. We have been able to detect co-localization between the early meiosis specific enzyme Dmc1 and the late crossing-over related enzymes Msh5 as well as Sgs1. Our data suggest that Msh5 and Sgs1 are recruited to some sites of Dmc1, and thus erases the gap between early and late RNs.