Homologous chromosome pairing in Schizosaccharomyces pombe.

Homologous chromosome pairing is a central feature of meiosis I, contributing to the correct segregation of chromosomes during meiosis. The fission yeast, Schizosaccharomyces pombe, has been widely used to study meiotic chromosome dynamics, partly because studies in this yeast are simplified due to the lack of post-pairing synaptic structures. Chromosome pairing in Sz. pombe occurs differentially throughout the genome. Telomeres cluster at the spindle pole body (SPB) at the onset of meiosis, imposing a spatial restriction on pairing events. Subsequently, centromeres dissociate from the SPB and pair in a recombination- and heterochromatin (Swi6)-independent fashion. Pairing of telomere distal regions occurs during meiotic prophase, concomitant with a dynamic association/dissociation of homologous regions, with interhomologue associations becoming increasingly stable. The stabilization of paired regions is enhanced by factors required for the initiation of meiotic recombination, suggesting that recombination stabilizes paired regions. However, substantial pairing is initiated in the absence of recombination; this is dependent upon another factor, the conserved Meu13 protein, demonstrating that recombination is not required for initial pairing interactions. During meiotic prophase Sz. pombe exhibits a pronounced dynein-dependent nuclear oscillation, which drives the pairing of centromeric and interstitial regions. Dynein is also required for the significant levels of achiasmate reductional segregation observed in Sz. pombe, possibly implicating the centromere-associated pairing with achiasmate homologue segregation. Whilst Sz. pombe does not form discernable synaptic structures continuously along the meiotic chromosomes, it does form proteinacious, meiosis-specific, linear structures (linear elements). However, the role, if any, of these structures in mediating homologue pairing is unknown.