The Synaptonemal Complex in Genetic Segregation

The assembly, rearrangement and disassembly of the synaptonemal complex during the stages from leptotene to anaphase I of meiosis are described. The assembly of the lateral component of the synaptonemal complex between the sister chromatids of the leptotene chromosome causes the undivided appearance of the leptotene chromosome in the light microscope. During zygotene the lateral component mediates the assembly of the synaptonemal complex between homologous segments of homologous as well as homoeologous chromosomes. Chromosome and bivalent interlockings are a regular feature during chromosome pairing at zygotene. They are resolved by breakage and precise repair of the chromosomes. The phase of synaptonemal complex formation between homologous chromosome segments is succeeded by a phase of rearrangements. In this phase, chromosome homology is no longer required for synaptonemal complex assembly. This reduces the number of univalent chromosome sections. Rearrangement of chromosome pairing with the synaptonemal complex is not expected to take place at positions where a crossover has occurred. Disassembly of the synaptonemal complex at diplotene is initiated at specific regions along the bivalent arms. Chiasmata are formed from retained pieces of the synaptonemal complex in which a recombination nodule is or had been present. A limit to the number of available recombination nodules and a mechanism which ensures that at least one nodule is placed in each bivalent arm are sources for the positive interference observed in linkage studies employing three-point tests. The time of crossing over relative to the zygotene assembly and pachytene rearrangement phases of the synaptonemal complex determines if multivalents or exclusively bivalents are found at metaphase I in amphidiploids and autotetraploids. The synaptonemal complex provides both the structural basis for the regular disjunction of chromosomes at meiosis and gene recombination between homologous or homoeologous chromosome arms.