Abstract
The intimate synapsis of homologous chromosome pairs (homologs) by synaptonemal complexes (SCs) is an essential feature of meiosis. In many organisms, synapsis and homologous recombination are interdependent: recombination promotes SC formation and SCs are required for crossing-over. Moreover, several studies indicate that initiation of SC assembly occurs at sites where crossovers will subsequently form. However, recent analyses in budding yeast and fruit fly imply a special role for centromeres in the initiation of SC formation. In addition, in budding yeast, persistent SC–dependent centromere-association facilitates the disjunction of chromosomes that have failed to become connected by crossovers. Here, we examine the interplay between SCs, recombination, and centromeres in a mammal. In mouse spermatocytes, centromeres do not serve as SC initiation sites and are invariably the last regions to synapse. However, centromeres are refractory to de-synapsis during diplonema and remain associated by short SC fragments. Since SC–dependent centromere association is lost before diakinesis, a direct role in homolog segregation seems unlikely. However, post–SC disassembly, we find evidence of inter-centromeric connections that could play a more direct role in promoting homolog biorientation and disjunction. A second class of persistent SC fragments is shown to be crossover-dependent. Super-resolution structured-illumination microscopy (SIM) reveals that these structures initially connect separate homolog axes and progressively diminish as chiasmata form. Thus, DNA crossing-over (which occurs during pachynema) and axis remodeling appear to be temporally distinct aspects of chiasma formation. SIM analysis of the synapsis and crossover-defective mutant Sycp1−/− implies that SCs prevent unregulated fusion of homolog axes. We propose that SC fragments retained during diplonema stabilize nascent bivalents and help orchestrate local chromosome reorganization that promotes centromere and chiasma function.
Highlights
The formation of gametes typically involves halving of the cellular chromosome complement from diploid to haploid
synaptonemal complexes (SCs) is known to be important for crossover recombination, details of its function remain enigmatic
We analyze mouse spermatocytes to investigate the interplay between SC, recombination, and centromeres
Summary
The formation of gametes typically involves halving of the cellular chromosome complement from diploid to haploid. This is achieved via two consecutive rounds of chromosome segregation during the process of meiosis [1]. SCs are proteinaceous structures with a zipper-like morphology [4,5,6,7,8]. The tripartite SC structure comprises two lateral elements, inferred to be elaborations of cohesin-based homolog axes, and a central element consisting of transverse filaments that interconnect the two lateral elements [7,8,9,10]. SC components show tendencies for self-assembly into ordered arrays and SC formation is believed to occur via polymerization from specific nucleation sites where the homolog axes have been brought into close proximity [6,11]
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